FINAL

Oceanic-Continental Convergent Plate Boundaries

-oceanic crust (which is denser) is subducted (forced) under the continental crust (which is less dense) & re-melted in the Asthenosphere -this creates an underwater trench along the plate boundary -continental crust is compressed, forming folded mountains and volcanic mountains where magma reaches the surface thru cracks and weak points -high earthquake and volcanic activity These plate boundaries are associated with volcanic arcs on the continents (called continental arcs) and trenches in the ocean where the two plates converge.

Oceanic-Oceanic Convergent Plate Boundaries

-subduction of one plate and re-melting of it -deep trenches created at boundaries -high level of volcanic & earthquake activity -formation of volcanic island archipelagoes (chains) These plate boundaries are associated with volcanic arcs in the ocean (called island arcs) and trenches in the ocean where the two plates converge. The diagram below shows an example of an island arc that is created at an oceanic-oceanic convergent plate boundary.

Earth's initial atmosphere

-was composed of gases that were leftover from the solar nebula: hydrogen and helium. These gases were blown away by the solar wind. The solar wind consists of electrically-charged particles that stream from the sun. -Once Earth differentiated into layers, Earth's atmosphere was protected! These gases, similar to gases emitted by volcanoes today, consisted of mainly water vapor, with small amounts of carbon dioxide, hydrogen, and other trace gases. This formed Earth's second (early) atmosphere. The composition of the Earth's atmosphere is not the same today, largely due to the evolution of life. Before life can evolve on Earth, however, we need to have oceans.

Paleoceanography

(the study of past climate) study of how ocean, atmosphere, and land interactions have produced changes in ocean chemistry, circulation, biology, and climate Sediments are interesting to scientists because they tell a story about the environmental conditions that prevailed when the sediments were deposited. Changes in the sediments that are deposited at the bottom of a body of water indicate a change in the depositional environment, such as a shift in temperature and/or precipitation patterns. In this way, oceanographers decipher Earth's history through studying sediments. This is the field of paleoceanography.

Mid-Ocean Ridge System

-At divergent plate boundaries, seafloor spreading creates new oceanic crust (basalt). The new basalt it hot, and is therefore less dense (more expanded) than older seafloor basalt, so it rises up off the seafloor higher than the surrounding seafloor. This creates the mid-ocean ridge system that rises 2.5 kilometers (1.5 miles) above the surrounding seafloor. -Common features along mid-ocean ridges are volcanoes, seamounts, and earthquakes. -

Convergent Plate Boundaries

-Convergent plates move together and may be oceanic-oceanic, oceanic-continental, or continental-continental. Old seafloor is destroyed when oceanic plates subduct. At oceanic-oceanic convergent plate boundaries a trench and volcanic island arc is created (example: Peru-Chile Trench and Andes Mountains). At oceanic-continental convergent plate boundaries a trench forms on the seafloor and a continental arc is created on land (example: Mariana Trench and Aleutian Islands). Continental collisions create tall mountains (example: Himalaya Mountains and the Alps). -Notice that ocean-continent and ocean-ocean convergent boundaries are similar in that they both involve the process of subduction. -Oceanic-continental: oceanic lithosphere subducts, causing partial melting of overlying rocks in the mantle. The magma rises and creates a continental arc on the land. A trench is created in the ocean along the plate boundary. -Oceanic-oceanic: oceanic lithosphere subducts, causing partial melting of overlying rocks in the mantle. The magma rises and creates an island arc on the surface of the oceanic plate that is not subducting. A trench is created in the ocean along the plate boundary. Continental-continental: a mountain range is created because continental crust is too low in density to subduct. Nearly all oceanic crust is returned to the mantle along convergent plate margins through the process of subduction.

Coral Reefs

-Coral reefs are composed of colonies of individual polyps. They are small benthic marine animals that feed with stinging tentacles similar to jellies. Coral secrete hard calcium carbonate structures (reefs) for protection. -Corals live symbiotically with photosynthetic microscopic dinoflagellate algae called zooxanthellae that live within their tissues. It is the zooxanthellae that give the coral their color. The zooxanthellae provide their coral hosts with a continual supply of food, and the corals provide the zooxanthellae with nutrients. Although coral polyps capture tiny planktonic food with their stinging tentacles, most reef-building corals receive up to 90% of their nutrition from symbiotic zooxanthellae algae. In this way, corals are able to survive in the nutrient-poor waters that are characteristic of tropical oceans. -Coral are temperature sensitive. They are restricted to warm, tropical waters (>64oF all year, but must not exceed 86oF for long). For this reason, they are mainly limited to tropical waters between about 40o north and 40o south latitude. -Besides warm water, other environmental conditions necessary for coral growth include: --Strong sunlight. The sunlight isn't needed by the corals themselves, which are animals and can exist in deeper water, but by the photosynthetic zooxanthellae that live within the coral's tissues and provide nutrients to the coral. --Strong wave or current action to bring nutrients and oxygen. --Lack of turbidity. Turbidity refers to suspended sediment in the water. The water will appear cloudy or muddy if it has many suspended particles. Suspended particles in the water absorb radiant energy, interfere with the coral's filter-feeding capability, and can even bury corals. For these reasons, corals are not usually found close to areas where major rivers drain into the sea. --Salt water. Corals die if the water is too fresh, which is another reason coral reefs do not form near the mouths of freshwater rivers. --A hard substrate for attachment. Corals cannot attach to a muddy bottom, so they often build upon the hard skeletons of their ancestors, creating coral reefs that are several kilometers thick. --When coral dies or breaks apart, the pieces become biogenous sediment. Most corals are found in shallow, warm waters, making them neritic biogenous sediment.

