Continental Drift

& Plate Tectonics

Alfred Wegener, a German scientist, proposed the

hypothesis of CONTINENTAL DRIFT, in 1912.

Hypothesis stated: All Earth’s continents were once

a single landmass (Pangaea) and then slowly

drifted apart.

Pangaea began to split 200 million years ago.

Evidence supporting Wegener’s Hypothesis

1. Fossils: a reptile, Mesosaurus, was found both in Eastern

South America and Western Africa only.

2. Climate changes: fossils of tropical plants have been found

on Greenland, indicating it might have once been close to the

equator.

3. Geology: Rocks found on one continent match up with rocks

on another continent, like a puzzle. Ex. Brazil to Western Africa.

THEORY OF PLATE TECTONICS

In the mid-1900’s, scientists

proved that the Earth was

made up of 18 moving

plates.

•Each plate is made of the

crust and the top part of the

mantle called the

LITHOSPHERE.

•The plates move on a layer

of hot, molten rock in the

rest of the mantle called the

ASTHENOSPHERE.

From Continental Drift came the....

http://curriculum.kcdistancelearning.com/courses/ENVSCIx-APU10/a/unit05/apes_5.c.4.html

http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/earth.htm

Convection Currents transfer heat energy from the inner core to

the crust.

The rocks within the asthenosphere rise toward the crust when

heated, because they are less dense. Rocks begin to cool and

sink back again into the asthenosphere, as they become more

dense.

Old crust is being destroyed at the same rate new crust is formed = Earth stays the same size.*

What causes

plates to move?

divergent boundaries

Spreading centersSpreading centersSpreading centersSpreading centers are where 2 plates are moving away from are where 2 plates are moving away from are where 2 plates are moving away from are where 2 plates are moving away from

each other. each other. each other. each other.

New crust is formed here, as magma rises from the New crust is formed here, as magma rises from the New crust is formed here, as magma rises from the New crust is formed here, as magma rises from the asthenosphere.asthenosphere.asthenosphere.asthenosphere.

Results on Oceanic Crust: Mid-ocean ridgesResults on Oceanic Crust: Mid-ocean ridgesResults on Oceanic Crust: Mid-ocean ridgesResults on Oceanic Crust: Mid-ocean ridges

(Oceanic Crust splits apart) (Oceanic Crust splits apart) (Oceanic Crust splits apart) (Oceanic Crust splits apart)

1. 1. 1. 1. Mid-ocean RidgesMid-ocean RidgesMid-ocean RidgesMid-ocean Ridges: longest chains of mountains on : longest chains of mountains on : longest chains of mountains on : longest chains of mountains on

EarthEarthEarthEarth

a. Mid-Atlantic Ridge – 11,000 Km from Iceland to Antarctica; where the North & South American Plates are moving away from the Eurasian & African Plates.

Results on Continental Crust: Rift Valleys (hot magma pushes up on

Continental Crust, and the crust cracks as it stretches apart)

As the plates continue to pull apart, the valley floor sinks and becomes thinner.

Once the valley floor is below sea level, the valley fills with water.

Ex. Red Sea created by Arabian & African plates moving apart.

Great Rift Valley of Africa is being created as the

African Plate splits apart.

divergent boundaries

Iceland is being split into 2 islands from the Mid-Atlantic Ridge.

3 Types of Convergent Boundaries3 Types of Convergent Boundaries

1. Continental-Continental Collision occurs

where 2 plates carrying continental crust push

together.•Both plates are the same density, so neither will sink

beneath the other.

•This causes the plates to crumple and fold.

•Continue folding creates mountains.

Mt. Everest (29,035 ft) – Among the Himalayas

where the Indian & European Plates collided.

European Alps result of collision between

African & European Plates.

Convergent BoundaryConvergent BoundarySUBDUCTIONSUBDUCTION

2. Oceanic – Oceanic Subduction: This occurs when an older oceanic plate sinks (subducts) below another younger

oceanic plate. The older plate is pulled back into the asthenosphere because it is colder and denser.

Results:1. Deep-ocean trenches●Deepest places in the oceans; ●Thinnest part of the crust.

2. Island Arcs●Chains of volcanic islands that formabove deep ocean trenches.

2. Oceanic – Oceanic Subduction: This occurs when an older oceanic plate sinks (subducts) below another younger

oceanic plate. The older plate is pulled back into the asthenosphere because it is colder and denser.

Convergent BoundaryConvergent BoundarySUBDUCTIONSUBDUCTION

3. Oceanic – Continental Subduction: This occurs when an older oceanic plate sinks (subducts) below another younger

Continental plate. The older plate is pulled back into the asthenosphere because it is colder and denser.

Results:1. Deep-ocean trenches●Deepest places in the oceans; ●Thinnest part of the crust.

2. Volcanic Mountains●Chains of volcanic mountains that form along the edge of the continent.

Transform Boundaries

•Crust is neither created Crust is neither created Crust is neither created Crust is neither created

or destroyed.or destroyed.or destroyed.or destroyed.

•Two plates scrape past Two plates scrape past Two plates scrape past Two plates scrape past

each other as they each other as they each other as they each other as they

move in opposite move in opposite move in opposite move in opposite

directions.directions.directions.directions.

San Andreas Fault line

runs from the Gulf of

California through San

Francisco.

At the rate of movement,

LA will be part of San

Francisco in 10 million

years.

HOT SPOTSHOT SPOTS

Hot spots are stationary areas where material from deep within the mantle rises and then melts, forming magma.●They can be in the middle of a plate or along a boundary.●Volcanos form above the hot spot when magma erupts through the crust and reaches the surface.

Ex: Hawaiian Islands and Yellowstone Caldera