Rocks (Encyclopedia of Science)
Rocks are composed of minerals, which are natural inorganic (nonliving) substances with specific chemical compositions and structures. A rock may consist of many crystals of one mineral or combinations of many minerals. Hundreds of different kinds of minerals make up hundreds of different kinds of rocks. Geologists, scientists who study Earth and rocks, divide rocks into three main groups: igneous rocks, sedimentary rocks, and metamorphic rocks. These distinctions are made on the basis of the types of minerals in the rock, the shapes of individual mineral grains, and the overall texture of the rock. All of these properties indicate the environment, pressure, and temperature in which the rock formed.
The first rocks on Earth were igneous rocks. Igneous rocks are formed by the cooling and hardening of molten material called magma. The word igneous comes from the Latin word ignis, meaning "fire." There are two types of igneous rocks: intrusive and...
(The entire section is 1340 words.)
Want to Read More?
Subscribe now to read the rest of this article. Plus get complete access to 30,000+ study guides!