Granite (rock)
Granite is a prominent igneous rock primarily composed of three essential minerals: quartz, feldspar, and varying amounts of ferromagnesian minerals such as biotite and hornblende. This rock forms from the cooling of magma beneath the Earth's surface, leading to the creation of large intrusive bodies known as batholiths. Granite is widely distributed across continents, often found in the exposed cores of mountain chains and highly eroded continental shields. Notable geological formations, such as Mount Rushmore and Half Dome, showcase granite's distinctive properties and resilience. Due to its strength and weather resistance, granite is commonly employed in construction as a durable building stone, with applications ranging from decorative monuments to structural components like sea walls. Variations in granite's texture and composition can lead to distinct types, including pegmatite, aplite, and graphic granite. Overall, granite plays a significant role in both geological formations and human utilization, making it a subject of interest for both geologists and builders alike.
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Granite (rock)
Granite is a medium- to coarse-grained igneous rock composed principally of interlocking grains of the light-colored silicate minerals—potassium feldspar, sodium-rich plagioclase, and quartz. The overall color may blend to reddish, pink, or white depending on which mineral predominates in the rock. Dark minerals may add a spotted appearance to the rock. Granite is an igneous rock formed at great depths in the Earth’s crust.
Definition
The three essential minerals in granite are quartz, which makes up 20 to 40 percent of the rock, and feldspars in which potassium feldspar is more abundant than plagioclase. Five to 10 percent ferromagnesian minerals, usually biotite or hornblende, or muscovite are common as accessory minerals. Garnet, tourmaline, corundum, or even pyroxene may be present in some granites.
Overview
The continents are primarily granite, with a thin veneer of sedimentary rocks. Granite is found in the exposed core of linear mountain chains and regions of highly eroded continental shields associated with regional metamorphism. The Sierra Nevada mountain range consists of a composite granitic batholith that is 640 kilometers by 110 kilometers. Granites also form such notable sites as Mount Rushmore, South Dakota; Half Dome in Yosemite National Park, California; and stone Mountain, Georgia.
Granite is used extensively as building stone. It is strong and weather resistant. Cut and polished slabs are used for internal and external facing, and polished or horned blocks are used for ornamental stones as tombstones and monuments. Large blocks are used in sea walls and jetties. Smaller blocks and crushed stone are used as rip-rap.
Variations in texture and composition give rise to distinctive varieties of granite. Pegmatite is an extremely coarse-grained rock of granite composition formed in the late, fluid-rich stage of magma crystallization. Individual crystals may reach several centimeters or tens of meters in length. Aplite is a fine-grained granite with a sugary texture. Graphic granite is conspicuous by its intergrowth of quartz within orthoclase crystals, which gives a pattern similar to cuneiform writing. Alaskite is a granite with no dark minerals. Charnockite is granite containing hypersthene as its chief ferromagnesian silicate.
Granite magma is formed by melting continental crustal rocks and thick prisms of sediments that form along the margins of convergent plates. The melt migrates upward in the crust through overlying rocks by assimilation of surrounding rocks and by forcefully pushing rocks out of the way. As the magma moves upward, blocks of overlying rocks are incorporated into the melt. If the melt is hot enough, the included rocks may be melted. If the magma has cooled sufficiently, the blocks are preserved as xenoliths (foreign rocks) within the magma. Granite magmas cool to form large intrusive bodies known as batholiths (bathos for deep, and lithos meaning rock) and smaller intrusions such as dikes and sills. As batholiths are emplaced fairly deep in the crust, the surrounding materials are usually high-grade metamorphic rocks such as schist and gneiss. Some granites may form by extreme metamorphism in which existing rocks are converted to granitic rock by recrystallization and chemical reaction with chemically active fluids.