Exam 1

mid-ocean ridges

A 2-km-high submarine mountain belt that forms along a divergent oceanic plate boundary.

volcanic arcs

A curving chain of active volcanoes formed adjacent to a convergent plate boundary

trench

A deep, elongate trough bordering a volcanic arc; a trench defines the trace of a convergent plate boundary.

African plate, Arabian plate

A great example of the birth of an ocean today is the Red Sea in north east Africa. The narrow Red Sea only recently formed when this part of Africa started extending as shown by the red arrows moving away from each other. The result over the past few million years is the formation of the narrow Red Sea and two plates, the Arabian microplate and the rest of the African plate.

magnetic inclination

the angle between a magnetic needle free to pivot on a horizontal axis and a horizontal plane parallel to the Earth's surface

magnetic declination

the angle between the direction a compass needle points at a given location and the direction of true north

atolls of the Maldives

Atolls are circular reefs like the one shown here in the upper left that sometimes occur next to each other in a straight line. In the 1800's Charles Darwin, on his famous Voyage of the Beagle, explained that atolls start out as reefs that surround or fringe an active volcano. When the volcano becomes extinct, subsides, and is eventually submerged, the marine organisms continue to grow and build upward, first forming a barrier reef around the sinking volcano and then forming an atoll when the volcano is completely submerged.

hot spots

A location at the base of the lithosphere, at the top of a mantle plume, where temperatures can cause melting. Deep seated volcanic flames or plumes that form independent tectonic plates are known as hot spots. Hot spots are thought to be rising plumes of hot mantle rock that originate from near the core. The rising plume penetrates through the overriding plate, tattooing it with volcanoes. Let's take a look at two famous hot spots, one which penetrated through continental lithosphere and one through oceanic lithosphere.

fracture zone

A narrow band of vertical fractures in the ocean floor; fracture zones lie roughly at right angles to a mid-ocean ridge, and the actively slipping part of a fracture zone is a transform fault.

fracture zones

A narrow band of vertical fractures in the ocean floor; fracture zones lie roughly at right angles to a mid-ocean ridge, and the actively slipping part of a fracture zone is a transform fault.

divergent boundary

A plate boundary where two plates move away from each other. mid-ocean ridges

transform boundary

A plate boundary where two plates move past each other in opposite directions san Andreas fault

convergent boundary

A plate boundary where two plates move toward each other so that one plate sinks beneath the other only oceanic lithosphere can subduct

triple junction

A point where three lithosphere plate boundaries intersect

pyroclastic flow

A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud)[1] is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that moves away from a volcano about 100 km/h (62 mph) on average but is capable of reaching speeds up to 700 km/h (430 mph).[2] The gases can reach temperatures of about 1,000 °C (1,830 °F). Pyroclastic flows are a common and devastating result of certain explosive eruptions; they normally touch the ground and hurtle downhill, or spread laterally under gravity. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope.

wadati-benioff zone

A sloping band of seismicity defined by intermediate- and deep-focus earthquakes that occur in the downgoing slab of a convergent plate boundary.

accretionary prism

A wedge-shaped mass of sediment and rock scraped off the top of a downgoing plate and accreted onto the overriding plate at a convergent plate margin. Peru-Chile Trench

Birth and Death of Oceans; Hotspots - Lecture Summary Part 1

All OCEANIC lithosphere is eventually recycled back into the mantle. Remember: Density is Destiny. Oceans are born and then die (a cycle known as the Wilson cycle). Four stages of the Wilson Cycle 1) Stage 1: Birth of an Ocean through Continental rifting (Great Rift Valley of east Africa today). 2) Stage 2: Growth of an Ocean (Atlantic ocean today; with its passive margins). 3) Stage 3: Closing of an Ocean (Pacific ocean today) - subduction on both sides of the Pacific. 4) Stage 4. Death of an Ocean by Continent-continent collision (Himalaya Mountains today).

continental rifting in the western US

Another great example of continental rifting is presently occurring in the western United States, in what's known as the Basin and Range Province. Most of Nevada and the western parts of Utah consists of thin mountain ranges separated by basins - giving the appearance of 'marching caterpillars'. This very wide area of continental rifting has been undergoing extension for 30 to 50 million years and yet it still has not broken apart to form a new ocean.

Mid-Atlantic ridge in cross-section

Another way to visualize the ocean floor topography is with a cross-section, or a vertical slice through the ocean. The bottom image is a cross-section slice that's oriented about east-west and extends from the U.S. east coast (near Florida) on the left (or the west) through the Atlantic ocean and over to Africa on the right. This cross-section shows the peak of the ridge in the middle of the Atlantic Ocean with both sides of the ridge gradually dropping off in elevation to the deepest and flat parts of the ocean. These vast flat regions of oceans are called abyssal plains, or I like to say abysmal plains, and represent some of the flattest regions found anywhere on Earth. Near the continental margins of the Atlantic, the edges of the oceans start to rise up and form a slope that extends to the continental shelf, or the shallow drowned margin of continents.

Ocean Crust and Continental Crust are made of different Earth Materials

Before we go over Wegener's land-based evidence that continents have moved, let's discuss briefly what oceanic and continental crust is made of. It's probably no surprise to you that the rocks that make up the Earth's crust beneath the oceans and on the continents are different rock materials. While continents are made up of many different rocks of different ages, overall it's fair to say that on average, continents have the composition of granite, a fairly light, low density crystalline rock. On the other hand, oceanic crust is much more uniform, consisting primarily of a rock known as basalt, a heavier, higher density crystalline rock. This simple fact is why Earth has oceans and continents - the oceanic crust lies low because it is much more dense and sinks into the underlying mantle more than the less dense continental crust does

the theory of plate tectonics

By the 1960's and 1970's the mounting evidence for plate tectonics was overwhelming and completely overturned existing geologic concepts. The evolution of ideas leading up to the Plate Tectonic theory, one of the most significant scientific discoveries of the 20th century, is an incredible story of discovery and a prime example of the scientific method of generating and testing hypotheses.

the Wilson cycle: birth and death of oceans

Canadian Geophysicist J. Tuzo Wilson was the first to propose the birth and death of oceans, which is why we call the birth and demise of oceans, the Wilson Cycle. There are four stages to the Wilson cycle, all of which are represented today in different parts of our world. Stage 1, the birth of an ocean, occurs by the process of rifting or breaking up of a continental landmass or lithospheric plate as shown sequentially in this diagram from top to bottom. A continent starts to rift apart where it becomes thinner due to stretching during extension. With enough extension, the underlying mantle rises or swells up until it actually breaks through the continental lithosphere and sea-floor spreading starts to occur. At this time a new ocean has formed and two tectonic plates exist and are moving away from one another.

convergent boundaries

Convergent boundaries are places where two plates in contact are moving toward one another as shown by the arrows on this block diagram. Here one plate dives or sinks back into the mantle, a process called subduction. When oceanic and continental lithosphere converge, the down-going subducting plate, which is ultimately destroyed and disappears, is always the more dense oceanic lithosphere. And when two oceanic plates converge, such as is occurring today in the Marianna Trench region, it's always the older, colder, more dense oceanic lithosphere that subducts. My phrase for this is "Density is Destiny." Since lithosphere floats on the asthenosphere, it tends to stay afloat as long as its less dense than the underlying mantle lithosphere, asthenosphere. So this is why the lower density continental crust keeps the continents afloat - it's kind of like having a low density life preserver. Once formed, continents stick around, moving around, sometimes crashing into each other and sometimes rifting apart, a messy dance in a way. The denser oceanic lithosphere always loses out, and gets destroyed at subduction zones where its old, cold, and dense portions occur. Subduction recycles oceanic lithosphere - which is why oceans are so young. Subduction also results in melting and formation of magma that rises to form volcanoes adjacent to deep sea trenches.

divergent boundaries

Divergent boundaries are places where two plates in contact are moving away from one another, as shown by the arrows on this block diagram. All oceanic ridges are divergent boundaries because they are locations where sea-floor spreading causes plates to move apart. In these regions new subsurface melt or magma rises up and fills in the gap and quickly cools into basaltic rock that becomes part of each plate. Because the plates are quite thin at the ridges, only very shallow earthquakes occur along divergent boundaries. Since these are areas on the Earth surfaces where plates are moving away from each other, these are also sometimes called rift zones (r-i-f-t) and also extensional zones, or areas where the lithosphere is extending horizontally. Most rifting along divergent boundaries is occurring along the oceanic ridges today, but there are a few places where continental lithosphere is rifting such as the Great Rift Valley in northeast Africa.

why is geology important

Earth processes have a huge impact on our environment, but also with so many people our modern societies today are using Earth's natural resources to a degree never seen before and in doing so humans are having a huge impact on the Earth's environment as well. Essentially humans are now agents of geologic change.

stage 3: closing of an ocean

Eventually, as a widening or growing ocean ages, its oldest portions adjacent to the continents become more dense than the underlying mantle and the old, cold, dense oceanic lithosphere starts to sink and subduct beneath the adjacent continental lithosphere. This results in the formation of active continental margins with volcanoes and earthquakes. The subducting oceanic lithosphere usually sinks fairly fast, thus causing the ocean basin to start to close, even though sea-floor spreading may still be occurring. At this stage three of the Wilson cycle, there are now four different tectonic plates as depicted in this simple cartoon.

Wegener matched Geology Across Oceans

Finally, Wegener examined geologic maps and recognized that rock types, structures, and ages matched up across continents and fit together very well in the absence of the Atlantic Ocean. The left image shows how the geologic core of South America and Africa match up when they are adjacent. And the right image shows how a single mountain belt exists when Africa and North America are in contact with one another.

the density of California is the Alaska terrain parking lot

For funsies, let's keep going with this thought experiment about the future western California terrain off the west coast. Where will it end up? Very likely it will end up in Alaska, which is essentially a terrain parking lot. It turns out that the whole of Alaska is made up of continental slivers or terrains that have come together over the past 80 million years or so. Actually, it's been kind of a terrain wreck! Haha, okay, let's get back to transform plate boundaries, and let's look more closely at the oceanic transforms, which you can see lots of along the ridges of the oceans. They are the big fractures that are perpendicular to the ridges.

driving mechanisms

Geologists think the plates drive themselves through density differences and gravity. Remember: Density is Destiny. The reason that oceanic plate subducts is because it's denser than continental plates and old, cold oceanic plates are more dense than younger warm oceanic lithosphere. So once subduction starts the dense sinking oceanic slab pulls the rest of the oceanic plate with it - like an anchor pulling down the anchor line. We refer to this driving process as "slab-pull" and it drives most of plate motion. So what drives the motion of the slower Atlantic Ocean half of the world where no subduction is occurring? There we think that plate motion is driven by ridge push forces, which exist simply because ridges lie at a higher elevation than the adjacent abyssal plains. Gravity causes the elevated lithosphere to push on the lithosphere farther from the ridge.

the 3 main types of plate boundaries

Here's the same United States Geological Survey cross-section showing the oceanic and continental litho-spheric plates, both overlying the asthenosphere (or upper mantle) layer shown in orange. Subduction of oceanic lithosphere is occurring on both sides of the cross-section and sea-floor spreading is occurring at an oceanic ridge shown by an upward pointing red arrow. Small block diagrams at the top show the three types of plate boundaries known as transform (on the left), divergent (in the middle) and convergent (on the right). These boundary types are defined by the relative motion of the two plates on either side of the boundary zone.

