SEA-FLOOR SPREADING

TXT: PG. 97CHAPTER 3, SECTION 4

Background

• Deep ocean parts don’t receive a lot of sunlight• Cold---temp near freezing• Areas where there is space between the plates

allows water down into the crust, then brings it back up.

• These warm areas provide a great area for life to thrive, and support information given by Wegener’s “continental drift” theory.

Mid-Ocean Ridges

• Ridge: Area of higher relief on the ocean floor (underwater mountain range).

• Sonar: Device used to bounce sound waves off of objects deep in the water.

• Time it takes for echo to return=distance to the object

• Ridges curve around the ocean floor, going into all oceans

• They connect in a system that forms the world’s longest mountain chain (60,000 km)

Sea-Floor Spreading

• Places where the sea floor spreads apart along an ocean ridge.

• Convection currents push new magma up into the crack.

• Magma cools at the surface, causing “new crust” to form and making the older ocean floor move.

Evidence

• Eruptions of molten material• Scientists used “Alvin”—small sub used to dive into high

pressure areas of the ocean• Found rocks that indicated molten material was present

and hardening quickly

• Magnetic stripes in the rock• Last reversal of the earth 780,000 years ago.• New layers of rock have formed above this reversal,

indicating new rock in the area.• Older rock lines up one way• New rock lines up opposite

• Ages of the rocks• Scientists used the Glomar Challlenger to drill into the

ocean floor• Scientists dated the rocks and determined that older

rocks were further away from the mid-ocean ridge.• The youngest of the rock was in the center of the ridge.

Deep-Ocean Trenches

• This is what keeps the ocean floor from getting wider and wider

• In this area, the ocean floor dips down• After tens of millions of years, the trench

allows part of the ocean floor to sink back to the mantle.

Subduction

• Definition: Process where the ocean floor sinks under a deep-ocean trench and back into the mantle

• As the new crust moves away from the ridge, it cools…what does that do to it’s density?

• Right, it gets more dense…eventually so dense that gravity pulls it down beneath the trench.

• This process changes the size and shape of the ocean floor…

• Ocean floor is renewed every 200 million years (time for new rock to become old, dense, and sink)

• Pacific ocean: New crust isn’t being produced as fast as it’s sinking-----the ocean is shrinking

• Atlantic Ocean: Old crust isn’t sinking as fast as new is forming---the ocean is growing causing the continents on it’s edges to move.