Maria Baunton, age 13, of Lethbridge, Alberta, Canada, for her question:

WHAT IS THE PLATE TECTONIC THEORY?

Tectonics is the study of forces within the earth that form mountains and ocean basins. Although tectonic forces cannot be fully explained, earth scientists believe they are produced by heat energy.

In the 1960s, scientists proposed that the earth's crust consisted of a number of rigid plates. Some of these plates do not follow continental boundaries, and some include both continents and oceans. The plates, the scientists' theory went, slowly slide on a soft plastic layer of rock called the asthenosphere. The plates move from one¬half to four inches each year.

Some scientists believe convection currents create the power that moves the huge plates. Others believe shrinking of the earth produces tectonic forces.