Mitchell L. Gabbert, age 10 of Phoenix, Ariz., for his question:

DO THEY KNOW WHAT CAUSES EARTHQUAKES?

Hurricanes struck without warning  until meteorologists learned how, when and where these disastrous storms happen. Nowadays, earthquakes strike without warning. But when seismologists learn all the causes behind the scenes, they will be able to predict just when and where these shuddering disasters will happen.

Earthquakes are part of the restless activity in the crust of the earth. Just a few years ago, world scientists agreed to accept a new theory which explains crustal behavior on a global scale. For example, we now know that the great continents drift around like lazy rafts  and also just how they do it.

Since earthquakes are part of this crustal activity, the new theory of continental drift may reveal more of their secrets.

The earth's crust, as we know, is the rocky shell which varies in thickness from five to more than 20 miles. However, it is not a one piece garment, fitting neatly around the globe. The earth's crust is cracked into large and small sections called tectonic plates.

There are about 20 large, rather rigid plates and many small fragments. They're not anchored firmly in place. Instead, they are in constant slow motion, always inching around the globe. And as the larger plates drift, they carry the continents on their backs, so to speak.

Meantime, a lot of local activity occurs where the edges of tectonic plates rub against each other. These are the weak crustal zones where a majority of earthquakes occur. Many of these crustal cracks also are regions of growing mountains and volcanic activity.

    We know by rubbing    a finger on a table that frictiongenerates heat. Imagine the stupendous heat generated when thesides of two enormous plates rub together, as each inches in adifferent direction. For a time, the crustal rocks strive toresist the heat and pressure. But when tensions build up to an impossible level, obviously something has to give.

With a sudden snap, the two sides of the crack adjust to new positions. A shuddering earthquake occurs. The ground heaves and shakes, and great cracks may open up in the surface. A major earthquake sends shivering vibrations clear through the planet.

Most earthquakes occur around certain well known zones on land and under the sea. These are the borders between crustal plates moving in different directions. But quakes are not limited to these zones. They can and sometimes do happen anywhere in the earth's restless crust.