Divergent Plate Boundaries

-Divergent plates move apart and may be oceanic-oceanic or oceanic-continental. New seafloor is created and this is called seafloor spreading. When a continent starts to split apart, this is called continental rifting. Seafloor features include mid-ocean ridges, volcanoes and young lava flows. Geographic examples include the Mid-Atlantic Ridge and the East Pacific Rise, the Red Sea, the Gulf of California, and the East Africa Rift Valley. -At divergent plate boundaries, seafloor spreading creates new oceanic crust (basalt). The new basalt it hot, and is therefore less dense (more expanded) than older seafloor basalt, so it rises up off the seafloor higher than the surrounding seafloor. This creates the mid-ocean ridge system that rises 2.5 kilometers (1.5 miles) above the surrounding seafloor.

Isostatic Adjustment

-Earth's layers float on each other due to their differences in density. This is based on the concept of isostasy. Isostasy is based on Archimedes' Principal, which states, "An object in a fluid is buoyed (lifted) up by a force equal to the weight of the fluid that is displaced." Essentially, an object sinks until it displaces a volume of fluid that has the same mass as the entire floating object. This means that: -An object will sink if it is heavier than the fluid it displaces (is denser than the fluid it displaces). -An object will rise if it is lighter than the fluid it displaces (is less dense than the fluid it displaces). -A floating object is balanced between sinking and rising.

Earth's Early Oceans

-Water vapor accumulated in Earth's early atmosphere due to outgassing, but also due to ice comets and asteroids that still frequently bombarded our Early Earth. As Earth cooled over time, this water vapor began to condense, forming clouds in Earth's early atmosphere. With continued cooling, the water droplets began to fall to Earth's surface, where it accumulated in the low areas (covered by basalt). This established weather patterns that continued for hundreds of millions of years, slowly filling the low-lying areas with water. By 4 billion years ago, Earth had early oceans! -The rain that fell was initially very acidic (due to lots of CO2 and SO2 in the early atmosphere). This acidic water was very efficient at dissolving ions in the rocks on Earth's surface. As the water flowed over the rocks to fill the low-lying areas, it dissolved the ions, making the ocean "salty."

Sediment Maturity

A sediment becomes more mature as it travels further from its source (just as we are said to get more "mature" as we grow older). As a sediment becomes increasingly mature: -Clay content decreases -Sorting increases -Non-quartz minerals decrease -Grains are more rounded (abraded)

Turbidite Deposits

A sediment or rock formed from sediment deposited by turbidity currents characterized by both horizontally and vertically graded bedding. (in submarine canyons at the edge of the continental shelf). The sediment spreads out at the base on the slope as deep-sea fans, which collectively make up the continental rise. Since these sediments are derived from land and are composed of broken down pieces of rock, they are lithogenous. Since they are found on the continental margins, they are neritic.

Active Continental Margins

Active continental margins are associated with a plate boundary that may be convergent or transform active. -Convergent Active An example of a convergent active continental margin is the west coast of South America, where the Nazca Plate subducts below South American Plate as shown below. Here, the continental shelf is narrow and the slope drops into the trench. There is no continental rise. -Transform Active A break in the subduction around the edges of the Pacific Ocean occurs along the coast of California, where the boundary between the Pacific and the North American plates is transform instead of convergent. This is where the Pacific Plate slides past the North American Plate. As illustrated in the figure below, the continental shelf here is marked by banks, basins and islands. It is called a continental borderland, and there is no trench because there is no subduction.

Cold Seeps

Area on the ocean floor where gases, nutrients, and other materials are released. The three main types of cold seeps are hypersaline, hydrocarbon, and subduction zone seeps. Notice in the map above that subduction zone seeps are associated with trenches, where water seeps from folded sedimentary rocks on one side of the trench. Hypersaline seeps occur at the base of the continental slope (where the rocks are fractured), such as the Florida Escarpment. Hydrocarbon seeps are more rare, and are found along continental slopes. All three types of cold seeps are associated with liquids that are rich in hydrogen sulfide and/or methane. These chemicals are used by bacteria to chemosynthetically produce food, thereby supporting a great diversity of marine life in deep ocean environments. This is a new frontier of research, and scientists are constantly learning more about life in the oceans.

Oceanic Crust

As Earth cooled, basalt formed. Basalt forms what we now call oceanic crust. Basalt is a dark-colored igneous rock that is very dense (3.0 g/cm3), but thinner than continental crust Oceanic crust "floats" lower due to its higher density

Satellite Altimetry

As you can imagine, sonar systems are fairly labor intensive. Scientists have to go out in a boat, and go back and forth across the ocean floor to map it out. For this reason, satellites are also used. A satellite measures the variation of ocean surface elevation, which is caused by gravitational attraction and mimics the shape of the seafloor. Essentially, these are small bulges on the ocean surface. They are too small to see with the naked eye, but satellites use radar pulses to detect anomalies (differences) in sea surface height.

Magnetic Striping on the Seafloor

As you hopefully gleaned from the animation, magnetic "stripes" are not actual stripes that you can see on the ocean floor. They are magnetic anomalies that are recorded in rocks and measured by a special instrument called a magnetometer. The magnetic anomalies are created due to changes in the direction of Earth's magnetic field (an "anomaly" is a departure from normal) that occur every few hundred thousand years. Magnetic stripes on the ocean floor are used to reconstruct motion at divergent plate boundaries, where seafloor spreading occurs.

