100 LESSON 1 Plate Tectonics

Did you ever wonder how mountains were formed or why theyare located where they are? Why do earthquakes happen?

The earth’s surface is not the same everywhere, and it keepschanging. One cause of these changes has to do with what’s hap-pening inside the earth.

Inside the EarthLook at Figure 3-1. It shows that the earth is made up of three

layers—the core, the mantle, and the crust. The core is locatedabout 4,000 miles (6,430 km) below the earth’s surface. It is madeup of iron and nickel. The middle layer is the mantle. This is alayer of hot, molten rock. The outer layer is the earth’s crust. Thislayer of rock ranges from about 2 miles (3 km) thick under theoceans to about 75 miles (121 km) thick under mountains.

The crust is broken into more than a dozen great slabs of rockcalled tectonic plates. These plates float on a partially meltedlayer of the mantle. The plates carry the earth’s continents andoceans. Map 3-1 shows the earth’s major tectonic plates and theirboundaries.

Plate TectonicsWhat You Will Learn

To explain how Earth’s majorphysical features were formed

Reading Strategy

Create a diagram like the one below.In the left box, write “platemovements.” In the right box,describe the effect this force has onthe earth.

Terms to Know

core, mantle, crust, tectonic plate,plate tectonics, divergent boundary,magma, transform boundary,convergent boundary

Lesson 11

Figure 3-1 Inside the Earth

How Do the Tectonic Plates Move?The theory of plate tectonics explains how most of the major

features of the earth’s surface were formed. According to this the-ory, tectonic plates have been moving and shaping the surface ofthe earth for 2.5 to 4 billion years. Map 3-1 shows the direction inwhich the plates are moving. Most of the time, plate movement isso gradual—only about 4 inches (10 cm) a year—that it cannot be

cause effect

Mantle

Crust

Inner Core

Outer Core

LESSON 1 Plate Tectonics 101

plates. Look at Map 3-1. Find the North AmericanPlate and the Pacific Plate. The North AmericanPlate moves in a northwest direction. The PacificPlate moves in a westward direction. The SanAndreas Fault in California is a boundary betweenthe two plates. Many earthquakes occur alongthis boundary.

At convergent boundaries, two tectonic platesmove toward each other and collide. One plate isoften forced below the other. When an ocean platecollides with a continental plate, the ocean plateslides under the continental plate and forms a deepocean trench. The Mariana Trench in the northPacific Ocean is an example of such activity. Whencontinental plates collide, they form major moun-tain ranges, such as the Himalaya. Look at Map 3-1.What two plates collided to form the Himalaya?

Sometimes a plate that is forced beneathanother reaches into the deeper regions of theearth where it is hot enough to melt part of theplate. The melted rock, or lava, rises back towardthe surface where it forms volcanoes and islands.The Hawaiian Islands were formed in this way.

felt. The way the plates move determines thekinds of landforms that are formed. It also deter-mines where earthquakes occur.

There are three kinds of movement that happenat the boundaries between the plates. At diver-gent boundaries, tectonic plates move apart, orrift, from each other. When a rift occurs on a con-tinent, it creates a gap into which water flows.This creates lakes and when the gap continues towiden, it creates seas. The Red Sea, for example,was formed when a rift occurred between theAfrican and Indo-Australian tectonic plates.

When a rift occurs in the ocean, liquefied rock,called magma, rises up to fill the gap between theseparating plates. The magma creates new cruston the edges of the two plates. The separation ofthe plates and the new crust helps to enlarge theocean floor. The built-up crust forms long under-water mountain ranges called ocean ridges.

At transform boundaries, plates slide pasteach other along what are called faults. Thismovement does not create landforms, but it doescreate earthquakes along the boundary of the two

Map 3-1 Plates and Plate Movement

N

S

EW

N

S

EW

Convergent BoundariesDivergent BoundariesPlate Movement

Earth's Tectonic Plates

EURASIAN PLATE

AFRICAN PLATE

CARIBBEANPLATE

NORTH AMERICANPLATEJUAN DE FUCA

PLATE

COCOS PLATE

NAZCA PLATE

PACIFIC PLATE

SOUTH AMERICANPLATE

SCOTIAPLATE

ANTARCTIC PLATE

ARABIANPLATE

INDO-AUSTRALIANPLATE

FIJIPLATE

PACIFICPLATEPHILIPPINE

PLATE

EURASIAN PLATE

AFRICAN PLATE

CARIBBEANPLATE

NORTH AMERICANPLATEJUAN DE FUCA

PLATE

COCOS PLATE

NAZCA PLATE

PACIFIC PLATE

SOUTH AMERICANPLATE

SCOTIAPLATE

ANTARCTIC PLATE

ARABIANPLATE

INDO-AUSTRALIANPLATE

FIJIPLATE

PACIFICPLATEPHILIPPINE

PLATE

Reviewing Key Terms

Match each term with its meaning. Draw a line from each term to itsdefinition.

1. tectonic plates

2. mantle

3. divergent boundaries

4. transform boundaries

5. convergent boundaries

6. crust

7. magma

8. core

Recalling Facts

Fill in the blanks to correctly complete the following sentences.

1. The earth’s is made up of iron and nickel.

2. According to the theory of plate tectonics, plates have been moving for

years.

3. The way that move determines the kindsof landforms that are created on the earth’s surface.

4. The Himalaya were formed at a boundaryof two tectonic plates.

5. Earthquakes in California often occur at the

boundary of two tectonic plates.

6. The Red Sea was formed at the boundaryof two tectonic plates.

7. When a rift occurs in the ocean, rises upto fill in the gap between the two plates.

8. The was formed at a convergent bound-ary of a continental plate and an ocean plate.

Mariana Trench

magma

divergent

transform

convergent

tectonic plates

2.5 to 4 billion

core

Using Your Skills

102 LESSON 1 Plate Tectonics

a. boundaries where tectonic plates move toward each other

b. a layer of hot, dense rock inside the earth

c. boundaries where tectonic plates slide past each other

d. slabs of rock that are part of the earth’s crust

e. boundaries where tectonic plates move away from each other

f. innermost layer of the earth

g. outer layer of the earth

h. liquefied rock

Practicing Map Skills

Use Map 3-1 on page 101 to answer the following questions.

1. On which tectonic plate is the United States located?

the American Plate

2. On which tectonic plate is Panama located?

the Caribbean Plate

3. On which tectonic plate is India located?

the Indo-Australian Plate

4. Which two plates make up a convergent boundary near South America?

the American Plate and the Nazca Plate

5. Which two plates make up a convergent boundary near Southeast Asia?

the Eurasian Plate and the Indo-Australian Plate

6. Which three plates make up a divergent boundary near the AtlanticOcean?

the American, Eurasian, and African Plates

7. Which two plates are responsible for the creation of the Himalayamountain ranges?

the Eurasian Plate and the Indo-Australian Plate

8. Which two plates are responsible for the creation of the Andes moun-tain ranges?

the American Plate and the Nazca Plate

LESSON 1 Plate Tectonics 103