Plate tectonics: flashcards, quiz and summary

Flashcards

1. 1What is the difference between the lithosphere and the crust?

   The crust is only Earth's outermost layer. The lithosphere is the rigid crust plus the uppermost part of the mantle, and it is broken into the plates that move.

2. 2What is the asthenosphere, and why does it matter for plate movement?

   The asthenosphere is the hot, soft upper mantle below the lithosphere. It is mostly solid rock, but it can flow slowly, which lets the plates above it move. It is not liquid magma.

3. 3What evidence did Wegener use to argue that the continents were once joined, and why was his idea rejected?

   He used the fit of continental coastlines, matching fossils such as Mesosaurus and Glossopteris, matching rock layers and mountain belts, and climate clues such as glacier scratches in tropical areas. It was rejected mainly because he could not explain what moved the continents.

4. 4How does seafloor spreading create new oceanic crust?

   Magma rises at mid-ocean ridges, cools into new oceanic basalt, and the new crust moves away from the ridge on both sides. Rock gets older and sediment gets thicker with distance from the ridge.

5. 5How do magnetic stripes on the seafloor support plate movement?

   As basalt cools at a ridge, it records Earth's magnetic field. The field reverses from time to time, so matching stripes of normal and reversed magnetism appear on both sides of the ridge, which fits new crust spreading outward.

6. 6What drives plate movement?

   Plates are driven mainly by mantle convection and slab pull, in which a sinking plate edge drags the rest of the plate along.

7. 7What happens at a subduction zone, and how does it produce volcanoes?

   The denser oceanic plate sinks into the mantle at a deep trench. Water released from the sinking slab lowers the melting point of the mantle above it, so magma forms and rises to build volcanoes.

8. 8Why do continent-continent collisions build high mountains but few volcanoes?

   Neither continental plate subducts easily, so the crust folds and stacks up into high mountains. Because little subduction happens, little magma is made, so there are few volcanoes. The Himalayas are an example.

9. 9How does a hot spot explain the ages of the Hawaiian islands?

   A plume of hot mantle stays roughly fixed while the Pacific Plate moves northwest over it. Islands form over the plume and are carried away, so they get older toward the northwest. The Big Island, over the plume, is the youngest and still active.

Practice quiz

Pick an answer to check it.

1. 1Which layer of Earth is the thickest?

   1. Mantle
   2. Crust
   3. Inner core
   4. Outer core
   Show the answer

   A. Mantle The mantle is the thickest layer. It is mostly solid hot rock that flows very slowly. The crust is much thinner, and the cores are smaller.

2. 2Why was Alfred Wegener's continental drift idea rejected?

   1. He could not explain what force moved the continents
   2. Glacier scratches proved the continents were always in their current places
   3. Scientists showed that the continents had never been joined
   4. His fossil evidence matched only within a single continent
   Show the answer

   A. He could not explain what force moved the continents Wegener gave good evidence that the continents had been joined, but he could not explain what moved them. Without a mechanism, many scientists rejected the idea.

3. 3What pattern in seafloor rock ages supports seafloor spreading?

   1. Rock is the same age everywhere on the seafloor
   2. Rock gets older with distance from the mid-ocean ridge
   3. Rock is oldest directly at the ridge crest
   4. Rock gets younger with distance from the mid-ocean ridge
   Show the answer

   B. Rock gets older with distance from the mid-ocean ridge Seafloor spreading moves new crust away from the ridge, so the crust farther from the ridge is older. Sediment is also thicker there because it has had more time to collect.

4. 4Which type of plate boundary formed the Himalayas?

   1. Transform boundary
   2. Convergent boundary where oceanic crust meets continental crust
   3. Divergent boundary
   4. Convergent boundary where two continental plates collide
   Show the answer

   D. Convergent boundary where two continental plates collide The Himalayas formed where India collided with Asia. Neither continental plate subducts easily, so the crust folded and stacked into high mountains.

