Patricia Ann Evans,, age 13, Of Richmond, va., for her question:

How thick is the earth's crust?

If some cosmic giant sliced our globe in half, he would see layers of minerals arranged somewhat like the skins of an onion. The heaviest minerals are in the huge., round central cor£; the lightest minerals form a thin crusty skin covering the entire g1obe. Compared with an orange, the earth's crusty skin is much thinner than the orange peel.

The surface of the crust is bumpy, and its texture varies from place to place. Some areas are covered with inches or perhaps a few feet of soil; others form deep basins to hold the seas. The thickness of the crust varies between 20 and 40 miles; some places are 10 miles and a few are 50 miles thick.