Jim Cochrane, age 12, of St. Catharine's, Ontario, Canada, for his question:
What really is under the earth's surface?
Earth scientists probe these buried mysteries from several angles. Deep wells, holes in the ground and plunging trenches in the ocean floor reveal a few secrets. Vibrations from earthquakes reveal bigger secrets as they sweep around the globe and plunge down through the planet. These studies yield provable data. But a lot of reasonable evidence is gathered indirectly from what we know of other facts of nature such as gravity and the physics of chemical substances.
Our planet seems to like to keep its own secrets. At the same time it seems to coax us to solve them. For example, here and there the earth gives us a peek at certain rocks that once were buried several miles below the surface. The busy Colorado River dug a plunging channel through our western mountains and created the scenic beauty of Grand Canyon. Its steep sides expose layers of rocks a mile below the surface. Most of them are crustal layers of pasty limestones and rainbow colored sandstones. The bottom layers are compressed and distorted by the crushing weight of rocks above.
Growing mountains heave up massive slabs, crack them and stack them in enormous rocky sandwiches. Their slopes also give us a glimpse of what other deeply buried layers are like. Well drillers bring up cores from a mile or more; mines reveal that things get warmer as they go deeper.
However, these glimpses merely dip into the earth's crust, the outer shell of lightweight rocks that wraps around the entire globe. Its thickness varies from five miles below the oceans to 20 miles or so under the continents. It fits some¬what like a loose egg shell, crisscrossed with cracks. Its massive crustal slabs are shifted by forces from above and below. They push and shove each other, constantly changing the geography of land and sea.
Certain earthquake vibrations plunge down through the planet, changing their pulses as they pass through denser materials. Some are stopped by molten or plastic materials. These vibrations indicate that the planet's interior is arranged in layers, somewhat like onion skins. From the surface to the center they increase in temperature, weight and density.
The mantle descends from below the crust to a depth of 1,800 miles. Its temperature increases from 1600 degrees Fahrenheit to 4000 degrees and its materials appear to be mostly common minerals and metals, such as silica, iron and aluminum. The two layers below the mantle form the planet's heavy core. The outer core is about 1,400 miles thick. Its densely packed materials seem to be mostly heavy iron and nickel, perhaps in a sort of molten or plastic state. ~ ~r
The inner core is a round ball and its center is the center of the planet, about 4,000 miles below the surface. It is the heaviest part of the earth and not much is known about it. Scientists suspect that it is a wad of densely packed iron and nickel in a more or less solid state and that its temperature reaches at least 9,000 degrees Fahrenheit.