Sea Floor Spreading

Mid-Ocean RidgesMid-ocean ridges are an undersea mountain

chain where new ocean floor is produced.In the mid-1900’s, scientists begin mapping

the mid-ocean ridges using SONAR.Mapping mid-ocean ridges, made scientists

more curious.What are these ridges?How did they form?

Sea-Floor SpreadingIn sea-floor spreading, the sea floor spreads apart

along both sides of a mid-ocean ridge as a new crust is added. The ocean floor moves like a conveyor belt, carrying

the continents along with them.Along these ridges, molten material that has

formed several km beneath the surface rises and erupts.As the molten material cools on the ridge, new solid

rock is formed, more molten material flows into the cracks and new strips of rock is formed.

Evidence of Sea-Floor Spreading

• Evidence from molten material

• In the 1960’s scientists sent Alvin, a submarine that could withstand the crushing pressures deep in the ocean.• Strange rocks were found. Rocks that looked like toothpaste squeezed from the tube. These types of rock only form when molten rock cools and hardens quickly after erupting.

More Evidence of Sea-Floor SpreadingEvidence from Magnetic Stripes

Evidence from Drill Samples

History shows that the earth’s magnetic field has reversed many times.

Rocks that make up the ocean floor reveal this pattern in magnetized stripes.

These rocks contain iron. As they cool and harden, the iron lined up in the direction of the Earth’s magnetic field.

Rock samples from drilling into the ocean floor are the final proof of sea-floor spreading.

The Glomar Challenger sent pipes 6 km deep in the ocean to drill hole sin the ocean floor.

Scientists found that the farther away from the ridge, the older the rock sample were.

Subduction at TrenchesDeep-ocean trenches are deep underwater

canyons, where the oceanic crust bends downward.

SUBDUCTION is a process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.

It takes about 200 million years for new rock to form, move across the mid-ocean ridge, move across the ocean, and sink into a trench.

Did you know the Pacific Ocean is Shrinking and the Atlantic ocean is expanding?Sometimes a deep ocean trench swallows more

oceanic crust than a mid-ocean ridge can produce.This is why the Pacific Ocean is shrinking

The opposite is happening to the Atlantic Ocean. More oceanic crust is being produced at mid-ocean

ridges that the deep ocean-trenches and swallow up.