Download Sea-Floor Spreading

• Using SONAR, geologist Harry Hess mapped
out the Mid-Ocean Ridge (MOR)
• The MOR is a massive underwater mountain
chain where molten material is rising from the
mantle, into the ocean
• The molten material cools, forming new crust
and pushing old crust – spreading out the sea
floor.
Mid-Ocean Ridge
Computer generated
image of MOR
• Pillow Basalts – rocks that cooled quickly,
indicating that magma is entering the ocean
• Magnetic Stripes – layers of rock with
alternating magnetic directions, indicating
different ages in the rock
• Drilling Samples – testing the age of rock
samples from the sea floor shows young rocks
near the MOR and old rocks near trenches.
• Subduction is the sinking of old, dense crust
back into the mantle
– Subduction occurs at deep-ocean trenches
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What the Mid-Ocean Ridge is
What Sea-Floor Spreading is
One piece of evidence for Sea-Floor Spreading
What Subduction is