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.