Chapter 10 Plate Tectonics Section 2 Continental Drift

Concept map that describes sea floor spreading along a divergent plate boundary.

convection currents circulate hot material along a divergent boundary ---- Older seafloor is forced apart ----- Magma rises to the seafloor ----- Magma cools to form new seafloor.

What evidence in rocks support the theory of seafloor spreading

magnetic reversals

Movement along the African - Rift Valley is about 2.1 cm per year. If plates continue to move apart at this rate; how much larger will the rift be (in meters) in 1,000 years? In 15,000 years?

0.0001 km/year, 10 cm/year

One step Equation. North America is moving 1.25 cm per year away from a ridge in the middle of the Atlantic Ocean. Using this rate, how much farther apart will North America and the ridge be in two million years?

1.25cm * 200 million years=250,000,000 cm

10 km of new seafloor has been created in 50,000 years, with 5 km on each side of a mid - ocean ridge. What is the rate of movement, in km per year, of each plate? In cm per year?

2 km; 150,000 years

How does new seafloor spreading form at the mid-ocean ridges?

As magma moves upward and out of the cracks in the seafloor. As it becomes solid, a new seafloor is formed. The older seafloor moves to the sides.

Think critically. The ideas of Hess, Wegener, and others emphasize the earth is a dynamic planet. How is seafloor spreading different from continental drift.

For continental drift, Wegener could not explain how the continents drifted. Seafloor spreading explained how the continents separated over time as the ocean floor grew larger.

Summarize How did Harry Hess's hypothesis explain seafloor spreading?

Hess hypothesis that hot, less dense material below Earth's crust rises toward the surface at the mid - ocean ridge. So the magma rises and moves the old crust over.

seafloor spreading

Hess's theory that new seafloor is formed when magma is forced upward toward the surface at a mid-ocean ridge.

Summarize what properties of iron bearing minerals on the seafloor support the theory of seafloor spreading.

Iron bearing minerals record the directions of Earth's magnetic field, when they were formed at the ridges over time.

How were mid-ocean ridges discovered?

Mid - ocean ridges were discovered using sound waves.

Describe how magnetism is used to support the theory of seafloor spreading.

Scientist discovered that the Earth's magnetic field has reversed itself many times in the past. Those reversals are recorded in the rocks that formed along the mid - ocean ridge.

Harry Hess

Scientist that came up with the theory of seafloor spreading.

Explain how the ages of the rocks on the ocean floor support the theory of seafloor spreading.

Scientist were able to find out that the youngest rocks are located at the mid - ocean ridge, while the older rocks are farther away.

Explain why some partly molten material rises toward the Earth's surface?

Some partly molten material rises toward the Earth's surface because it is less dense than the surrounding rock. It is forced upward toward the surface.