Plate Tectonic - Plate Tectonics (3)
Pre Lab 

  • Discovering that there are many types of stress.
  • Exploring stress on the outside portion of the Earth.
  • converging
  • crust
  • diverging
  • inner core
  • mantle
  • outer core
  • stress
  • transform
  • blocks of wood (optional)

Students learn about stresses in the Earth.


According to the theory of plate tectonics, the Earth's crust and upper mantle are broken into moving plates. The lithospheric plates are solid rock. There are several very large plates, each consisting of both oceanic and continental portions. There are a dozen or more smaller plates. The plates average about 80 kilometers (50 miles) in thickness. All of the plates are moving. They are slow, moving at speeds of centimeters to tens of centimeters per year. They slide along on top of an underlying mantle layer called the asthenosphere, which is composed of a rock that is fluid-like.

Geologists have distinguished three main internal subdivisions of the Earth, based on the behavior of seismic waves and laboratory experiments. The outermost layer is the crust. Underlying the crust is the second layer, the mantle. It comprises the largest portion of the Earth. Like the crust, it is also composed of silicate minerals. The innermost portion of the Earth is the core. It is composed of metallic elements, primarily iron and nickel. The core is subdivided into two parts. The outer core is liquid metal, while the inner core is solid.

Volcanoes and earthquakes help define the boundaries between the plates. Volcanoes form mostly at converging and diverging plate boundaries, where much magma is generated. 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.

Many lines of evidence indicate that the plates are moving. What is less clear, however, is why the plates move. There are two main scientific ideas for explaining plate movement: gravity and convection currents. All objects on and in the Earth are pulled towards its center by the force of gravity. This may effect the plates at converging plate boundaries in areas called subduction zones, where one plate sinks into the mantle. This is shown in the left picture below. Some evidence suggests that gravity pulls the sinking plate down. The rest of the plate is dragged along behind it. This is physically similar to slowly pushing a piece of paper off a table; it eventually bends, and slides off, pulling the rest of the paper behind it.

The other reason for plate motion relates to convection currents within the upper part of the mantle. Convection is the heat-driven circulation of a fluid. The inside of the Earth is much hotter than its surface. Heat thus moves from the interior towards the surface. In the mantle, heat from deeper in the Earth causes the overlying mantle to circulate. The mantle can circulate because it contains a little magma (molten rock); it is a very hot, thick (viscous) fluid. Mantle convection currents move very, very, slowly. It is possible that as the mantle convects, it drags the overlying plates along with it. Where convection currents come together, a converging plate boundary is present. Where they move apart, a diverging plate boundary forms. An example is shown above. This picture portrays convection of the entire mantle, but it some scientists think that is more likely that just the upper part of the mantle convects. Convection and gravity contribute to the movement of the plates.


  1. This unit reinforces the concept that the movements within the Earth cause stress in the plates. Most of the stress is caused by movement with the Earth and gravity. A portion is also due to external forces such as the Earth’s rotation.

  2. Draw the diagram on the board. Discuss the crust, mantle, outer core, and inner core. These terms are discussed in the Earthquakes portion of the Plate Tectonic Cycle.

  3. Prepare the students to think about what lies inside of the Earth. Have them guess how we know what is inside of the Earth. Many of the students will think we can drill deep into the Earth, but we can not. We cannot even drill through the crust. Refresh their memories on seismic waves from the previous unit on Earthquakes. Scientists use variations in the speed and behavior of seismic waves to distinguish the different parts of the Earth’s interior.

  4. Explain the meaning of converging, diverging, and transform plate motion to the class.

  5. Draw the diagram below. Ask the students where the stress is. In the diverging picture it is near the pull-apart area; in the converging picture it is where the plates bump; and in the transform motion picture it is where the plates are slipping past each other.

  1. You may want to use blocks of wood to demonstrate the different motions of the plates.

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