Yes, Rocks can be Folded

Here's a beautiful photo that proves that yes, rocks can be folded. Rock folds are some of the most spectacular features of Earth structures. They are common features in many belts and they occur at all scales - I have hand size samples of rocks in my office that have beautiful folds. There are larger size folds that are about the size of road cuts, and there are very large folds that are the size of mountains. I ask this question to show you how answers to questions in Geology often come from simple observations of the rock record. I also ask it to get you thinking about how folds form and the tremendous forces that must be involved to cause rocks to fold.

san Andreas fault

Here's a great aerial photograph of the San Andreas transform fault which looks like a large straight fracture or slice through the land surface. The San Andreas Fault occurs as this incredibly straight and dramatic feature that pilots used to use to navigate when flying between Los Angeles and San Francisco. So faults are fractures or fracture zones along which there has been movement of the blocks on either side of the fault. The rocks on the left side of this image belong to the Pacific plate and are moving northward or away from us, whereas the rocks on the right are part of the North American plate and are moving toward us. You can imagine adding arrows to this photograph drawn parallel to the San Andreas Fault showing the relative motion between these two plates.

letting off some steam

Hotspot volcanism represents another way that the Earth cools over time. Heat from deep in the Earth's interior is transferred upward by the rising of hot plumes, which penetrate through the Earth's lithosphere, and releases the heat through the formation of volcanoes.

this cross-section through lithosphere and asthenosphere

Let's look again at this cross-section through the lithosphere and asthenosphere. Now that we have some understanding of plate motions and the three types of boundaries, let's think about how many different tectonic plates are depicted in this diagram. Are there two, three, four, maybe five different plates? Keep in mind that a plate boundary is where two plates are in contact with one another - so one way to determine the number of plates is to consider the number of plate boundaries. On the left and right sides we see two convergent plate boundaries, so we know that there must be at least three different plates - the ones on the far left and far right and the oceanic region in between. But we also see that there's a divergent plate boundary in the middle of the diagram, so the oceanic region is actually made up of two different plates, one on the left and one on the right. So the correct answer is four - there are four different tectonic plates depicted in this diagram.

intermediate and deep earthquakes

Let's look at subduction zones in the third dimension - in cross-section - at depth. You can imagine the oceanic lithosphere moving along the ocean floor and starting to subduct underneath continental lithosphere. At the region of contact the down going plate starts to bend and break under great stress and shallow earthquakes occur. However, the entire plate continues to break along the subduction zone, because the plate is still cool and rigid there. So intermediate and deep seated earthquakes occur all along the subduction zone - because so much more area of plate is deforming. The deepest earthquake ever recorded originated at 660 kilometer depths (about 400 miles down) in the Pacific Ring of Fire. How are these deep earthquakes located on two-D world maps? Seismologists define the location of earthquakes on maps by their epicenters, or the place on the surface directly above its point of origin (or focus). Just imagine drawing vertical lines straight up to the surface from the deep earthquakes in this diagram. You can see that these epicenters would all occur together in a narrow region where the volcanic arc is located.

plates with arrows indicating motion direction

Let's look at the USGS world map of tectonic plates. The red arrows show the relative motion of the plates at their boundaries, with arrows pointing away from one another at the divergent oceanic ridges and arrows pointing together at the convergent subduction zones. Recall how the age of the sea floor map revealed that the Pacific Ocean has been spreading much faster than the Atlantic ridge over the past 180 million years. And measurements today of plate motions show that it's still moving fast. So tectonically, Earth can be divided into a fast half and a slow half. And the fast half of the world tectonically is where subduction is occurring - where the Pacific Ring of Fire is located. But the slow half of the world has no subduction zones.

one year of earthquakes across the world

Let's look at this USGS image of only one year of earthquakes across the world. You can see how there are not enough earthquakes in a year to provide an outline of each tectonic plate. For instance, note the sparsity of earthquakes along the oceanic ridges. However, there are clearly lots of earthquakes around and along the plate margins surrounding the Pacific Ocean - the area of the Pacific Ring of Fire. So why is it that some plate boundaries experience many more earthquakes than other plate boundaries? I'll let you think about this until the next lecture.

mt st helens

Let's look in more detail at one location where subduction, volcanism and earthquakes are occurring today. Off the northwest coast of the United States the Juan de Fuca oceanic plate (shown in yellow) is subducting beneath the North American continental plate forming a line of volcanoes known as the Cascade Range from northern California, through Oregon and Washington. This is the tectonic setting responsible for Mt. St Helens which has historically been one of the most active volcanoes in the Cascades, having erupted numerous times during the past 4000 years. Other famous volcanoes in the Cascades include the beautiful Mt. Rainer near Seattle Washington, Crater Lake in Oregon, and Mt. Shasta in California.

Pacific Ocean basin showing ht eHawaiian Ridge-Emperor seamount chain

Looking at the world ocean floor map, we can see that the Hawaiian Island Chain actually continues underwater as seamounts for hundreds of kilometers. The Hawaiian Ridge is a line of extinct underwater volcanoes that formed over the last 22 million years. What's really interesting is that the seamounts chain continues for another 23 million years as the Emperor Seamount Chain but with a very different orientation. Why did the orientation of the seamounts change direction about 22 million years ago? Remember that the seamounts formed above a stationary hotspot, so their orientation represents the direction of Pacific plate motion over the past 45 million years. So geologists infer that the Pacific plate was moving almost due north between 45 million years and 22 million years ago, and that it changed direction at about 22 million years and started moving toward the northwest and has continued moving in that direction right up to today.

whats the destiny of California

Looking at this large scale map of the San Andreas Fault transform plate boundary, the thick blue arrows show the relative direction of motion between the North American plate, which is what most of the United States is located on, and a sliver of California, which is part of the Pacific plate to the west. Note that LA is on the Pacific plate and San Francisco is on the North American plate. So what's the destiny of California? Will part of it eventually fall into the Pacific Ocean? Just imagine that the transform, or sideways, motion along the San Andreas Fault keeps going into the future. Eventually a sliver of land, western California, will separate from mainland United States, just like Baja California has already separated from mainland Mexico to the south. Eventually Los Angeles will be located just across the plate boundary from San Francisco - which would be great for sports rivalries - and the sliver of land will become a very long, thin island or terrain that exists off the west coast, perhaps much like the present elongate islands of New Zealand located in the south Pacific.

Pangea (250 my ago) = "All Land"

On the left is a simple cartoon image of Wegener's proposed Pangea Supercontinent reconstruction annotated with the names of the continents. And on the right is an amazing more realistic image of what Wegener's supercontinent-superocean world may have looked like around 250 million years ago. So I have question - do you think Wegener makes a strong argument for moving continents and the existence Pangea?

global positioning system (GPS)

Past motions of plates were largely determined from the rock record, primarily using paleomagnetic inclination data from the continents and magnetic stripes preserved on the ocean floor. With today's advanced technology, we can use GPS, Global Positioning System satellite data to precisely measure the motion of the plates today. Such measurements are being continuously made around the world to monitor potential volcanic activity and to track movement along major highly populated plate boundaries, such as along the San Andreas fault in western California and the islands of Japan. So what is the rate of plate motion today? Essentially the plates are moving today on average at about the same rate that your finger nails grow, about 1 to 2 cm per year, which might not seem very fast over our lifespan, but over millions of years would add up to very long fingernails indeed

two ways to form volcanic island chains

Perhaps it's appropriate to end our topic on Plate Tectonics with this image of the world ocean floor map in the northern Pacific region. We started this topic with the discussion of the Pacific Ring of Fire, noting that volcanic island chains, like the Aleutian Islands, surround much of the Pacific Ocean. Today I've mentioned a second way that chains of volcanic islands form - those associated with plate motion above a hotspot. It's important to realize that the volcanic islands that form above subduction zones and next to the deep sea trenches consist of many active volcanoes (shown here by the red dots). In contrast, hot spot island chains (and their seamount extensions) are only active directly above the hot spot (shown by the red dot on the big island of Hawaii). This explains why most of Earth's cooling occurs by plate motion and convection of the underlying mantle and much less cooling is associated with hotspots today.

Understanding Plates and their Motions - Lecture Summary Part 1

Plate tectonic theory: Earth's outer shell (the lithosphere*) is broken into a few large and a few small RIGID plates that move. Lithosphere: literally 'rocky world' (=plates). The plates include the outermost layer of the crust PLUS a bit of the underlying mantle. Crust, Earth's thin outermost rocky layer, overlies the more dense and very thick mantle (made up primarily of a rock called peridotite), which overlies the most dense Iron/Nickel core. There are two types of lithosphere: continental and oceanic. Tectonic plates can be all oceanic (Pacific Plate) or continental (Saudi Arabian plate) but many consist of both continental and oceanic lithosphere (like the North American plate). The Lithosphere overrides the Asthenosphere: literally "flowing world" - the weak flowing mantle directly beneath the lithosphere (but it's a solid!). *Lithosphere consists of the crust and upper most part of the mantle, whereas the Asthenosphere is only mantle. Plate motion defines three types of plate boundaries Divergent boundaries: Sea-floor spreading causes plates to move apart. Less dense magma wells up to fill the gap. Magma cools, adding material to each plate. Shallow Earthquakes occur at ridge Convergent boundaries: Where lithospheric plates move toward one another. One plate dives back into the mantle (subduction). The subducting plate is always the more dense oceanic lithosphere. Subduction results in melting and formation of magma that rises to form arcs of volcanoes adjacent to oceanic trenches. Subduction recycles oceanic lithosphere. Why so many more earthquakes at subduction zones than at oceanic ridges? Only shallow earthquakes occur at ridges, whereas shallow, intermediate and deep Earthquakes all occur at subduction zones (the entire subducting plate is breaking under stress). Earthquake locations are based on their epicenters: the point directly above the earthquake origin below the surface (focus)

the theory of plate tectonics

Plate tectonics is the idea that Earth has an outer rocky shell that is broken into only a few large plus a few small strong or rigid plates that move. The plates interact with each other where they are in contact at their boundaries and the motion between two bounding plates describes or defines the type of boundary it is - and there are only three types. Plate tectonics is so important because it explains so many aspects about Earth including 1) the distribution of earthquakes and volcanoes, 2) changes in past positions of continents and ocean basins, 3) the origins of mountain belts and seamount chains, 4) the origin and ages of ocean basins, 5) why there are marine fossils at the top of Mt. Everest, and 6) even why insurance premiums are so high in California today.

faulting create offsets in the landscape

So how do we know which direction blocks on either side of the San Andreas Fault move? Well faulting causes natural or built features such as small valleys or roads to be offset - or displaced by the fault plane. For instance, the inset photo here shows two pair of parallel road lines that are clearly offset from one another across a fault. The pavement farthest from us has moved to the left compared to the pavement closest to us. Similarly, looking at the larger photo, we can see that the San Andreas Fault that runs horizontally across the middle of the photo has offset a drainage valley. By finding offset features like these we can then use arrows like those drawn next to the fault to show the direction of fault motion.

convection of solid (but flowing) mantle is why plates move

So why do plates move at all? Think of Earth as an initially very hot sphere that has been cooling since it first formed. How has it been cooling? Ultimately, the plates move because Earth is cooling by convection. You may or may not know about 'lava lamps' - which were real popular some decades ago. These lamps had a heat source at the base and a material inside that would float up or sink as its temperature changed. When heated at the base, the warming material becomes less dense until it starts to rise; but as it rises it's also cooling and becoming more dense. So after a while, that same material starts to sink. Of course when it's rising and sinking it's becoming lighter and darker and it's also changing shape, which is mesmerizing and why lava lamps are so cool. This is convection - the transfer of heat by warm material rising, cooling off at the top and sinking. Essentially the entire solid mantle is slowly convecting or flowing as depicted in this simple diagram. The heat source is the hot iron core beneath the mantle.

a decade of earthquakes

So why do some plate boundaries have lots of earthquakes whereas others have many fewer earthquakes? By far, the majority of earthquakes (and volcanoes) occur around the Pacific Ocean in what's called the Pacific Ring of Fire. You can see this quite dramatically represented by the deep red zone of earthquakes - which we now know coincide with subduction zones.

