Chapter 10, lesson 3, Plate Tectonics


Before You Read

Have you ever been swimming and noticed some areas in

the water are colder and other areas are warmer? Why do

you think this happens?


Plate Tectonics

In the 1960s, scientists developed a new theory that

combined continental drift and seafloor spreading.

According to the theory of plate tectonics (tek TAH nihks), Earth’s crust and part of the upper mantle are broken into plates, or sections, that move around on a plastic like layer of the mantle.


What are plates?

Plates are large sections of Earth’s crust and upper

mantle. These plates float and move around on a plasticlike layer of the upper mantle. You can think of the plates as rafts that float on this layer.


What are the lithosphere and the asthenosphere?

Together, the crust and the rigid upper mantle form the

lithosphere (LIH thuh sfihr). The lithosphere is about

100 km thick. The layer below is called the asthenosphere (as THE nuh sfihr). The asthenosphere is plasticlike. The rigid plates of the lithosphere float and move around on the plasticlike asthenosphere.



Plate Boundaries

When plates move, several things can happen. They can

move closer together and converge, or collide. They also can pull apart or slide by one another. When plates move, the result of these movements shows up at plate boundaries.

Movements at one boundary means that changes must happen at other boundaries. The figure above shows the major plates, the way these plates are moving, and plate boundaries.



What is a divergent boundary?

The area where two plates meet is called a boundary. When two plates are moving apart, the area between them is called a divergent boundary. In the figure above, find the North American Plate. Now find the African and Eurasian plates. The arrows show the North American Plate moving away from the Eurasian and African Plates. The divergent boundary between these plates is called the Mid-Atlantic Ridge. It is a midocean ridge. As the plates pull apart, magma pushes up and becomes new seafloor. At some divergent plate boundaries, rift valleys form as the plates pull apart and crust sinks.



What is a convergent boundary?

When two plates converge, or come together, they form a

convergent boundary. What happens to the plates when they come together? One plate can sink and disappear under the other plate. For example, oceanic plates are denser than continental plates. When an oceanic plate converges with a less dense continental plate, the denser oceanic plate sinks under the continental plate.


What is subduction?

The area where an oceanic plate subducts into the mantle

is called a subduction zone. As the plate subducts into the

mantle, it begins to melt. The melting rock becomes magma.

The newly formed magma is forced upward along these

plate boundaries. Volcanoes form above these subduction

zones. Subduction zones occur at convergent boundaries.

The Andes mountain range in South America is located at

a convergent boundary. The Nazca and the South American Plates converged to form them. There are many volcanoes in the Andes mountain range. When the Nazca Plate subducted, newly formed magma was forced upward, creating these volcanoes.

The figure above shows a subduction zone and what can

occur at a convergent boundary between oceanic and

continental plates. The denser oceanic plate is sinking under the less dense continental plate. High temperatures cause the rock to melt around the subducting slab as it moves under the other plate.


What happens when oceanic plates converge?

When two oceanic plates converge, the colder, older, denser plate bends and sinks down into the mantle. A subduction zone forms where these plates collide. Volcanoes can form and, over time, some volcanoes form islands. The Mariana Islands in the western Pacific Ocean are a chain of volcanic islands that formed where two oceanic plates collided.


What occurs if continental plates collide?

When two continental plates collide or converge, neither

of the plates sinks under the other. Subduction usually

doesn’t occur. The continental plates are less dense than the asthenosphere below them. As a result, when these two plates collide, they fold and crumple to form mountain ranges. Earthquakes are common at these convergent boundaries. Volcanoes do not form because there is no, or little, subduction.

The Indo-Australian Plate is colliding with the Eurasian

Plate. These converging plates are forming the Himalaya in Asia.


What is a transform boundary?

The third type of plate boundary is called a transform

boundary. Transform boundaries occur when two plates

slide past one another. In one type of transform boundary,

two plates slide past each other in opposite directions. In

another type, two plates are moving in the same direction,

but at different rates. When one plate moves past another,

earthquakes occur.

As shown below, the San Andreas (an DRAY us) Fault in

California is part of a transform plate boundary. The Pacific Plate is sliding past the North American Plate. Both plates are moving in the same direction, but at different rates. As a result, this area has many earthquakes.

Causes of Plate Tectonics

Scientists don’t know exactly why Earth’s plates move.

They hypothesize that plates move by the same basic process that occurs when soup is heated in a pan.


What is convection current?

