What are the plates of Earth's lithosphere?
What causes them to move?
In what directions do these plates move?

In the Film

Animated graphics portray the planet's mantle churning with heat. On Earth's surface, leading-edge, cool pieces of the lithosphere are shown sinking into the mantle dragging their plates behind them. Other animation shows the Baja California peninsula being pulled away from mainland Mexico and the peninsula drifting northwestward.

Concept

Plates (sections) of Earth's lithosphere move slowly over the mantle, pressing against one another, sliding by each other, or pulling apart.

Objective

To demonstrate movements at plate boundaries

Content

Science, language arts

Background

Earth's lithosphere (crust and upper mantle) is cracked into pieces, called plates. These plates, driven by convection currents, drift across the underlying mantle. Some plates pull apart (diverge), and some collide (converge). One plate may slide by another in a lateral (transform) movement. These movements cause earthquakes and contribute to the formation of mountains, volcanoes, and seas. The theory of plate tectonics explains the concept of Earth's lithosphere as being constructed of moving plates.

Seafloor spreading, a divergent activity, takes place at mid-oceanic ridges as magma pushes up through cracks in the crust, pushing plates apart and forming new ocean crust. The mid-Atlantic ridge is the best known spreading zone. A smaller spreading zone is found in the middle of the Gulf of California, moving Baja California away from mainland Mexico.

Subduction, a convergent activity, occurs as a plate with heavier ocean crust collides with a plate of lighter, continental crust. The ocean crust pushes under and sinks into the mantle. Ocean trenches, volcanoes, and island arcs are associated with subduction zones.

Another type of convergent activity occurs when two plates carrying continental crust converge. Mountains may be thrust up at the boundaries. An example would be the formation of the Himalayan mountains.

At transform boundaries one plate slips and grinds against another. It is a place where earthquakes occur as pressure builds, and then releases with the shifting crust. The San Andreas Fault, which extends from Northern California to the Gulf of California, is one of the most famous transform faults. It is along this fault that Baja California and part of Southern California are shifting northwestward.

On the Web

Plate Tectonics: How Baja California and the Sea of Cortés were formed—an illustrated step-by-step summary showing this process beginning in the Late Jurassic period—140 million years ago.
Quicktime video of animation from the film. (not available yet)

Part A   Plate Boundaries
No materials

Part B   Seafloor Spreading
Per group of students: 1 plastic cup of snack-style chocolate pudding, 2 sugar wafer cookies (or any rectangular-shaped cookies), one 6-inch square of thin cardboard, scissors, tape

Part C   Transform Faults
Per group of students: 2 sugar wafer cookies, 5" x 8" piece of paper, pencil, small amount of frosting

Part A   Plate Boundaries (whole class)

• Review or discuss the plates of Earth's crust and their various motions. Include in your discussion: seafloor spreading (divergence), subduction (convergence), and transform or lateral movement.
• Hand movements can be used to illustrate these motions (see illustrations).

Part B   Seafloor Spreading (small groups)

• Cut a hole in the center of the cardboard about 1" x 1/2". Poke a small hole in the top of the pudding cup. Tape the cardboard to the top of the pudding cup, centering the slit in the board over the hole in the cup. The pudding represents magma.
• Center cookie wafers plates, lengthwise, over the hole in the cardboard.
• Keep the set-up on the table. One student steadies the cardboard with the cookies, while another student gently squeezes the pudding cup. Observe and record the results. Compare the movement of the pudding with the magma welling up at mid-oceanic ridges.

Part C   Transform Faults (small groups)

• Discuss the San Andreas Fault. Tell what and where it is.
• Mark directional points (north, south, east, west) on a piece of paper. On the paper, line up two cookie wafers side by side, lengthwise in a northwesterly direction. The cookies represent the Pacific Plate and the North American Plate. Spread a line of frosting horizontally across the cookies to represent a road.
• One student gently pushes the cookie on the west side (Pacific Plate) in a northwesterly direction while a second student presses the other cookie (North American Plate) in a westerly direction. Do the plates stick? Is there sideways motion? What happens to the road? Do the edges crumble?
• Discuss your results. Compare them to the movement of the Pacific and North American Plates along the San Andreas Fault.

Local Connection
Do you live near the edge of a plate? Have you ever felt an earthquake? What type of landforms are around you? Mountains? Volcanoes? Hills? Plains? Tell how they might be related to the movement of Earth's crust.

Key Words
lithosphere, convection currents, seafloor spreading, diverge, subduction, converge, transform, San Andreas Fault

Continue to Activity 4: Convection Currents