Autotrophs: Chemosynthesis and Photosynthesis

Autotrophs, those that could produce their own food from sunlight (through photosynthesis) or chemicals (through chemosynthesis), likely evolved after the first primitive heterotrophs. It is believed that anaerobic bacteria probably evolved first, using chemosynthesis near hydrothermal vents on the deep ocean floor near mid-ocean ridges (oldest evidence 3.2 billion years ago), then photosynthetic bacteria evolved in warm, shallow waters (stromatolites by 3.5 billion years ago).

Quartz-rich Sand

Beaches in California are dominated by quartz-rich sand. This is because the source material (the rock that is weathering to produce the sand) is mainly granite (continental crust). Granite contains many different minerals (mainly orthoclase feldspar, quartz, plagioclase feldspar, mica, and amphibole); however, the non-quartz minerals break down as the rock breaks apart and is transported from its source in the mountains. Neritic Lithogenous

Black Sand

Black sand is created when basalt weathers. Basalt is common on volcanic islands like Hawaii and Iceland. Neritic Lithogenous

Oceans Versus Seas

Compared with the ocean, seas are smaller, shallower, still salty, and usually enclosed by land We now include the Indian Ocean as an ocean, not a sea. Additional seas include the Baltic Sea, Bering Sea, North Sea, South China Sea, Sea of Okhotsk, and the Caribbean Sea.

Glacial Deposits

Glacial deposits are most common on the high-latitude continental shelves, such as along the coast of Greenland. These deposits are composed of poorly-sorted sediment called glacial till and may include large drop stones that were carried out to the ocean while frozen in icebergs ("ice rafting").

Alfred Wegener

Continental Drift, which we now think of as the precursor to plate tectonic theory, was first proposed by Alfred Wegener in 1912. Although Wegener had a wealth of evidence to support the idea that the continents have not always been in their present position, he could not provide a mechanism that would have caused the massive continents to move. For this reason, he was ridiculed by the scientific community, and it wasn't until much later (after his death) that further evidence supported his claims. Alfred Wegener noticed many compelling arguments that suggest that the continents have not always been in the position that they are in today. In fact, they used to be connected in a supercontinent called Pangaea. Over time, the continental landmasses broke apart and drifted to their present locations. Despite all of the compelling evidence outlined above indicating that the continents have not always been in their present positions, no one could explain the mechanism responsible for moving such large masses of rock. Alfred Wegener was unable to convince the scientific community of continental drift during his lifetime. He died on an expedition to Antarctica in 1930. Further supporting evidence revealed itself decades later, when new observations shed light on the processes occurring on the ocean floor.

Continental Margins

Continental margins are the edges of the continents where Earth's crust transitions from continental crust (mainly granite) to oceanic crust (basalt). Continental margins are classified as active or passive depending on presence or absence, respectively, of a plate boundary at the edge of the continent.

Coral Importance

Coral are important because their reefs provide shelter, food, and breeding grounds for numerous other species. Coral reef ecosystems are characterized by a diversity that surpasses even tropical rainforests. Coral cover <0.5% of the ocean's surface area, but are home to 25% of all marine species. They are important to humans because healthy reefs teeming with marine life support tourism, fisheries, and they prevent shoreline erosion.

Turbidity Currents

Downslope movements of dense, sediment-laden water; underwater landslides. They are water-saturated slurries of sediments that break off the continental shelf and fall down to the base of the continental slope due to gravity.

Abyssal Clay

Due to the slow suspension settling of these fine particles, the abyssal plains are covered by a thick blanket of fine sediment called abyssal clay. Abyssal clay is composed of at least 70% (by weight) fine clay-sized particles from the continents. Recall from Chapter 3 that the abyssal plains are extremely flat due to the slow accumulation rate of abyssal clay. Abyssal clay is also called red clay because it often contains iron that has been oxidized, giving it a red or buff color. Abyssal clay is abundant when other sediments are absent. In the sections that follow, we will learn about other sediments that might be found on the deep ocean floor, but only when conditions are just right. For this reason, we can think of abyssal clay as the default sediment on the deep ocean floor unless specific conditions exist for the accumulation of other types of sediment. The photograph to the right captures the deep ocean floor, covered by fine abyssal clay.

Proto-Earth

Early Earth looked very different from the Earth we are familiar with today. Proto-Earth was larger than Earth today. It had no life, no oceans, and it was homogeneous (the same throughout). Shortly after the Earth's formation, the moon formed after a large asteroid collided with Earth. Heat from collisions and internal heat from radioactive decay caused early Earth's surface to melt.

Density Stratification (differentiation)

Earth became chemically and physically layered (stratified) when the denser materials sank to middle, and the less dense materials floated to surface. In this way, the initially homogeneous Earth became separated into layers based on density. The process of separating into layers is called differentiation, because the Earth is differentiating (or separating) into layers.

East Africa Rift Valleys

Examples of the early stages of divergent plate boundary formation occur in East Africa. The map and photograph on the left, below, illustrate how rifting in East Africa is pulling apart the continental crust. In this region, there are numerous deep lakes (in the rifts, or down-dropped sections of crust) and active volcanoes that form when the magma underneath erupts on the surface. The same system of mantle plumes that created the East Africa Rift Valleys also created the Red Sea and Gulf of Aden that separated the Arabian Peninsula from Africa. The Red Sea and the Gulf of Aden are further along in the process of plate boundary formation (juvenile stage) than is East Africa.

