Julie Velasco, age 12, of Tucson, Arizona, for her question:

What exactly is a fault line in the earth?

The earth's crust is a bumpy, rocky skin with an average thickness of about 20 miles. Compared with the rest of our bulky planet, it is a very small item. However, it is the surface layer that supports us and everything else alive. This is one reason why earth scientists find it so fascinating. Another reason is its mysterious restlessness. They are still trying to figure out the planetary forces that keep it shifting in slow motion. Until these dynamic facts are defined in detail, we can only suggest theories to explain things such as fault lines.

Present evidence suggests that the rocky surface fits somewhat like a loose jacket. It is cracked into huge crustal blocks, agitated by forces from the earth's interior. Some of them push against each other, shoving in opposite directions. Here we get weak crustal areas called fault lines, where immense stresses tend to cause earthquakes, mountain making and volcanic activity. The earth's fault lines tend to fogy curving global belts of restless activity, usually many thousands of miles long.