Lesson 2: Sea-Floor Spreading

What are mid-ocean ridges?

long chains of mountains that wind across the floors of the oceans

List the evidence for sea-flooring spreading.

magnetic stripes in the ocean floor rock, earthquake patterns in subduction zones, and the fact that the age of the ocean floor increases with distance from mid-ocean ridges

Describe mid-ocean ridges and deep-ocean trenches.

Mid-ocean ridges are part of a large, volcanic mountain range that winds through Earth's oceans; deep ocean trenches are deep canyons in the ocean floor beneath which oceanic lithosphere sinks into the mantle through subduction.

What happens during subduction?

Older parts of the ocean floor sink into the mantle at trenches.

What is a Wadati-Benioff zone?

A Wadati-Benioff zone is the belt in which earthquakes occur along a slab of subducting ocean floor.

Explain what occurs during sea-floor spreading.

During sea-floor spreading, magma erupts along a mid-ocean ridge to form new oceanic lithosphere. The floor then moves away from the ridge, often toward a trench at the edge of the ocean basin.

Would earthquakes occur at a depth of over 700 kilometers? Why or why not?

Earthquakes would not occur below 700 km because at that depth mantle rock is too soft and hot to store the elastic energy needed to cause an earthquake.

What is paleomagnetism?

Paleomagnetism is the magnetization of a rock that reflects the polarity and direction of Earth's magnetic field at the time that the rock formed.

How do strips of magnetized rock on the ocean floor provide evidence of sea-floor spreading?

Strips of ocean-floor basalt record the polarity of Earth's magnetic field at the time the rock formed. These strips form a pattern that is the same on both sides of the mid-ocean ridge. The pattern shows that ocean floor forms along mid-ocean ridges and then moves away from the ridge.

Why are the oldest parts of the ocean floor less than 200 million years old?

The ocean floor is less than 200 million years old because that is the maximum amount of time it takes sea-floor spreading to move oceanic lithosphere from the mid-ocean ridges where it forms to the trenches where it is subducted.