Harry Hess (1906-1969)

Geologist Harry Hess (1906-1969) was a U.S. Navy captain during World War II and inadvertently recorded detailed ocean depth data while out at sea. Analysis of this data revealed massive undersea mountain ranges (mid-ocean ridges) in the middle of ocean basins and deep, narrow features (trenches) towards the edges. This led to his proposal of sea floor spreading in 1962, along with mantle convection as the driving force behind the motion. This major advance in understanding could finally provide a mechanism for the movement of the continents, and the formation of new ocean basins.

Hotspots

Hotspots are created by mantle plumes (plumes of heat that rise from deeper within the Earth). We don't know precisely what causes them, but they are thought to arise from convection currents in the mantle. Some hotspots may have a shallow mantle source, while others have a deeper mantle source. Because they originate deeper in the mantle, lithospheric plates move over the hotspot. -As an oceanic plate moves over hotspot, a chain of volcanoes is produced that may create islands. This is because oceanic crust is thin, and the magma can pass right through it to Earth's surface. This is what is responsible for the creation of the Hawaiian Islands-Emperor Seamount Chain. -As a continental plate moves over hotspot, rifting can occur (especially if multiple hotspots are aligned). Rifting is the process where crust is thinned and pulled apart. The magma can seep to the surface through fissures that crack the Earth open. This is what is happening in East Africa today. -The present-day hotspot that is responsible for creating the Hawaiian Islands is located just off the SE coast of Hawaii. The islands are located in the middle of the Pacific Plate, which is moving over the hotspot. Older islands and seamounts are located further from the modern-day hotspot as the plate moves to the northwest.

Hydrothermal Vents

Hydrothermal vents are also associated with mid-ocean ridges. Hydrothermal vents are sea floor hot springs that are created when cold seawater seeps through cracks and fractures in the ocean crust and approaches an underground magma chamber. The water becomes superheated (since it can't boil and evaporate at such high pressures in the deep ocean). The superheated water dissolves minerals and ions in the rocks when it makes it way back up toward the ocean floor. -The temperature of the water that rushes out of a particular hydrothermal vent determines its appearance: -Warm-water vents have water temperatures below 30°C (86°F) and generally emit water that is clear in color. -White smokers have water temperatures from 30° to 350°C (86° to 662°F) and emit water that is white because of the presence of various light-colored compounds, including barium sulfide. -Black smokers have water temperatures above 350°C (662°F) and emit water that is black because of the presence of dark-colored metal sulfides, including iron, nickel, copper, and zinc.

Iceland's Rift Valley

Iceland is on the divergent plate boundary along the mid-Atlantic Ridge. In fact, Iceland was created by basaltic magma that erupted from the ridge. This has been a particularly active section of the ridge, so the basalt accumulated to such a degree that it formed an island.

Barrier Reef

In a barrier reef, the volcano has become extinct and begins subsiding as the rocks cool and contract. The coral continue to grow upward on the skeletons of their ancestors to maintain their access to sunlight. The world's largest and most famous barrier reef is off the east coast of Australia, the Great Barrier Reef The southern part of the Great Barrier Reef has the most recent coral development and is located along the Tropic of Capricorn (23.5 degrees south latitude). As Australia continues to move north, the Great Barrier Reef will continue to expand southward along Australia's east coast.

Age of Discovery

The Age of Discovery was the period from 1492 to 1522, during which time the voyages of Columbus and Magellan were undertaken. Magellan's voyage was the first to circumnavigate the globe. the Western European world realized the vastness of Earth's water-covered surface

Continental Hotspots: East Africa and Yellowstone

The East Africa Rift Valleys, discussed previously, are though to have been created by multiple hotspots that are aligned in this region underneath East Africa (see map below). Yellowstone National Park is also moving over a continental hotspot. Yellowstone is known as a supervolcano, because is massive magma chamber is deep under the surface, and there is no volcanic peak that rises above the surface. Yellowstone supervolcano experiences massive eruptions every 600,000 years or so. It has been 640,000 year since its last major eruption.

Mendocino Fracture Zone

The Mendocino Fracture Zone illustrates that transform plate boundaries that create offsets in the mid-ocean ridge system have active and inactive portions. The active segment is called the transform fault (and it is a plate boundary). The inactive segment is called the fracture zone (and it is not a plate boundary, it is just a scar left on the rocks by the transform fault). The Mendocino Fracture Zone is created where the San Andreas Fault cuts into the Pacific Ocean from northern California.

Ocean Trenches and Volcanic Arcs

Trenches and volcanic island arcs also occur in the deep ocean when the tectonic setting is conducive to their formation. In the Pacific Ocean, subduction occurs along the edges of the ocean basin, creating the Pacific Ring of Fire with active volcanic arcs.

Continental Transform Faults

Motion along transform faults cause displacements of surface features

Multibeam Sonar

Multibeam sonar utilizes multiple frequencies of sound to generate detailed images of the ocean floor along a strip up to 60 km wide. The sound waves travel from the acoustic instrument down to the ocean floor, they are reflected off the ocean floor, and then they return to the instrument. We know the speed of sound in water (1507 meters/second), so we can calculate the water depth using: Distance = (Speed of sound in water) x (Travel time/2). We have to divide the travel time by two to account for the sound waves traveling down the seafloor and back again. These data allow oceanographers to determine the depth, shape, and even composition of the seafloor.

