Sea-Floor Spreading

Mapping the Mid-Ocean Ridge

The East Pacific Rise is just one part of the mid-ocean ridge, the longest chain of mountains in the world.

The mid-ocean ridge curves like the seams of a baseball along the sea floor, extending into all of Earth’s oceans.

Most of the mountains in the mid-ocean ridge lie hidden under hundreds of meters of water.

A steep-sided valley almost twice as deep at the Grand Canyon, splits the top of the mid-ocean ridge for most of its length.

Scientists used sonar to map the mid-ocean ridge and made them curious as to how it got there.

Evidence for Sea-Floor Spreading

Harry Hess, an American geologist, was one of the scientists who studied the mid-ocean ridge.

He studied the mid-ocean ridge and began to think about the ocean floor in relation to the problem of continental drift.

When looking at the ocean floor, he realized that maybe Alfred Wegener’s theory of continental drift was in fact, true!

Hess suggested in 1960 that the ocean floors move like conveyor belts, carrying continents along with them.

This movement begins at the mid-ocean ridge, which forms along a crack in the oceanic crust.

At the mid-ocean ridge, molten material rises from the mantle and erupts.

The molten material then spreads out, pushing older rock to both sides of the ridge.

As the molten material cools, it forms a strip of solid rock in the center of the ridge and then more molten material flows into the crack.

This material splits apart the strip of solid rock that formed before, pushing it aside.

Hess called the process that continually adds new material to the ocean floor sea floor spreading.

http://emvc.geol.ucsb.edu/forteachers/flashmovies/Spreading.swf

 

Several types of evidence from the oceans supported Hess’s theory of sea floor spreading:

1. Evidence from molten material

2. Evidence from magnetic stripes

3. Evidence from drilling samples

Subduction at Deep-Ocean Trenches

Hess wondered how can the ocean floor keep getting wider and wider without the Earth getting larger?

The ocean floor plunges into the deep under water canyons called deep-ocean trenches.

A deep-ocean trench forms where the oceanic crust bends downward.

Where there are deep-ocean trenches, a process called subduction takes place.

Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle to be recycled.

Convection currents under the lithosphere push new crust that forms at the mid-ocean ridge away from the ridge and toward a deep-ocean trench.

New oceanic crust is hot but as it moves away from the mid-ocean ridge it cools and becomes denser.

Eventually gravity pulls this older, denser oceanic crust down beneath the trench.

At deep-ocean trenches, subduction allows part of the ocean floor to sink back into the mantle under the continental crust.

Check These Out!!

http://www.classzone.com/books/earth_science/terc/content/visualizations/es0902/es0902page01.cfm?chapter_no=visualization

http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_subduction.html

Review

1. What is the role of the mid ocean ridge in sea-floor spreading?

2. What is the evidence for se-floor spreading? 

3. Draw a diagram of the processes of sea-floor spreading and subduction. Label the trench, mid-ocean ridge, oceanic crust, continental crust and indicate with arrows the direction of sea-floor spreading and subduction.