Hawaiian hot spot

The line of islands making up Hawaii are formed by a deep-seated hot spot that has seared through the middle of the Pacific oceanic plate. Active volcanism occurring today on the big island is depicted by red. The remaining islands are no longer active and get progressively older towards the northeast, with the oldest Hawaiian island having formed about 5 million years ago.

stage 4: death of an ocean

Stage 4 of the Wilson Cycle is the closure or death of an ocean basin by the process of continent-continent collision. Eventually subduction results in complete closure of the ocean and the pile-up of two continents, both of which are too buoyant to subduct. When continents collide a wide zone of crumpled, thickened lithosphere forms exceptionally high mountains.

ring of fire

The Cascade Volcanoes are part of the Pacific Ring of Fire with its numerous volcanoes, island arcs, earthquakes, and deep-sea trenches, all of which are the result of oceanic plate subduction. So we've gone full circle from my introduction of plate tectonics, but we still have the third plate boundary to discuss as well as a few other interesting things before closing out this topic.

plate boundary

The border between two adjacent lithosphere plates 12 major plates and several microplates

magnetic reversal

The change of the Earth's magnetic polarity; when a reversal occurs, the field flips from normal to reversed polarity, or vice versa.

magnetic anomaly

The difference between the expected strength of the Earth's magnetic field at a certain location and the actual measured strength of the field at that location.

marine magnetic anomalies

The difference between the expected strength of the Earth's main dipole field at a certain location on the sea floor and the actual measured strength of the magnetic field at that location.

global seismicity map

The final dataset that confirmed the theory of plate tectonics was the collection of very detailed earthquake locations during the 1960's and 1970's. It was the Nuclear Age and the early part of the Cold War and countries were very interested in verifying whether other countries were developing and testing nuclear bombs. It turns out that seismographs - instruments which detect seismic waves - can be used to distinguish between energy released by natural earthquakes and energy released by a nuclear bomb. With this realization, seismograph stations proliferated across the globe and the World Wide Seismic Station Network (or WWSSN) was created. With so many seismographs, seismologists (researchers who study earthquake waves) were able to produce very detailed global maps of the earthquake locations compiled over several years. This image of earthquake locations compiled over 10 years clearly depicts the current outline of the Earth's tectonic plates, which interact with one another along their boundaries. If you were to connect the earthquake dots you would essentially produce a 2-D map of today's plates. Compare this image with the following map.

slab pull force

The force that downgoing plates (or slabs) apply to oceanic lithosphere at a convergent margin.

seafloor spreading

The gradual widening of an ocean basin as new oceanic crust forms at a mid-ocean ridge axis and then moves away from the axis.

deep sea challenge

The history of ocean research I've just described consisted of lots of remotely collected data, such as Sonar mapping, using magnetometers, etc. Of course there have been lots of interest in direct observations of the oceans using submersibles, a few of which are shown here. The Alvin was an early U.S. submersible that was able to house 3 people for short expeditions. And the Shankai 6500 was once the world's deepest-diving peopled submersible. In 2012 the Canadian filmmaker and National Geographic explorer, David Cameron, did a solo dive in the Deepsea Challenger submersible to the lowest point on Earth, along the Marianna Trench in the western Pacific. Pretty cool. National Geographic created an IMAX 3D movie about the expedition which you might want to check out sometime.

geologic time scale

The last 541 million years comprise the Phanerozoic Eon, and all time before that makes up the Precambrian. The precambrian can be further divided into three main intervals names, from oldest to youngest: the hadeon, the Archean, and the Proterozoic eons. the Phanerozoic eon can be divided into three main intervals named, from oldest to younger: the Paleozoic, mesozoic, and Cenozoic eras

asthenosphere

The layer of the mantle that lies between 100-150 km and 350 km deep; the asthenosphere is relatively soft and can flow when acted on by force.

vents in the seafloor spew out hot, mineral-saturated water

The ocean ridges are dark and hot regions of the ocean floor once thought to be void of any life forms. However, exploration using submersibles in the 1970s and 1980s discovered chimneys along on the ridges that vented hot black mineral-saturated water and these chimneys are dubbed black smokers. It turns out that these vents provide chemical energy to the ocean ridge regions that supports life in a complex ecosystem, including bacteria, shrimp and worms.

continental rift

The process by which a continent stretches and splits along a belt; if it is successful, rifting separates a larger continent into two smaller continents separated by a divergent boundary. A linear belt along which continental lithosphere stretches and pulls apart East African Rift

subduction

The process by which one oceanic plate bends and sinks down into the asthenosphere beneath another plate convergent boundaries

collision

The process of two buoyant pieces of lithosphere converging and squashing together. a convergent boundary ceases to exist when a piece of relatively buoyant crust moves into the subduction zone, and jams up the system

lithosphere

The relatively rigid, nonflowable, outer 100- to 150-km-thick layer of the Earth, constituting the crust and the top part of the mantle.

stage 2: growth of an ocean

The second stage of the Wilson Cycle is growth or widening of the young ocean with continued sea-floor spreading at the oceanic ridge. Growing oceans are bordered by continents with passive or quiet margins, such as what we presently see along the margins of the Atlantic Ocean. Since Pangea started to break-up around 200 million years ago, the Atlantic Ocean has been slowly expanding and the Americas have been getting further and further away from Africa and Europe.

paleopole

The supposed position of the Earth's magnetic pole in the past, with respect to a particular continent.

why do some volcanoes occur in the middle of plates

The theory of plate tectonics very nicely explains the existence of the Pacific Ring of Fire where most of the world's active volcanoes exist. Subducting plates at active margins result in the formation of melts, which rise up and form necklaces or arcs of volcanoes along plate boundaries. But some very well-known volcanoes, such as Yellowstone and Hawaii and others depicted by the orange circles on this map occur in the middle of the plates far from plate boundaries. Why?

age of the sea floor

There's also been lots of research involving deep sea drilling through the ocean floor to collect cores of sediment and the basaltic crust underneath the sedimentary blanket. The young age of the sea floor has been confirmed by this direct sampling and radiometric dating of oceanic basalts.

earth's tectonic plates

This USGS image of Earth's tectonic plates constructed from the previous earthquake data reveals that the outer layer of the solid Earth consists of a few large and rigid (that is not deformable) plates. I encourage you to look over this image and to learn the names of the major plates.

basins and rangers formed by extensional faulting

This blown up Google Earth image shows the individual narrow ranges separated by sediment filled valleys, which have been dropped down between faults on both sides of the valley. The block diagram is a simplified rendition of how faulting during continental extension results in high-standing ridges (labelled horsts) and grabens (which is the German word for ditch), which represent the interleaving down-dropped blocks making up the basins. We'll talk more about faulting later in the course.

hotspot trail reveals the motion of overlying plate

This cross-section through the Hawaiian Islands shows how the hotspot currently beneath the big island has left a trail of volcanic islands that initially formed above the hotspot and then became inactive as the Pacific plate moved over and away from the rising plume. The orientation of the islands represents the direction of plate motion, from the youngest to the oldest islands.

distribution of ash fall from the mount st helens eruptions

This is a map of the 1980 ash layer erupted from Mt. St Helens. The thickest ash layer rained down and was deposited in much of Washington State (shown in dark grey) while thinner layers of ash (generally less than 2 inches thick) covered much of the northwest states over to the Minnesota border. My aunt and uncle lived in western Washington at the time and they have a jar of Mt. St Helens' ash that they collected sort of a souvenir of that event. You can imagine the mess that a blanket of two inches or more of ash rain would leave. And size wise, the Mt. St Helens eruption was a firecracker compared to the largest historical eruptions.

plate boundaries

This is a world map of the outline of today's tectonic plates derived from a decade of earthquake data. Notice how the margins of continents sometimes match up with the plate boundary (the west coast of South America for example), but more often than not the continental-ocean margins do not match up with a plate boundary, like along the east coast of the United States. In fact, the entire Atlantic Ocean is surrounded completely by passive or quiet continental margins (labeled with the letter P), that are not plate boundaries. The only plate boundary in the Atlantic Ocean is the mid-Atlantic ridge itself. But on the Pacific Ocean side of the world we see that active continental margins surround nearly the entire Pacific Ocean. There, the edges of continents are essentially all plate boundaries, making them very active margins indeed - I've labeled these with the letter A.

where would earthquakes occur along this plate boundary

To figure out where earthquakes would occur, let's look at a fictional map of three ocean ridge segments, shown by the red lines that are separated by two transform faults shown by the dashed lines. We would expect earthquakes to occur along the ridge segments during sea-floor spreading, so all up and down the red lines, but where would earthquakes occur along the transform faults? You may think that earthquakes would occur all along the transform faults from end to end, but that's not quite right. See how between the ridge segments the transform faults have arrows on both sides that are pointing in opposite directions? That's the part of the transform that is drawn with a solid black line - and that's where earthquakes would be expected to occur because rocks are sliding past one another. But note how the dashed segments of the transform faults (for instance where the words 'fracture zone' are), the arrows on either side are pointing in the same direction. We would not expect earthquakes along the dashed lines because the blocks on either side are moving together in the same direction there. So we would expect earthquakes along the divergent ridges and along the transforms only between the ridge segments. Essentially we would expect earthquakes to occur everywhere along the plate boundary separating the grey oceanic plate on the left from the blue oceanic plate on the right. So this map nicely shows two plates and the earthquake locations would exactly outline the boundary between them.

the Pacific Ocean today is shrinking

Today, the Pacific Ocean is the one example of an oceanic basin that reached its maximum width and is starting to shrink. Although sea-floor spreading is forming new oceanic lithosphere beneath the Pacific Ocean, a greater amount of older oceanic lithosphere at its borders is being destroyed and therefore the Pacific Ocean has started to close. Looking at this world ocean floor map, I've added numbered locations of modern examples of the first three stages of the Wilson Cycle; the birth of an ocean in northeast Africa (stage 1), the growth of an ocean represented by the Atlantic Ocean (stage 2), and the closing of an ocean represented by the Pacific Ocean (stage 3). In fact, if plate motion continues at the same rate as today, we can determine that the Pacific Ocean will close completely in about 300 million years at which time a new supercontinent will form with the coming together of the Americas and Asia. This future supercontinent will be called Amasia

Understanding Plates and their Motions - Lecture Summary Part 2

Transform boundaries: Where lithospheric plates slide past one another sideways; not created or destroyed. Many transforms offset spreading ridge segments. Some transforms cut through continental crust (San Andreas Fault) Characterized by shallow Earthquakes and absence of volcanism. San Andreas fault in California is a continental transform fault. Eventually a sliver (terrain) of California will be faulted away from the rest of North America and be transported north to Alaska (the terrain parking lot). Most transform faults occur along oceanic ridges (divergent boundaries). Shallow earthquakes along ridge spreading centers occur at the divergent ridge and along the segment of the transform that separates two ridges. What Drives Plate Motion? Plate subduction only occurs around the Pacific ocean and that's where the plates are moving very fast. Therefore we think the plates drive themselves through density differences and gravity. Slab-pull - Gravity pulls a dense subducting plate downward (like a stone sinks in water) Ridge-push - Elevated mid-ocean ridges push lithosphere away. Earth formed as a hot sphere 4.6 billion years ago and has been losing heat (cooling) by convection since then. Convection - heat transfer that results when warmer, less dense material rises whereas cooler more dense material sinks (think lava lamps!).

transform plate boundaries

Transform plate boundaries occur when two lithospheric plates slide past one another horizontally. In this case, lithosphere is not being created or destroyed, so there is no associated volcanism along this boundary zone and earthquakes that occur because of breaking and sliding along the fault zone are all shallow, extending only as far down as the thickness of the plate. Most commonly transform boundaries appear to offset oceanic ridge segments; I initially described them to you as great Oceanic Fracture zones that seem to slice up the ocean mountain chains. There are a few transform faults that cut through continental lithosphere, perhaps the best known being the famous San Andreas Fault in western California.

lithosphere and asthenosphere

What are the tectonic plates made of? Harry Hess was right about the processes of seafloor spreading and subduction but he was wrong about what was spreading and subducting. He speculated that it was only the basaltic oceanic and less dense granitic continental crust that moved. However, we now know that each plate is a piece of brittle or breakable lithosphere (where lith means rock and sphere means world) and the lithospheric plates are really made up of a top crustal layer plus a part of the mantle layer beneath it. The entire mantle is that thick middle layer of Earth that underlies a very thin crust and overlies or mantles the Earth's very dense Iron-Nickel core. The mantle is made up of primarily peridotite (p-e-r-i-d-o-tite) a rock that is more dense than the oceanic crust basalt but less dense than the Iron-Nickel core - remember that the Earth is layered by increasing density. The lithosphere moves or floats in a sense on top of a soft, more plastic portion of mantle peridotite that we call the asthenosphere (where astheno means flowing in Greek) - so this is a weaker softer flowing part of the mantle. This diagram is a cross-section (or vertical slice) through both the thicker continental lithosphere and the thinner oceanic lithosphere (with its thinner crust). I should point out that this diagram shows a single plate part oceanic and part continental lithosphere. This explains why continental margins are not equal to plate boundaries. Here we have a passive continent-ocean margin in the middle of a single lithospheric plate.

Himalayas start forming

When India slammed into Asia, the attached oceanic lithosphere broke off and sank into the mantle and rocks and sediment that once lay between the two continents were squeezed into a 8 kilometer high welt that we now call the Himalayan mountains. This explains why fossil rich limestone, originally deposited in the shallow oceans, can now be found at the top of Mount Everest, the highest point on earth.

India-Asia 60 million years ago

When Pangea broke up, India, which was located near the South Pole, started moving northward at an incredible rate of plate motion toward Asia. It was so fast that India really should have been given a plate tectonic speeding ticket. Rapid subduction consumed the ocean separating Asia and India, until India slammed into the southern margin of Asia about 45 million years ago.

Birth and Death of Oceans; Hotspots - Lecture Summary Part 2

Why do some volcanoes occur in the middle of plates? Like Hawaii and Yellowstone? They are plumes of very hot mantle (Hotspots) that rise from the core and sear the overlying lithospheric plate forming tracks of volcanic islands that get older in the direction the plate is moving. Both Yellowstone and Hawaii have such tracks. So formation of 'hotspots' are another way that the Earth cools (in addition to mantle convection via plates).

