Earth ScienceEarth Science

Evidence for the

SLOW CHANGES

we can’t see

Continents- Do they Continents- Do they REALLY move?REALLY move?

• RECAPPING:

• What were the two observations Alfred Wegener made to

support his claim of Continental Drift?

• 1) The continents look like they fit together; and

• 2) There are fossils found of the same

type on opposite sides of the ocean

(which nicely line up when you push

the continents back together).

Continents- Do they Continents- Do they REALLY move?REALLY move?• RECAPPING:

• After the undersea mountains (ridges) were discovered, what did Harry

Hess notice about them?

• They have big cracks in them, called rifts.

• What was happening in the rifts?

• Lava was slowly pouring

out, forming new crust

and causing SEAFLOOR

SPREADING

Continents- Do they Continents- Do they REALLY move?REALLY move?

• RECAPPING:

• Other support for Hess’ theory:

• Magnetic particles in the rock, called magnetites were

• arranging themselves according to

changes in the magnetic field of

the Earth, changing direction.

Continents- Do they Continents- Do they REALLY move?REALLY move?

• And yet more evidence:

(write this down)

• The AGE of the rocks got

older the further away

from the ridge, and it was

perfectly symmetrical on

both sides.

Continents- Do they Continents- Do they REALLY move?REALLY move?

• Also, by dating the rocks in the sea floor, as well as on

the continents, it was discovered that the further away

from these ridges, the older the rocks.

• The oldest seafloor rocks are only about 200 million

years old- that sounds OLD!! But not compared to

• Some rocks on the continents are BILLIONS of years old.

So, how do plates move?So, how do plates move?

• Notice the upper layer

in the mantle- called the

ASTHENOSPHERE it’s

the liquidy bit.

• Now notice the crust

and the solid bit under it

• Called the

LITHOSPHERE

So, how do plates move?So, how do plates move?

• Convection currents form in the magna, moving plates above.

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

• It happened with the Red Sea.• It’s getting wider, just like Iceland!