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Rock (mineral), naturally occurring solid material consisting of one or more minerals. Minerals are solid, naturally occurring chemical elements or compounds that are homogenous, meaning they have a definite chemical composition and a very regular arrangement of atoms. Rocks are everywhere, in the ground, forming mountains, and at the bottom of the oceans. Earth’s outer layer, or crust, is made mostly of rock. Some common rocks include granite and basalt.
TYPES OF ROCKS Rocks are divided into three main types, based on the ways in which they form. Igneous rocks are made of old rocks that have melted within the earth to form molten material called magma. Magma cools and solidifies to become igneous rocks. Sedimentary rocks form as layers of material settle onto each other, press together, and harden. Metamorphic rocks are created when existing rocks are exposed to high temperatures and pressures, and the rock material is changed, or metamorphosed, while solid.
Pegmatite is a variety of igneous rock with extremely large crystals; individual crystals can be as large as a bathtub. Pegmatites are the last rocks to crystallize from a solidifying body of magma. The slow rate of cooling and the presence of large amounts of water dissolved in the magma account for the large size of the crystals.
Igneous rocks commonly contain the minerals feldspar, quartz, mica, pyroxene, amphibole, and olivine. Igneous rocks are named according to which minerals they contain. Rocks rich in feldspar and quartz are called felsic; rocks rich in pyroxene, amphibole, and olivine, which all contain magnesium and iron, are called mafic. Common and important igneous rocks are granite, rhyolite, gabbro, and basalt. Granite and rhyolite are felsic; gabbro and basalt are mafic. Granite has large crystals of quartz and feldspar. Rhyolite is the small-grained equivalent of granite. Gabbro has large crystals of pyroxene and olivine. Basalt is the most common volcanic rock.
Sedimentary rock, such as this sandstone, forms when layers of sediment are compacted and cemented together. Nearly 5% of the top 15 km (10 mi) of the earth’s crust is made of sedimentary rock. However, sedimentary rock constitutes nearly 75% of the exposed rock on the earth’s surface.
Sedimentary rock forms when loose sediment, or rock fragments, hardens. Geologists place sedimentary rocks into three broad categories: (1) clastic rocks, which form from clasts, or broken fragments, of pre-existing rocks and minerals; (2) chemical rocks, which form when minerals precipitate, or solidify, from a solution, usually seawater or lake water; and (3) organic rocks, which form from accumulations of animal and plant remains. It is common for sedimentary rocks to contain all three types of sediment. Most fossils are found in sedimentary rocks because the processes that form igneous and metamorphic rocks prevent fossilization or would likely destroy fossils.
The most common types of clastic rocks are sandstone and shale (also known as mudrock). Sandstone is made from sand, and shale is made from mud. Sand particles have diameters in the range 2.00 to 0.06 mm (0.08 to 0.002 in), while mud particles are smaller than 0.06 mm (0.002 in). Sand and mud form when physical or chemical processes break down and destroy existing rocks. The sand and mud are carried by wind, rivers, ocean currents, and glaciers, which deposit the sediment when the wind or water slows down or where the glacier ends. Sand usually forms dunes in deserts, or sandbars, riverbeds, beaches, and near-shore marine deposits. Mud particles are smaller than sand particles, so they tend to stay in the wind or water longer and are deposited only in very still environments, such as lake beds and the ocean floor.
Sedimentary rock forms when layers of sand and mud accumulate. As the sediment accumulates, the weight of the layers of sediment presses down and compacts the layers underneath. The sediments become cemented together into a hard rock when minerals (most commonly quartz or calcite) precipitate, or harden, from water in the spaces between grains of sediment, binding the grains together. Sediment is usually deposited in layers, and compaction and cementation preserve these layers, called beds, in the resulting sedimentary rock.
The most common types of chemical rocks are called evaporites because they form by evaporation of seawater or lake water. The elements dissolved in the water crystallize to form minerals such as gypsum and halite. Gypsum is used to manufacture plaster and wallboard; halite is used as table salt.
The most common organic rock is limestone. Many marine animals, such as corals and shellfish, have skeletons or shells made of calcium carbonate (CaCO3). When these animals die, their skeletons sink to the seafloor and accumulate to form large beds of calcium carbonate. As more and more layers form, their weight compresses and cements the layers at the bottom, forming limestone. Details of the skeletons and shells are often preserved in the limestone as fossils.
