Plate Tectonic - Plate Tectonics (5)

  • Testing models of crustal movement.
  • Experimenting with materials to make mountains.
  • contracting
  • expanding
  • mountain building
  • compression
  • tension
  • shear
  • round balloons
  • flour
  • water
  • lab sheet
  • wax paper
  • aluminum foil
  • worksheet

Students test models for the origin of stress in the Earth.

This fault is an example of shearing stress within the Earth.


Understanding the movement and behavior of the Earth's outermost layers has been a painstakingly long scientific process. The theory of plate tectonics is our current "best explanation" and working model for answering these questions. Plate tectonic theory has developed slowly and progressively since it was developed in the 1960s. It is a theory that truly has the entire world as its experiment.

Scientific ideas are subject to change as new information is gathered. This is why there are no "facts" in science. Every interpretation can be improved upon and altered as new research is completed. Scientific interpretations are continually tested and updated.

In this lab, students will test plate tectonics against two other hypotheses: 1) the Earth is expanding in size, and 2) the Earth is contracting in size. Both of these are real hypotheses proposed by earth scientists. They are not accepted, because, as the students will see, they do not accurately explain the behavior of the Earth’s crust.

This lab focuses on testing the ability of each of ideas to explain the stresses observed in the plates (especially the crust). Three types of stress are common. Compression is when a squeezing force is applied to a rock. Compression or converging plates builds mountains, which often occur in parallel ranges, with valleys between them. Diverging plates or extension is a when a pulling apart force effects a rock; it is the opposite of compression. Diverging also creates sets of parallel valleys. Transform motion along plate boundaries produces a shearing force is when two forces are applied to a rock, but in parallel and opposite directions.

In the lab, the students will discover that a contracting Earth only produces converging and that an expanding Earth creates only diverging. Only plate movement can produce all three types of motion.

  1. Review the types of motion that are common in the Earth’s crust. Make sure the students understand the differences between stress caused by converging, diverging, and transform.
  2. Have the students work with a lab partner. Demonstrate lab procedures to the class before they begin to work. Emphasize to the students that the experiments should be done slowly. Also emphasize that they should carefully observe the types of stress created in each experiment, and why they occur. Make sure they record their findings on the worksheet.
  3. For theory 1 and 2 students use the flour mixture to simulate the lithosphere (crust and upper mantle) moving on top of the balloon (asthenosphere). Students can try to be realistic about making continents out of the flour mixture. In the expanding Earth, the flour mixture will move apart, making cracks and wrinkles that are parallel. The contracting earth will have the lithosphere moving together causing parallel mountains and valleys. Plate tectonics will also give you parallel features.
  4. For theory 3, the arrows indicate the direction of pressure. In 1, the students demonstrate compression. They should first squeeze a piece of aluminum foil with equal pressure from both hands, and then with one hand exerting more pressure. In 2, the students experiment with shear. They should move one hand diagonally down while the other hand goes diagonally up. This is similar to movement on the San Andreas Fault in California. 3 and 4 represent a more realistic models of the Earth where different densities of respond differently to stress. Key observations are the creation of parallel mountains and valleys.
  5. When they are finished, review the students’ findings. Focus on Theory 3 (Plate Tectonics) so that they get an understanding of mountain building, earthquakes, and volcano formation as they relate to crustal movement. There are many reasons why the contracting and expanding Earth theories are not good explanations. They do not fit all the data that geologists have accumulated over the years. Geologists can measure the Earth's diameter, and the Earth does not seem to have gotten larger nor smaller. Students should discuss various tests. Even if they come up with some silly answers, it gets them thinking about the pros and cons of the different models.

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