Chapter 10

Mid-Ocean Ridge

A broad, symmetrical ridge that crosses an ocean basin

Passive margin

A continental edge that lacks tectonic activity

Old and new ideas about continental drift

A crucial weakness of Wegner's hypothesis was that he could not explain how or why the continents moved. Wegner imagined that continents plowed through or over oceanic crust in The same way that a ship plows through the ocean. Scientist of his day, however, could demonstrate that this mechanism was not feasible. Continental crust is not strong enough to survive the forces needed To move a large mass across such a great distance while pushing aside oceanic crust.

Driving force

A force that drives the motion. Must exceed resisting forces

10.12 What features form at Continental hotspots?

A hotspot within a continental plate is marked by high elevations, abundant volcanism, and continental rifting. Hotspots can facilitate complete rifting and separation of a continent into two pieces and can help determine where the split occurs. Several continental hotspots are active today. Continental hotspots are associated with certain characteristics, including high elevations, volcanism, and the presence of rifts. Two examples are the Afar region of east Africa and the Yellowstone region of western US.

Tectonic Terrane

A piece of exotic crust added to a continent. For terrane to become attached to a continent, it typically enters a subduction zone, where it is scraped off the subducting plate and tectonically added to the continent. Through this process of adding terranes along subduction zones, a continent grows. Such terranes generally represent pieces that were sliced or riffed off another continent and then tectonically transported until they collided with the edge of a continent.

Rule of thumb for elevations

A rule of thumb is that increasing the thickness of the crust by 6 km will result in an increase in elevation of 1 km (about 3,300 ft)

Continental Drift

Am argument but Alfred Wegener that states that the continents were once joined together but later drifted apart.

10.6A What Happens at Mid-ocean Ridges?

As plates move apart, solid mantle in the asthenosphere rises toward the surface. It partially melts as it rises, because decreasing pressures can no longer confine the rock as a solid. The molten rock (magma) rises along narrow conduits, accumulates in magma chambers beneath the rift, and eventually becomes part of the oceanic lithosphere. The elevation of seafloor decreases away from the ridge because the rock cools and contracts, and because the less dense atmosphere cools enough to become part of the denser lithosphere.

When oceanic plates converge

As the plate subducts, its temperature increases, releasing water from minerals in the downvoting plate. This water causes melting in the overlying atmosphere, and the resulting magma is buoyant and rises into the overlying plate.

10.3B Is the Distribution of Fossils Consistent with Continental Drift?

Continental drift also explained why identical plant and animal fossils are found on different continents. The plants and animals were originally on a single huge supercontinent that later split into separate smaller continents. Two continents could share plants and land animals before they split, but not after. Other fossil data suggests that Antarctica was once farther north, away from the South Pole. Such data include coal beds interpreted to have formed from plants that grew in warm-weather swamps. One explanation is that Antarctica moved to its present polar location after the coal formed more than 150 Ma.

10.14 How diff continents join and split apart?

Continents shift their positions overtime in response to plate tectonics. They have riffed it apart and collided only to rift apart again. Where were the continents located in the past in which mountains resulted from their motions? The figures below illustrate which continents were joined and how they were separated. We start with 600 million years ago and work forward to the present, as if watching a movie of earths history.

Subduction Zone

Convergence of two oceanic plates forms an ocean-ocean convergent boundary. One plate bends and slides beneath the other plate along an inclined zone. The process of one plate sliding beneath another is SUBDUCTION, and the zone around the downward- moving plate is a SUBDUCTION ZONE

Island Arc

Crossing the seafloor are curving chains of islands called ISLAND ARC. Most of the islands in an island arc are volcanoes, and many are active and dangerous. Most island arcs are flanked on one side by an ocean trench.

Mid-ocean Ridges/Spreading Centers

Divergent plate boundaries where the new oceanic lithosphere forms as two oceanic plates move apart. Also called SPREADING CENTERS because of the way the plates spread apart.

10.9D Is The age of the seafloor consistent with plate tectonics?

Drill core samples reveal that sediment is then or absent on the ridge but becomes thicker away from the ridge. Age determinations on fossils in the sediment and from underlying volcanic rocks show that oceanic crust gets systematically older away from mid ocean ridges. Such drilling results strongly support the theory of plate tectonics The map below shows the age of the seafloor, with letters marking The position of some mid ocean ridges R and trenches T, Purple represents the oldest areas, about 180 million years old, In the darkest orange represents very young oceanic crust.