Yellowstone super volcano

Yellowstone ranks as a supervolcano, being 1,000 times more powerful than Mt. St. Helens. This map shows the deposit of significant ash that spread east of Mt. St Helen's 1980 eruption. Compare the size of that deposit to the huge ash deposits spread across much of the western U.S. from the three most recent eruptions of Yellowstone. Wow. These were erupted at 2.1 million years, 1.4 million years and again at 670,000 years.

the Yellowstone hot spot track

Yellowstone today lies at the northeast end of a track or tail of extinct volcanoes that started forming about 16 million years ago and has been migrating toward the northeast to its present position in northwest Wyoming. These tracks are characteristic of hot spots and represent where the plume of hot rock seared or burned through the overlying plate as the plate was moving above the plume.

Yellowstone, WY: America's first national park

Yellowstone, America's first national park in northwest Wyoming, is famous for hosting 90% of the world's geysers, as well as spectacular mud springs, hot springs, volcanic deposits and hydrothermal terraces.

transform faults at Mid-Ocean ridge

You can see in this block diagram the divergent plate boundaries along the ridges themselves (depicted with large blue arrows pointing away from each other) and you can see a single transform boundary that separates or offsets these ridges with this smaller arrows showing the relative motion across the transform fault. Now that you know something about transform faults you can perhaps appreciate the cartoon on the right about faults and tectonic relationships. Before we leave this slide, I have a question about the block diagram on the left: Where would you expect earthquakes to occur in oceanic mountains like these?

oceans are born and then die

You've learned that there are two types of lithosphere (or rocky outer world) that make up the tectonic plates: oceanic and continental. And because of its higher density, the oceanic lithosphere doesn't survive very long, and is ultimately recycled back into the mantle by subduction. Back when the supercontinent Pangea existed, there was a superocean consisting of relatively young oceanic lithosphere. None of this oceanic lithosphere exists anymore - it's all been recycled back into the mantle. And all of the current oceanic lithosphere we have today formed after Pangea. How does this happen? How does this cycle of oceans being born and then dying occur?

mantle plum

a column of very hot rock rising up through the mantle

active continental margin

a continental margin that coincides with a plate boundary

passive margin

a continental margin that is not a plate boundary

apparent polar wander

a path on the globe along which a magnetic pole appears to have wandered over time the continents drift, while the magnetic pole stays fairly fixed

ridge push force

a process in which gravity causes the elevated lithosphere at a mid-ocean ridge axis to push on the lithosphere that lies farther from the axis, making it move away

seamounts

an isolated submarine mountain once volcanoes but no longer erupt

earthquakes and subducted plates

at convergent boundaries the down going plate grind along the base of the overriding plate, process generates large earthquakes. they occur fairly close to earths surface

earthquake epicenters

majority occur in relatively narrow, distinct belts (seismic belts). these belts define the position of plate boundaries because of the fracturing and sipping that occurs along plate boundaries generates earthquakes.

bathymetric maps reveal several features

mid-ocean ridges, fracture zones, deep-ocean trenches, seamount chains

rifting

rift- a linear belt along which continental lithosphere stretches and pulls apart most new divergent boundaries form when a continent splits and separates into two continents

absolute plate velocity

the movement of a plate relative to a fixed point in the mantle

relative plate velocity

the movement of one lithosphere plate with respect to another

paleomagnetism

the record of ancient magnetism preserved in rock

plate interiors and earthquakes

they remain relatively earthquake free because they do not accommodate as much movement

What is Geology? - Lecture Summary Part 1

1) Plate Tectonics - an introduction to how plate tectonics, the unifying theory that explains much of how the Earth works; has shaped the Earth over geologic time. 2) Earth Materials and Resources- formation and composition of major rock types and minerals, especially economically and societally important rock resources. 3) Geologic Hazards - processes that alter Earth's surface in catastrophic ways (volcanism, earthquakes). 4) Geology of Water - how freshwater sculpts and shapes landscapes; how humans have created a global water crisis through the overuse and misuse of water resources. 5) Energy Resources and Environmental Impacts- origin, migration, trapping, exploration and extraction of fossil fuels that are critical to modern civilizations. 6) Climate Change- what was climate like in the geologic past and how and why is it changing today?

Criticism of Continental Drift Hypothesis

After publishing his book and when presenting his idea that continents move slowly or drift to the scientific community, Wegener's proposal was ridiculed and rejected by nearly everyone. He convinced essentially no one at the time and although he kept looking for further evidence, he ultimately died in 1930 at the age of 40 on an ice sheet in Greenland when he was on a supply expedition. And since he was the sole advocate for continental drift, the drift hypothesis 'drifted away' and died with him. Now why couldn't Wegener convince anyone of continental drift? Some people point out that being a German at the time probably didn't help him (during and after World War I); others argue that being a meteorologist also probably didn't help him either since he was not trained and educated as a Geologist. But certainly the most important reason however, was that Wegener couldn't explain how continental crust could drift or plow through oceanic crust. He lacked a mechanism for continental drift.

Earth Materials and Riches in Rocks

Another topic we will cover is the formation and characteristics of rocks and the mineral and rock resources that come from them. Most of the resources that humans use come from geologic materials and people living in developed societies use these resources extensively - on average every American born will use almost 3 million pounds of minerals, metals, and fuels in their lifetime. Earth's resources are not infinite and some countries have already run out of some of these economically important resources. We will learn about the processes by which rocks are formed and the conditions which concentrate valuable resources, including minerals and natural ores.

Another view of Earth's History

Another way to view Deep Time (the last 4.6 billion years) is to compress it into a single calendar year - which is a manageable amount of time for us. So Earth's birthday would be January 1, the oldest dated rocks would be about mid-February, single cell organisms would come on the scene in early March, but the first fossils of animals with hard parts wouldn't arrive until middle October. And the first dinosaurs, which lived on the order of 1-200 millions of years ago, would not arrive until December 11 and they would disappear December 26. The first modern humans would come on the scene only 23 minutes before midnight, the last day of the year.

harry hess recognized that volcanic islands moved

As Captain Harry Hess was moving around the south Pacific during World War II, he recognized that volcanic islands moved horizontally as they progressively sank and evolved into atolls. He noted that submerged extinct volcanoes often had flat tops from being eroded when they were islands. Flat topped submerged volcanos are now known as guyots (g-u-y-o-t-s). Perhaps most importantly, Captain Hess noticed that seamount chains always moved away from ocean ridges. As a prestigious Geology professor, Hess was well aware of the new magnetic inclination data that resurrected Wegener's continental drift. But now he correctly deduced, in an Oh-Ah moment, that oceanic crust was also slowly moving along the earth's surface and that everything was moving away from the oceanic ridges.

Wegener Used Climate Evidence

As a meteorologist, Wegener recognized that Earth has climate zones in which different land and marine deposits and life forms exist. Coal deposits and marine limestone reefs form in warm tropical deposits, for instance, whereas salt deposits and sand dunes form in hot and dry climates. Since his Pangea supercontinent extended from the South Pole to the equatorial regions, Wegener expected to find different rock types within different climate belts across Pangea - and that's what he found. For instance, he found coal and limestone deposits that would be predicted to form in the tropics of Pangea shown here in the green.

Glacial Till

As an Arctic climate scientist who worked extensively on ice sheets in Greenland, Wegener knew that glaciers are rivers of ice that flow across the land surface. He recognized that glaciers carry rock debris that gets laid down or deposited on the Earth's surface when the glaciers melt. The rock debris left behind is typically very chaotic with a mixture of different rock fragments and of different sizes (a jumbled mess essentially) called glacial till. The jumbled, mixed nature of the different fragments, such as you can see in this photo, are evidence that this rock from South Africa was deposited by a glacier

magnetic reversal timescale

By dating easily accessible piles of layered volcanic rocks on continents like shown in this drawing, and measuring their polarity, the paleomagicians were able to reconstruct a timescale of all of the magnetic reversals that have occurred in the past several hundred million years of Earth history. This drawing shows the intervals of normal (in grey) and reverse (in white) polarity extending back to about 2.3 million years ago.

Continental Drift

Continental Drift is the hypothesis that continents move. Despite the recognition of the 'fit of continents', in the early 1900s most scientists believed the continents were permanently fixed on the Earth's surface and had never moved. In 1915 Alfred Wegener, a German meteorologist, published a short book, The Origin of the Continents and Oceans, in which he argued that the continents once fit together into a Supercontinent that he called Pangea (which means all land), and that they had separated or 'drifted' apart to their present positions. Although his idea met with lots of resistance until after his death in 1930, today Alfred Wegener is recognized as the father of Plate Tectonics - which makes him a real true Rock Star. Wegener's evidence for Pangea came solely from the continents.

harry hess

Dr. Harry Hess is another rock star that I want to introduce to you. He was a Princeton University Geology Professor and during World War II he served as Captain of the Cape Johnson in the south Pacific. He was keen on using the new Sonar technology to study depth variation of the ocean floor so he would do sonar mapping as he moved from battle to battle. Amazing. He also made some key observations of the ocean floor sediment and of volcanic island chains that I will come back to later in this presentation.

Paleomagnetism - Lecture Summary Part 1

Earth behaves like a giant dipole magnet - because of its circulating liquid iron-rich outer core. Aurora Borealis, the northern lights, are a visible representation of Earth's magnetic field, which shields earth from dangerous solar winds. Geographic and magnetic north do not coincide, but neither are they widely different. The magnetic north pole wanders around the geographic North Pole over time. Magnetic declination is the angle between geographic north and magnetic north. Curved magnetic field lines around Earth cause a magnetic needle and magnetic minerals in rocks to tilt. The angle between magnetic field line and horizontal is called magnetic inclination. Magnetic inclination varies with latitude, changing from zero or low inclination at the equator, increasing gradually toward the poles, and reaching a maximum of 90 degrees or vertical at the poles themselves. Paleomagnetism - some rocks record the earth's magnetism at the time and location where they form. A rock's magnetic inclination tells us what latitude that rock formed at - very important!

The Earth's Magnetic Field

Earth is essentially a huge dipole magnet - meaning it has force field lines that go around the Earth and enter and exit Earth at the magnetic poles. You can see how the shape of the Earth's magnetic field is thought to be pretty much the same as the magnetic field surrounding a bar magnet - which is also dipolar, consisting of two poles.

Grasping 4.6 billion years

Earth is not only large, but it also extends almost unimaginably far back in time. So how can we fathom or comprehend 4. 6 billion years of time? Again, let's go backwards in time using factors of 10 years. Most of us have no problem thinking in timescales of 1 year or 101 years ago, but what was happening say about 100 years ago, or 102 years ago? The first cars were being mass produced. 1000 years ago? That was the Middle Ages and people thought that the sun and planets orbited earth. About 10,000 or 104 years ago was the dawn of civilizations. The rise of Homo Sapiens occurred on the order of 100,000 or 105 years ago, and early hominids first appeared about one million or 106 years before that. 10 million years ago was the Age of Mammals, 100 million years ago was the Age of Dinosaurs, and 1,000 million years ago (1 billion years) only single celled organisms existed. And Earth's history started 4.6 billion years ago which is Earth's birthday - woohoo!

new maps of earthquakes

Finally, new maps of earthquake locations produced in the 1950s revealed that oceanic earthquakes were localized in distinct narrow belts along the ocean ridges and at deep sea trenches. This 1954 map nicely shows how the Atlantic Ocean earthquakes occur north-south, right along the curved Mid-Atlantic Ridge on the left side of this map.

Geology of Water

Freshwater is earth's most important natural resource. This topic covers where freshwater occurs, how freshwater sculpts and shapes landscapes and transports earth materials from the continental interiors and mountains to the margins of the oceans. Especially important is how humans extract and use freshwater and how people have created a global water crisis through the overuse and misuse of water resources. For example, Lake Erie has been experiencing harmful algal blooms that are toxic and contaminating an important freshwater resource. The increase in harmful algal blooms is being studied by scientists who have determined that the heavy use of pesticides and fertilizers in Ohio and other places is at least part of the reason why there are more blooms.

Geologic History

Geologic history is often depicted with a vertical geologic timescale with the top of the scale being the present and time going further and further backwards going down the scale. This is a simple geologic timescale that highlights the most recent 542 million years. These 542 million years are divided into three parts, the Paleozoic Era where 'Paleo' means ancient and 'zoic' means life. This 200 million year era was when hard-shelled organisms evolved, and also fishes and plants. The Mesozoic or middle life period was another approximately 200 million years from 250 to 65 million years ago when dinosaurs wandered earth. And this is the Age of Dinosaurs. The extinction of the dinosaurs 65 million years ago was the start of the Cenozoic Era, the Age of Mammals. I know that's a lot of information to take in about geologic time, but hopefully it didn't just go into one Era and out the other Era. Note how the lowest segment is the Precambrian - which is the first 4 billion or 7/8 of geologic time, lumped into one word called the "Precambrian." That doesn't seem quite right but geologists tend to know much more about the more recent 500 million years of Earth history than we do about the Precambrian, so maybe that's OK.

