Gravity causes objects to attract one another. The greater the mass of the objects, the more strongly gravity pulls them together. As the mass of the planets increased, gravitational forces increased. The atoms were pulled more and more strongly toward each other. The centers of the planets began to heat up: if the planet was large enough, the rocks melted and the planet became a spinning ball of molten minerals.

     Now let's think about this for a minute. When you put objects into a liquid, the heavy ones sink and the light ones float. If the whole earth is liquid rock, where can the heavier elements sink to? Down is toward the center of the earth, and the heavier elements sank down through the liquid toward the center.

     Eventually the inner planets cooled enough so that their surfaces became solid. The interiors, however, remained, and remain, very hot. Scientists have developed theories about the structure of the earth, based on information gathered through studying earthquake waves and the earth's magnetic field. So far as we know, this is how the earth is structured.

     The earth has a solid inner core made mostly of iron and nickel. It is solid because it is under so much pressure: all the rest of the earth is pressing down on it. Its temperature is about 7000 degrees Kelvin (about 12,000 degrees Fahrenheit) -- hotter than the surface of the sun!

     The outer core is also made of nickel and iron, but is liquid. Currents in this liquid metal generate the earth's magnetic field. The temperature of this molten metal is about 3000-6500 degrees Kelvin (roughly 5000- 11,250 degrees Fahrenheit). The lower temperature is above the boiling point of iron on the surface of the earth. This is still too hot for us to really imagine. The pressures holding this hot layer down are still immense.

     84% of the volume of the earth is in the part called the mantle. This is above the hot core, and made mostly of iron (yes, more iron!) magnesium, aluminum, silicon and oxygen. The rock in the mantle is hot, and somewhat plastic, like very, very thick cookie dough. This area does not conduct heat well, which explains why the interior of the earth is still so very hot.

     We live on the crust. The crust is very thin compared to the rest of the earth, thinner than the skin on an apple by comparison. It is made of granite and basalt, minerals that are lighter than iron. It is rich in silica, a light weight element. The rock on the deep sea bottom is only about 5 to 7 kilometers thick. The rocks that make up the continents are about 30 to 70 kilometers thick, thicker than the deep sea bottoms. The continents are made mostly of granite, which is light compared to the rest of the materials that make up the earth. I think of the continents as floating like islands of soap bubbles floating on water.

     Even the rocks that make up the surface of the earth are still pretty warm. When miners go down as deep as a mile, heat from the earth becomes a problem for the miners, making work there very uncomfortable.  Every time that you go down a mile, the termperature increases by 72 degrees Fahrenheit.

     The crust of the earth is not all one piece, like the outside of a tennis ball. As the surface cooled, the rocky surface cracked. (Think of how the surface of a pan of brownies cracks as it cools.) It cracked into twelve main plates. These plates float on top of the mantle. The continents are on top of these plates. The plates move very slowly as the plastic rock in the mantle moves under them. Where they collide we get mountains, volcanoes, and earthquakes. Sometimes one plate will slide down underneath another plate. This is called subduction. Subduction is an important process because it allows elements locked in the rocks to be heated up and returned to the surface by volcanoes. Of course, this takes millions of years.

     Here you see the crust floating on top of the mantle. You can see the weight of mountain ranges pushing down on the mantle rocks. It is hard to imagine rock being plastic, but we are thinking about enormous forces here.

     In this picture the mountains on the continents are much higher in proportion to the curve of the planet than mountains really are. If you could hold the planet in your hand it would seem quite smooth.

     Other planets in our solar system have magnetic fields, so it seems reasonable to assume that they also have iron cores. In our solar system, rocky planets are near the sun, and the big gas giants are further out, where gases will solidify in the cold temperatures of space. We can guess that rocky planets, being made from the same elements as earth, and having been formed by similar processes, will probably have much in common with our own world.