10.1 What is Inside Earth?

Earth consists of concentric players that have different compositions. The outermost layer is the CRUST, which includes the CONTINENTAL CRUST AND OCEANIC CRUST. Beneath the crust is the MANTLE, Earth's mood voluminous layer. The molten OUTER CORE and the solid INNER CORE are at Earth's center.

10.10A What causes earths magnetic field?

Earth has a metal iron core, which is composed of a solid inner core surrounded by a liquid outer core. The liquid core flows and behaves like a dynamo (electrical generator), creating a magnetic field around earth Many Times in the geologic past, the magnetic field A REVERSED POLARITY, so that a compass needle would point south. The switch between normal polarity and reversed polarity is a magnetic reversal and may only require as little as 100 years to occur.

Earthquakes and Plate Boundaries

Earthquakes are a better guide to plate boundaries then are volcanoes. Most but not all volcanoes are near plate boundaries but mini plate boundaries have no volcanoes. Some earthquakes occur in the middle of plates, indicating that the situation is more complicated than a simple plate-tectonic model, in part because some parts of a plate are weaker than others. Forces can be transmitted through the strong parts, causing weaker parts to break and slip, generating an earthquake within the plate. Generally, most tectonic activity occurs near plate boundaries.

For each major plate, note the types of boundaries between this plate and other plates it contacts. Then use the various maps to determine whether each type of plate boundary has the following features:

Earthquakes, volcanoes, mountain belts, mid-ocean ridges, ocean trenches

tectonic plates

Earths strong outer layer, the LITHOSPHERE, is broken into a dozen or so fairly rigid pieces called TECTONIC PLATES

Oceanic Crust

Exists beneath the deep oceans and has an average composition that is the same as basalt, a common dark lava rock. Has an average thickness of about 7km (4 mi), which is much thinner than can be shown.

Ocean-continent convergent boundary

Formed by the convergence of an oceanic and continental plate. Along this boundary, the denser buoyant plate subducts beneath the more buoyant continental plate.

Oceanic Trench

Forms as the subducting plate moves down. Sediment and slices of oceanic crust collect in the trench. This sheared, scraped-off material generally remains completely submerged, but is exposed in a few islands, like Barbados, in the eastern Caribbean.

Hotspots and continental outlines

Geoscientist conclude that hotspot has helped define the outlines of the continent by shipping the boundary along which continent separate from one another. The best example of this is the N-word the end of the western coast of Africa near Cameroon. This bend occurs at the intersection of three arms of a rift, two of which led to the opening of the south Atlantic Ocean. The third failed rift cut north east into Africa and is the site of several major rivers

Oceanic Plateaus Form

Geoscientist interpret ocean plateaus as forming at hotspots, above rising hot mantle, called MANTLE PLUMES. The plumes travel through the mantle a solid masses not liquids Immense volumes of basalt, As much as 50,000,000 km³, irrupt into the seafloor over millions of years this volcano is a creates a broad, high oceanic plateau

Continental Crust

Has an average composition similar to granite. Continental crust averages 35 to 40 km (20 to 25 mi) in thickness.

Western Canada

Has been a convergent margin for most of the last hundred Ma. It's mountain ranges over like rust second from major faulting, from magnetic additions, and from collisions with island arcs and pieces of Continental material

10.8 What Happens Along Transform Boundaries?

In the oceans, transform faults are associated with mid ocean ridges. Transform faults combined with spreading centres to form a zigzag pattern on the seafloor. A transform fault can link different types of plate boundaries, such as mid ocean ridge and an ocean trench. Some transform boundaries occur beside or within a continent, sliding one large crustal block against another.

Density

Influences regional elevations. Denser= floats lower A region of earth underlain by especially dense crust or mantle is lower in elevation than a region with less dense crust or mantle, even if the two regions have similar thickness of crust.

10.10 How are magnetic reversals expressed at mid-ocean ridges, and how do magnetic patterns on the seafloor help us study plate tectonics?

Is the oceanic plates spread apart at a mid ocean ridge, Basaltic lava erupts onto the surface or solidifies at depth. As the rocks cool, the orientation of earths magnetic field is recorded by the iron rich mineral magnetite. If the magnetic field reverses,any new rocks that form will acquire a reversed polarity. Once the rocks have cooled, they retain their original magnetic direction, preserving the magnetic polarity in the seafloor. The new reversely magnetized rocks form a stripe along the mid ocean ridge, and the previously formed, normally magnetized rocks have been split into stripes and have moved away from the ridge in the opposite directions. We can calculate rates of seafloor spreading: RATE OF SPREADING FOR STRIPE= WIDTH OF STRIP/TIME DURATION

