Sea- floor Spreading• Scientists have mapped the ocean floor and

mid-ocean ridge using S.O.N.A.R. Sound waves bounce off the underwater objects and then records the echoes of these sound waves. The time it takes for the echo to return is the distance away from the object.

The longest chain of mountains in the world is the Mid-Ocean Ridge.

The Island of Iceland is part of the ridge that rises above the surface in the Atlantic Ocean. Down the middle of the ridge is an area called the rift valley – It is almost twice as deep as the Grand Canyon

Sea Floor Spreading Theory• Sea Floor Spreading: 1960’s Harry Hess defined the process that

continually adds new material to the ocean floor while pushing rocks away from the ridge.– proposed that the ocean floors move like conveyor belts, carrying the

continents along with them.– New Rock forms along the cracks in the ocean crust as molten material

erupts from the mantle spreading out and pushing older rock to the sides of the cracks.

– New ocean floor is continually added by the process of Sea floor spreading.

This movement begins at the Mid- Ocean Ridge.At the ridge, the molten material rises from the mantle and erupts (much like a volcano).The oldest rock is farthest from the center of the ridge.

ORNRNR

OR

How does the ocean floor keep getting wider

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

• The ocean floor plunges into deep under water trenches.

• A deep ocean trench forms where the oceanic crust collides with continental crust. Oceanic crust melts in the mantle.

Evidence for Sea-Floor Spreading:

• Evidence #1• Molten Material. The submersible ALVIN was sent down

to study the rift valley.

Evidence #2• Rock Sampling. The Glomar Challenger was

a drilling ship the gathered samples from the ocean floor.

• The scientists determined the age- of the rocks in the samples and found that the farther-away from the ridge, the older the rocks were.

• The oldest rocks are about 4 million years old.• The newer rocks were always in the center of

the ridges

Evidence #3• Magnetic Stripes.

How does the pattern of matching stripes show evidence of sea-floor spreading?

FYI

• Most ocean trenches are found in the Pacific Ocean.

• The deepest part of the ocean is the Marianna’s Deep. It is located in the Pacific Ocean

• The Pacific Ocean is shrinking, but it is still the Largest ocean