Mid-Ocean Ridges

• Used to think the ocean floor was flat.

• Harry Hess used sonar to discover there were mountain ranges, canyons, and deep trenches.

• Long system of mid-ocean ridges extends around the world.

Harry Hess Discoveries

• Analyzed core samples of the sea floor.

• Ocean floor becomes OLDER as you move FURTHER from the ridge.

• YOUNGEST ocean floor is CLOSEST to the ridge.

Expanding Seafloor

• Ocean floor is made from cooling lava.

• Lava comes from the MANTLE at the ridge and cools.

• As more lava erupts, the crust moves away from the ridge.

• This is called SEAFLOOR SPREADING

What about the Old Ocean Floor?

• Old ocean floor moves edges of continents

• Trenches push ocean floor back to mantle

• RECYCLED!

• SUBDUCTION: crust pushing down into the mantle

Continental Drift

• Hess's idea of sea floor spreading proved Alfred Wegener's idea of CONTINENTAL DRIFT.

• 1960 - THEORY OF PLATE TECTONICS

• The Earth's LITHOSPHERE or CRUST is divided into tectonic plates.