Convection (kun VEK shun) currents can be found in a

pan of soup that is cooking. As it heats, some of the soup

becomes hotter and less dense. Some of the soup is cooler

and more dense. This difference in temperature causes

movement in the soup.

The cooler soup sinks and forces the hotter soup to rise

to the top of the pot. As the hot soup reaches the surface, it cools and sinks back down into the pan. This happens in a cycle, over and over. This cycle of heating, rising, cooling and sinking is called a convection current.


Are there convection currents inside Earth?

What causes Earth’s plates to move? A type of convection

current is occuring inside Earth. Materials deep inside Earth have different amounts of heat and density. The colder, denser materials force the hotter, less dense materials towards Earth’s surface. The arrows in the figure below show the rise and fall of materials in Earth’s mantle.

One hypothesis suggests the transfer of heat inside Earth

provides the energy to move plates and causes many of

Earth’s surface features. All of the hypotheses use convection currents in some way to explain the movements of plates.

Features Caused by Plate Tectonics

Earth is an active planet with a hot interior. The heat

inside Earth causes convection that powers the movement of Earth’s plates. When the plates move and interact, they

produce forces that cause Earth’s surface to change. These changes may happen over millions of years.


How do normal faults and rift valleys form?

If forces are pulling Earth’s crust in opposite directions,

the crust will stretch. These pull-apart forces are called

tension forces. As the crust stretches, large blocks of crust

will break and slip down the broken surface of the crust.

When rocks break and move along surfaces, a fault forms.

Faults move rock layers out of place. In the process,

mountains can form. Usually faults that form this way are

called normal faults. In normal faults, the rock layers above the fault move down when compared with the rock layers below the fault. Look at the figure below. The arrows show how tension forces stretch Earth’s crust causing the movement of rock along normal faults. A range of mountains, called fault-block mountains, can form in the process.


Tension forces also cause rift valleys and mid-ocean

ridges. Rift valleys and mid-ocean ridges are large cracks

that form where Earth’s crust separates. One example of a

rift valley is the Great Rift Valley in Africa. Valleys also

occur in the middle of mid-ocean ridges. The Mid-Atlantic Ridge and the East Pacific Rise are two examples of mid-ocean ridges.


How do mountains, reverse faults, and

volcanoes form?

Compression forces squeeze objects together. Where

Earth’s plates come together, compression forces produce

several effects. As continental plates collide, compression

forces cause rock layers to fold and fault. Mountains can

form. The Himalaya (hih muh LAY uh) are mountains

being formed where two plates are colliding and forcing

huge sections of rock to fold and break. The figure above

shows compression forces forming a mountain range.

Usually compression forces cause a reverse fault. In a

reverse fault, rock layers above the fault surface move up

when compared with the rock layers below the fault. This is the opposite of a normal fault.

As you read earlier, when two oceanic plates converge, the denser plate will sink under the less dense plate. If an

oceanic plate converges with a continental plate, the denser oceanic plate slides under the continental plate. Mountains and volcanoes can form as a result of the folding and faulting that occurs at the plate boundaries.


What are strike-slip faults?

Strike-slip faults occur where two plates stick, or strike,

and then slip by one another. Strike-slip faults occur at

transform boundaries. A transform boundary is where two

plates slide past one another. The plates can slide by in

opposite directions or they may slide by in the same direction, but at different rates. When the plates move suddenly, they cause vibrations inside Earth that we feel as earthquakes. The San Andreas Fault is a strike-slip fault.


Testing for Plate Tectonics

Only recently have scientists been able to measure exact

movements of Earth’s crust. All the early methods to check for plate movements were indirect.


What are indirect methods of testing?

You have been reading about indirect methods of testing

plate movements in this chapter. One method is studying

the magnetic properties of rocks on the ocean floor.

Scientists also could study volcanoes and earthquakes. These methods supported the theory that plates have moved and are still moving. However, these methods did not prove that plates are moving.


How is plate movement measured?

There is a new method of measuring small amounts of

plate movement that uses lasers and satellites. The figure

below shows the Satellite Laser Ranging System. From the ground, scientists aim laser pulses at a satellite in orbit. The pulse reflects off of the satellite and returns to Earth. With this new technology, scientists can measure exact amounts of movement of Earth’s plates. This new method shows that the plates move at rates between 1 cm to 12 cm per year. For example, Hawaii is moving toward Japan at a rate of 8.3 cm per year.