Rock (geology)
A rock has no crystalline structure when it is volcanic glass, yet geologists still count it among the building materials of a planet. In geology, a rock, also called a stone, is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It forms the Earth's outer solid layer, the crust, and most of the interior too. The only exceptions are the liquid outer core and pockets of magma in the asthenosphere. How does one ordinary word stretch to cover both a continent's foundation and a chip of glass? What separates one rock from another when the categories blur into each other with no hard boundaries? And how did a material this old become the thing that built civilizations? The answers run from the chemistry of silica to the quarries of ancient Egypt.
Magma cooling in the Earth's crust, or lava cooling on the ground surface or the seabed, produces igneous rock. The name comes from the Latin igneus, meaning of fire, from ignis meaning fire. This magma may be drawn from partial melts of pre-existing rocks in either a planet's mantle or crust. Three processes typically trigger that melting: an increase in temperature, a decrease in pressure, or a change in composition.
Sedimentary rocks take a slower, gentler path. They form by diagenesis and lithification of sediments, which themselves come from the weathering, transport, and deposition of existing rocks. Clastic pieces or organic detritus settle and accumulate, or minerals chemically precipitate from solution as evaporite. The particulate matter then undergoes compaction and cementation at moderate temperatures and pressures.
Metamorphic rocks are made by force rather than melting. Subject any rock type to high enough pressures and temperatures and it transforms without significant melting. The word metamorphism means to change in form. The original rock, known as the protolith, recrystallizes into other mineral types or other forms of the same minerals. These three classes, igneous, sedimentary and metamorphic, are linked by a geological model called the rock cycle, which can turn one type into another over time.
Silicate minerals dominate most rocks, compounds that carry silica tetrahedra in their crystal lattice. They account for about one-third of all known mineral species and about 95% of the Earth's crust. The proportion of silica in a rock or mineral is a major factor in determining its name and its properties. Rocks themselves are composed primarily of grains of minerals, crystalline solids built from atoms bonded into an orderly structure.
Magma differentiation explains why silica matters so much for igneous rock. Magmas tend to grow richer in silica as they rise toward the surface. Minerals low in silica crystallize out as the magma cools, a sequence known as Bowen's reaction series. The rising magma also assimilates some of the crustal rock it passes through, the country rock, which tends to be high in silica. Silica content is therefore the most important chemical criterion for classifying igneous rock, with the content of alkali metal oxides next in importance.
No hard-and-fast boundaries separate allied rocks from one another. By increasing or decreasing the proportions of their minerals, they pass through gradations from one kind to the next. The distinctive structures of one rock can be traced gradually merging into those of another. The names adopted simply correspond to selected points in a continuously graduated series.
About 65% of the Earth's crust by volume consists of igneous rocks. Within that share, 66% are basalt and gabbro, 16% are granite, and 17% are granodiorite and diorite. Only 0.6% are syenite and just 0.3% are ultramafic. The oceanic crust is 99% basalt, an igneous rock of mafic composition, while granitoids like granite dominate the continental crust.
Sedimentary rocks make up about 7.9% of the crust by volume, a thin skin compared with the igneous mass beneath. Of that fraction, 82% are shales. The remainder splits into 6% limestone and 12% sandstone and arkoses. These rocks form under the influence of gravity, settling into horizontal or near horizontal layers called strata, which is why they are sometimes called stratified rocks.
Metamorphic rocks compose 27.4% of the crust by volume. The conditions that make them are always more extreme than anything at the surface: temperatures greater than 150 to 200 degrees Celsius and pressures greater than 1500 bars. Such conditions arise, for example, when continental plates collide. The collisions that crumple mountains also forge the rock that records the crust's deepest pressures.
Schists are foliated metamorphic rocks built primarily from lamellar minerals such as micas. Whether a metamorphic rock is foliated, meaning it possesses a texture, or non-foliated places it in one of two general categories. The minerals present then determine the rock's name. A gneiss shows visible bands of differing lightness, with the granite gneiss as a common example.
The formation mechanism gives metamorphic rock its three major classes. An intrusion of magma that heats the surrounding rock causes contact metamorphism, a transformation dominated by temperature. Burial metamorphism happens when sediments are buried deep, with pressure dominant and temperature playing a smaller role; it can produce jade. Where heat and pressure both matter, the mechanism is regional metamorphism, typically found in mountain-building regions.
Marble, soapstone, and serpentine are familiar non-foliated rocks, while slates, phyllites, and mylonite join schists and gneisses among the foliated. Quartzite, a metamorphosed form of sandstone, also sits in this branch alongside hornfels. Quartzite carries one of the highest strengths of any rock, a tensile strength in excess of 300 megapascals.
