Rock Types 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.
Igneous rocks Igneous rocks are rocks formed from a molten or partly molten material called agma.Magma forms deep underground when rock that was once solid melts. Overlying rock presses down on the magma, and the less dense magma rises through cracks in the rock. As magma moves upward, it cools and solidifies. Magma that solidifies underground usually cools slowly, allowing large crystals to form.
Magma that reaches Earth's surface is called lava. Lava loses heat to the atmosphere or ocean very quickly and therefore solidifies very rapidly, forming very small crystals or glass. When lava erupts at the surface again and again, it can form mountains called olcanoes.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; gabbros 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 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 ypes 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 classic rocks are sandstone and 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 he sediment when the wind or water slows down or where the glacier ends. Sand dunes usually form 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. The most common types of chemical rocks are called evaporates 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 (CaC03).
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 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).In a metamorphic rock, one ineral 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 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, Waw patterns from the pressure.
