Continental Drift and

Plate Tectonics

The Theory of Continental Drift

What you will learn about the theory of Continental Drift.

• Alfred Wegener and the theory of Continental Drift (PANGEA!)

• Pieces of evidence found to support the theory of continental drift.

• The theory of Plate Tectonics (how continental drift happened)

Close examination of a globe often results in the observation that most of the continents seem to fit together like a puzzle: the west African coastline seems to snuggle nicely into the east coast of South America and the Caribbean sea; and a similar fit appears across the Pacific.

In 1912 Alfred Wegener (1880-1930) noticed the same thing and proposed that the continents were once compressed into a single large continent which he called Pangaea (meaning "all lands"), and over time they have drifted apart into their current position. He believed that Pangaea was intact until about 300 million years ago, when it began to break up and drift apart.

In the next few slides we will discuss the evidences that Alfred Wegner and his followers used to prove that the Continents indeed drifted apart.

1. Wegener noted the jigsaw fit of South America and Africa, especially, but also elsewhere.

Making Connections: Canada’s Geography. Clark & Wallace. Prentice Hall Ginn, 1999.

Four Main Pieces of Evidence

2. He found fossils that were the same on both continents.

Making Connections: Canada’s Geography. Clark & Wallace. Prentice Hall Ginn, 1999.

3. He found that on both sides of the Atlantic, mountains were the same both in terms of age and structure.

Making Connections: Canada’s Geography. Clark & Wallace. Prentice Hall Ginn, 1999.

4. He found that ice sheets covered parts of Africa, India, Australia and South America 250 million years ago. How could this happen in places that are so warm today?

Making Connections: Canada’s Geography. Clark & Wallace. Prentice Hall Ginn, 1999.

Problem? Wegener's hypothesis of continental drift lacked a geological mechanism to explain how the continents could drift across the earth's surface.

….thus, the theory of PLATE TECTONICS!

It wasn’t until the the 1960s that the theory of plate tectonics was advanced to explain how the continents could separate.

The main features of plate tectonics are:

• The Earth's crust is broken into a series of plates or pieces.

• The plates are continually moving.

The Surface of the Earth with no water!http://www.ngdc.noaa.gov/mgg/image/mggd.gif

A map of the PLATES!

So let’s explain what’s happening!

• the outer surface of the Earth is a thin crust of fragile rock, fractured like the cracked shell of an egg

• the pieces of the shell are Earth's tectonic plates -- there are 12 major ones -- and they float along on top of a layer called the asthenosphere

1. The plates move apart, new magma wells up to the surface, forming new crust. (volcanoes occur). New land is being created at this point. This type of movement is called Divergent.

The plates have three basic movements…

2. When two plates collide, one plate may go underneath (subduction) the other creating huge valleys or oceanic trenches. These trenches are as deep as 35,000 feet below the ocean surface, are long and narrow, and run parallel to and near the shoreline of continents. They are associated with and parallel to large continental mountain ranges as well. This type of plate movement is called Convergent.

When one plate goes underneath the other. The plate that goes under will begin to melt as it approaches the mantle. This new hot molten rock will rise and if it breaks through the lithosphere it will create a volcano. It is important to note that since part of the lithosphere is being lost at this point, there must be some other point on the earth where new land is being created, remember divergent zones!

3. In some parts of the world plates are rubbing up against each other creating tremendous friction. When these plates actually slip (move) they release this energy in the form of earthquakes. Eg. The San Andreas fault. This type of movement is called Transform.

The three basic movements of plates.

Notice that the trench is parallel to the shoreline and the mountain range

The Mid-Atlantic Ridge is a divergent zone.

Iceland: On the Mid-Atlantic Ridge – Geo-Thermal Energy

Indian Plate collides with Eurasian Plate. This is an example of a Convergent Zone.

Mountains!

The result: the Himalayas and Mt. Everest

Earthquakes and Volcanoes! The Pacific Ring of Fire

Transform plate margins: where two plates slip past one another.

Earthquakes!

The San Andreas Fault, California

http://sts.gsc.nrcan.gc.ca/page1/geoh/quake/figures.htm

Tectonic setting of western British Columbia and Washington state. The oceanic Juan de Fuca plate is moving beneath the continental North America plate at a rate of about 4 cm/year. Great earthquakes occur along part of the boundary between the two plates.

Summary• Theory of continental drift and evidence to support it.

(Pangea!)

• Different movements of plates, geologic processes and associated landforms– Diverging: ridges/volcanoes– Converging: trenches, mountains, island arcs– Subducting: same as above– Slipping/Transform: faults and earthquakes– Earthquakes and Volcanoes: along major tectonic plate

boundaries

The End!