The discovery of radioactive decay in 1896 gave geologists a clock. It allowed the radiometric dating of rocks, a way to read time directly from the stone. The formal science of rocks itself was developed during the 19th century, even though human theories about rocks and their origins had persisted long before. Plutonism took shape as a theory during that century.
Petrology studies the character and origin of rocks, while mineralogy studies the mineral components that create them. Both sit within geology, the study of Earth and its components. The work may be limited to rocks found on Earth, or it may extend into planetary geology, which studies the rocks of other celestial objects. Understanding of plate tectonics arrived only in the second half of the 20th century.
The study of rocks has reached well beyond geology itself. It has fed the archaeological understanding of human history and the development of engineering and technology in human society. Meteorites that fall to Earth, typically heavier than rocks here, give evidence of extraterrestrial rocks and their composition, with samples also returned by space missions such as the Hayabusa mission.
For at least 2.5 million years, humans and other hominids have used rock, making lithic technology one of the oldest and most continuously used technologies. The Stone Age was a period of widespread stone tool use. Early tools were simple implements like hammerstones and sharp flakes. Middle Stone Age tools gained sharpened points for use as projectile points, awls, or scrapers, and Late Stone Age tools carried craftsmanship and distinct cultural identities before copper and bronze largely replaced them.
Quarrying turned rock into architecture. Relatively soft, easily worked sedimentary rock was quarried for construction as early as 4000 BCE in Egypt. Stone built fortifications in Inner Mongolia as early as 2800 BCE. The soft rock tuff, common in Italy, served the Romans for many buildings and bridges, while limestone was widely used across Europe in the Middle Ages and stayed popular into the 20th century.
Mining pulled the value out of the ground, extracting base metals, precious metals, iron, uranium, coal, diamonds, rock salt, potash, and dimension stone. The mining of rock for its metal content has been one of the most important factors of human advancement. It progressed at different rates in different places, partly because of which metals the local rock could yield. Such operations can harm the environment for years after mining ceases, which is why most of the world's nations have adopted regulations to manage those effects.
Concrete is a human-made rock, built from natural and processed rock and developed since Ancient Rome. It belongs to a category called anthropic rock, synthetic or restructured rock formed by human activity. Rock can also be modified with other substances to make new forms, such as epoxy granite. Artificial stone has appeared too, including Coade stone.
For a sense of scale, quartzite reaches a tensile strength above 300 megapascals while some sedimentary rock is so soft it can be crumbled with bare fingers, the property called friable. Structural steel, by comparison, has a tensile strength of around 350 megapascals. Geologist James R. Underwood has proposed anthropic rock as a fourth class of rocks, standing alongside igneous, sedimentary, and metamorphic as humanity's own addition to the rock cycle.
Common questions
What is a rock in geology?
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 it formed. Rocks make up the Earth's crust and most of its interior.
What are the three main types of rock?
The three main groups of rock are igneous, sedimentary, and metamorphic. Igneous rocks form when magma or lava cools and solidifies. Sedimentary rocks form by diagenesis and lithification of sediments, and metamorphic rocks form when existing rocks are transformed by high pressure and temperature without significant melting.
How much of the Earth's crust is igneous rock?
About 65% of the Earth's crust by volume consists of igneous rocks. Of these, 66% are basalt and gabbro, 16% are granite, and 17% are granodiorite and diorite. Sedimentary rocks make up about 7.9% and metamorphic rocks 27.4% of the crust by volume.
Why is silica important in classifying rocks?
Silica content is the most important chemical criterion for classifying igneous rock, with alkali metal oxide content next in importance. Silicate minerals account for about 95% of the Earth's crust and about one-third of all known mineral species.
What conditions are needed to form metamorphic rock?
Metamorphic rock forms at temperatures greater than 150 to 200 degrees Celsius and pressures greater than 1500 bars, always higher than those at the Earth's surface. Such conditions occur, for example, when continental plates collide. The original rock, called the protolith, recrystallizes without significant melting.
How long have humans used rock?
Humans and other hominids have used rock for at least 2.5 million years, making lithic technology one of the oldest continuously used technologies. Soft sedimentary rock was quarried for construction as early as 4000 BCE in Egypt, and stone built fortifications in Inner Mongolia as early as 2800 BCE.
What is anthropic rock?
Anthropic rock is synthetic or restructured rock formed by human activity, such as concrete, which has been developed since Ancient Rome. Geologist James R. Underwood has proposed anthropic rock as a fourth class of rocks alongside igneous, sedimentary, and metamorphic.