Chapter Introduction

Lesson 1 Plate Tectonics

Lesson 2 Earthquakes and Volcanoes

Lesson 3 Weathering, Erosion, and Deposition

Chapter Wrap-Up

1. Continents do not move.

2. Earth’s mantle is liquid.

3. Earthquakes occur and volcanoes erupt only near plate boundaries.

4. Volcanoes erupt melted rock.

5. Rocks cannot change.

6. Sediment can be transported by water, wind, and ice.

Do you agree or disagree?

• What is the theory of plate tectonics?

• What evidence do scientists use to support the theory of plate tectonics?

• How do the forces created by plate motion change Earth’s surface?

Plate Tectonics

• plate tectonics• continental drift• convergent

boundary• divergent

boundary

Plate Tectonics• transform

boundary• subduction

zone• compression• tension• shear

• The theory of plate tectonics states that Earth’s crust is broken into rigid plates that move slowly over Earth’s surface.

• The rigid plates are called tectonic plates.

• North America is part of the North American Plate.

Plate Motion

• The movement of one plate is described as either moving away from or toward another plate, or sliding past another plate.

• Plates move at speeds of only a few centimeters per year.

• At this rate, it takes moving plates millions of years to make new continents, new mountain ranges, or other landforms.

Plate Motion (cont.)

Long before geologists proposed the theory of plate tectonics, they discovered evidence of continental movement.

Plate Motion (cont.)

Photo by G.K. Gilbert, U.S. Geological Survey

• In 1912, Alfred Wegener developed the hypothesis that continents move, called continental drift.

Plate Motion (cont.)

• One piece of evidence is the shape of Earth’s continents.

• The outlines of South America and Africa match up like pieces of a puzzle.

Plate Motion (cont.)

• Wegener used different types evidence to back up his hypothesis:

– Geological evidence

– Fossil evidence

•Fossil and rock evidence from the Gondwana continents also supports the hypothesis of continental drift.

Plate Motion (cont.)

Geologists have discovered the same types of fossils on continents that are now separated by vast oceans.

•Scientists have also found geological evidence.

•Rocks that are made of similar substances and mountains that formed at similar times are present on continents that are now far apart.

Plate Motion (cont.)

•Wegener’s hypothesis of continental drift was not accepted for more than 50 years after it was proposed.

•The main reason for this was because he could not explain how the continents could move.

Plate Motion (cont.)

Plate Motion (cont.)

• Convection is the circulation of particles within a material caused by differences in thermal energy and density

• Convection affects the mantle underneath tectonic plates.

• Hotter mantle rises toward Earth’s surface and cooler mantle sinks deeper into the mantle.

Plate Motion (cont.)

As the Earth’s mantle moves, it pushes and pulls tectonic plates over Earth’s surface.

• The edges of tectonic plates are called plate boundaries.

• A convergent boundary is where two plates move toward each other.

• A divergent boundary is where two plates move apart from each other.

• A transform boundary is where plates slide horizontally past each other.

Tectonic Plate Boundaries

Convergent Boundaries

• When two plates come together, the denser oceanic plate usually is forced down into the mantle and the less dense continental plate remains on Earth’s surface.

• The area where one plate slides under another is called a subduction zone.

• When two continents collide at a plate boundary, both continents remain on the surface.

• As two continents push together, the crust rises up and large mountains form.

Convergent Boundaries

• When plates move apart at divergent boundaries, a rift forms between the two plates.

• A rift can form within continents when continental crust moves in opposite directions.

Divergent Boundaries

• A rift also can form at divergent boundaries on the ocean floor.

• As plates separate, molten rock can erupt from the rift.

• As the molten rock cools, it forms new crust.

Divergent Boundaries

• Tectonic plates slide past each other at transform boundaries.

• The two sides of the boundary move in opposite directions.

• This can deform or break features such as fences, railways, or roads that cross the boundary.

Transform Boundaries

• Forces within Earth cause plates to move.

• The three types of plate boundaries experience different types of forces.

Forces Changing Earth’s Surface

• The squeezing force at a convergent boundary is called compression.

• The pulling force at a divergent boundary

is called tension.

• The side-by-side dragging force at transform boundaries is called shear.

• Even though plates move slowly, the forces at plate boundaries are strong enough to form huge mountains and powerful earthquakes.

• Tensional forces pull the land apart and form rift valleys and mid-ocean ridges.

• Compressional forces form mountains.

Forces Changing Earth’s Surface (cont.)

• Evidence of continental drift includes fossils, mountain ranges, rock types, and the shapes of continents.

• Earth’s crust is broken into pieces called tectonic plates. Convection currents in the mantle cause the plates to move.

• There are three types of tectonic plate boundaries. Movement occurs at all three boundaries.

Which boundary is between two plates moving toward each other?

A. convergent

B. divergent

C. subduction

D. transform

A. convergent

B. divergent

C. subduction

D. transform

Which zone describes the boundary where one plate slides under another plate?

A. shear

B. tension

C. compression

D. continental drift

Which is the side-by-side dragging force at transform boundaries?

1. Continents do not move.

2. Earth’s mantle is liquid.

Do you agree or disagree?