Age of Ocean Crust

Notice that a pattern is evident, with the youngest crust along the axis of the mid-ocean ridge system, and age progressively increases with distance from the ridge. Furthermore, the oldest oceanic crust is only 180 million years old. The Earth is 4.6 billion years old, so oceanic crust is much younger and newer than the Earth itself. This confirmed the idea that the ocean floor is constantly renewing itself. According to plate tectonic theory, new ocean floor is created at mid-ocean ridges due to seafloor spreading. The new basalt is pushed off the ridge into deeper water, and it becomes older as it moves further away from the ridge. Eventually, this oceanic crust is pushed back into the mantle at subduction zones, where it is recycled back into the mantle

ocean size largest to smallest

Pacific, Atlantic, Indian, Arctic The Pacific Ocean is the world's largest and deepest ocean. The Atlantic Ocean is the second-largest ocean, and it is about half the size of the Pacific. The Indian Ocean is only slightly smaller than the Atlantic, and it exists mostly in the southern hemisphere (the area of Earth that is south of the equator). The Arctic Ocean is the smallest and shallowest ocean. The Southern Ocean is actually composed of portions of the Pacific, Atlantic, and Indian Oceans, but is defined by the "Antarctic Convergence" zone where ocean currents meet. The oceans hold 97% of Earth's water, leaving only 3% of Earth's water as freshwater that is found on land and in groundwater. A great majority (~99%) of Earth's freshwater is frozen. The Atlantic Ocean is Saltier than the Pacific Ocean. This is due to its closer proximity to Land, higher evaporation rates, and wind patterns that carry water vapor from the Atlantic Ocean basin into the Pacific Ocean basin.

Passive Continental Margins

Passive continental margins are not associated with a plate boundary. There is simply a passive transition from the continental shelf to the continental slope to the continental rise. Examples of passive continental margins are the east coast of North America and the east coast of South America, where no plate boundaries exist at the edges of the continents

Earth's surface

Roughly 70.8% of Earth's surface is covered by ocean, leaving only 29.2% above sea level. Notice that Earth's ocean is all connected, so it is really just one big ocean that we divide geographically.

Rounding

Rounding refers to the shape of the individual sediment grains. More rounded sediment grains have experienced more abrasion (rubbing against each other like sand paper) than more angular grains. For this reason, more rounded sediment particles indicate that the sediment grains have traveled further from their source.

Scientific Method

Science is based on the understanding that natural phenomena are governed by physical processes, and physical processes that operate today are similar to the physical processes that occurred in the past. Observations, Testing Ideas, Drawing Conclusions The scientific method is not linear

Seamounts

Seamounts are created by underwater eruptions of (basaltic) lava. If only a small amount of lava bursts through the seafloor, an abyssal hill may form. Continuous eruptions will build the peak, making it larger over time as long as the source of magma remains active. Once the source of magma is no longer underneath it, the seamount becomes extinct. Most seamounts on the ocean floor are therefore extinct volcanoes that rise hundreds or thousands of feet from the seafloor. seamounts are commonly found near oceanic-oceanic subduction zones, divergent plate boundaries, and at oceanic hotspots. At mid-ocean ridges, plates are spreading apart and magma rises to fill the gaps. Once this magma breaks through the surface of the seafloor as lava, it piles up and can form a seamount. seamounts fertile habitats for diverse communities of marine life, including sponges, crabs, sea anemones, commercially important fish, and deep-sea corals.

Frederick Vine and Drummond Matthews

Shortly after Harry Hess published his History of Ocean Basins outlining the processes above, Frederick Vine and Drummond Matthews confirmed the processes with additional findings. In 1963, they connected the symmetric pattern of seafloor magnetic "stripes" on either side of mid-ocean ridges with the process of seafloor spreading; This evidence strengthened our understanding of seafloor spreading as a mechanism to force plates apart, and create new oceanic crust (basalt) along the axis of the ridge.

Side-scanning Sonar

Side-scanning sonar also utilizes multiple frequencies of sound to generate detailed images of the ocean floor. Here, a side-scanning sonar instrument (called "GLORIA") is towed behind a survey ship and can map a strip of ocean floor with a gap in data directly below the instrument. The ship goes back and forth across the ocean floor (in swaths) to map ocean depths and features.

Purple Sand and Heavy Mineral Sand

Some beaches have colorful sand grains. in Big Sur, California. The purple streaks in the sand are concentrated garnet crystals that have weathered from the local metamorphic rocks. streaks of purple are often interspersed with streaks of black. These streaks are created by waves. when waves wash up onto shore (called swash), the water also pushes the sand grains towards the shore. When the water returns to the ocean (called backwash), the lighter minerals are carried away, but the heavier minerals stay behind. This process leaves streaks of heavier minerals in the sand. In this case, the purplish pink grains are garnet, and the black grains are the iron-rich minerals that are weathering from the rocks in the nearby mountains. Neritic Lithogenous

Red Sand

Some sands are red, which would indicate the presence of oxidized iron. Red sands may be created by weathered sandstone (the cement that holds the sand grains together may be iron oxide and/or the sandstone may contain feldspar that is reddish in color), or by weathered basalt (the iron in basalt is oxidized once exposed to the air). The photograph below illustrates what a red sand beach might look like. This image is from Hawaii, where basalt is the source rock (note the black basalt boulders) Neritic Lithogenous

Oceanic Ridges Versus Oceanic Rises

Some segments of the mid-ocean ridge system spread more quickly than others. Fast-spreading segments of the mid-ocean ridge system are called rises because they have a different shape than their slower-spreading counterparts. In general, ridges have a slow rate of spreading. They have steep sides and a prominent rift valley in center. Rises have a faster rate of spreading. They have broader, more gently sloping sides and no prominent rift valley.

