Sea Floor Spreading

Chapter 1-4

Mid Ocean Ridges

• Mapped by sonar in mid 1900’s

• Curve like seams on a baseball along the sea floor

• Extend across all oceans• Some are completely

under water• Some poke through – Iceland

How were these ridges formed?

• Studied by American geologist Harry Hess in 1960’s

• Thought Wegner may have been right

• Proposed idea of sea floor spreading

Sea Floor Spreading• Sea floor spreads apart at

mid ocean ridge as new crust is added.

• Molten material erupts from ridge, cools into a strip of new rock

• Older rock moves outward as it is replaced by new molten rock

• Cause ocean floors to move like conveyor belts carrying continents with them

Evidence from Molten Material

• Pillow shaped rocks found along mid ocean ridge by the crew of the submarine Alvin.

• Rocks can only be formed by molten material that cools quickly under water.

Evidence from Magnetic Stripes

• Scientists pattern of magnetized “stripes” of rock on ocean floor

• Hold record of reversals of Earth’s magnetic field

Evidence from Drilling Samples

• Drilling ship Glomar Challenger brought up samples from ocean floor that showed rocks further away from ridge were older and closer rocks were younger

Why doesn’t the ocean floor keep on getting wider

• Part of the ocean floor sinks back into the mantle at deep ocean trenches by the process of subduction

Subduction

• As new, hot oceanic crust moves away from the mid ocean ridge, it cools down and becomes more dense.

• Gravity causes denser crust to sink down into trenches

Subduction and the Earth’s Oceans

• Ocean floor is renewed about every 200 million years because of seafloor spreading and subduction.

• Pacific Ocean is shrinking because it’s numerous trenches are swallowing more crust than is being formed

• Atlantic ocean is growing because it has less trenches