Coal is another common organic rock. Coal comes from the carbon compounds of plants growing in swampy environments. Plant material falling into the muck at the bottom of the swamp is protected from decay. Burial and compaction of the accumulating plant material can produce coal, an important fuel in many parts of the world. Coal deposits frequently contain plant fossils.
Metamorphic rock forms when extreme temperatures and pressures deep within the earth alter the mineralogical or structural aspects of existing rock and change it into metamorphic rock. This temperature and pressure alteration within rock causes its set of minerals, or mineral assemblages, to change into other minerals. The change from one mineral assemblage to another is called metamorphism.
Metamorphic rock forms when pre-existing rock undergoes mineralogical and structural changes resulting from high temperatures and pressures. These changes occur in the rock while it remains solid (without melting).
The changes can occur while the rock is still solid because each mineral is stable only over a specific range of temperature and pressure. If a mineral is heated or compressed beyond its stability range, it breaks down and forms another mineral. For example, quartz is stable at room temperature and at pressures up to 1.9 gigapascals (corresponding to the pressure found about 65 km [about 40 mi] underground). At pressures above 1.9 gigapascals, quartz breaks down and forms the mineral coesite, in which the silicon and oxygen atoms are packed more closely together.
In the same way, combinations of minerals are stable over specific ranges of temperature and pressure. At temperatures and pressures outside the specific ranges, the minerals react to form different combinations of minerals. Such combinations of minerals are called mineral assemblages.
In a metamorphic rock, one mineral assemblage changes to another when its atoms move about in the solid state and recombine to form new minerals. This change from one mineral assemblage to another is called metamorphism. As temperature and pressure increase, the rock gains energy, which fuels the chemical reactions that cause metamorphism. As temperature and pressure decrease, the rock cools; often, it does not have enough energy to change back to a low-temperature and low-pressure mineral assemblage. In a sense, the rock is stuck in a state that is characteristic of its earlier high-temperature and high-pressure environment. Thus, metamorphic rocks carry with them information about the history of temperatures and pressures to which they were subjected.
The size, shape, and distribution of mineral grains in a rock are called the texture of the rock. Many metamorphic rocks are named for their main texture. Textures give important clues as to how the rock formed. As the pressure and temperature that form a metamorphic rock increase, the size of the mineral grains usually increases. When the pressure is equal in all directions, mineral grains form in random orientations and point in all directions. When the pressure is stronger in one direction than another, minerals tend to align themselves in particular directions.
In particular, thin plate-shaped minerals, such as mica, align perpendicular to the direction of maximum pressure, giving rise to a layering in the rock that is known as foliation. Compositional layering, or bands of different minerals, can also occur and cause foliation. At low pressure, foliation forms fine, thin layers, as in the rock slate. At medium pressure, foliation becomes coarser, forming schist. At high pressure, foliation is very coarse, forming gneiss. Commonly, the layering is folded in complex, wavy patterns from the pressure.
IIITHE ROCK CYCLE The rock cycle describes how rocks change, or evolve, from one type to another. For example, any type of rock (igneous, sedimentary, or metamorphic) can become a new sedimentary rock if its eroded sediments are deposited, compacted, and cemented. Similarly, any type of rock can become metamorphic if it is buried moderately deep. If the temperature and pressure become sufficiently high, the rock can melt to form magma and a new igneous rock.
Rock Cycle • The rock cycle shows how each of the three principal types of rocks (sedimentary, metamorphic, and igneous) can evolve into either of the two other types of rock, or even into other rocks of its own type. Sediments that are compacted and cemented form sedimentary rocks, rocks that are subjected to heat and pressure form metamorphic rocks, and rocks that are melted and then cool form igneous rocks.
Rock Formation Rock formations are caused by erosion in regions of soft, unconsolidated rock where short periods of heavy rain alternate with long periods of drought. The little vegetation that grows during the dry period is insufficient to check erosion and is washed away with the soil by the rains.
Red Rock Canyon, Wyoming Red Rock Canyon, near the town of Lander in central Wyoming, is an example of the kind of land formations found in the lowlands separating the state’s numerous mountain ranges. The famous Pony Express Trail, a treacherous route used for delivering mail during frontier days, runs along the canyon.
