Mark Koester, ape 11, of Monroe, Michigan, for his question:

Why is the center of the earth so hot?

Drillings have shown that the temperature of the earth increases as we go down. The rock at the base of a three mile deep oil well in California was found to be 400 degrees Fahrenheit. Countless samplings from wells and mines show that temperature increases with depth. However, our deepest probings have gone no deeper than 1/1000th of the distance to the center of the globe. We cannot assume that things continue to get hotter all the way to the middle. And earthquake vibrations suggest that they do not. We can assume only that the crustal layer gets hotter with depth.

Geologists of the past century attributed the earth's inner heat to the pressure of heavy layers above. Then radioactivity was discovered. We now think that the earth's inner heat, or most of it, comes from radioactive substances in granites and other crustal rocks. On the average, the temperature of the crust increases about one degree with every 60 feet. If this geothermal gradient continued on down, the core of the earth would be 350,000 degrees. This, we now think, is not so. The inner earth, it seems, may be quite a bit cooler than most people suspected.