Earth ScienceEarth Science

Continents DriftingContinents Drifting

Alfred Wegener in 1912Alfred 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

Alfred WegenerAlfred Wegener

• Same fossils and rocks found in different parts of the world

• He died before new evidence proved him right

Sea floor spreadingSea 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.

Sea floor spreadingSea 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.

Some spreading starts on landSome 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

Why spread?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

More evidenceMore 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

Now we will make our own “spread”.