Hot spots

Linear chain of islands and seamounts, and most clusters of islands in the ocean, Have two key things in common: They were formed by volcanism and they are near sites interpreted to be above unusually high temperature regions in the deep crust and upper mantle-HOT SPOTS If the plate above the hotspot is moving relative to the hotspot, the volcanic island can move off the hotspot. As it does, that part of the plate cools and subsides, so volcanos that start out as islands Masing beneath the sea to become seamounts. As an island sinks, erosion can level of its top, forming a flat top seamount If a plate is not moving or is moving very slowly, the hotspot forms a cluster of volcanic islands in seamounts instead of a linear chain

Continental Shield

Many continents, including North America, have a central region called a CONTINENTAL SHIELD. A shield consists of relatively old metamorphic and igneous rock, often simply called crystalline rocks or the crystalline basement. The rocks exposed in the shield represent the kinds of rocks that underline much of the continent. In North America, the continental shield is mostly in Canada and called the CANADIAN SHIELD.

10.12B How did Continental hotspots evolve?

Mini continental hotspot underwent a similar sequence of events. They started with doming and ended with the formation of a new continental margin in a new ocean formed by seafloor spreading

10.9C Is there a way to measure plate motions directly?

Modern technology allows direct measurement of plate motions using satellites, lasers, and other tools.

Common Feature of Continents

Most continents have at least one edge we are elevation of the land slowly decreases toward the coast until it flattens out in a low plain barely above sea level. This low region is a coastal plain and is underlain by sedimentary units, many of which were deposited quite recently in geologic terms. In the US, the coastal plain warps around the southern and eastern edge of North America, from Texas to Florida to new

Continental Rift

Most divergent plate boundaries are beneath oceans, but a divergent boundary may also form within a continent. This process creates a CONTINENTAL RIFT, such as the Great Rift Valley in East Africa. Rifting can lead to seafloor spreading and formation of a a new ocean basin, following progression shown here.

10.5 what causes tectonic activity to occur in belts?

Most earthquakes and volcanoes occur in belts Around earths surface. Between these belts are vast regions that have comparatively little of this activity. Where are these belts of concentrated activity? What explains the spatial distribution? What underlying processes cause these observed patterns? These and other questions help lead to the THEORY OF PLATE TECTONICS

Seamounts

Most mountains on the seafloor do not reach sea level and are termed SEAMOUNTS.

10.15C What happens during erosion of mountain belts?

Mountains, once formed, are subjected to weathering and erosion. These processes where mountains down but our countered by uplift related to isostasy. Uplift is driven by buoyancy due to the root of underlying thick crust. Through simultaneous erosion and isostasy, the mountain is eroded down and the thick crustal root is gradually reduced in size. Material eroded from the mountain ends up in adjacent basins, increasing the crustal thickness beneath the basins.

Abyssal Plain

Much of the ocean floor is moderately deep-3 to 5 km (9,800 to 16,000 ft)- and had a fairly smooth surface. Such a smooth, deep part of the seafloor is an ABYSSAL PLAIN.

10.2 What Are the Major Features of Earth?

Oceans cover around 70% of earths surface. Seven continents make up most of the rest of the surface, and islands account for less than 2%. Features of the ocean floor, not generally seen by people, are just as diverse and include deep trenches and submarine mountain ranges.

10.11C What Is the distribution of hotspots, linear island chains, and oceanic plateaus?

On this map, red dots show the locations of likely hotspots, many of which are located at the volcanically active ends of the linear island chains. There is a great debate however about which areas are really hot spots and how hotspots form The dark gray areas in the oceans represent linear island chains, clumps of islands, and oceanic plateaus, similar to this high area around Iceland which is over a hotspot

10.9B How fast and in what direction to do plates move relative to one another?

Plates move at 1 to 15 cm per year, about as fast as your fingernails grow

10.15A In which tectonic settings do regional mountain belts form?

Regional mountain ranges are hundreds or thousands of kilometres long. They are large enough that they can only be explained by major variations in the thickness and temperature of the crust and lithosphere. Most ranges occur near convergent plate boundaries or where there has been large scale movement of material in the mantle.

10.3C How Did Continental Drift Explain Glacial Deposits in Unusual Places?

Rounded outcrop in South Africa has polished and scratched surface that is identical to those observed at the bases of modern glaciers. This observation is surprising because South Africa is currently a fairly warm and dry region without any glaciers.

Continental Shelves

Some continents continue outward from the shoreline under shallow seawater for hundreds of kilometers, forming submerged benches, known as CONTINENTAL SHELVES

Island Arc

Some magma erupts, initially under the ocean and later as dangerous, explosive volcanoes that rise above the sea. With continued activity, the erupted lava and exploded volcanic fragments construct a curving belt of islands in an ISLAND ARC

Slab pull

Subducting oceanic lithosphere is more dense than asthenosphere, so gravity pulls the plate down word into the asthenosphere. Slab pull is a significant force, and A plate being subducted generally moves faster than a plate that is not being subducted.

