1. Check your notes for key ideas!
1.4 Sea-floor Spreading
Check your notes for key ideas!

2. 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!

3. 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!

4. The mid-ocean ridge is the longest chain of mountains in the world!
This is a sonar image of the mid-ocean ridge

5. 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!)

6. ANIMATION!

7. 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

9. 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!

10. ANIMATION!
VIDEO!

11. 3. Evidence for sea-floor spreading
C. 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.

13. 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.

15. Why does oceanic crust sink below continental crust?
Why does older oceanic crust sink below younger oceanic crust?

16. 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

17. Pacific Ocean: Atlantic 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