• Distinct properties define Earth’s three main layers:

crust, mantle, and core.

• The crust is the rigid outer layer that makes up the

continents and sea floors.

• The lithosphere is a region formed by the crust and

the rigid outer layer of the mantle.

• The asthenosphere is the fluid-like layer of mantle

beneath the lithosphere.

• Earth’s centre has two layers:

a liquid outer core and

a solid inner core.

Structure of Earth17.1

VOCABULARY

crust

mantle

lithosphere

asthenosphere

core

outer core

inner core

seismic wave

• Scientists obtain information about the structure of

Earth through direct and indirect observations.

• Direct observations include analyzing rock directly

from Earth’s interior.

• Indirect observations include measuring gravitational

force differences around the globe to infer the density

of material beneath Earth’s surface.

Structure of Earth17.1

VOCABULARY

crust

mantle

lithosphere

asthenosphere

core

outer core

inner core

seismic wave

• The surface of Earth moves constantly.

• The lithosphere is divided into massive tectonic plates

that are pushed and pulled over the asthenosphere.

• Continental drift theory argues that the continents

have moved slowly since Earth formed.

• Fossil and geological evidence support

the continental drift theory.

• At one time, the continents formed

Pangaea, a single huge landmass.

Evidence of a Dynamic Earth17.2

VOCABULARY

tectonic plate

continental drift theory

paleoglaciation

• Sea-floor spreading occurs at mid-ocean ridges and subduction occurs at deep ocean trenches.

• Heat within Earth creates convection currents in the mantle that help move the plates.

• Radioactive dating of core samples confirmed evidence that the sea floor is older the further it is from the ridges.

• Magnetic striping patterns in sea floor rock are similar on both sides of an oceanic ridge, suggesting new ocean floor is being created at the ridge.

New Evidence of a Dynamic Earth17.3

VOCABULARY

mid-ocean ridge

sea-floor spreading

ocean trench

• The theory of plate tectonics states that the

lithosphere is divided into 12 large sections (plates)

and about 20 smaller ones.

Theory of Plate Tectonics17.4

VOCABULARY

theory of plate

tectonics

divergent boundary

rift valley

convergent boundary

subduction zone

oceanic–oceanic

convergent boundary

oceanic–continental

convergent boundary

continental–continental

convergent boundary

transform boundary

strike–slip fault

• Three types of boundaries exist where tectonic plates meet. These create Earth’s geological features and events.

• Ridges, rifts, volcanoes and earthquakes are created at divergent boundaries, where plates are moving away from each other.

• Earthquakes and strike-slip faults are created along transform boundaries, where plates move past each other in opposite directions.

• At convergent boundaries (where plates move toward each other), we find mountains, trenches, subduction zones, volcanoes, and earthquakes, depending on the types of plates involved.

Theory of Plate Tectonics17.4

VOCABULARY

theory of plate

tectonics

divergent boundary

rift valley

convergent boundary

subduction zone

oceanic–oceanic

convergent boundary

oceanic–continental

convergent boundary

continental–continental

convergent boundary

transform boundary

strike–slip fault

The Dynamic Earth

• Earth is composed of layers.

• The three main layers are the crust,

mantle, and core.

• Earth’s outer layer has moved.

• At one time, the continents formed

Pangaea, a single huge landmass.

17CHAPTER

The Dynamic Earth

• Plates of the outer layer move continuously.

• Plate tectonic theory explains the mechanisms by which tectonic plates move.

• This movement explains Earth’s geological events and features, such as

volcanoes and mountains.

17CHAPTER

The Dynamic Earth

Activity• Scientists use both direct and indirect observations/measurements to

solve real-world problems. A direct measurement means you measure exactly what you mean to measure, while an indirect measurement measures one thing to get information about something else.

• Try the following: • Indirectly determine the area of one circle if the square is

10 cm by 10 cm.• Directly determine the area of one circle if you measured

the radius to be 2.5 cm (Area of a circle = 3.14r2).• What can you infer about direct and indirect

measurements? Which is better? Is one more accurate?• Which type of measurement would you use to get information about

• the weight of a car?• the size of an atom?• the temperature inside a freezer?

• Find out how scientists use both direct and indirect observations and measurements to find out about Earth’s structure on pages 494–495.

17CHAPTER

10 cm

10 cm

The Dynamic Earth17

Key Ideas

• Earth is composed of layers.

• Earth’s outer layer has moved.

• Plates of the outer layer move continuously.

• This movement explains Earth’s geological features and events.

CHAPTER