Plate Tectonic - Plate Tectonics (4) Lab
 OBJECTIVES: Exploring plate movements. Observing the effects caused by plate movements. VOCABULARY: converging plates diverging plates transform movement MATERIALS: sand clay wax paper box Students explore plate boundary motions and predict geologic events. A converging plate boundary - the plate on the left subducts under the right hand plate.
 BACKGROUND: Using a variety of instruments, geologists have measured the directions and rates of plate movement at the surface of the Earth. They have found that plates move in three basic ways. In some places, two plates move apart from each other; this is called a diverging plate boundary. Elsewhere two plates move together; this is a converging plate boundary. Finally plates can also slide past each other horizontally. This is called a transform plate boundary. All of the plates move slowly. Their speeds vary from a few millimeters per year to a maximum of 15 centimeters per year. On average, the plates move about as fast as human fingernails grow. Volcanoes and earthquakes help define the boundaries between the plates. Earthquakes occur at all three types of boundaries. Because the plates are rigid, they tend to stick together, even though they are constantly moving. When the strength of the rocks at the plate boundary is exceeded, they move rapidly, "catching up" with the rest of the plates. We feel this release of energy as an earthquake. At diverging plate boundaries, earthquakes occur as the plates pull away from each other. Volcanoes form between the plates, as magma rises upward from the underlying mantle. Two situations are possible at converging plate boundaries. First, only earthquakes occur when two plates collide (obduct), building a mountain range. Second, both volcanoes and earthquakes form where one plate sinks under the other, instead of colliding. This process is called subduction. Transform plate boundaries commonly have only earthquakes. PROCEDURE: Review the interior structure of the Earth with the class. Make sure that they also understand that the plates are composed of the crust and uppermost mantle - the lithosphere. In Exercise I below, the students will draw the basic internal structure of the Earth.    In Exercise II, the students will simulate plate motions at each type of plate boundary. Have the students imagine that they are "Mother or Father Earth," who are feeling the pain of the moving Earth. As they move the sand or clay, have them try to capture the slowness of movements in the real Earth. We use clay and sand to represent the many types of rocks that make up the plates, as well as the Earth's surface. Here are some guidelines for Exercise II:   The slower the students do this exercise the better their results will be. They will form parallel "mountains" by compressing and sliding the sand or clay. At transform boundaries, mostly earthquakes are produced; any mountains will be very small. Diverging plate motion is difficult to show. Make sure that you use a tub of sand that is large enough for the students to be able to move their hands in it. If done slowly, the students will realize that you cannot pull the earth apart without the "tear" replacing itself with "lava," here represented by sand. Diverging plate boundaries thus produce both earthquakes and volcanoes. Converging plates will produce mountain chains. In this case, you will get earthquakes.    Conclude with the class that plate motions are associated with earthquakes and volcanoes. Explain that scientists (geologists) use plate tectonics to explain the movement of the Earth's crust. Illustrate the three different crust movements on the chalkboard and a wall map to point out some of the boundaries.    ANSWERS: Transform: clay: cracks in clay, sand: valleys and hill; model of San Francisco area, San Andreas Fault Diverging: forms mountains, volcanoes; example Iceland Converging: clay forms mountains and valleys that are parallel; sand forms same structural form; example: parts of California's Coast Ranges     Conclusion: (1) B; (2) A, B, C; (3) B