.

Plate Tectonics• Theory that the Earth’s

crust is made of rigid plates that float on the molten layer of the mantle.

• Comes from the Greek word meaning “builder”.

• Explains nearly all of Earth’s major surface features and activities: faults and earthquakes, volcanoes and volcanism, mountains and mountain building, and the origin of the continents.

Continental Drift

• About 50 years earlier, German geophysicist Alfred Wegener developed a related theory known as continental drift.

• His theory is supported by the fact that the world’s continents fit together like pieces of a puzzle.

• This super continent is called PANGAEA.

Rodinia

History of Plate Tectonics• Wegener’s idea was

supported by:

• the fit of the continents

• the distribution of fossils

• a similar sequence of rocks at numerous locations

• ancient climates

• the apparent wandering of the Earth's polar regions

• Wegener used his observations to hypothesize that all of the present-day continents were once part of a single super continent called Pangaea.

Fossil Record

Ancient Climates

Problems with Wegener’s Theory

• Wegener’s Theory was not accepted by most geologists.

• They said that ocean currents or winds could scatter the ancient plants and animals.

• They also thought that maybe it was possible for the poles to wander.

The Big Problem

• Wegener could not provide an explanation for how the continents could move.

• He couldn’t explain what forces could actually cause the continents to move.

The Big Solution• Arthur Holmes, (Scottish geologist) made this proposal in 1928.

• He believed heat trapped in the Earth caused convection currents, areas where fluids beneath the Earth's crust rise, flow laterally, and then fall. The currents would rise beneath continents, pushing them into each other.

• Unfortunately, Wegener died in 1930 while exploring the Greenland ice cap. He never had the opportunity to adapt Holmes' ideas to his views of continental drift.

Theory of Plate Tectonics

• Totally revolutionized our understanding of the Earth, past and present.

• Theory ranks with the theories on evolution, relativity, the Big Bang, and Newton’s Laws.

Tectonic Plates

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Visual Concept

Section 1 Inside the EarthChapter 7

Plate Structure

• The lithosphere is broken into rigid slabs of rock called tectonic plates.

• These plates are moving about on top of the asthenosphere.

• Resembles a giant jig saw puzzle.

Plate Movement

• The plates are pushed along by convection currents in the mantle.

• Plates move at rates of about 1 inch per year.

• The fastest move more than 4 inches a year.

Causes of Tectonic Plate Motion

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Visual Concept

Chapter 7 Section 3 The Theory of Plate Tectonics

Convection Currents Inside the Earth

The tectonic plates float on the magma that lies in the core of the earthThis magma is just one big convection currentThese currents push the continental plates around.

Plate Boundaries• This is where all the action is:• Volcanoes• Earthquakes• New crust being created, old crust being

destroyed

Plate Interaction• Tectonic plates can

interact in three ways• Converge (→←),

diverge (←,→) and transform(↑↓).

• Boundaries where plates meet are known as plate margins.

Continental Drift

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Visual Concept

Section 2 Restless ContinentsChapter 7

Tectonic Plate Boundaries

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Visual Concept

Chapter 7 Section 3 The Theory of Plate Tectonics

Converging Boundary O-O• When two plates

converge, one of the plates subducts or sinks underneath the other forming a deep depression called an ocean or deep sea trench. Here crust is destroyed.

• The subducted plate sinks downward into the mantle and begins to melt.

• The molten rock rises towards the surface and forms a chain of volcanic islands.

Converging Boundary O-O

Converging Boundary O-C

• Instead of a volcanic arc, you get a volcanic mountain range.

•Subduction zone

Converging Boundary O-C

Converging Boundary C-C

• Continental crust is too light to be carried downward into a trench.

• Two continental plates converge to form mountain ranges.

• Andes and Himalayans are examples.

Converging Boundary C-C

Transform Boundary• A transform boundary is

when two plates slip past each other, moving in opposite directions.

• Earthquakes often happen in areas with transform boundaries.

• The San Andreas fault in California is a good example of this type of fault

• Tend to have the most earthquakes.

• Crust is not created or destroyed

Transform Boundary

Divergent Boundaries• These are places where the

plates are moving away from each other.

• Magma from deep in the mantle is pushing the plates apart. This creates new crust.

• They cause the mid-ocean ridges. They are the longest mountain ranges on the planet.

• Mid-ocean ridges on land are called rift valleys.

• The rift valley in east Africa is about 3,000 km long.

• Could be the beginning of a new ocean.

• This is called sea floor spreading.

• The mid-ocean ridges wrap almost completely around the Earth!

Magnetic Reversals and Sea-Floor Spreading

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Visual Concept

Section 2 Restless ContinentsChapter 7

Rifting

Hot Spots• Some volcanoes can occur

in the middle of plates.• Sometimes the plates

move over an area where the magma is close to the surface.

• Volcanoes occur at the same place, but because the plate is moving, a chain of volcanoes can occur.

• Hawaiian Islands were formed this way.

Future Earth

• What do you think the world will look like in a million years???

• It is predicted that the continents will eventually crash together on the other side of the world, creating a new Pangaea.

Future World

150 million years in the future

Return to Pangaea