In this topic you will learn about how to tell different kinds of rocks apart so we can get what we need from them.

A rock is any naturally formed solid in the crust made up of one or more kinds of minerals. You can often see mineral crystals in a rock. A rock's color, density, and texture result from how the rock was formed. Rocks can be grouped into three types according to how they were formed.

Molten rock deep below Earth's surface is called magma. Magma is less dense than the material surrounding it, so it rises toward the surface. Before it reaches the surface, however, it may cool and harden into solid rock. Rocks that form when melted rock material cools are called igneous rocks.

Sedimentary rocks are made of small bits of matter joined together. Most sedimentary rocks are formed when sediment is compacted or cemented together. The weight of layer upon layer of sediment on top of each other compacts, or squeezes, sediments together. Sedimentary rocks are very useful in helping to piece together Earth's history. They often contain fossils, which are clues to life long ago. A fossil is any remains or imprint of living things of the past. Almost all fossils are found in sedimentary rocks.

Great heat and pressure can change one rock into another rock. A rock formed under heat and pressure is called a metamorphic rock. In the process, the original rock does not melt under heat and pressure. The mineral grains in the original rock may flatten and line up. The minerals may change their identities. The minerals in the original rock may separate into layers of different densities. In each case the result is a different rock.

Soil is a mixture of many things. The main ingredient in soil is weathered rock. Soil may also contain water, air, bacteria and humus. Humus is decayed animal or plant material in the soil. Soil is Earth's greatest treasure. All rooted plants need soil to grow. People depend on soil. However, people often get rid of hazardous wastes by burying them in soil. They also spray chemicals on the soil to kill unwanted plants and animals. These kinds of materials add up to pollution. Pollution means adding any unnatural substances to Earth's land, water, or air.

All rocks come from other rocks. Rocks are changing from one form to another in a never-ending series of processes called the rock cycle. Rocks are a nonrenewable resource.