Sam Andreas Fault

The San Andreas transform fault extends from the north of San Francisco to south east of Los Angeles. The part of California west of the fault is on the Pacific plate and is moving approximately 5 cm per year to the northwest relative to the rest of north America. South of this map area, the transform boundary continues across southern California and into the Gulf of California.

Mantle Convention

The asthenosphere, although a solid, is capable of flow. It experiences convection, where hot material rises due to its lower density, while cold material sinks because it is more dense. Hot material rises at mid ocean ridges, calls, and eventually sinks back into the asthenosphere at a subduction zone

Isostasy

The concept of different thickness of crust riding on the mantle. Explains most of the variations in elevation from one region to another, and it is commonly paraphrased by saying "mountain belts have thick crustal roots"

Seafloor spreading

The idea that oceanic crust was spreading apart at underwater mountain belts, carrying the continents apart. Proposed by Hess and Dietz

Ridge push

The mid ocean ridge is higher than the seafloor away from the ridge because lithosphere near the ridge is thinner and hotter. Gravity causes the plate to slide away from the topographically high Ridge and push the plate outward.

Transform Fault

The openings are linked by breaks, or faults, where the two parts of the pizza simply slide by one another. There are no gaps along these brakes, only horizontal movement of one plate sliding past another. Arrows show the direction of relative motion. A fault that accommodates the horizontal movement of one tectonic plate past another is a transform fault. The spreading direction must be parallel to the transform fault in perpendicular to the spreading segments so a zigzag pattern is required to allow a plate boundary to be curved.

Lithospheric Mantle

The part of the uppermost mantle that is in the lithosphere.

Triple Junction

The place we are three plate boundaries meet. The Medocino Triple junction is the meeting place of two different transformer faults in a subduction zone.

10.1C Why Do Some Regions Have High Elevations?

The primary factor controlling the elevation of a region is the thickness of the underlying crust.

10.9 Why and how to plates move?

The process of plate tectonics circulate material back and forth between the asthenosphere and the lithosphere. Some asthenosphere becomes lithosphere at mid ocean spreading centres and then takes a slow trip across the ocean floor before going back down into the asthenosphere at a subduction zone. Besides creating and destroying lithosphere, this process is the major way that Earth transport heat to the surface.

Gondawa

The theoretical combination of the southern continents into a single large supercontinent.

Lithosphere

The uppermost mantle is relatively strong and solidly attached to the overlying crust. The crust and uppermost mantle together from rigid upper layer called the LITHOSPHERE. Averages about 100 km (about 60 mi) in thickness.

Resisting forces

Those forces that resist movement, such as friction and any resistance from other material that is in the way.

Continental Collision/ Continent- Continent convergent boundary

Two masses may converge along a CONTINENT-CONTINENT CONVERGENT BOUNDARY. This type of boundary is commonly called a CONTINENTAL COLLISION, and it produces huge mountain ranges, like the Himalaya and Mount Everest. When the converging continent arrives at the subduction zone, it may partially slide under the other continent or simply clog the subduction zone as the two continents collide. Because the two continents are thick and have the same density, neither can be easily subducted beneath the other and into the atmosphere. Continental collisions form enormous mountain belts and high plateaus, such as the Himalayas, and Tibetan plateau of southern

Divergent Boundary

Two plates move apart relative to one another. In most cases, magma fills the space between the plates

Transform boundary

Two plates move horizontally past one another, as shown by the white arrows on the top surface.

Convergent boundary

Two plates move toward one another. A typical result is that one plate slides under the other.

Volcanoes formation

Volcanoes form on the surface of the overriding continental plate in the same way the volcanoes form in an ocean-ocean convergent boundary. These volcanoes erupt, often violently, producing large amounts of volcanic ash, lava, and mudflows, which pose a hazard for people who live nearby. Examples include large volcanoes of the Andes of South America and the Cascade Range of Washington, Oregon, Northern California, and southern British Columbia.

Pacific Ring of Fire

Volcanoes that surround the Pacific Ocean. The volcanoes extend from southwestern Pacific through the Phillippines, Japan, and Alaska, and then down the western coasts of North and South America. The Ring of Fire results from subduction on both sides of the Pacific Ocean.

Accretionary Prism

a wedge-shaped mass of material that was scraped off oceanic plates that were subducted along a trench