Plate Tectonic - Plate Tectonics (2)

  • Exploring the results of movement on the Earth's crust.
  • Discovering that the physical fit of continents is one piece of evidence.
  • continents
  • plate tectonics
  • stress
  • worksheet
  • crayons
  • scissors
  • world map for reference

Students reconstruct the super continent Pangaea.

A map of the plates


According to the theory of plate tectonics, the Earth's crust and upper mantle are broken into moving plates of "lithosphere." The Earth has two types of crust. Continental crust underlies much of the Earth’s land surface. The ocean floors are underlain by oceanic crust. These material have different compositions; the continental crust is like the igneous rock granite, and the oceanic crust is like basalt, another igneous rock.

Students and many adults often equate the geographic continents, i.e., land, with the plates. This is incorrect. The Earth’s various units of continental crust are actually embedded into plates. You may wish to explain this to your students by saying that the continental crust "ride on the back" of a plate. Moreover, continental and oceanic crust are often part of the same plate. For example, the North American plate has continental crust (essentially the land area of North America) at its core and is surrounded on most sides by oceanic crust.

As they move, plates interact at their edges or boundaries. There are three basic directions or types of boundary interactions. In some places, two plates move apart from each other; this is called a diverging plate boundary. Elsewhere two plate move together, which is called a converging plate boundary. Finally plates can also slide past each other horizontally. This is called a transform plate boundary. 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. This builds up stress in the rocks at the plate boundary. When the strength of the rocks is exceeded, they move rapidly, "catching up" with the rest of the plates. We feel this release of energy as an earthquake.

One of the first observations used to suggest that the outer portion of the Earth is mobile is the fit of the continents, particularly the west coast of Africa against the east coast of South America. This observation predates plate tectonics. It was first noticed in the 18th century, and most recently proposed by a German scientist, Alfred Wegener in 1912. Wegener called his theory "continental drift", referring to the apparent movement of continents alone. However, "continental drift" is a only historical term. We now know it is not the continents that move, but the plates, in which the continents are embedded. South America and Africa were once together, but were split apart by the formation of a diverging plate boundary. This is also confirmed by matches between the rocks and fossils of the two continents. The two continents are still moving away from each other today.

A map of Pangaea shortly after it began to split. 160 million years ago

This exercise looks at the continents of North America, South America, Africa, Antarctic, and Australia, and how they have moved over the last 200 million years. At that time, these five continents were all part of a single large super continent, called Pangaea. Starting about 180 million years ago, Pangaea began to break up; new diverging plate boundaries formed within it. This eventually created the continents we see today. In this exercise, the students will reconstruct Pangaea. They will use the fit of the continental crust to put Pangaea back together.

  1. Remind the students of the information they learned in the Pre Lab. Explain again that the plates are moving, due to convection and gravity. Explain that this movement causes stress within the plates, which generates earthquakes and volcanoes. You may want to show students a map of the plates.
  2. Review the composition of the plates with the class. Make sure the students understand that the continents make up the non-oceanic part of the crust. Discuss with them that the edges of the continents look as if they may have fit together at one time.
  3. Have the students label, color, cut out, and fit the continents together. The lines and numbers make this puzzle a little easier. You may want your students to work in pairs. Matching up the continents is not as easy as it looks.
  4. Once the students have placed the continents together have them move the pieces apart very slowly. They are to move the pieces until they reach their present positions.
  5. Ask students if they think this movement could have happened. Let them come up with stories about why it took place. Remind them of convection and the moving of the plates. This is a difficult concept to get across to the students.

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