BACKGROUND:
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. 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.
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 lighter, similar to granite, and the oceanic crust is
denser like basalt, another igneous rock. Continental and oceanic crust can
both be part of the same plate. For example, the North American plate has
continental crust (essentially the land area of North America) at its core;
this is surrounded on most sides by oceanic crust. A geographic
"continent" does not equal a plate.
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.
PROCEDURE:
- Introduce the phrase "plate tectonics" to the class. Explain
that it describes the movement of the Earth's outer portion (the crust
and upper mantle). Illustrate the crust of the earth by using a globe or
an orange (the peel is the crust and the fruit inside is the rest of the
Earth.)
- Show the students a globe. Point out the continents on the Earth's
surface. Explain that scientists have evidence that the continents have
moved great distances during the course of the Earth's history. Ask the
students this question: if one continent broke up and moved apart, would
the pieces resemble one another and fit together? They may answer yes or
no. Both answers are actually correct; sometimes split continents still
match up, i.e., South America and Africa. In other cases, such as
Europe, Greenland, and North America, the match-up is very
obscure.
- To understand plate tectonics, students must be familiar with the
globe. Point out the continents and the oceans by making the class
repeat the names of the continents. Explain that the continents are
merely the crust exposed above sea level, and that the solid surface of
the Earth below sea level is also crust. Tell the class that the crust
is broken into pieces which are called "plates." The
continents are the exposed portions of the Earth's plates. However, some
continents may be composed of the exposed sections of more than one
plate. Therefore "continent" does not equal a plate. This may
be a confusing point for adults and children alike.
- Have the students complete the worksheet. Instruct them to color Moppy
and Moppa, the continents, and the oceans. When they have finished
coloring they should cut Moppy and Moppa out and fit them together once
again. You can create a story about Moppy and Moppa being together on a
continent that was riding on a single plate. The plate broke apart when
they had a fight. Now Moppy and Moppa have made up and want the plate to
come together again.
- Have the students observe that the edges of the plates fit. Make sure
they see that there is only one part of the plate on which Moppy and
Moppy can meet (the continent).
