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!

This is a sonar image of the mid-ocean ridge

The mid-ocean ridge is the longest chain of mountains in the world!

2. What is Sea-Floor Spreading• Harry Hess, and American geologist, studied mid-

ocean ridges and developed the idea of sea-floor spreading.– Sea-floor spreading: process by which new rock is

continuously added to the sea-floor

IDEA: – 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!)

ANIMATION!

3. Evidence for sea-floor spreadingA. 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 mid-ocean ridge, which suggests it is spreading from both sides!

ANIMATION!

VIDEO!

3. Evidence for sea-floor spreadingC. Evidence from Drilling Samples– Glomar Challenger gathered drill samples– Determine age of rocks

Findings: • Rock closer to the mid-ocean

ridge is younger.

• Rock farther away from mid-ocean ridge is older.

The old rock has moved away from the ridge.

4. Subduction at Trenches

• Deep-ocean trenches are underwater canyons.• 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 trench.

Why does oceanic crust sink below continental crust?

Why does older oceanic crust sink below younger oceanic crust?

Mid-Ocean Ridge: New rock

surfaces

Old rock pushed aside

Deep Ocean Trenches: Old rock sinks into

mantle

Rock becomes magma in

asthenosphere.

Magma rises through cracks

in crust

• Pacific Ocean: – Shrinking. – Old crust is subducting faster than new crust is being

made.• 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