Aurora Borealis from Space

How do we know the Earth has a magnetic field? One way we know is the existence of the amazing northern and southern lights - the Aurora Borealis and Aurora Australis. This is a beautiful photo of the Aurora Borealis taken from space. These are the lights seen above the magnetic poles of the northern hemisphere.

Geologic Processes take place across Vast Scales of Space and Time

Geologic materials exist at many different scales of space from the size of sand grains (and smaller) to the size of mountains and tectonic plates. Likewise geologic processes occur across vast scales of time - on the order of seconds and minutes for earthquakes to millions of years for the construction of mountains. One way to think about different scales of space or size is to think about size variations by multiples or factors of 10, which scientists also refer to as an order of magnitude. Now humans are about 1-2 meters in length (a meter is 3 feet). Something that's about 10 times our length might be a 3 story apartment building - that would be a one order of magnitude different or 101 . A very tall structure, say the Eiffel tower, would be closer to 100 times our length or two orders of magnitude or 102 . What about Mount Rushmore, the giant sculpture of Presidents carved into granite in the Black Hills of South Dakota? We would say that's roughly 1000 times larger than us, or 3 orders of magnitude larger, or 103 . This image nicely depicts scales of space using multiples of 10 from a carbon atom (10-10 meters across) to the size of the universe (1026 meters across). In this physical geology course we will refer to scales about as small as coffee beans (10-2 meters or 100 times smaller than us) up to the size of earth which is on the order of 107 meters across.

Oceans and Continents

Geology is an outside science - it's the study of our physical world and therefore it's helpful to know very broadly about the primary geographical features of Earth - namely the oceans and the continents and where some of the major geologic features are located - like mountains. Now I know that everyone is familiar with the names and locations of the four oceans, the Pacific, Atlantic, Indian and Arctic Ocean at the top of the world. And the continents Australia, Asia, Africa, Europe, North and South America and Antarctica at the bottom of the world. Now can you locate these geologic features, the Rocky Mountains, Denali, the Alps, the Himalaya Mountains, the Great Rift Valley, and the Andes Mountains? We've already talked about the Rocky Mountains, but while you are thinking about the others, you should know that Denali is the new name for what was formerly called Mt. McKinley, originally named after President McKinley from Ohio

Geology is the study of Earth Materials, Processes and History

Geology, the study of Earth, has changed a lot since I was a student learning about Earth Science. The older view of Geology focused on identifying Earth materials (rocks, minerals, fossils), boots on the ground mapping of the earth's surface, and development of the Plate Tectonic theory as the over-riding paradigm for understanding our planet. Modern views of Earth Sciences focuses much more on human environmental impacts; recognizing that people are a major force on earth, causing significant environmental changes and dramatically changing Earth's landscape. Energy and mineral resource extraction and usage are exceedingly important to societies. And Earth is studied today by remote sensing techniques (satellites, radar, infrared cameras, etc.) and is viewed as an interconnected system (the Earth System).

Answer these Questions

Here are the questions. The age of the Earth is a) 4600 years, b) 46000 years, c) 46 million years, d) 4.6 billion years, and e) 46 billion years. Rocks can be folded: True or False? Continents are older than oceans: True or False? The Rocky Mountains are older than the Appalachian Mountains: True or False? Ice is a mineral: True or False? And lastly, the largest historical earthquake in the lower 48 states occurred in a) California, b) Oregon, c) Missouri, d) Ohio. Now the answers: The age of the Earth is d) 4.6 billion years. The reason I start with this question in this course is to emphasize that the Earth is really old, so old in fact that it's hard for humans to fathom the vast amount of geologic time that encompasses Earth history. In fact scientists give a name for the past 4.6 billion years -Deep Time, a topic we will come back to soon. Another reason I start with this question is that it allows me to follow up with another question - namely how do we know the Earth is 4.6 billion years old? Interestingly, there are no known rocks or minerals on earth that are 4.6 billion years old; the oldest radiometrically dated rocks and minerals are about 4 billion years old and there are very few of those. Most rocks are much, much younger. We don't have the time or the background to go into how we know the Earth's age right now, but my point is this: "it's not so much what you know, but how you know it."

Mapping the Invisible

Here's a great example of using remote sensing to document an environmental problem that occurred near Los Angeles. In 2015 a leak was discovered in a natural gas storage facility near Los Angeles. The gas leak was invisible and no one new how extensive it was, although residents in a nearby neighborhood were experiencing significant health problems and many residents had to be evacuated.

U.S. Seismic Hazard Map

Here's a map of seismic hazard in the United States. The hotter red and orange colors represent regions where there is very high potential of earthquakes and hence high seismic hazard risk, and the cooler colors, the blues and greys, are areas of few or no earthquakes historically and therefore low seismic hazard. Very likely you are well aware of the high seismic hazard along the entire west coast (shown in the red), but you may be surprised to see that there is also high seismic hazard potential in the great plains area along what is known as the New Madrid seismic zone (the bulls-eye pattern in the interior). It turns out the historically largest earthquake (2 earthquakes actually) in the lower 48 states actually occurred in Missouri in 1811 and 1812, in the interior of the North American plate. So although most earthquakes do occur along the edges of geologic plates, some significant earthquakes have occurred in the middle of plates and we will talk about why that is when we cover geologic hazards. Another interesting point about earthquakes is that humans are now causing earthquakes by the process of removing gas from the subsurface via fracking.

hess's hypothesis: continents "drift"

Hess's elegant Essay in Geopoetry is famous for instantly providing the long-sought explanation for how continental drift occurs. The idea is that continents are attached to the oceans and passively pushed across the Earth's surface by the formation and destruction of oceanic crust. They don't plow through the ocean floor but instead ride along with the ocean floor as it does all the work of spreading.

: Earth System Science

I want to emphasize the point of viewing the Earth as a System. When I was learning about Geology as a student some time ago, I learned primarily about the solid components of Earth -Geosphere - primarily Earth Materials, Earth's crust and underlying mantle, Earth's tectonic plates, etc. Now students of Geology understand our changing Earth as consisting of four realms or components all influencing and interacting with one another. In addition to the Geosphere there's the atmosphere, the mixture of gases that surround Earth; the hydrosphere, all of the water on Earth including the oceans, rivers, lakes, but also including water in the subsurface and in the atmosphere; and lastly the biosphere, the realm of all living organisms, including mankind importantly. Within the Earth System certain materials cycle among rock, sea, air, and living organisms and this cycling and interaction of the different spheres or worlds is energized by the Sun, by Earth's internal heat, and by the force of gravity.

thin ocean sediments and high heat flow

I want to return to this close-up image of mid-ocean ridge to mention two other observations of the ocean floor: 1) that it's covered by a thin layer of ocean sediments and 2) that lots of heat is rising from below the ridges. Ocean sediment is formed by clay and shell-like material from organisms constantly raining down and settling on the ocean floor. Since the ocean sediment was quite thin, Hess realized that the oceans must be fairly young. If the oceans were very old, then there would be more time for sediment to settle and accumulate on the ocean floor and the sediment should be very thick. He also observed that the thin sedimentary layer on the ocean floor was a little thicker moving away from the ridges, which suggested to him that the ridges were younger than the abyssal plain regions which had been around long enough to collect more sediment. In fact the abyssal plain regions are flat (as seen in the right side of this image) because the sediment there blankets the oceanic floor there and smooths out the rough topography that makes up the ridges. The high heat flowing out of the ridge area suggested to Hess that the ridge area was underlain by liquid magma which was erupting volcanic basalt onto the ocean floor at the ridges.

Curved Magnetic Field Lines

If we look again at Earth's magnetic field we see that the field lines are curved - that is they are about parallel to the Earth's surface near the equator and then start to tilt downward toward the surface moving away from the equator. Since your compass needle wants to align itself with the magnetic field lines, this curving of the magnetic field causes your compass needle to tilt - and this tilt is the magnetic inclination. Now practically compass needles, like the Brunton Compass, are weighted so that they can't tilt, because the needle needs to be kept horizontal so that it can spin freely.

sea-floor spreading and subduction

In 1960, Professor Harry Hess wrote an elegant explanation for the many new observations about the ocean floor that he dubbed an "Essay in Geopoetry." He proposed that molten magma formed beneath ocean ridges erupted and solidified to form new oceanic crust. Earthquakes along the ridges suggested that the new crust was cracking and splitting at the ridges and was moving away from the ridge on both sides. The process Hess described is now known as Sea-floor Spreading, the gradual widening of an ocean basin as new oceanic crust forms at and moves away from an ocean ridge. Since new ocean floor was being created at the ridges, Hess realized that it had to be consumed somewhere else, and he proposed that it was being consumed or sinking back into the earth at the trenches, a process now known as subduction.

ocean ridge, great fracture zones, and seamounts

In addition to ocean ridges and great fracture zones (labeled here as transform faults), Sonar mapping revealed isolated lines or chains of underwater extinct volcanoes such as these shown in the upper right of this beautiful seafloor image. Lines of active volcanic islands in the middle of oceans have been known for about for a long time - such as the Hawaiian Islands in the middle of the Pacific. But sonar revealed lines of extinct underwater volcanoes called seamounts.

How do continents plow through oceans?

In his writings and at conferences Wegener wasn't able to provide a mechanism or process by which the continents could move and he couldn't explain the great forces that would be needed to move huge masses of continents.

Scientific Theories are ideas Supported by Multiple Lines of Evidence

In science, a theory is a scientific idea supported by an abundance of evidence; it has passed many tests and failed none. So what are the abundant lines of evidence for plate tectonics and what are the evolution of ideas that led to the Theory of Plate Tectonics? It's a fascinating story that starts with the idea of continental drift, involves evidence stored in rocks about the Earth's magnetic field (called Paleo, or ancient, magnetism) and some really important observations about the ocean floor - a feature that we knew very little about until after WWII. We will see that the evidence and ideas leading to Plate Tectonics started with land-based evidence coming from the rock record preserved on several continents written about by Alfred Wegener in 1915, but I want start with evidence first mentioned a couple of centuries before that.

Recent Lava Flows at Hawaii Volcanoes National Park

Lava is liquid or molten melt, from the subsurface, that flows onto the land surface or the ocean floor, cools quickly, and crystallizes into volcanic rock. As lava cools like this one in Hawaii, magnetic crystals that form within the melt align themselves with the magnetic field much like iron filings do when placed next to a bar magnet. Active lava flows today are in a sense 'freezing-in' magnetic inclinations that will be permanently recorded in the volcanic rock. So here's a question, what do you expect would be the ballpark magnetic inclination angle for Hawaiian basalt flow like these? Would you expect a low, medium, or steep magnetic inclination to be preserved? The right answer would be a low inclination since Hawaii is located in the tropics at low latitudes.

oceanic fracture zones

Let's look a little more closely at those fracture zones that segment the ocean ridges. The map on the left is an image of the south Pacific ridge that is cut by two very long and closely spaced fractures that are about perpendicular to the ridge segments. Note how the ridge seems to be offset or displaced by each fracture. The right ridge shows an oblique view created by multibeam sonar showing the offset segments of ocean ridges (in orange) between two fracture zones, which consist of broken up fractured oceanic crust. The blue regions on both those images represent the flatter deeper parts of the ocean floor, located away from the ridges.

world ocean floor topography map

Let's return to this amazing image of the world's ocean floor topography, which represents one of the most important datasets collected about Earth. We now know that the underwater mountains or ridges are where new oceanic real estate is forming and you can perhaps imagine the pattern of magnetic zebra stripes which would be parallel to these ridges. The presence of past magnetic reversals permanently recorded in the sea floor meant that they could be used to date the entire sea floor without the need to sample any oceanic rocks. The magnetic reversal timescale obtained from the easily accessible continental volcanic rocks could be correlated with the record of ocean floor magnetic reversals.

