Plate Tectonics

Overview

I. The Theory of Plate Tectonics The Earth’s surface is divided into plates that move and

interact with one another.

The plates include Continental Crust and Oceanic Crust.

The plates are made up of brittle slabs called the Lithosphere.

This includes the crust and the uppermost region of the upper mantle

The lithospheric plates slide around on top of a plastic-like layer of the upper mantle, called the Asthenosphere.

Convection

Material moves from regions of high heat (low density) to regions of low heat (high density).

This creates convection currents in the mantle

This movement causes the overlying lithospheric plates to move!

II. The Mechanism that Drives Plate Tectonics

Divergent

Lithospheric plates are moving apart

New crust and lithosphere is “created”

Associated with ocean ridges, volcanism, earthquakes and high heat flow

Most commonly found on the seafloor

III. Plate Boundary Interactions

Examples found on Earth: Mid-Atlantic Ridge, African Rift Valley, Iceland

Convergent

Plates are moving towards each other

Associated with trenches, island arcs and mountains

Old crust and lithosphere is “recycled”

Three types of convergent boundaries exist:

1. Continental - Continental (colliding)

2. Continental - Oceanic (subducting)

3. Oceanic - Oceanic (subducting)

Examples found on Earth: Himalayas, Ring of Fire, Andes Mountains, Marianas Trench

Sliding

Lithospheric plates slide horizontally past each other

Crust is deformed or fractured

Characterized by long faults and earthquakes

Examples found on Earth: San Andreas Fault

Plate Interactions Overview

IV. Evidence For Plate Tectonics

A. Continental Drift

In 1912, Alfred Wegener proposed that the Earth’s continents had once been joined as a large landmass called Pangaea.

• Continents fit together like a puzzle.

• Fossil remains of Mesosaurus (lived 270 million years ago) are found in Brazil and in South Africa, but nowhere else in the world.This hypothesis was rejected

because he could not propose an explanation for how this happened!

B. Earthquakes

Do not occur randomly throughout the world

Occur in the same regions as volcanoes

C. Volcanoes and Hot Spots

Do not occur randomly throughout the world

80% occur at convergent plate boundaries

15% occur at divergent plate boundaries

The remaining 5% do not occur at a plate boundary and form as the result of hot spots.

D. Magnetism

The Earth’s magnetic poles have often been reversed.

Rocks in the Earth’s crust contain magnetic minerals that provide a record of the direction of Earth’s magnetic poles at the time the rock was formed.

Magnetic reversals reveal themselves in symmetrical banding patterns in the rocks on the ocean floor.

What do you notice about the age of the rocks?

E. Age of the Ocean Floor

Further analysis of rocks on the sea floor revealed that the ages of the rocks vary.

The rocks are youngest near ocean ridges and get older as you move away in opposite directions

F. Heat Flow

Heat flow is a measure of the amount of heat leaving the rocks of the lithosphere.

The values are unusually high near ocean ridges and decrease as you move away from the centers in either direction.

The values are unusaully high in the subduction zone where plates converge.

What conclusions can be drawn from this evidence?

Harry Hess compiled all the evidence together and proposed the theory of Seafloor Spreading in 1962.

The theory states that new ocean crust is formed at ocean ridges and is destroyed at deep-sea trenches.

The Theory of Seafloor Spreading

A compilation of all this evidence and past theories has lead scientists to develop the current theory of Plate Tectonics.

V. Examples of Land Formation due to Plate Tectonics

A. Formation of the Andes Mountains

Nazca plate subducts beneath the South American

plate

B. Formation of Japan

Pacific Plate subducts beneath the Eurasian

Plate

C. Formation of Iceland

North American plate and Eurasian plate diverge

D. Formation of the Galapagos Islands

Nazca plate and Cocos Plate diverge

Is the Geology of the Galapagos more similar to

Iceland or Hawaii?