Grain Size Sorting

Sorting refers to the range of grain sizes in a sediment. If the sediment is composed of all the same grain size, it is well sorted. A well-sorted sediment can be any size, as long as all of the grains are the same size. If a sediment is composed of a wide range of different grain sizes, it is poorly sorted. The diagram below illustrates grain size sorting. Grain size sorting is an indication of the selectivity of transportation and deposition processes. For example, wind can generally only pick up the finer particles (dust, clay, sometimes sand or larger if the wind is strong). Sediment that is carried and deposited by normal wind patterns therefore tends to be well-sorted. Water is also a very efficient sorting mechanism. The grain sizes that can be carried by moving water depends on the energy of flow. A slow trickling stream can only carry fine particles. A raging flood can carry all.

Stromatolites

Stromatolites are photosynthetic cyanobacteria that built bulbous structures in warm shallow ocean waters with carbonate sediment. Although these are some of the earliest fossils on Earth, these were probably not the first life forms on Earth

Continental-Continental Convergent Plate Boundaries

Subduction does not occur when two continents collide, because the continental crust is too buoyant to descend into the mantle. Once the oceanic crust that separated the continents is completely subducted and the continental crust collides, collisional mountains are created as the crust thickens and is pushed upwards. An example of a continental collision is illustrated below, with the Indian subcontinent colliding with Eurasia.

San Andreas Fault

The San Andreas Fault in southern California is a local example of a continental transform plate boundary. As mentioned above, the bend of the San Andreas Fault in southern California has created hundreds of individual faults in the region. However, not all of them are transform, and not all of them are plate boundaries.

Ocean Depth

The average depth of the world's oceans is 3682 meters (12,080 feet). The average height of land is only 840 meters (2756 feet). This difference will be discussed shortly, but is due to the different type of rocks that make up the ocean floor (oceanic crust: basalt) compared with the continents (continental crust: granite).

black smoker

The cloud of suspended minerals formed where hot water spews out of a vent along a mid-ocean ridge; the dissolved sulfide components of the hot water instantly precipitate when the water mixes with seawater and cools.

Abyssal Plains

The deep-ocean basins are covered by abyssal plains. Abyssal plains are created by the very slow suspension settling of fine sediments (mainly clay, dust, and ash) blown into the ocean by wind or carried into the deep ocean by deep ocean currents. When conditions are right, other types of sediments may accumulate on the deep ocean floor as well (such as hydrogenous manganese nodules or biogenous silicious ooze).

Core

The densest material (iron with nickel and sulfur) sank to the middle, forming the core. The inner core is solid because it is under so much pressure, and the outer core is liquid. Convection (rotation) in the outer core generates the magnetic field.

Atoll

The final stage in coral reef development occurs when the extinct volcano is completely submerged under the ocean's surface. The reef continues to grown upward, forming a ring around a lagoon.

heterotrophs

The first life on Earth were probably early heterotrophs who fed on non-living organic molecules. Heterotrophs are organisms that cannot produce their own food (humans are heterotrophs).

Seismic Profiling

The last main method for studying ocean bathymetry is seismic profiling. This method also uses multiple frequencies of sound, but it differs from the other methods in that it provides information about sub-surface materials and layers below the seafloor. A boat releases an explosion, or a boom, that emits low frequency sounds that can penetrate bottom sediments and rocks layers. The sound waves travel through the water and down below the ocean floor into underlying sediment and rock layers. Different frequencies bounce off different layers, and are received by a receiver that is attached to the boat. Scientists analyze the reflected sounds, and interpret the layers of rocks and sediments below the seafloor.

Crust

The lighter silicate materials floated to Earth's surface, creating the crust. The crust is very thin, like the skin of an apple. Rocks in the crust are rigid and brittle.

Oceanography

The modern field of oceanography has evolved since the term oceanography was first coined in the 1870s. Oceanography has emerged as an interdisciplinary science that draws on multiple other fields. This is because Earth's spheres, including the geosphere (rocks), atmosphere (gases), hydrosphere (water), biosphere (life), and cryosphere (ice; sometimes included in the hydrosphere) are all connected. A change in one of Earth's spheres will affect all other spheres.

Collecting Marine Sediments

The photographs below illustrate some methods that scientists use to collect sediment samples. Sediments are usually collected as a core sample, which is a cylinder of sediment. Essentially, an empty pipe is driven into the sediments at the bottom of the body of water, and the sediments are captured inside the pipe.

Mantle

The thick middle layer of Earth is the mantle. This layer is made of silicate materials rich in iron and magnesium. The lower mantle is called the mesosphere, and it is solid. The upper mantle is called the asthenosphere. The asthenosphere is closer to the surface of the Earth, so it is under less pressure (but it is still hot). For these reasons, rocks in the asthenosphere can bend and flow (but they are still mostly solid). The asthenosphere is plastic, which is a texture like silly putty. Above the asthenosphere is the lithosphere. The lithosphere includes the uppermost mantle and the crust. The lithosphere is rigid because the rocks here are cooler and under less pressure.

isostatic adjustment

The vertical movement of crust to accommodate additional weight or removal of weight

Age of the Earth

To put it simply, scientists know from the isotopic dating of rocks that the Earth is 4.6 billion years old. Isotopic dating uses radioactive elements to precisely determine how long it has been since the rock formed.

transform fault

Transform faults along the mid-ocean ridge are plate boundaries. They are where the two plates are sliding past each other locally. On either side of the transform fault is an area called a fracture zone, where the rocks are cracked and uneven. The diagram below illustrates a transform fault with a fracture zone on either side Conversely, if you stand on one side of the transform fault, you are moving in one direction. If you step across the transform fault, you will be moving in the opposite direction. This is because rocks on either side of a transform fault are on different sides of the ridge axis. In the image above, the top side of the transform fault is moving to the left, and the bottom side of the transform fault is moving to the right. One example of a transform fault and fracture zone is the Eltanin in the South Pacific Ocean between the Pacific Plate and the Antarctic Plate. In the past, the East Pacific Rise was a linear spreading center.

