Plate Tectonics Unit:

Composition of the Earth:

Layers of the Earth: 1. Crust: 5-100km thick. a. Oceanic crust: thin and

more dense, mostly basalt

b. Continental crust: thicker and less dense, made up of more granite than oceanic crust

Layers, Continued:

2. mantle: made of molten (melted rock), very thick, 2,900 km, and likely to have convection currents.

3. Outer Core: molten iron, 2,200 km thick

4. Inner Core: solid but very hot iron, 1,230 km thick

The core makes up 33% of Earth’s total mass

Tectonic Plates:

Because of convection currents in the mantle, plates of the earth’s crust move around as they “float” on the mantle. Over many millions of years, the location of the oceans and continents have changed dramatically, and surface features like valleys and mountain ranges are continually created and destroyed.

Wegener’s Theory of Continental Drift:

In 1910, a German scientist named Alfred Wegener proposed that the continents had, in the very distant past, been joined together in one supercontinent he called Pangea, and had since drifted apart into separate continents. He called this theory Continental Drift.

Wegener’s arguments in support of his theory:

1. Evidence from landforms: • Continents along the Atlantic Ocean appear to

fit together like puzzle pieces. • Mountain ranges in South Africa match up

with mountains in South America. • Coal deposits in Europe match up with coal

deposits in North America.

2. Evidence from fossils: • Fossils of the same species of land animals and land plants

had been found on different continents; they could not have crossed oceans.

• Fossils of tropical plants were found on the island of Spitsbergen, which is well north of the Arctic Circle. Wegner proposed that this island had once been close to the equator.

• Evidence of glaciers had been found in South Africa, which currently has a subtropical climate. Wegner thought South Africa must have been much farther south millions of years ago.

Unfortunately, no one believed Wegener at the time. His evidence was not totally convincing, and he could not explain what force could cause whole continents to move.

Plate Tectonics: Wegener was Right!

Starting in the 1960’s, as scientists started mapping the ocean floor, they discovered that the ocean floor was spreading out along the Mid-Ocean Ridge. This very easily explained how continents could move.

How Sea-Floor Spreading Works:

Evidence for Sea-Floor Spreading: 1. Rock samples from the ocean

floor close to the mid-ocean ridge are younger than rock samples farther away.

2. Magma can actually be observed oozing up from the mid-ocean ridge, and cooling into rock.

3. Iron samples from the ocean floor alternate their patterns, showing that some parts of the ocean floor were formed at times when the magnetic poles were reversed.

Subduction at Deep Ocean Trenches:

The Ultimate Cause of Plate Movement? Convection!

Tectonic Plates of the Earth:

Homework:

LO: Describe the theory of plate tectonics SLE: Communicate ideas clearly and articulately

1. Read p. 108-111 2. Answer review questions on p. 111