Tectonics

Question 1

Alfred Wegener’s Theory of Continental Drift

Answer 1

Wegener proposed that continents were once joined in a supercontinent (Pangaea) and drifted apart over time.

Question 2

Evidence for Continental Drift

Answer 2

The continents fit together like a jigsaw and fossils of the same prehistoric species were found in continents a long way apart.

Question 3

Hess’s theory of sea floor spreading.

Answer 3

Hess proposed that new oceanic crust is created at mid-ocean ridges and move outward. He also proposed the process of subduction where crust sinks back down into the earth, forming ocean trenches.

Question 4

Magnetic striping

Answer 4

Magnetic striping on the ocean floor shows symmetrical patterns of magnetic reversals.

Question 5

Age of sea floor

Answer 5

Sea floor is younger near the ridges and older away. This supports the idea that new crust forms at the ridges and spreads outward over time.

Question 6

Sediment thickness

Answer 6

The thickness of sediment on the ocean floor increases with distance from the mid-ocean ridges. Closer to the ridges, where the sea floor is younger there is little time for sediment to accumulate. Further away, the older seafloor has had more time to collect sediment, supporting the idea of seafloor spreading over time.

Question 7

Tectonic plate movement: convection currents

Answer 7

In the mantle hot magma rises toward the earth’s surface, cools and then sinks back down, creating a continuous cycle that drags plates along.

Question 8

Tectonic plate movement: Slab pull

Answer 8

Occurs at convergent boundaries where one tectonic plate is being forced under another in a process called subduction. The denser, older plate sinks into the mantle because it is heavier. As the edge of the subducting plate sinks into the mantle, it pulls the rest of the plate along with it.

Question 9

Tectonic plate movement: ridge push

Answer 9

Occurs at divergent boundaries where tectonic plates are moving apart. At these boundaries, magma rises from beneath the earth’s surface and forms new crust at mid-ocean ridges. This new crust is hotter, less dense, and sits higher compared to older crust. Gravity causes the newly formed, elevated crust to slide down and away from the ridge, pushing the older cooler crust outward.