What Is the Earth and What Is It Made Of? Earth is a planet, which is a large round object in space. Like the other planets, it orbits the sun, which means that it spins around the sun along a circular path. The group of planets around the sun are together called the solar system . Earth is the third planet away from the sun in our solar system and is the only planet we yet know of that has life, like plants, animals, and humans. Most of Earth is made up of rock, but depending on where in the Earth you look, the rocks can be very different. Rocks on the Earth''s surface (the kind you can find and pick up) are cold, hard, and brittle, which means that even though they are very tough, they can break. But many of the rocky areas inside the Earth are very hot--so hot that the rocks can be soft and will bend rather than break! The Earth gets hotter and hotter toward the center, and at the very middle is a core formed of solid metal. The Earth stays hot inside because its rocks are very good at holding heat and also because gravity inside the Earth and radioactive minerals (special crystals that release energy) make new heat.

If we were able to cut open the Earth, we''d see that it is made of several layers . Some layers are hard and others are soft, but they all get hotter as they get closer to the middle of the planet. The layer most important to us is the crust , which is the very top layer, on the outside of the planet. It''s the coldest and hardest layer. The crust is where all the rocks you''ve ever seen are found, and it''s the layer we live on. But the other layers below it can affect and change how the crust looks--they can even make the crust break and move! We will discuss the other layers and how they all interact later on in this book. So now we know that the Earth is formed of rocks that are arranged into different layers and that each layer is composed of different kinds of rocks. But what are rocks, and what are they made of? Rocks and Minerals Rocks and minerals are the materials that make up the Earth, and they can be found anywhere beneath your feet.

But what''s the difference between them? A mineral is a special kind of hardened natural material. Minerals form when certain chemicals combine together and harden. For example, common table salt is actually a mineral called halite (say it "hay-lite"). Halite forms when two elements (special materials) called sodium and chlorine combine together to make a chemical called sodium chloride . When sodium chloride hardens, it becomes halite. (If this sounds complicated, don''t worry! We''ll explain all of these things). What makes minerals special is that when they harden, they form crystals , which have a special shape depending on the mineral. Halite forms crystals shaped like cubes, or blocks.

(If you look at table salt through a magnifying glass, you may see little cubic crystals.) Rocks are also hard, solid, natural materials, but they are made up of mixtures of minerals. Different kinds of rocks can have totally different mixtures of minerals in them. Some rocks have lots of different minerals in them while others have only a few. Sometimes the minerals in a rock are big enough to be seen as spots of color, but in other rocks they may be too small to see. Rocks make up all the landscape that we can see and are always underfoot. And if you can''t see any rocks, like in a grassy park, that just means that lots of dirt and plants have buried it. When you dig down deep enough, you''ll eventually hit bedrock , which is the first layer of hard rock found below all the loose dirt, soil, and mud.

Minerals form inside rocks, both as part of the rock and in the spaces and cracks hidden in the rock. You can find minerals as the little spots of color that are stuck tightly in a rock, but if you''re lucky, you''ll also find nice mineral crystals hiding in a hole or crack in a rock. When they form inside a hole, they may be standing up off the walls of the opening, and you''ll be able to see their full crystal shape. We''ll look at all of these things in detail, beginning with minerals. Because the Earth is made of rocks, and because rocks are made up of minerals, it''s very important to understand minerals and how they form.