Chapter 2 – Plate Tectonics

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What statement best describes Wegener’s idea of continental drift?

Continents were formerly in different positions on the Earth and have shifted to their present locations over time. **Wegener believed that the continents had once been part of a supercontinent that he named Pangaea.

How was the fossil record of ancient life used to support Wegener’s hypothesis about Pangaea?

Fossils of identical organisms were found on several continents, indicating the continents were once joined. **When organisms are separated into two smaller groups by a physical barrier (i.e., the Atlantic Ocean), then they follow different evolutionary paths. The organisms Wegener cited were identical on both continents.

Late Paleozoic sedimentary rocks often contain extensive coal seams that were used to support the existence of Pangaea. What would the climate have been at that time and at the location where the coal deposits were formed, and what would it indicate about the continent’s past latitude?

The climate would have been a warm, humid climate near the Equator. **Swamp vegetation needs a lot of moisture and plenty of warm temperatures to thrive. The conditions would have been (and are today) present near the Equator.

When matching up once-joined continents such as South America and Africa, the coastlines do not fit together as nicely as the continental shelves. Why might this be the case?

The continental shelves are the edges of the continent. The shoreline simply shows where the sea level is in relation to topography. **Depending on sea level, continental shelves are often underwater and below the erosive effects of wave action.

What evidence supports that the glaciers on the southern continents were once part of a single, massive ice sheet?

Striations **Striations are scratches or grooves created by glacial debris as it moves. The striations found on the southern continents all radiate from the center of the south part of Pangaea.

Which scientist developed the idea of Continental Drift?

Alfred Wegener

What is the name of the supercontinent proposed by Alfred Wegener?

Pangaea

Which statement about the theory of plate tectonics is correct?

The rigid lithosphere plates overlay the hotter and weaker asthenosphere.

Which tectonic boundary results when two tectonic plates pull apart from each other?

Divergent boundaries **Divergent boundaries form because of tensional stress.

Divergent boundaries are not one, smooth ridge of separation. Instead, they are broken into a series of segments, with different segments of fault offset from others. What features allow for the offset of divergent boundaries?

Transform faults **Transform faults account for the lateral offset that is seen along divergent boundaries.

Crustal rifting is currently occurring in East Africa. Which feature would you expect to find there?

Normal Faults, long-wide vallies, and lava flows **All of these physical features are evidence that the landscape is experiencing tensional stress from rifting.

What is convection?

Convection is a cycle of moving material formed by the rise of less-dense material and the sinking of denser material.

What accounts for most tectonic plate motion?

Forces at subduction zones account for most tectonic plate motion.

What is the relationship between temperature and density?

In general, warmer materials are less dense, and colder materials are more dense.

Which term best characterizes up-welling in the Asthenosphere?

Diffuse

What is a plume?

A plume is a zone of up-welling material originating at the outer core-mantle boundary.

What features at the surface provide evidence of plumes?

Hot spots provide evidence of plumes.

How do plates move at divergent plate boundaries?

Plates move apart at divergent plate boundaries.

How do plates move at convergent plate boundaries?

Plates move toward one another at convergent plate boundaries.

How do plates move at transform plate boundaries?

Plates slide past one another at transform plate boundaries.

In general, where do earthquakes AND volcanic eruptions occur?

In general,earthquakes and volcanic eruptions occur at both divergent plate boundaries AND convergent plate boundaries.

What forms at divergent plate boundaries?

New oceanic lithosphere form at divergent plate boundaries.

In general, where do volcanoes form in subduction zones?

In general, volcanoes form on the overriding plate, away form the convergent boundary.

Why are volcanoes NOT found at transform boundaries?

Volcanoes are not found at transform boundaries because they do not cause changes to the pressure, temperature, or composition of the mantle.

The East African rift is a divergent plate boundary that is splitting the continent of Africa into two pieces. What will eventually form around this divergent boundary?

an ocean

____ is an example of a feature associated with an oceanic-continental convergent plate boundary?

The Cascade Mountain Range

Which type of tectonic boundary is the result of two tectonic plates colliding with each other?

Convergent boundary **Convergent boundaries form as a result of compressional forces pushing two plates together.

At a subduction zone where oceanic lithosphere meets continental lithosphere, which would subduct and why?

The oceanic lithosphere would subduct because it has a higher density at 3.0 grams per cubic centimeter. **Oceanic lithosphere, which is basaltic, has a higher density than continental lithosphere.

Why are volcanic chains associated with convergent boundaries?

Partial melting occurs due to subduction. **Partial melting is triggered by water loss in the plate. This melts mantle material, which rises to the surface.

Plate A has oceanic lithosphere on its leading edge that is 10 million years old. Plate B has oceanic lithosphere on its leading edge that is 200 million years old. Which plate will be subducted if they collide with each other, and why?

Plate B will subduct because it is older—and therefore colder—and more dense. **The older an oceanic plate is, the colder it is, making it more dense.

In a collision between two continental plates, can a continental plate subduct? Why or why not?

Continental plates are too buoyant to subduct. **Continental lithosphere has a density of 2.7 grams per cubic centimeter, making it unlikely to subduct.

____ is an example of a transform plate boundary.

The San Andreas Fault

Identify each type of convergent plate boundary.

Since oceanic plates are denser than the continental plates, they will subduct to great depths when they converge with continental plates. When two oceanic plates converge, the denser of the two will subduct; whereas when two continental plates converge, neither will subduct. Therefore, the resulting geologic feature depends on the types of plates that converge.

Depending on the type of plate convergence, different features could result. Examples of these features include volcanic island arcs, continental volcanic arcs, and a collisional mountain chain.

Once the volcanoes become completely dormant, they can be called mountains. Similarly, when the volcanic activity in the volcanic arc islands stops, they would appear and have the characteristics of islands.

Which part of the Earth is responsible for generating the planet’s magnetic field?

Core **The field is generated in the outer part of the core, which is believed to be a liquid iron alloy.

Using the figure above, when was the last time the Earth had a magnetic reversal?

700,000 years ago

How is information about magnetic fields recorded in rocks?

Iron particles in magnetically susceptible minerals within basaltic lava align with the magnetic field of the Earth while the lava is still liquid and then freeze in position when the lava solidifies. **Particles of iron will align while the material is still molten, but are locked into place once the lava hardens.

How were scientists able to determine the date of the magnetic reversals during seafloor spreading?

Small marine fossils in deep-sea sediments give the age of the portions of the seafloor **Fossils are very useful in determining geologic ages.

Which type of plate boundary is most closely associated with the formation of new ocean floor?

divergent

Which of the following statements about Earth’s magnetic field is most accurate?

Earth’s magnetic field switches polarity at irregular time intervals.

How often does the Earth’s magnetic field switch polarity?

every 100,000 to several million years

How are normal polarity and reverse polarity of Earth’s magnetic field different?

A compass points toward the North Pole during normal polarity and to the South Pole during reverse polarity.

In general, what is our best evidence that the orientation of Earth’s magnetic field has changed over time?

A record of Earth’s magnetic field is recorded in oceanic rocks, which show a clear pattern of changes in Earth’s magnetic polarity.

Where would you expect to see alternating bands of rock with different magnetic polarities?

on both sides of the mid-ocean ridge

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