Igneous rock is formed from magma in two ways. Because magma is liquid and less dense than surrounding matter, it flows upward into the cracks, or fissures, of Earth's surface. There the magma slowly cools and hardens into rock. Igneous rock formed in this way is described as intrusive. Sometimes magma flows upward and is forced out onto Earth's surface during a volcanic eruption. Magma that reaches the surface of Earth is called lava. When it cools and hardens, it also forms igneous rock. Igneous rock that is formed on Earth's surface is described as extrusive.

As magma cools, the minerals in it form tiny grains, or crystals, almost like ice crystals forming in water that gets colder and colder. It is these mineral grains that give igneous rock its distinctive appearance.

Sedimentary rocks can form other ways as well. For instance, salts dissolved in water can fill open spaces between solid grains. The grains can be sand, clay, or other tiny particles. When the salty filling dries, it cements the grains together.

Shale is a sedimentary rock formed by both compacting and cementing. It is also one of the most common types of sedimentary rock. Large sheets of slate form on lake and ocean bottoms. Each sheet consists of clay and other particles too small to see.

Most sedimentary rock shows layering. Within these layers one can find the remains of ancient times. Split the layers apart. You may find a lot of fossil shells. Often, each layer of a sedimentary rockrepresents a period of time. For instance, a river may flood its banks each spring. Each flood lays down a layer of mud. Eventually, each mud layer becomes a visible stripe in a sedimentary rock.

Metamorphic rocks emerge at the base of great mountains. In the United States, they occur in the Appalachian Mountains, Rocky Mountains, and Sierra Nevada. Elsewhere, they arise in the Andes, Alps, and Himalayas.

As metamorphic rocks form, they undergo chemical changes as well. The heat and pressure cause minerals inside the rock to break apart. They re-form into new substances, with new textures and colors. The powerful baking also drives out water. The result is a very strong kind of stone.