Rock (geology)
Introduction
Rock, in geological context, refers to a naturally occurring solid aggregate of one or more minerals or mineraloids. Rocks have been used by humans for millions of years, from early tools and weapons through to various construction materials. There are three major types of rock: igneous, sedimentary, and metamorphic.
Classification
Rocks are classified according to characteristics such as mineral and chemical composition, permeability, texture of the constituent particles, and particle size. These physical properties are the end result of the processes that formed the rocks.
Igneous Rocks
Igneous rocks are formed when magma cools and solidifies. They may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This process produces a very diverse group of rocks that include granite, basalt, and pumice.
Sedimentary Rocks
Sedimentary rocks are formed by the accumulation of sediments. There are three types: clastic, organic (biological), and chemical. Clastic sedimentary rocks like sandstone are composed of rock fragments. Organic sedimentary rocks like coal form from hard, biological materials like plants, shells, and bones. Chemical sedimentary rocks form when minerals precipitate from a solution, like limestone.
Metamorphic Rocks
Metamorphic rocks are formed by subjecting any rock type—sedimentary rock, igneous rock or another older metamorphic rock—to different temperature and pressure conditions than those in which the original rock was formed. This process is called metamorphism, and it can result in rocks such as slate, marble, and quartzite.
Rock Cycle
The rock cycle is a basic concept in geology that describes the time-consuming transitions through geologic time among the three main rock types. It is the process by which rocks of one kind change into rocks of another kind. The cycle can be simple or complex and includes the processes of weathering, erosion, transportation, deposition, compaction, and cementation.
Economic Importance
Rocks have been used by humans for millions of years. They are used in the construction industry, for making roads, buildings, and bridges. Rocks are also used in the production of many products, including glass, ceramics, and metals. Precious and semi-precious gemstones are also found in certain types of rocks.