Rock (geology)
Naturally occurring solid aggregate of minerals or mineraloid matter.
Adrian Diack · CC BY-SA 2.0
In geology, a rock (also called a stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's outer solid layer, the crust, and most of its interior, except for the liquid outer core and pockets of magma in the asthenosphere. The study of rocks involves multiple subdisciplines of geology, including petrology and mineralogy. It may be limited to rocks found on Earth, or it may include planetary geology that studies the rocks of other celestial objects.
- field
- Geology
- subdisciplines
- Petrology, Mineralogy
- main groups
- Igneous, Sedimentary, Metamorphic
- crust composition by volume
- Igneous ~65%, Metamorphic ~27.4%, Sedimentary ~7.9%
- key process
- Rock cycle
Lore & Background
Rocks are usually grouped into three main groups: igneous rocks, sedimentary rocks and metamorphic rocks. Igneous rocks are formed when magma cools in the Earth's crust, or lava cools on the ground surface or the seabed. Sedimentary rocks are formed by diagenesis and lithification of sediments, which in turn are formed by the weathering, transport, and deposition of existing rocks. Metamorphic rocks are formed when existing rocks are subjected to such high pressures and temperatures that they are transformed without significant melting.
Reader's Guide
Humanity has made use of rocks since the time the earliest humans lived. This early period, called the Stone Age, saw the development of many stone tools. Stone was then used as a major component in the construction of buildings and early infrastructure. Mining developed to extract rocks from the Earth and obtain the minerals within them, including metals. Modern technology has allowed the development of new human-made rocks and rock-like substances, such as concrete. The study of rocks and their components has contributed to the geological understanding of Earth's history, the archaeological understanding of human history, and the development of engineering and technology in human society. While the history of geology includes many theories of rocks and their origins that have persisted throughout human history, the study of rocks was developed as a formal science during the 19th century. Understanding of plate tectonics developed in the second half of the 20th century.
Did You Know?
- About 65% of the Earth's crust by volume consists of igneous rocks.
- Sedimentary rocks often contain fossils.
- Metamorphic rocks compose 27.4% of the crust by volume.
- Most rocks contain silicate minerals, which account for about 95% of the earth's crust.
Gallery
Frequently Asked Questions
What exactly counts as a rock in geology?
A rock is any naturally occurring solid mass made up of one or more minerals or mineraloid material bound together. Unlike a single mineral crystal, a rock is an aggregate of multiple crystalline components, and it is classified by its mineral content, chemistry, and formation history.
What are the three main rock groups and how do they differ?
Geologists sort rocks into igneous, sedimentary, and metamorphic categories based on their origin. Igneous rocks solidify from molten material, sedimentary rocks build up from accumulated particles or precipitates, and metamorphic rocks form when pre-existing rocks are reshaped by heat and pressure.
How does the rock cycle work?
The rock cycle is the ongoing loop in which rocks are melted, cooled, eroded, compacted, or metamorphosed, turning one type into another over geological time. No rock type is permanent; every rock can eventually be recycled into a different category.
What percentage of Earth's crust is each rock type?
By volume, igneous rocks account for roughly 65% of the crust, metamorphic rocks about 27.4%, and sedimentary rocks around 7.9%. This distribution reflects the fact that most of the crust originally solidified from magma.
Which subdisciplines of geology focus on studying rocks?
Petrology deals with the origin, structure, and classification of rocks, while mineralogy examines the individual mineral components that make them up. Together these two fields form the core of rock science within geology.
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