Randolph Johnson, age 13, of Portland, Me  for his question:    

What exactly is the earth's core?

The earth, of course, is a solid ball and its core begins about 2,000 miles beneath our feet. It is a central ball around which the outer layers are enfolded, somewhat like the skins of an onion: No one has been down to the core of the earth to get first hand information: We can only form theories about it from outside information.

We can estimate the weight of the core and this tells us a great deal. First, we find the weight of the whole earth from the effect of its gravity upon other heavenly bodies. The overall weight of the earth is about five and a half times heavier than water. But the average weight of the minerals in the crust and outer layer is only two to three times heavier than water. To make the overall weight come out right, then, the core must be made of much heavier materials. It may be twelve times as heavy as water,

We know that everything in the universe is made from a hundred or so basic elements. The elements in the earth's core must be very heavy ones but which ones? Here we get a clue from outer space. Our earth is constantly bombarded with meteorites which end their days of space travel when they hit the ground. There are stony meteorites similar to the minerals in the earths crust and outer layers. There are heavy metal meteorites of iron and nickel which could be similar to the materials in the earths core. At any rate, if the core were made of iron and nickel, the overall weight of the earth would be what it is.

On all sides, the core of the earth is capped with layers some 2,000 thick and made mostly of stony materials. These outer layers exert a tremendous pressure. Every square inch of the corers surface is under a weight of some 24,500 tons. This pressure generates tremendous heat. Down there it is certainly hot enough to melt iron.

This weight lead us to suppose that the core is a ball of liquid, molten metal. But we are not sure that .his is entirely true. The bulk of evidence comes, of all things, from shuddering earthquakes. Quakes send vibrations both through and around the earth. Vibrations travel at different speeds through gases, liquids and solids. When an earthquake occurs, its vibrations are recorded at different stations, sometime clear around the world.

Certain of these vibrations cannot pass through fluid materials, and go down, down to a point about 2,000 miles below our feet and then veer away. Other vibrations change speed when they go through fluids. They can bring us information clear through the earth. They indicate that the surface of the core is fluid, but the center is hard and solid.