Transform Plate Boundaries

Transform plates slide past each other. Faults are created where earthquakes occur, but seafloor is neither created nor destroyed. Examples include the San Andreas Fault and the Alpine Fault in New Zealand. -Transform plate boundaries may exist on a continent, or they may exist on the ocean floor, as offsets in the mid-ocean ridge system. However, not all transform faults are plate boundaries

Coral Reef Threats

Unfortunately, coral reefs are in rapid decline. Only about 30% are healthy now, and this decline is mainly due to humans. Human threats to corals include disease, pollution, bleaching, ocean acidification, destruction, overfishing (causes overabundance of algae that smother the reef), predation by species that are increasing because of humans, and sea level rise (reduces access to sunlight).

Coral Reef Development

Volcanic peaks on the seafloor create platforms that can support coral reefs. On his now-famous voyages, Charles Darwin noticed a progression of stages in coral reef development. Coral reefs form in warm, shallow water and migrate upward as ancient volcanoes subside (sink). Darwin published his hypothesis in The Structure and Distribution of Coral Reefs in 1842. Major advances in our understanding of plate tectonics in recent decades support Darwin's early hypothesis. -A fringing reef forms along the edge of an active volcano. On an active volcano, coral growth builds up very close to the shore. Free-swimming coral larvae attach to submerged rocks along edges of land. This process can take thousands of years. -The volcano becomes extinct and subsides, the reef grows upward and forms a barrier reef (with ocean between the subducting volcanic cone and the reef). This process can take hundreds of thousands of years. -An atoll is formed as the landmass/volcano completely submerges and the reef continues to grow upward to stay close to the surface. A lagoon is located in the center of the atoll. This process takes millions of years

Volcanic Peaks on the Seafloor

Volcanic peaks poke through the sediment cover of the abyssal plains. These peaks include small abyssal hills (<1km high), larger seamounts (>1km high), tablemounts (also called guyots), which form from eroded islands that have subsided back into the ocean, and volcanic islands. Volcanic peaks like these are created by a source of magma beneath the ocean floor. Common sources of magma include mid-ocean ridges, oceanic hotspots (such as Hawaii), and oceanic-oceanic subduction zones (such as the Mariana Islands and Japan).

Pillow Basalt

When hot basaltic lava spills onto the ocean floor, it is exposed to cold sea water that quickly chills (solidifies) the outer layers of the lava, creating a crust. The hot inside is still hot and runny, so the lava continues to flow, sometimes breaking through the thin crust that was just created. This creates "pillow lavas" or "pillow basalts," that are smooth, rounded lobes of rock that resemble a stack of bed pillows (but they are not soft!). Pillow lava is covered by organisms including clams, crabs, sea anemone, deep-water octacorals, rattail smoker fish, and giant tubeworms.

Pebble Beaches

While we are exploring beach deposits, it is important to note that not all beaches have sand on them. Remember that sand is just a grain size. Some beaches are composed of pebbles, while others are covered by cobbles and boulders. It all depends on where the sediment is coming from, and how far it has traveled from its source. One example of a California beach that has a range of grain sizes is Spooner's Cove (Links to an external site.) in Montaña de Oro State Park

White Sand

White sand is common on tropical beaches where shells are common. Shells are typically composed of calcium carbonate that the organism absorbs from the warm ocean water. We'll learn more about carbonate sediments when we cover biogenous sediments in the next section, but this sand is worth mentioning here while we are covering beach deposits. The photograph below shows how white sands differ from our typical California beach sands. This sand is brighter white, and it is composed of weathered shells and coral that are more common in warm, tropical waters.

mantle plumes

a localized column of hot magma rising by convection in the mantle, believed to cause volcanic activity in hot spots, such as the Hawaiian Islands, away from plate margins. Hotspots are created by mantle plumes (plumes of heat that rise from deeper within the Earth).

Fracture zones

are not plate boundaries. If you stand on one side of the fracture zone, and step across the fracture zone, you are still moving in the same direction. The rocks here are on the same side of the ridge axis, regardless of what side of the fracture zone they are on.

Archaea

are primitive single-celled organisms that resemble bacteria but have chemical similarities to multicelled organisms. Archaea thrive on sea floor chemicals, most notably hydrogen sulfide, and perform chemosynthesis.

Lithogenous sediments

composed of broken down pieces of rock. They are carried to the ocean by streams (all sizes), wind (usually only fine particles), glaciers (all sizes), and gravity (all sizes). They deposit on the seafloor by settling through the water column in a process called suspension settling. Because they originate from land, the greatest quantities of lithogenous sediments are found at continental margins. Coarser-grained sediments are found closer to shore (in the neritic environment), and finer-grained sediments are found further from shore (in the pelagic environment). Lithogenous sediments are dominated by the mineral quartz (SiO2) because it is most resistant to weathering.

Green Sand

composed primarily of olivine crystals that erode out of basaltic lava flows. The crystals are heavier than most other sand types on the beach. The olivine crystals become concentrated when waves wash away the lighter grains (this is the same process that was described above as being responsible for creating the purple or black streaks in the quartz-rich sand)

Submarine Canyons

deep, steep-sided valleys cut into the continental slope; Submarine canyons are created by turbidity currents, which are underwater landslides.