5. 5Why do the Hawaiian volcanoes form in the middle of the Pacific Plate?

   1. The plate is pulling apart along a rift valley
   2. A transform fault is sliding the plate past another plate
   3. A stationary plume of hot mantle sits under the moving plate
   4. The plate is subducting beneath a trench there
   Show the answer

   C. A stationary plume of hot mantle sits under the moving plate Hawaii sits over a hot spot, a plume of hot mantle that stays roughly fixed while the Pacific Plate moves over it. The volcanoes are not at a plate boundary.

6. 6What does water released from a sinking oceanic slab do?

   1. It cools the mantle above and stops melting
   2. It makes the lithosphere denser so it sinks faster
   3. It stops the plate from moving by adding friction
   4. It lowers the melting point of the mantle above, creating magma
   Show the answer

   D. It lowers the melting point of the mantle above, creating magma Water from the slab lowers the melting point of the mantle above it. The mantle partly melts, and the magma rises to form volcanoes above the subduction zone.

Test traps

What teachers like to test on this, and the mistakes students make.

• Confusing the lithosphere with the crust. The lithosphere includes the rigid uppermost mantle, so it is thicker than the crust alone.
• Calling the asthenosphere liquid magma. It is solid rock that flows slowly, and that flow is what lets plates move.
• Saying that Wegener's idea was rejected because the evidence was wrong. The evidence was good; the missing piece was a mechanism for moving continents.
• Mixing up the direction of the age pattern. Seafloor rock gets older with distance from a ridge, not younger.
• Assuming all convergent boundaries make volcanoes. Continent-continent collisions build high mountains with few volcanoes because little subduction occurs.
• Thinking Hawaii's volcanoes sit on a plate boundary. They come from a hot spot in the middle of the Pacific Plate, and the islands get older toward the northwest.

Summary

• Earth's rigid outer shell is broken into plates that move a few centimeters per year.
• Earth has four main layers: crust, mantle, outer core and inner core. The lithosphere is the rigid crust plus the uppermost mantle, and the asthenosphere below it is solid rock that flows slowly.
• In 1912 Alfred Wegener proposed that the continents were once joined as Pangaea, using fit, fossil, rock and climate evidence, but he could not explain what moved the continents, so the idea was rejected.
• Seafloor spreading explains how new oceanic crust forms at mid-ocean ridges and moves outward, with rock getting older and sediment thicker farther from the ridge.
• Magnetic stripes on the seafloor record reversals of Earth's magnetic field, leaving matching patterns on both sides of a ridge.
• Plate boundaries are divergent, convergent or transform, and the boundary type determines whether crust is made, destroyed or only slides past.
• Subduction of oceanic plates creates trenches and volcanoes, while continent-continent collisions fold crust into high mountains with few volcanoes.
• Hot spots are plumes of mantle that stay roughly fixed while a plate moves over them, which explains why the Hawaiian islands get older toward the northwest.

Key terms

Lithosphere

The rigid crust plus the uppermost mantle, broken into plates.

Asthenosphere

The hot, soft upper mantle below the lithosphere. It is mostly solid but flows slowly, which lets plates move.

Pangaea

The supercontinent that Wegener proposed the continents were once joined into.

Seafloor spreading

The process in which magma rises at mid-ocean ridges, cools into new oceanic crust, and moves away from the ridge on both sides.

Subduction

The sinking of an oceanic plate under another plate into the mantle at a deep trench.

Divergent boundary

A plate boundary where plates move apart, forming new crust, rift valleys, shallow earthquakes and volcanoes.

Transform boundary

A plate boundary where plates slide past each other. Crust is neither made nor destroyed, earthquakes are common and volcanoes are rare.

Hot spot

A plume of hot mantle that stays roughly fixed while a plate moves over it, producing volcanoes in the middle of a plate.

Focus and epicenter

The focus is the point underground where slip starts during an earthquake. The epicenter is the point on the surface directly above the focus.

How to study it

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