Plate Tectonic - Plate Tectonics (3)

  • Discussing that the Earth's surface is constantly under stress.
  • Discovering that there are many ways substances react to stress.
  • converging
  • diverging
  • fault
  • plates
  • stress
  • transform
  • worksheet
  • glue ball
  • clay

Students learn about stress in the Earth’s crust.


Stress is an important geologic concept to understand. It is important to distinguish the geologic meaning of this word from its other meanings, such as anxiety and tension. In geology, a stress is a force that is applied to a geological material, whether it is a tiny mineral crystal or a gigantic plate.

Stresses that build from plate motion eventually causes earthquakes. When the earthquake occurs, some of the stress is relieved. If the stress accumulates a rupture or fault will occur. As the plates continue to move, stress builds up in the rocks again. More earthquakes will take place, but the ruptures this time will tend to happen along the fault. It is an existing zone of weakness in the rocks.

There are three basic ways stress is applied to rocks within the lithosphere. First, rocks can converge or be compressed. Second, rocks can be pulled apart, or diverging. This is the opposite of converging. Finally, rocks can be slip slide pass each other. Squeezing is most common at converging plate boundaries, pulling apart at diverging plate boundaries, and shearing at transform plate boundaries.

  1. Define "plate" to the class. Explain that plates are large areas of the Earth's outer portion (crust and upper mantle) that move together.
  2. Explain the concept of stress in rocks to the class. Define the three basic types of stress to the students. You can demonstrate these with the wooden blocks from the Pre Lab, or use a sheet of paper. Converging a piece of paper from both sides crumbles the paper toward the center. Pulling on opposite sides of it is extension. Holding opposite sides and moving one hand up and the other down is shear.

    Explain that once a fault exists, it is a weak point in the crust. It is likely to become the location of future crustal breakage.

  3. Here are answers and information for the lab exercise:
    EXERCISE I. Have the students follow the directions on the lab sheet. The glue ball represents a portion of the Earth. The glue ball is made of a polymer used to insulate telecommunication cables. This material "has memory". In other words, it will return to its original shape after a stress is applied to it. Emphasize to the students that the stresses applied to the glue balls will cause the "fault" to reappear.
    EXERCISE II. This exercise illustrates that under similar stresses, different substances respond differently. For example, ask the students if the rocks they observed in previous class would react differently if subjected to similar stresses. The answer is no, with respect to the stress a human can put on a rock, but yes in terms of geologic stresses.
    EXERCISE III. It is important for the students to start hearing the vocabulary words associated with stress and plate boundaries. Stress is very important to understand but hard to explain. Letting them experience stress by playing with the clay will help them to understand the concept.
    This illustrates the 3 major types of stress effecting the plates by using hand motion and clay.
    1. Simulates "plates" moving past each other at a transform plate boundary such as the San Andreas Fault.
    2. Simulates "plates" diverging from each other, such as the Mid Atlantic Ridge.
    3. Simulates "plates" converging toward each other such as the Himalaya Mountains, where the Indian and Asian plates collide.

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