In the 1950's and early 1960's, the idea of continental drift began to gain support among geologists.  The main reason for this new support was that geologists began to have ideas about what might cause the continents to drift.  Most of the information for how continents drift came from two areas of research:

•     studies of how earthquake waves travel through the entire Earth

•     studies of the ocean floor

Throughout the 1950's and 1960's, numerous geological symposia were held.  Scientists presented their information and debated the importance of that information. Gradually, a big idea started to form that helped explain all the new information.  A new theory was born.

The new theory was called plate tectonics.  Here are the major pieces of that theory:

(see also "A Synopsis of Plate Teconics") - http://geollab.jmu.edu/Fichter/PlateTect/synopsis.html

•     Earth's lithosphere (the hard "outer shell" that includes the crust) is divided into many pieces, the way that the shell of a hard-boiled egg might break into many pieces.  These pieces are called lithospheric plates.

•     Earth's lithospheric plates "float" on the aesthenosphere, a hot, semi-liquid layer that includes the outer part of the mantle.  This layer is kept hot and liquid by the radioactive decay of elements in granite.

•     Very slow movements ("currents") in the aesthenosphere cause movements of the lithospheric plates.  This movement causes the plates to collide with each other, move apart from each other, or slide past each other.

•     In some places, new crust is created and spreads apart.  In other places, crust is forced under other crust and is destroyed by melting.  This maintains the size of the Earth - it neither shrinks nor grows through plate tectonics.

Use your textbook or any other resource that your teacher wants to loan you to help you answer all of the following questions.  You may discuss these as a group; however, each of you should write the answers to these on a separate sheet of paper.

1. At the start of all of this discussion of continental drift, you were asked why you think there are mountains, volcanoes, and earthquakes.  Use the theory of plate tectonics to explain why there are mountains, volcanoes, and earthquakes.

2. Plates may slide past each other, collide with each other, or move apart. Draw cross-section diagrams showing how this takes place.

3. What would happen if scientists did not get together at symposia to present and discuss their research?

4. Based on the evidence presented at the symposium, does plate tectonics qualify as a theory according to the National Academy of Sciences definition? Explain why or why not.

Updated June 25, 2001