Map view of Aleutian Islands

Let's take a closer look at the Ring of Fire that surrounds the Pacific Ocean by zooming in on the northern rim of the Pacific Ocean and looking at the Aleutian Islands. The Aleutian Islands are a necklace of active volcanic islands extending from Alaska to Russia. This arc of volcanic islands is directly adjacent to another great geologic feature, a deep sea trench that is parallel to the arc of volcanic islands - visible as a dark blue line just south of the islands. The deep sea trenches are one of the most dramatic topographic features on earth - but they are not well known because they are occur under the oceans and largely invisible to us. To get a sense of how deep these linear features extend, imagine taking the tallest mountain in the world, Mt. Everest, turning it over, and putting it into a trench. Believe it or not, the peak of Mt. Everest would not touch the bottom of the deepest trench. In other words, the ocean trenches are deeper than the tallest mountains on earth.

Basalt Flow, Grand Coulee, WA

Let's take a look at another sequence of basalt flows in a cliff near Grand Coulee, Washington. And let's say, just for funsies, that paleomagnetic scientists, I sometimes call them paleo-magicians, measured the magnetic inclination in three flows, the highest one, one in the middle of the sequence and one near the bottom of the cliff. And let's say the highest flow has a very gentle magnetic inclination - a tilt of only 5 degrees. The middle flow, however, has somewhat steeper inclination of about 30 degrees and the lowest flow has the steepest inclination, say 60 degrees. Since these flows are all from the same geographic location today, why are their magnetic inclinations different? The answer is that the latitude of these rocks changed as they were erupted over time. The lowest oldest flow formed when North America was at higher latitudes, and over time, the continent moved southward toward the equator, as suggested by the decreasing magnetic inclination. And since, eruption of the highest, youngest flow, this part of the continent has migrated northward to its present location at an intermediate latitude.

Grand Tetons, Wyoming

Let's take a look at some mountains out west. Here's a favorite photo of mine of Geology majors attending our summer field course in the Rocky Mountains - great looking bunch of people I might add - and behind them are the spectacular Grand Tetons, which are majestic mountains in Wyoming that are only about 2 million years old. If you get a chance to go west, I encourage you to visit the Grand Teton National Park - you won't be disappointed. It's one of my favorite places!

Climate Change

No doubt you have heard and read a lot about climate change in the present. But what was climate like in the geologic past and how do Earth Scientists recognize what climate was like just one-two million years ago during the ice ages but hundreds of millions of years ago when Earth may have been completely immersed in ice? And what earth processes might have been responsible for past climates and how and why is climate changing today?

normal, revers

Looking again at the pile of volcanic flows from Grand Coulee, Washington, we can now add the additional information of normal and reverse polarity in this volcanic sequence, with N being the abbreviation for normal polarity and R being the abbreviation for Reverse polarity. The discovery that the magnetic poles have flipped positions fairly regularly in the geologic past was a key element in recognizing that it was not just the continents that moved but that oceanic crust also moved as I'll explain later.

Paleomagnetism - Lecture Summary Part 2

Magnetic Inclination is preserved in volcanic liquid flows (lava) that cool and crystallize quickly (freeze) at the surface or along the ocean floor. As lava cools, newly formed magnetic minerals align themselves with the magnetic field where the lava forms. Inclination angle is related to the geographic latitude (distance from the equator) that the rock formed at. Gentle near equator, steep near the poles. Magnetic inclinations in ancient rocks that don't match with their current latitude, could be explained if continents moved - which was Wegener's Continental Drift hypothesis! Earth's magnetic field changes polarity (it reverses itself) fairly regularly. That is, the N and S poles flip, so that during 'normal' polarity (like we have today), compasses point north, but during 'reverse' polarity, compasses would point south. Polarity flips happen quite suddenly (in geologic time). Volcanic rocks permanently record the polarity that existed when the lava cooled and crystallized. Because volcanic rocks can be radiometrically dated, they can be used to determine when a polarity flip (or reversal) occurred. Magnetic Reversal timescale - generated by dating easily sampled volcanic rocks erupted on land, but applied widely to the oceanic volcanic crust (basalt)

Natural Hazards: When Nature Strikes!

Many geologic phenomena affect society, often in catastrophic and devastating ways. In this topic on Geologic Hazards, we will learn about the hazards of volcanoes, earthquakes, and landslides, why these hazards are increasing, and about what can be done to mitigate their impacts. This is an aerial photo of the Oso landslide (seen in the upper center) in Washington state. In March 2014 the landslide catastrophically engulfed 49 homes, killing 43 people, and dammed the river, causing extensive flooding upstream. Excluding landslides caused by volcanic eruptions, earthquakes or dam collapses, the Oso slide is the deadliest single landslide event in the United States history.

Ice is by Definition a Mineral

Many students are surprised to learn that ice is indeed a mineral- so these beautiful glaciers could be considered flowing rivers of rock, all made of a single mineral - ice. Now everyone knows that ice melts and becomes water. So is water a mineral? Actually no, because by definition minerals are solid. So what about mineral water? Heehee. Mineral water is water that is obtained from mineral springs and that contains dissolved solids and gases. OK, so what about ice cubes? Are they minerals? Actually, no! Because ice cubes are not naturally occurring, and a mineral by definition must also be naturally occurring. Anyway, we'll talk more about ice and water in this course as both are important components of the Earth's System.

The Ocean Floor - Lecture Summary Part 1

Marine Geology - became super important during and after WWII (note important role of Captain Harry Hess, Princeton Geology Professor). Topography of the ocean floor determined by Sonar mapping (echo sounding). Sonar revealed a rich underwater world of physical features including. : Ocean ridges - Very long submarine mountain ranges that rise ~2 km above the vast and flat abyssal plains and are roughly symmetrical. Great fracture zones - closely spaced, narrow bands of fractured, broken up rock that are perpendicular to the ocean ridges. They appear to segment the ridges. Deep-ocean trenches -8-12 km deep troughs that occur along the edges of some oceans and border volcanic arcs (curved chains of active volcanoes) Seamount chains - underwater extinct volcanoes that rise up from the seafloor in isolated areas (Hawaiian-Emperor Seamount is most famous example). Captain Harry Hess - recognized that volcanic islands moved horizontally as they sank and developed into guyots (flat-topped submarine volcanoes), then barrier reefs, then atolls (circular reefs surrounding lagoons). And Hess noticed that seamount chains always moved away from ocean ridges! Additional new evidence from ocean floor: o Thin layer of ocean sediment atop ocean crust gets thicker further away from ridges o Lots of heat rises from below the ridges; flow of heat from ocean floor decreases away from ridges. o Earthquakes occur in narrow belts along ocean ridges and deep sea trenches Sea-Floor Spreading (Harry Hess and his 'Essay on Geopoetry') - Harry Hess hypothesized that new oceanic crust (basalt) is formed through volcanic activity at ocean ridges and then gradually moves away from the ridge. This process provides the missing explanation for how continents drift. Subduction - the process by which old oceanic floor sinks down into the mantle

The Pacific Ring of Fire

Mt. St. Helens eruption and the Great East Japan earthquake both occurred along segments of the Pacific Ring of Fire that includes over 160 active volcanoes. The Pacific Ring of Fire is a region of high volcanic and earthquake activity that surrounds the majority of the Pacific Ocean Basin. The Ring of Fire is over 40,000 kilometers long and touches 4 of the world's continents as well as major island chains. About 90% of the world's earthquakes and 80% of the world's largest earthquakes occur along the Ring of Fire as well as 80% of the world's active volcanoes. Why?

Mount St. Helens, May 1980

Mt. St. Helens is an active volcano along the Pacific Northwest coast of the U.S., in Washington State, well known for its ash explosions and pyroclastic flows. It is most notorious for its explosive eruption on May 18, 1980, the deadliest and most economically destructive volcanic event in the history of the United States. Fifty-seven people were killed; 250 homes, 47 bridges, 15 miles of railways, and 185 miles of highway were completely destroyed. A massive avalanche triggered by a 5.1 magnitude earthquake caused an eruption that reduced the elevation of the mountain's summit by 1300 feet, replacing it with a 1 mile wide horseshoeshaped crater.

Normal and Reversed Polarity

Normal polarity of Earth today is represented by a dipole magnet with an arrow pointing toward the South Pole with force field lines exiting the South Pole and entering at the North Pole (as shown on the left). During reverse polarity the arrows point in the opposite directions (exiting the North Pole and entering the South Pole shown on the right). All volcanic rocks formed in the last 700,000 years or so record normal polarity like we have today. However, volcanic rocks formed between 700,000 years and about 1.2 million years or so show reverse polarity.

Geographic and Magnetic Poles do not Coincide

Now the geographic pole, the location where the Earth's rotational axis intersects Earth's surface, does not coincide exactly with the magnetic pole. Today, the magnetic pole is several hundred miles away from the geographic pole and measurements over time show that the magnetic pole moves around a bit, but it's never too far from the geographic pole. Since compasses point toward magnetic north, people need to correct their compasses to point toward true north. The angle between the magnetic pole and the geographic pole is known as the magnetic declination, which varies depending on where you are on Earth. If you go to magnetic-declination.com you will see that its 8 and 1/2 degrees west in Kent, whereas in Duluth Minnesota, where I grew up, its only about 1 degree west. So when Minnesotans go hiking in the northwoods, they don't really need to worry about magnetic declination since the angular difference is so small, but here in northeast Ohio the magnetic declination is quite significant!

Yes, Continents are older than the Oceans

Ok, which do you think is older; continents or oceans? Continents are indeed older than ocean basins - in fact they are much older. For instance, the North American continent started forming about 2 billion years ago and was largely complete 1 billion years ago. In comparison, the Atlantic Ocean that borders North America to the east started forming less than 200 million years ago. I ask this question to get you thinking about why there are oceans and continents, and to stress that they are fundamentally different geologic features. Oceans are born and die all the time, but when continents form, they tend to stick around. This map of the U.S. is also helpful in answering which mountain chain is older - the Appalachians on the eastern side of the U.S. or the Rocky Mountains to the west on the other side of the Great Plains? The answer lies in how tall the mountains are - note how the Rocky Mountains stand tall compared to the Appalachian Mountains to the east. Older mountains have had lots of time to be eroded, so they tend to be worn down and have a more subdued topography. In contrast, younger mountains tend to be higher and can often have dramatic topography.

Mountain Locations

Okay, here are the locations of those geologic features. Denali is in Alaska and is the highest point in North America. The Rockies (RM) are in the western US (we knew that already). The Andes Mountains (AM) are the long mountain chain on the western side of South America. The Alps are young spectacular mountains in southern Europe. The Himalaya Mountains (HM) are the huge mountains in Asia formed by the collision of India into Asia about 45 million years ago (and still growing today). And lastly, the Great Rift Valley (GRV) is in the northeastern part of Africa and it represents a place where the African continent is breaking apart. Okay, very good.

Tsunami during 2011 Japan Earthquake

On March 11, 2011, a large undersea earthquake occurred off the east coast of Japan in the western Pacific. It was the most powerful earthquake ever recorded to have hit Japan, and the fourth most powerful earthquake in the world since modern record-keeping began in 1900. The earthquake triggered powerful tsunami waves that reached heights of up 133 feet and travelled up to 6 miles inland, causing massive death and destruction.

Intro to Plate Tectonics - Lecture Summary Part 1

Pacific Ring of Fire - a belt around the Pacific Ocean where most of Earth's active volcanoes, earthquakes, and deep sea trenches occur. One of the primary lines of evidence for Plate Tectonics. Plate Tectonics - the theory that Earth's outer shell consists of few large rigid plates that move and interact with each other at the edges of the plates. The theory began with Alfred Wegener's idea/hypothesis that continents have moved - called Continental Drift The Origin of the Continents and Oceans (1915) - small book written by Wegener which presented land-based evidence that the continents had once been united into a single Supercontinent named Pangea ("All Land") Why Oceans and Continents? They are composed of different rock materials. Continental crust = granitic rocks (on average); Oceanic crust = basaltic rock Granite is light colored, coarse-grained crystalline, low density rock (made of minerals quartz and feldspar) Basalt is dark colored, fine-grained crystalline, higher density rock.

Iron Filings surround a Bar Magnet

So what does the magnetic field look like? Now you may have done this interesting experiment in a secondary school science class. If you take a bar magnet and place it on a paper thinly coated with randomly oriented iron filings, the filings will move and orient themselves into the direction shown here, and what they are doing is aligning themselves parallel to the magnetic force field that surrounds the bar magnet. So in essence this allows us to see the shape of the normally invisible magnetic field. Now many rocks have tiny magnetic minerals in them, magnetite being perhaps the best known magnetic mineral. And these magnetic minerals in rocks align to Earth's magnetic field just like those iron filings aligned to the magnetic field of the bar magnet.

