Plate Tectonics

• Three factors cause the movement of Earth’s tectonic plates:

mantle convection, ridge push, and slab pull.

• Forces at plate boundaries produce landscape features such as

mountains, trenches, and island arches.

18CHAPTER

Plate Tectonics

• The interactions of

tectonic plates cause

volcanoes and

earthquakes.

• Earthquakes send

waves through Earth.

18CHAPTER

Plate Tectonics

Activity

• Consider a pot of water sitting on a hotplate. Using your knowledge

of convection (Ch.14), draw a labelled diagram and use arrows to

indicate the rising of warm, less dense water and the falling of cool,

more dense water.

• What would happen if you placed a cork in the

middle of the pot of water? Would it move or

stay in the middle? Why? What force is acting

between the water and the cork causing the cork

to move?

• The cork floating on the water is analogous to

Earth’s crust “floating” on the mantle. Read

page 517 to learn how mantle convection currents

can move tectonic plates.

18CHAPTER

Plate Tectonics18

Key Ideas

• Three factors cause the movement of Earth’s tectonic plates.

• Forces at plate boundaries produce landscape features.

• The interactions of tectonic plates cause volcanoes and earthquakes.

• Earthquakes send waves through Earth.

CHAPTER

• Scientists now believe that three forces work together

to move the tectonic plates:

1. mantle convection

2. ridge push

3. slab pull

Causes and Effects of Plate Movement18.1

VOCABULARY

mantle convection

ridge push

slab pull

• Uplifted mountains, volcanic belts, and island arcs

are found parallel to convergent boundaries.

• Ocean trenches lie along convergent boundaries.

• Mid-ocean ridges and rift valleys mark divergent

boundaries.

Causes and Effects of Plate Movement18.1

VOCABULARY

mantle convection

ridge push

slab pull

• Volcanoes are produced over lithosphere cracks and mantle hot spots.

• Hot spots are rising plumes of hot mantle magma. As tectonic plates move over the hot spot, a chain of progressively younger volcanoes is formed opposite to the direction of plate movement.

• Volcanic island chains and volcanic belts are created on the overriding plate, parallel to a convergent boundary.

Geological Events18.2

VOCABULARY

volcano

hot spots

volcanic belt

volcanic island arc

earthquakes

fault

focus

epicentre

shallow-focus

intermediate-focus

deep-focus

seismic wave

body wave

primary wave (P–wave)

secondary wave

(S–wave)

surface wave

• Earthquakes result when tectonic forces overcome the friction between plates. Earthquakes are categorized based on how far beneath the surface the focus is located.

• The sudden movement of the lithosphere during an earthquake sends seismic waves (vibrations) through Earth.

• Primary and secondary body waves travel through Earth, starting at the focus, the site of the earthquake within Earth.

• Surface waves travel along the outside of Earth. They cause the most destruction.

Geological Events18.2

VOCABULARY

volcano

hot spots

volcanic belt

volcanic island arc

earthquakes

fault

focus

epicentre

shallow-focus

intermediate-focus

deep-focus

seismic wave

body wave

primary wave (P–wave)

secondary wave

(S–wave)

surface wave