Donna Joan Tennyson, age 10, of Van Nuys, Calif., for her question:

How big is the earth's core?

The solid earth is almost 8,000 miles in diameter. Its rocks get heavier, denser and hotter toward the center. The outer layer is the earth's crust. Its cold, rather light weight rocks reach to a depth of 30 to 50 miles.

Below the crust is a shall of denser rocks reaching to a depth of perhaps 600 miles. Below this lager is a region where the material seems to be as dense as steel and very, very hot. This layer is thought to be about 1,200 miles in depth.

This brings us down over 1,800 miles, still over 2,000 miles from the center of the solid earth. About here is where the core of the earth is thought to begin. It is a ball, probably 4, 000 miles in diameter, maybe larger. This means that it occupies a half' of the earth's volume or more. There is, however, no need to worry about it breaking forth to the surface. It is buried under some 2,000 miles of outer rocky layers and each square inch of the hot core is held under a pressure of about 22,000 tons.