The Ocean Floor - Lecture Summary Part 2

Passive Continental Margins - have broad shelves, a slope, and a rise. Cut by submarine canyons. Example: east coast of North America. NOT a plate boundary, so little to no earthquakes and no volcanism. Active Continental Margins - narrow shelf, deep trench which borders volcanic islands or continent with active volcanoes. IS a plate boundary so lots of earthquakes. Example: Pacific Ring of Fire margins. Marine magnetic stripes on ocean floor - symmetrical pattern of normal and reverse magnetic polarity (zebra stripes) preserved on ocean floor (parallel to the ridges). Results of sea-floor spreading process at ocean ridges. Age of the Ocean Floor - magnetic reversals that are dated by sampling vertical stacks of volcanic rocks on continents can be used to determine age of sea floor without any sampling of oceanic rocks! Ocean floor gets older away from ridges on both sides and oldest preserved oceanic crust is only about 180 million years old - remarkably young compared to continental rocks! Direct Studies of the Oceans - submersibles and drilling of ocean floor from research vessels Seismology and the New Global Tectonics - Nuclear age and cold war period in 1960s and 1970s led to rapid increase in seismometers around the world and much improved maps of earthquake locations. Belts of earthquake zones around the world reveal the current outline of large tectonic plates today. But why do some plate boundaries exhibit many more earthquakes than other plate boundaries? Plate Tectonic Theory - outer rocky layer of Earth consists of a few large and rigid (strong) plates that move and interact with one another at their boundaries.

marine geology

Prior to World War II, we knew very little about the ocean floor and about the oceanic crust other than it was made of primarily basaltic volcanic rocks. During World War II, navy battles in the South Pacific and the presence of German U-boats off the east coast of the United States, resulted in renewed political and scientific interest in the oceans. Use of submarines in the future would require knowledge of the variation of ocean floor depths (or ocean topography) and this and other information from the oceans was key for discovering plate tectonics.

volcanic sequence

Remember that volcanic sequences preserved on continents record normal and reverse magnetic polarity and indicate reversals or flips of the Earth's magnetic field in the past. The volcanic sequence on the left represents a vertical stack of volcanic rocks with the oldest 2.3 million year flow on the bottom and the youngest 300,000 year old flow on the top. N stands for normal polarity like we have today and R stands for reversed polarity. On continents, volcanic rocks get younger vertically from bottom to top (as younger flows erupt atop older flows on the land surface). But according to the sea-floor spreading hypothesis, oceanic flows get younger horizontally towards the ridges where new oceanic flows are erupting today. To show this on the right I have tipped the volcanic continental sequence sideways with the oldest flow on the left side and youngest on the right side. Looking down on this sideways sequence hopefully you can now see how the ocean floor would appear with zebra stripes recording past magnetic reversals.

Studies of Magnetic Inclination

So it was studies of magnetic inclination in volcanic rocks in the 1950's that resurrected Wegener's hypothesis of Continental Drift, although scientists still did not know how they could drift. More information or data was needed and that would come, in part, from learning about the second key aspect of Earth's magnetic field, that it changes polarity or reverses itself fairly regularly.

Granite vs Basalt

So what are granite and basalt? Granite is made of large interlocking crystals of primarily quartz and feldspar minerals, making it a light colored crystalline rock as seen here on the left. There's a penny for scale in this photo - so clearly most of the crystals are much smaller than a penny. To a geologist these crystals are still large or coarse in size, since they can be seen by the naked eye without the use of a hand-lens or microscope. In contrast, the rock basalt is made of tiny crystals that are so fine-grained that the individual crystals can't be seen with the naked eye. The mineral crystals that make up basalt are primarily dark and are heavier than the quartz and feldspar minerals in granite. So now you know that rocks of the continental crust are made up in large part of granite (and other rocks of similar composition), and that rocks of the oceanic crust are made up of primarily basalt.

Why Does Earth have a magnetic field?

So why does Earth have a magnetic field? It turns out that internally Earth is layered, kinda like a layer cake, and that it has a very dense iron rich core, part of which is liquid iron, that's the outer part of the core. The motion of earth spinning through space causes circulation of the outer liquid iron core, which generates a magnet field that surrounds and shields Earth from dangerous solar winds. Just as a sidebar, this graphic nicely shows how Earth is layered, with its outermost layering being atmosphere, the oceans and ice caps forming the hydrosphere on the surface beneath the atmosphere, the thin outer shell of granitic continental and basaltic oceanic crust below that, which is underlain by a very thick and dense layer of mantle, which in turn surrounds the densest iron core. Each layer I just described is more dense or heavier than the previous layers, so we know that Earth is layered by density.

It's Relevance Today...Soil Degradation

Soil is another critical natural geologic resource that humans are using like never before. We'll go over how soils form later in the course but it's important to realize that a welldeveloped soil can take a few hundred years to form, and if humans degrade the existing soil faster than it can be replenished naturally, well then we have a problem. This map uses colors to visualize the state of soil on earth with the warm or hot color red showing the location of very degraded soil, orange showing somewhat degraded soils, and the cream colored regions showing stable soils. The United Nations has determined that 10s of millions of acres of cropland are lost annually to soil degradation especially in Africa, Latin America, and Asia. Even in the U.S., our soil is being eroded at a rate 10 times greater than it can be replenished! Wow.

the Mid-Atlantic ridge

Sonar studies in the Atlantic Ocean revealed a 60,000 kilometer (or 40,000 mile) long mountain range which splits nearly the entire Atlantic Ocean from north to south as shown in this map illustration. These ocean floor highs are not tremendously tall mountains; they rise about 2 kilometers above the deeper flat plains of the ocean floor, which is why we call them ridges, and they are roughly symmetrical. Clearly these ridges are not straight features but curve, maintaining its presence in the middle of the Atlantic Ocean.

sonar mapping (echo sounding) of the ocean floor

Sound Navigation and Ranging or Sonar mapping is also known as echo sounding, which is the same method that bats use to navigate. Sound waves from a ship bounce off the ocean floor and return as an echo. The time it takes for a sound wave to echo back is related to depth, the distance to the ocean floor. By cruising back and forth with multibeam sonar as shown here, physical features of the ocean floor can be mapped in detail. Today, these types of maps are made even more quickly using satellite data.

Which moves - the Continents or the Magnetic Poles

Studies of magnetic inclinations in ancient volcanic rocks in the 1950's revealed that many flows preserved inclinations that were very different from the expected inclination based on their current distance from the equator (or the latitude). Geologists realized that these unexpected inclinations could be explained if the continents had moved since the volcanic flow initially formed millions of years ago - and hence these studies inadvertently resurrected Wegener's hypothesis of Continental Drift. Now it had been 25 years since Alfred Wegener died and about 40 years since his book about Pangea and Continental Drift was published. Many of these young scientists had never even heard of Wegener, so you can imagine their surprise when they rediscovered his idea of Continental Drift. As a short aside, the paleomagnetic results generated a debate about what actually moves - the continents or the magnetic poles? These diagrams show how the new results can be explained by a) assuming a fixed pole and a drifting continent or by b) assuming a fixed continent and a wandering pole. Suffice it to say that with further study, the scientists learned that the magnetic poles move only slightly and that it's the continents that have somehow drifted substantially.

one dramatic impacts humans have had on earth's most important resource, water

The Aral Sea, in western Asia, was once the 4th largest lake in the world. In the 1960s, the Soviet Union undertook major water diversion projects on nearby rivers, capturing water that once fed into the Aral Sea for irrigation purposes. Now this worked great for crop production in the surrounding area but it was a disaster for the natural freshwater lake. These images show the shrinking of the Aral Sea from 1977 on the left to 1989 in the middle and to 2006 on the right. Today the Aral Sea is virtually gone. Wow...and it only took humans about 4 to 5 decades to do this.

ocean ridges from the longest single mountain chain on earth

The Pacific Ocean also has a ridge, although it's not in the middle of the Pacific but is located closer to the Americas in the east Pacific. This spectacular world ocean floor map shows how the ridges wind their way between the continents much like the seam on a baseball forming the longest single mountain chain on Earth. Note also how the ridges appear segmented or chopped up by closely spaced fracture zones.

New Evidence after WWII

The argument for continental drift disappeared and was largely forgotten about until after World War II when new evidence came to light largely driven by renewed political and scientific interest in the oceans. Remember, Wegener used only land based evidence. The oceans and the Earth's magnetic field were two other key elements in the Plate Tectonic story.

methane

The center photo of these 3 photos shows the Aliso Canyon area during the gas leak. Obviously it looks very normal. However, the images on the left and right were taken by infrared cameras which reveal a huge cloud of methane gas. The purple stuff on the left and the orange cloud and vent on the right. The image on the right clearly shows the gas pouring out of the ground like an erupting volcano - you could view this essentially as a human made methane volcano. These types of remote sensing images enabled scientists to measure the amount of methane being released and to understand the magnitude of the problem. The Governor of California declared a state of emergency for this area and sometime later the leak was capped, but not before a tremendous amount of methane was released into the atmosphere. Methane is a very potent Greenhouse Gas, nearly 100 times, or two orders of magnitude, more potent than carbon dioxide. The methane released from this one storage facility was thought to be about equivalent to greenhouse gases released by 7 million cars driven for an entire year.

Paleomagnetism

The idea of continental drift, the notion that large masses of continents are somehow drifting through oceans of basalt, died with Wegener in 1930, but was inadvertently rediscovered after World War II in the 1950s with renewed interest in studying rock magnetism and especially the record of ancient magnetism preserved in rocks. In order to understand Paleo-magnetism, we first have to understand the Earth's magnetic field.

Aurora from Earth

The lights are even more amazing to see from the ground, they are these amazing green lights that dance around the sky. They exist because a magnetic field permeates the space around Earth. The dancing lights are actually collisions between electrically charged particles from the sun that enter the Earth's atmosphere. These charged particles flow toward Earth's magnetic poles and cause gases in the atmosphere to glow. Of course the other way we know Earth has a magnetic field is because we can use compasses, which point toward the magnetic North Pole.

deep sea trenches bordering volcanic island

The other physical feature about the Pacific Ocean that's different from the Atlantic Ocean is that it's bordered by deep ocean trenches. These Google images show a deep trench adjacent to the Aleutian volcanic islands in the north Pacific and the Japanese volcanic islands in the west Pacific. These narrow trenches at the edge of the Pacific are 8 to 12 kilometers deep troughs that occur adjacent to arcs of volcanic islands or directly adjacent to continents like South America.

2nd key discovery from oceans

The second key discovery from the ocean studies was the documentation of magnetic stripping along the ocean floors like that shown here off the Pacific Northwest coast. The bold dashed lines represent oceanic ridges and the colored zones or stripes represent patterns or stripes of alternating magnetic strength preserved on the ocean floor. These stripes are always oriented parallel to ocean ridge segments and the pattern of stripes are the same on both sides of the ridges. This symmetrical magnetic stripping pattern exists in all oceanic crust and was documented when magnetometers towed behind research vessels recorded changes in magnetic strength across the ocean floor.

strength of magnetic field varies away from mid-ocean ridges

The top diagram shows how magnetic strength swings up and down from high to low away from ocean ridges. Note how the broad high or positive magnetism in the middle of the diagram coincides with mid-ocean ridge and how the positive and negative (or up and down) swings are the same on either side going away from the ridge. If we color the positive high strength stripes dark and the negative (or low) strengths light, the magnetic pattern appears like stripes on a zebra oriented parallel to ridges. These magnetic zebra stripe patterns, which are recorded on all ocean floor, were interesting but very mysterious patterns until the paleomagicians recognized that the magnetic field reverses itself or flips.

Geology of Energy and Environmental Impacts

There has been a dramatic increase in use of and demand for energy resources with industrialization and global population growth. This topic covers the nature and origin of the most widely used energy sources (oil, gas and coal, the fossil fuels), the discovery and search for fossil fuels, and the methods and environmental impacts of extracting and burning these energy sources.

