Sharalyn Kokaska, age 12, of Westmont, Ill.,,for her question:
How do we know what the inner earth is made of?
Scientists cannot prove what materials exist in the deep heart of our planet. But they can use indirect evidence to build a sketchy picture of this inner earth. We have no facts to prove this theory wrong, but we need far more facts to list the buried materials with certainty.
Project Mohole may bring up direct evidence of conditions below the earth's crust. So far, no one has been down more than a mile or so to make an on the spot examination of buried materials. Our picture of the inner earth is assembled from data gathered by mathematicians, earth scientists, astronomers, physicists, chemists and astrophysicists.
The basic problem may seem simple. If you know the size and weight of an object, you can figure whether its materials are light or heavy. The size of the round earth was measured by Eratosthenes in 230 B.C. Its weight is figured from the pull it exerts upon other heavenly bodies, and this had to remain a mystery until Newton figured out the laws of gravitation. Its weight is 6,600 quintillion tons, and its volume is 260 billion cubic miles. Its total volume is five and one half times heavier or denser than an equal volume of water.
This might suggest that our planet is made mostly of vanadium, germanium, arsenic and other materials of similar densities. But these materials are rare, and tests of surface rocks show the average weight of the crustal layer is only three times the density of water. We must look for much heavier material below to balance the picture,
Evidence to bear out this idea comes from earthquakes that send shock¬wave vibrations both through and around the earth. Such waves travel faster through denser materials, and they are checked by world wide seismology stations.
Penetrating earthquake vibrations give an indirect picture of the density of the materials in the inner earth. They indicate that the planet is built in denser and denser layers around a core that is 10 times denser than water.
Most of the core is thought to be iron, with perhaps some nickel, chromium and molybdenum. On the surface, iron has a density of 7.6, but under the terrific heat and pressure of the inner earth molten iron is estimated to reach 10 times the density of water.
The same steps are taken to estimate the materials of the mantle below the crust. This heavy layer begins anywhere from 10 to 40 miles below the surface and reaches down 1,800 miles. Its density indicates that it is made from a mixture of assorted rocks and metals. But no one can identify the separate materials of this middle layer of the earth.