Harold Carney, Age 14, Of Dunwoody, Ga.., for his question:

What is moho?

The splendid ranges of central europe are an inspiration to artists and nature lovers. They are a challenge to climbers and scientists. Perhaps this is why the theory of the ice ages and many other basic geological ideas sprang from scientists who lived their lives in these mountainous regions.

Almost 100 years ago an Austrian geologist suggested that our solid globe is arranged in concentric layers made from denser and denser materials toward the center. Earth scientists are still probing the mysteries of the inner globe, but so have they learned nothing to contradict this basic theory. The shells of various minerals may be compared to the skins of an onion, and ane of the thinnest of them is called the moho.

Most of our knowledge of the deep earth is figured from earthquakes that send throbbing shock vibrations through and around the globe. They are recorded as primary and secondary seismic waves. The secondary waves only go through solids, and they travel faster through denser materials. In 1909 the balkans were shaken by a shuddering quake, and the Yugoslav geophysicist Mohorovicic noticed that, at a certain depth, its seismic waves speeded up with a sudden jerk.

Mohorovicic, whose name rhymes with chick, suggested that a thin layer of material rests between the earth's outer crust and the mantle layer below. This strange shell, much denser than the crust and less dense than the deep mantle, is called the mohorovicic discontinuity, or the moho. It may be the upper border of the thick mantle, which is thought to be mostly iron and manganese. It may be eclogite, a denser form of the heavy gabbro rocks found in the earth's outer crust.

Under the continents, the moho lies between 10 and 30 miles below the surface.

Under the floors of the seas, where testing is limited, it is usually about two and one half miles belaw the muddy sediments. Igy scientists found areas in the pacific,

However, where the moho nearer the surface and a few sites where it does not exist.

The outer crust varies from 10 to 20 miles in thickness. As the secondary seismic waves pass through this layer, they speed up from three to four miles a second. At the moho level, which may be less than one half a mile thick, they suddenly accelerate to five miles a second. As they continue down through the deep mantle, they speed up to eight and one half miles a second.

A modern theory suggests that mountain making and other crustal unrest is caused by convection currents, streams of heat welling up from the bowels of the earth. Igy scientists found more heat in spots where the moho was thin. The fascinating mohole project is a plan to drill down to the moho with the hope of solving some of the mysteries that keep the surface of our planet in such a state of upheaval.