Continental Drift and Sea Continental Drift and Sea Floor SpreadingFloor Spreading

Chapter 17Chapter 17

Continental DriftContinental Drift

• In 1912 Alfred Wegener came up with the hypothesis

– The continents were all one

– Called Pangaea

Continental DriftContinental Drift• Pangaea

– Super continent– Existed during the

Permian Period– North half called

“Laurasia”– South half called

“Gondwanaland”

Continental DriftContinental Drift

• Wegener had 4 major pieces of evidence for his theory

1. Similar rocks found in Africa and South America

2. Glaciation and Ancient Climates

3. Fossils

4. Continents seem to fit like puzzle pieces

Continental DriftContinental Drift

1. Similar Rocks in Africa and South America– Rocks on both coasts are the same kind– 2 continents must have been joined when

rocks formed

Continental DriftContinental Drift2. Glaciation/Ancient

Climates• During

Carboniferous period

• Glaciers in India, South Africa, South America and Australia

• Northern hemisphere was warm and tropical

Continental DriftContinental Drift

3. Fossils• Mesosaurus = aquatic (shallow) reptile

Continental DriftContinental Drift

3. Fossils• Glossopteris = fern

that liked cold climates that has large seeds that do not spread out well

Continental DriftContinental Drift4. Continents fit like

puzzle pieces• First looked at

South America - looked like it would fit like a puzzle piece into Africa

• Most continents fit together

Continental DriftContinental Drift

• Wegener tried to explain why and how the continents moved

• He thought that the continents plowed through the ocean floor

• This was physically impossible

• So the other scientists of the time didn’t believe him

Sea Floor SpreadingSea Floor Spreading• Enter Harry Hess (1962)

• Naval Officer• Used Fathometer

(measures depth)• Took a map of the sea

floor• Noticed mountains

and valleys in the ocean

Sea Floor SpreadingSea Floor Spreading

• He proposed a theory to explain mid-ocean ridges– The magma wells up from deep inside the Earth

and forms new rocks.– Some rocks contain minerals that are magnetic.– When the rock forms it records the direction of

Earth’s magnetic field.– Like a compass, the rocks “point” towards

magnetic north.– From the rocks we know that the earth’s

magnetic field has reversed over time.

Sea Floor SpreadingSea Floor Spreading

• From this theory, he could estimate age of land

• And the speed of movement of plates• Noted that if new crust is formed, there must

be a place where old crust is destroyed• It is a continuous cycle• Even though new crust is forming, the Earth

remains the same size!