Sea Floor Spreading

p. 331-337

Mid-Ocean Ridge

• Longest chain of mountains in the world (50,000 km long!)

• Mostly underwater• Iceland part of Mid-Ocean Ridge• We use sonar to map the ocean floor

How was the Ridge Made?

• Harry Hess, studied the mid-ocean ridge

• 1960 he suggested ocean floor like conveyor belts, carrying continents with them

• Movement begins at mid-ocean ridge

What Happens?

• At the mid-ocean ridge, molten material rises from the mantle and erupts.

• The molten material then spreads out, pushing older rock to both sides of the ridge.

Sea-Floor Spreading

• Sea-floor spreading: the process that continually adds new material to the ocean floor.

• Evidence for Sea-floor spreading:– Molten material– Magnetic stripes– Drilling samples

Evidence: Molten Material

• Rocks shaped like pillows found – only forms when molten material hardens quickly

Alvin

Scientists inside

4 km deep

Evidence: Magnetic Stripes

• Magnetic field around Earth• Poles have reversed themselves (last

time 780,000 years ago)• Magnetized “stripes” on ocean floor

hold record of reversed poles

Evidence: Drilling Samples

• Rocks drilled from ocean floor• Age of rocks determined• Further away from ridge, older the

rock• Youngest rock found in the center of

the ridge

Subduction at Deep-Ocean Trenches

• How can the ocean floor keep getting wider and wider?

• Floor plunges into deep underwater canyons called deep-ocean trenches

• These occur where oceanic crust bends downward

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