ROCKS

It might come as a surprise to learn that geologists regularly use an unscientific-sounding term, rocks. Yet as is almost always the case with a word used both in everyday language and within the realm of a scientific discipline, the meanings are not the same. For one thing, rock and stone are not interchangeable, as they are in ordinary discussion. The second of these two terms is used only occasionally, primarily as a suffix in the names of various rocks, such as limestone or sandstone. On the other hand, a rock is an aggregate of minerals or organic material. Rocks are of three different types: igneous, formed by crystallization of molten minerals, as in a volcano; sedimentary, usually formed by deposition, compaction, or cementation of weathered rock; and metamorphic, formed by alteration of preexisting rock.