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1.4 Sea-floor Spreading
Check your notes for key ideas!
1. Mid-Ocean Ridges
• An underwater mountain range stretching all
over Earth.
• Scientists used sonar to map the ocean floor
• Mapping the mid-ocean ridge made scientists
more curious about what is happening on our
ocean floor!
Sonar is a device that bounces sound waves
off of under-water objects and then records
the echoes of these sound waves. The time it
takes for the echo to bounce back indicates
the distance to the object. The object in this
case is the ocean floor!
The mid-ocean ridge
is the longest chain
of mountains in the
This is a sonar image
of the mid-ocean ridge
2. What is Sea-Floor Spreading
• Harry Hess, and American geologist, studied midocean ridges and developed the idea of sea-floor
– Sea-floor spreading: process by which new rock is
continuously added to the sea-floor
– At mid-ocean ridges, molten material pushes up
through cracks in the crust.
– The new rock pushes old rock away from the ridge.
– The ocean floor moves like a conveyor belt and carries
the continents with it!
(ALFRED WEGENER was on to something!)
3. Evidence for sea-floor spreading
A. Evidence from Molten Material
– Scientists dive to ocean floor in submarine, Alvin
– Find rocks shaped like pillows. This rock shows
that molten material has erupted again and again
along the mid-ocean ridge
3. Evidence for sea-floor spreading
B. Evidence from Magnetic Stripes
– As the rock cools, it becomes magnetized parallel
to Earth’s magnetic field at the time.
– Earth’s magnetic field has reversed many times.
– Scientist discover that the rock making up the
ocean floor lies in a pattern.
• Reversed; Normal; reversed; Normal
– This pattern is the same on each side of the midocean ridge, which suggests it is spreading from
both sides!
3. Evidence for sea-floor spreading
C. Evidence from Drilling Samples
– Glomar Challenger gathered drill samples
– Determine age of rocks
• Rock closer to the mid-ocean
ridge is younger.
• Rock farther away from midocean ridge is older.
The old rock has moved away
from the ridge.
4. Subduction at Trenches
• Deep-ocean trenches are underwater
• At these trenches, OLD ocean floor bends
down and sinks back into the mantle.
• Subduction is the process by which the ocean
floor sinks.
• It takes rock about 200 million years to go
from the mid-ocean ridge to the deep ocean
Why does oceanic crust sink below continental crust?
Why does older oceanic crust sink below younger oceanic crust?
Ridge: New rock
Magma rises
through cracks
in crust
Rock becomes
magma in
Old rock pushed
Deep Ocean
Trenches: Old
rock sinks into
• Pacific Ocean:
– Shrinking.
– Old crust is subducting faster than new crust is being
• Atlantic Ocean:
– Expanding.
– Lots of new rock forming and very little place for old
rock to go.
– Connected to continents. As it spreads, it moves the
continents and gets wider