Martin Hancock, age 15, of Cumberland, Md., for his question:

WHAT IS THE PLATE TECTONIC THEORY?

In the 160s, earth scientists suggested that the earth's crust, called the lithosphere, consisted of a number of rigid plates and that some of these plates did not follow the continental boundaries. The lithosphere, which is about 60 miles thick, appears to be in continual motion.

Earth scientists say that the lithosphere plates slowly slide on a soft plastic layer of rock called the asthenosphere. The plates move from one half to four inches each year.

Tectonic activity seems to happen chiefly along the edges of the earth's plates. If one plate pushes against another, it either crumples and forms mountains, or bends toward the earth's mantle, the layer beneath the crust and above the core.

Two plates spreading apart, according to plate tectonic theory, form ocean floors and long underwater mountains called oceanic ridges. Major earthquakes and fractures in the earth's crust happen in places where two plates slide past each other. Such fractures are called faults.

Some of the earth's major crustal features occur at the edges of the lithosphere plates. Such features include mountains, ocean floor trenches, volcanoes and volcanic islands.

Most earth scientists believe convection currents create the power that move the huge plates. According to this theory, convection currents in the earth's mantle carry molten rock up from the asthenospshere. The rising molten rock adds to the ocean floor. Convection currents in the rock carry the newly formed crustal plates away from the ridge, as if it were riding on a conveyor belt.

Some scientists still want more proof that the convection currents really exist and that they produce the enormous power needed to move the plates.

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

Some scientists believe that the earth once was a molten ball and that it has been cooling ever since. As the earth becomes cooler, these scientists say, it shrank. The shrinking produced tectonic forces.

Still other scientists say that the earth started as a cold mass and was warmed by heat from radioactive material inside the planet. As the earth became hotter, it expanded and created forces that fractured the crust into large blocks.

These blocks, some suggest, became the continents, and the regions between the continents became the basins of the oceans.