Fit of Continents

These are images made in 1858 by geographer Antonio Snider-Pellegrini of two world maps showing his version of how the American and African continents may once have fit together and later separated. In a way, this is the oldest pictorial representation of continental drift in the Atlantic basin. Snider-Pellegrini was not the first to notice the fit of continents - Leonardo Da Vinci in the 1500s, Sir Francis Bacon in the 1600s, and even Benjamin Franklin in the 1700s, all noted the fit of continents in their writings.

age of the sea floor

This beautiful color image of sea floor age was created with using magnetic ocean floor reversals. It reveals how the youngest sea floor in red occurs at the ridges with oceanic crust getting progressively older away from the ridges in both directions. The coolest color blue represents the oldest oceanic crust preserved on Earth today which is only 180-200 million years old, the middle of the Mesozoic Era when dinosaurs roamed the Earth. You can see that the oldest (deep blue colored) oceanic crust occurs in the western Pacific Ocean region and on the continental margins of the Atlantic Ocean. Note how this image of the age of the sea floor mimics the world ocean floor topography map. Their remarkable similarity tells us that the ocean mountains stand high because they are the warmest parts of the ocean floor (recall that lots of heat emanates from the ridge regions) - and therefore they are less dense than the off-ridge portions of the ocean floor. Essentially they are thermal mountains elevated by their lower density compared to the rest of the older, cooler parts of oceanic crust. Density is a key aspect of Geology that we will discuss more later. Two more things about this Age of the Sea Floor map - I've added white double arrowed lines from the Pacific and Atlantic ridges to their margins. Both lines represent the amount of oceanic crust produced by sea-floor spreading in the past 180 million years or so. Clearly the rate of sea floor production has been much faster in the Pacific Ocean than the Atlantic Ocean during this same time period. We know from this that plate motions are not uniform. The second point I want to emphasize is that oceanic crust is incredibly young, will all existing sea floor having formed in the last 180 million years. What did the world look like before this? Well, the ocean floor magnetic stripes are recording sea floor spreading from 180 million years forward to the present. How about we reserve the recording and in a sense play it back? What happens to the position of the continents when we do that? Remarkably, they all come together into a single large landmass or supercontinent - Pangea! So Wegener was right. This is a great example of the robustness of a scientific theory - a theory is supported by many very different datasets or tests. The data from the continents and from the oceans are very different - yet both support the existence of Pangea.

passive continental margin

This block diagram nicely summarizes the physical features making up the ocean floor. The ridges, great fracture zones, abysmal plains, and seamounts constitute the ocean floor interior. Note however that there are two different types of submerged continental margins where the oceans and continents meet. The continental margin on the left side consists of a broad shallow shelf that steepens to form a slope that extends down to where the ocean floor starts to rise up from the abysmal plain region. Continental shelf-slope-and-rise regions are sometimes down cut by narrow and deep valleys called submarine canyons. Broad continental shelves form along passive continental margins that are not plate boundaries and thus lack earthquakes and volcanoes. An example is the east coast of North America. In contrast, active continental margins have a narrow drowned shelf that slopes steeply into a trench which typically borders active volcanic islands or continents with active volcanoes. Active continental margins do constitute a plate boundary with lots of earthquakes and examples include the continental margins which surround the Pacific Ocean and make up the Pacific Ring of Fire.

formation of magnetic stripping on ocean floor

This diagram shows the creation of oceanic crust by sea floor spreading over time. New oceanic basalt forming continuously at the ridges records or freezes in the polarity of the Earth's field at the time of crystallization of the melt. At is moves away from the ridge on both sides, the oceanic crust and sea floor becomes increasingly older and the magnetic stripes preserve the time of prior periods of normal and reverse polarity in the geologic past. This explanation for the magnetic stripping recorded in the ocean regions was an extraordinary test of and perfectly consistent with Hess's sea-floor spreading idea.

Magnetic Inclination

This image shows how the tilt of a compass needle would change moving from the equator to the North Pole if it weren't artificially weighted, that is held horizontal. The inset circle in the lower right shows that inclination is the angle of tilt down from the horizontal. Looking at the Earth's surface near the equator, the tilt or inclination of the needle is zero as the field lines there are parallel to Earth's surface near the equator. In contrast, at the North Pole, the inclination is 90 degrees or vertical, as the needle would be perpendicular to the Earth's surface, just as the magnetic field lines are oriented. And in-between the equator and the pole, the inclination steepens gradually going from the equator to the pole. The take-home message about magnetic inclination is that it varies from zero (or no tilt) at the equator and increases with increasing latitude toward the polar regions, until it's very steep or even vertical (90 degrees). When certain rocks form, they freeze in the magnetic inclination at the location where the rock formed - so rocks with low magnetic inclination must have formed near the equator, and rocks with high or steep inclination must have formed at high latitudes near the poles.

key discoveries from the oceans

This is one of the early sonar images of a segment of a ridge in the Pacific Ocean. Warm red colors represent shallow water depth or ocean ridges and cooler blues depict deeper depths or lower ocean floor elevations. These types of images were the first to show that the ocean floor was not merely flat like a shallow bowl or saucer - sonar revealed a rich underwater world of dramatic physical features. In addition to underwater mountains like the Pacific ridge, other key discoveries included great fracture zones (which are some of the largest faults on Earth), deep sea trenches (which I already mentioned earlier with regard to the Pacific Ring of Fire), and seamounts.

The Magnetic Memory of Rocks

This photo shows a stack or sequence of multiple basalt flows that erupted millions of years ago in Washington State. In sequences like this, where each layer flowed onto the surface sequentially, we know that the oldest visible flow would be at the base of the sequence, at about lake level, and that younger flows formed on top of older flows in sequence with the youngest visible flow seen at the top of the cliff and along the flat plateau extending away from the cliff in the background. Now Washington State is at an intermediate latitude today, but the magnetic minerals in these ancient basalt flows do not align with the Earth's current magnetic field - they aligned to the field that existed when the rock originally cooled from lava millions of years ago. In a sense, rocks have a magnetic memory from when they initially formed, from their birthdays.

Volcanic Rock Layers permanently record magnetic reversals

This sketch of a volcano shows the different volcanic layers that erupted onto the surface as the volcano grew. The purple lines represent the magnetic field lines at the volcanoes' location on Earth. The angle of tilt of these lines represents the magnetic inclination angle (shown here as moderately steep) and the arrowheads on these lines reflect periods of normal polarity and reverse polarity (with arrows pointing in the opposite direction). We see that the youngest layer on top, which is 0.4 million years old or m.y. (or 400,000 years old) has normal polarity, but the 800,000 year old volcanic rock below it has a reverse polarity. We therefore know that there must have been a polarity flip or reversal between the eruption of these two rock layers. And we know from the third and oldest sample that there must have been another reversal again between 1.2 million years and 800,000 years.

volcanic ash

Volcanic ash consists of fragments of pulverized rock, minerals and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter.[1] The term volcanic ash is also often loosely used to refer to all explosive eruption products (correctly referred to as tephra), including particles larger than 2 mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere. The force of the escaping gas shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. Ash is also produced when magma comes into contact with water during phreatomagmatic eruptions, causing the water to explosively flash to steam leading to shattering of magma. Once in the air, ash is transported by wind up to thousands of kilometers away. Due to its wide dispersal, ash can have a number of impacts on society, including human and animal health, disruption to aviation, disruption to critical infrastructure (e.g., electric power supply systems, telecommunications, water and waste-water networks, transportation), primary industries (e.g., agriculture), buildings and structures. Volcanic ash is formed during explosive volcanic eruptions, phreatomagmatic eruptions and during transport in pyroclastic density currents.

plate tectonics

We will begin the course on the topic of Plate Tectonics, the theory that the outer layer of Earth consists of rigid moving plates. Plate Tectonics is one of the top 10 scientific discoveries of the 20th century and it serves as the foundation for understanding most geologic phenomena. This image outlines in black the boundaries of the large plates that constitute the rigid outer shell of earth. And it shows the location of active volcanoes on earth with red dots. We see that the majority (about 80%) of earth's 1900 active volcanoes occur around the outer rim of the Pacific. This Pacific "Ring of Fire" is a direct result of active Plate Tectonics.

Wegener used Fossil Evidence

Wegener also used fossil evidence to connect continents that are now widely separated. Since land animals and plants cannot cross oceans, they evolve independently on different continents. But when continents are together, land animals and plants can migrate across them. By examining the late Paleozoic and early Mesozoic fossils, Wegener was able to show that certain species had indeed lived on several continents, including species of ferns and reptiles as shown in this reconstruction.

Glacial Deposits and Striations fit together near the South Pole in Wegener's Pangea

Wegener hypothesized that these ancient glacial features, which are now widely separated from one another, and even extend to north of the equator today, could have been a single ice sheet located near the South Pole at the end of the Paleozoic (around 250 to 260 million years ago).

Wegener's used Fit of Continents too

Wegener noted the jigsaw puzzle fit of the continents as others had before him, but with a twist. Instead of using the coastlines of the continents, Wegener realized that the true edge of continents actually extend out a ways from the coastline, to where the underwater continental shelves start to steepen or slope down into the ocean. By including the continental shelves as the drowned true edge of continents, and as shown in the cyan color bordering the continents of this map, Wegener was able to show that the continents fit together even better, with fewer gaps between the puzzle pieces.

Wegener used Glacial Evidence

Wegener recognized and mapped 260 million year old Paleozoic glacial till deposits in parts of South America, Africa, southern Australia, and even in tropical India, places today that are much too warm to harbor glaciers! He also recognized glacial grooves and scratches on rocks (called striations) that were carved by rocks embedded in the base of the glacier as it was moving. And he noted that on a map these striations were oriented so that they appeared to be radiating outward from a location in southern Africa.

Intro to Plate Tectonics - Lecture Summary Part 2

Wegener's Land-based Evidence for Pangea and its breakup by continental drift included: 1) Fit of continents - including continental shelves (drowned margins of continents) 2) Glacial Evidence - 260 million year glacial deposits (till) - mapped in Australia, Africa, South America and India - suggest these continents plus India where under a single ice sheet located at the South Pole. 3) Climatic Evidence - sedimentary and marine deposits around Wegener's proposed Pangea supercontinent match with their climate zones. Glacial deposits are found in the southern polar region of Pangea whereas coal and reef limestones are found in the tropical regions of Pangea - just like one would expect. 4) Fossil Evidence - late Paleozoic and early Mesozoic species exist on southern continents now widely separated. This suggests those continents were in contact with each other then. 5) Matching Geologic features - including rock units, rock structures (mountains) and rock ages Wegener's idea of Continental Drift and Pangea was rejected by nearly everyone in the scientific community, largely because Wegener couldn't explain how continents moved through oceanic crust

Paleomagnetism: Ancient Magnetism Preserved in Rocks

Welcome back! There are two aspects about Paleomagnetism, the study of ancient magnetism preserved in certain rock types. The first is that rocks have a magnetic inclination, or tilt, that is related to the latitude where the rock formed. And the second, which we will cover today, is that rocks record regular reversals of the magnetic field, that is the flipping of the N and S magnetic poles in the past. So what type of rocks preserves a record of the Earth's magnetic field when it forms?

observations of the ocean floor

Welcome back. Ocean studies were critical to the discovery of plate tectonics. Recognition of major physical features of the ocean floor, including ocean ridges, great fracture zones, deep sea trenches, and seamounts led to Dr. Hess's idea of sea-floor spreading as the mechanism for continental drift. Another key discovery from the oceans related to magnetic reversals provided a key test for the concept of sea-floor spreading and cemented the realization that the continents and the ocean floor together make up the mobile plates.

observations of the ocean floor

What would the ocean floor look like without water? Prior to World War II people knew very little about the ocean floor. Military needs during World War II gave a boost to sea floor exploration. With submarines and warfare, suddenly countries became very interested in mapping the ocean floor. Prior to World War II, researchers used plumb lines (that is a weight at the end of cable) to determine the ocean depth at one location (a pinpoint), an incredibly tedious task, but multibeam sonar technology greatly enhanced ocean floor observations.

Ring of Fire

Zooming back out to this image of the entire Pacific ocean, the reddish shaded regions outline the location of most of the worlds earthquakes and volcanoes, extending along the west coasts of North and South America, the Aleutian Islands in the north Pacific, and the Japanese and Philippine Islands in the northwest Pacific and southward down to the islands of New Zealand in the southwest Pacific. Across nearly the entire Ring of Fire, we see that the chains and islands of volcanoes are parallel to the deep sea trenches, shown as blue lines. We also see that the deep sea trenches occur everywhere on the ocean side of the volcanic islands. Today, we recognize the Pacific Ring of Fire as the active boundary zone between large tectonic plates, where the oceanic plates are being subducted into the earth beneath the surrounding continental regions. The Pacific Ring of Fire is one of the primary lines of evidence that the outer shell of earth consists of a few large rigid plates that move and interact with each other at the edges of the plates. This is the Theory of Plate Tectonics, the unifying idea that explains much of the way the Earth works.

what do geologists study

they are interested in what Earth is made of - its materials. Mostly solid materials like minerals, rocks, and fossils, but also liquid materials like magma, lava, surface water, subsurface fluids (oil and gas), and even vapors or gases such as those released during volcanic eruptions or released by the burning of fossil fuels Earth processes, such as the processes by which rocks, mountains, oceans, and even continents form and the processes by which rocks, mountains, oceans, and continents are destroyed. the history of Earth including the history of life on Earth.