1. Earth Science Continents Drifting

2. Alfred Wegener in 1912
Wegener thought whole sections of the crust moved (plate tectonics)
Some people did not understand and thought the continents floated around in the oceans (continental drift)
He thought this was true for two reasons:
The continents looked like they once fit together, and

3. Alfred Wegener
Same fossils and rocks found in different parts of the world
He died before new evidence proved him right 

4. Sea floor spreading
Even before Wegener, in 1872, people were laying telegraph cables on the ocean floor so that America and Europe could communicate.
While surveying the ocean floor (using long wires that touched the bottom) they discovered undersea mountains (ridges)!
These ridges were confirmed in 1925 when sonar was invented.
In 1953, it was discovered how extensive these ridges were- right around the Earth.

6. Sea floor spreading
In 1962, Harry Hess discovered cracks in the mountains, called “rifts”.
He proposed that new crust was forming and moving outward, away from the rifts.

7. Some spreading starts on land
Large continents begin to crack and split apart
The gaps fill with water
Small seas become oceans
The mid ocean ridge continues to produce new crust

8. Why spread? Why is the Atlantic still getting wider
The plates are pulled apart by convection currents in the mantle below
Caused by heat released from natural radioactive processes
At the mid Atlantic ridge molten rock from below rises up to fill the gap with new basaltic rock

9. More evidence
“Magnetites” in the sea floor crust show that the Earths North and South pole have flipped many times
These leaves magnetic ‘stripes’ in rock containing iron minerals

10. Now we will make our own “spread”.