Earthquakes Concentrated Along Mid-ocean Ridges and Deep-ocean Trenches

earthquakes do not occur randomly across the Earth. They are concentrated in distinct belts around the world and are centered along mid-ocean ridges, deep-ocean trenches, and continental mountains. These earthquake belts helped scientists identify the edges of the tectonic plates, shown in the map below.

tip of the iceberg

ice is 90% (9/10) as dense as liquid water. "Just the tip of the iceberg" therefore means that 90% of the iceberg is below water! Earth's internal structure follows the same principles.

Stanley Miller's experiment

in 1952 illustrated that the precursors to life (organic molecules) could be formed by ultraviolet light, an electrical spark (lightning), and a mixture of water, carbon dioxide, hydrogen, methane, and ammonia, which were the components of Earth's early atmosphere. It is widely thought that the first life forms evolved in shallow ocean waters to eat these organic molecules that are believed to have been present on earth by about 4 billion years ago. How and precisely when the first living organisms emerged from this "prebiotic soup" is one of the most tantalizing questions in science today.

Bathymetry

is the study of the ocean floor, including ocean depth and the features on the ocean floor (similar to topographic maps on land that show mountains, valleys, hills, depressions, etc.). Section 3.1 in your textbook describes the four main methods that scientists use to study ocean bathymetry: 1) Multibeam Sonar; 2) Side-scanning Sonar; 3) Satellite Altimetry; and 4) Seismic Profiling.

continental crust

light colored igneous rock called granite that is less dense (2.7 g/cm3) and thicker than oceanic crust. Granite forms what we now call continental crust. continental crust "floats" higher due to its lower density.

Chemosynthesis

occurs in the absence of sunlight. In chemosynthesis, hydrogen sulfide combines with water and carbon dioxide to form sugar and sulfur compounds. The sugar is used as a source of energy for chemosynthetic organisms. a process whereby organisms create their own energy, manufacturing carbohydrates (sugar) from water, carbon dioxide, and dissolved oxygen.

The Wilson Cycle

opening and closing of ocean basins -During the embryonic stage, a complex system of linear rift valleys form on a continent. This is currently occurring in East Africa. In the juvenile stage, narrow seas are created as the crust thins and pulls apart through rifting (thinning and cracking of the crust as it bulged upward in the embryonic stage), and the down-dropped crust is flooded by seawater. Examples today are the Red Sea and the Gulf of California. -In the mature stage, the ocean basin has continued to spread apart to become a well-developed ocean basin with continental margins on the edges and a mid-ocean ridge in the middle. Examples today include the Atlantic Ocean and the Arctic Ocean. -During the declining stage, the ocean basin becomes smaller due to subduction of oceanic lithosphere along the margins. This creates island arcs and trenches around the edges of the ocean basin (continental arcs will also be created if the oceanic plate is subducting below a continental plate). The Pacific Ocean is currently in the declining stage due to subduction around the edges creating the Pacific Ring of Fire. -During the terminal stage, the process of subduction causes the ocean basin to shrink and become narrow again. Young mountains are created as the continental crust on either side of the narrow sea starts to converge and uplift. The Mediterranean Sea is currently in this stage, as the African and Eurasian Plates converge. In the final suturing stage, the ocean is completely gone (subducted), and the continental crust converges and uplifts to form young to mature mountain belts. The Himalaya Mountains were created due to convergence of the Indian subcontinent with Asia.

asthenosphere

plastic; The asthenosphere is composed of hot rock material that has high-viscosity plastic properties (has the ability to flow slowly over time).

Deep-sea fans

re created at the base of the continental slope by the turbidity currents. The deep-sea fans are composed of layers of graded bedding, and together, the fans create the continental rise.

Pacific Ring of Fire

region around the Pacific Ocean where most of the volcanoes and earthquakes on Earth occur regularly the majority of ocean trenches are along the margins of the Pacific Ocean, where plates are being subducted. Most of the world's large earthquakes (due to subduction) and active volcanoes (as volcanic arcs) occur around the Pacific Rim, which is why the area is called the Pacific Ring of Fire (red shading)

Hypsographic Curve

shows relationship between height of land and depth of ocean Bathymetric data and topographic data (land elevations above sea level) are used to create the Hypsographic Curve. The Hypsographic Curve illustrates the relationships between the height of land on Earth and the depth of the oceans. The difference in ocean depth versus land elevation results from the greater density and relative thinness of oceanic crust compared to continental crust

Moon Formation

the moon formed after a large asteroid collided with Earth. Heat from collisions and internal heat from radioactive decay caused early Earth's surface to melt.

Nebular Hypothesis

the theory that a solar system evolved from a hot gaseous nebula

Wentworth Scale

used to describe sediment grain size Largest to Smallest:: boulder, cobble, pebble, gravel, sand, silt, clay

Seamounts and Tablemounts

volcanic peaks in the abyssal plains. seamounts being upside down ice cream cone shaped and tablemounts being flat topped Seamounts and tablemounts (also called guyots, pronounced GEE-oh) form along mid-ocean ridges and at hotspots. These volcanic peaks are created where magma reaches Earth's surface and erupts on the seafloor (where it is then called lava). Seamounts are small underwater volcanoes that formed at volcanic centers such as the mid-ocean ridge. Islands are volcanoes that extend above the ocean's surface. Tablemounts are flat on the top because they are eroded by wave action. Through seafloor spreading, seamounts and tablemounts that formed along the axis of the mid-ocean ridge are transported into deeper water, sometimes carrying with them evidence that their tops once reached shallow water.

photosynthesis

which is when plants (including microscopic phytoplankton) take in carbon dioxide and water in the presence of sunlight to produce sugar.