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basalt

basalt (bəsôlt´, băs´ôlt), fine-grained rock of volcanic origin, dark gray, dark green, brown, reddish, or black in color. Basalt is an igneous rock, i.e., one that has congealed from a molten state. Basaltic magma is derived by partial melting of the peridotite that is found in the asthenosphere which reaches the mid-ocean ridges, such as the Mid-Atlantic Ridge, and forms the new oceanic crust, the uppermost layer of the lithosphere. Because molten basalt is lighter than peridotite, it rises more rapidly. Basaltic magmas contain around 50% silica; they are the most common extrusive rocks and comprise more than 90% of all volcanic rock. It forms mostly lava flows, including present-day Hawaiian flows, and the ancient Columbia River plateau of the NW United States. Basalt dominates the mid-ocean islands and surrounding regions of the Hawaiian Islands and Iceland, as found by samples of lava flows found in drill cores recovered by vessels of the Deep Sea Drilling Project and the now defunct Project Mohole (see Mohole, Project). Basalt contains a high percentage of iron and magnesium. Some basalts are porphyritic, i.e., they contain large crystalline structures called phenocrysts embedded in a matrix called a groundmass (see porphyry). Phenocrysts are usually formed in the molten lava before eruption and are often composed of the minerals olivine and pyroxene. Where molten basalt cools rapidly, as at the earth's surface, fine-grained rocks are formed. Basalt may be compact or vesicular, i.e., porous because of gas bubbles contained in the lava while it is solidifying. If the vesicles become subsequently filled with secondary minerals, e.g., quartz or calcite, the rock is called amygdaloidal basalt. Basalt may form as columns of rock, such as the Devil's Tower in Wyoming; or it may form as twisted coils of rope, or cinders of jagged rock, called "pahoehoe" and "aa," respectively. Gabbros are similar in composition to basalt, but gabbros are coarse-grained rocks formed by slow cooling in large underground masses, common in New York's Adirondack Mts. When subjected to metamorphism, i.e., high temperatures and great pressures, basalt is transformed into various kinds of schists including hornblende schist. Fine and coarse-grained crystalline rocks returned from various regions of the moon by Apollo astronauts were similar in many respects to terrestrial basalts. Fine-grained basaltic lunar rocks are vesicular, with glass-lined pits on exposed surfaces that have been interpreted as micrometeorite impact scars. Lunar rocks differed from terrestrial basalts in lacking water and organic compounds, and were higher in titanium, magnesium, and iron.

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Basalt

Basalt

Basalt is a mafic volcanic rock consisting primarily of plagioclase feldspar and pyroxene minerals . Common accessory minerals can include other pyroxenes, olivine , quartz , and nepheline. Basalt is the volcanic equivalent of the plutonic rock gabbro, and as such has a low silica content (48%52%). Like other volcanic rocks, basalt cools quickly after it erupts and therefore generally contains less than 50% visible crystals floating in a matrix of glass or microscopic crystals. Pillow basalt, consisting of lobes of lava emplaced and solidified on top of each other, is the result of undersea eruptions such as those along divergent oceanic plate boundaries. Basalt is also known to occur on the moon .

Because of its low silica content, which translates into a high melting point and low viscosity, basaltic lava erupts at a higher temperature (2,0122,192°F; 1,1001,250°C) and flows more easily across low slopes that do more silicic lava types. Under some conditions, basaltic lava can flow more than 12.5 miles (20 km) from the point of eruption. The low viscosity of molten basalt also means that dissolved volcanic gasses can escape relatively easily as the magma travels to the surface and confining pressure is reduced. Thus, basalt eruptions tend to be quiet and effusive (as typified by Hawaiian volcanoes) as compared to the explosive eruptions often associated with more viscous and silica-rich lava (as typified by Mount St. Helens). Lava fountains can, however, reach heights of several hundred meters during basaltic eruptions.

Lava flows that solidify with a smooth or ropy surface are often described using the Hawaiian term pahoehoe, whereas those which solidify with a jagged or blocky surface are described by the Hawaiian term aa. The former is pronounced "pa-hoy-hoy" and the latter is pronounced "ah-ah."

Another characteristic of many basalt flows is the presence of polygonal columnar joints, which are understood to form by contraction of the lava as it cools. The result is a system of nearly vertical joints that form a polygonal pattern when viewed from above and break the rock into slender prismatic columns.

See also Divergent plate boundary; Extrusive cooling; Joint and jointing; Rate factors in geologic processes; Rifting and rift valleys; Sea-floor spreading

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basalt

basalt A dark-coloured, fine-grained, extrusive, igneous rock composed of plagioclase feldspar, pyroxene, and magnetite, with or without olivine, and containing not more than 53wt.% SiO2. Many basalts contain phenocrysts of olivine, plagio-clase feldspar and pyroxene. Basalts are divided into two main types, alkali basalts and tholeiites, with the tholeiites being subdivided into olivine tholeiites, tholeiites, and quartz tholeiites. Petrographically (see PETROGRAPHY), alkali basalts have as their groundmass pyroxene titanaugite (an augite rich in titanium), whereas tholeiites have pigeonite (a calcium-poor pyroxene). Also, for similar concentrations of SiO2, alkali basalts have a higher content of Na2O and K2O than tholeiites. Basalt flows cover about 70% of the Earth's surface and huge areas of the terrestrial planets, and are therefore arguably the most important of all crustal rocks. They are formed by partial melting of mantle peridotite. Alkali basalts are typically found on oceanic islands and on the continental crust in regions of crustal upwarping and rifting. Tholeiites are typically found on the ocean floor and on the stable continental crust where they form large basalt plateaux such as the Deccan Traps of India.

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basalt

basalt Hard, fine-grained, basic igneous rock, which may be intrusive or extrusive. Its colour can be dark green, brown, dark grey or black. It can have a glassy appearance. There are many types of basalt with different proportions of elements. It may be compact or vesicular (porous) because of gas bubbles contained in the lava while it was cooling. If the vesicles are subsequently filled with secondary minerals, such as quartz or calcite, it is called amygdaloidal basalt. Basalts are the main rocks of ocean floors, and form the world's major lava flows, such as the Deccan Trap, India.

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basalt

ba·salt / bəˈsôlt/ • n. a dark, fine-grained volcanic rock that sometimes displays a columnar structure. ∎  a kind of black stoneware resembling such rock. DERIVATIVES: ba·sal·tic / -tik/ adj. ORIGIN: early 17th cent. (in the Latin form): from Latin basaltes (variant of basanites), from Greek basanitēs, from basanos ‘touchstone.’

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basalt

basalt XVII. — L. basaltēs, var. of basanītēs — Gr. basanī́ēs, f. básanos touchstone.

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basalt

basalt •gestalt • asphalt •belt, Celt, dealt, dwelt, felt, gelt, knelt, melt, misdealt, pelt, Scheldt, smelt, spelt, svelte, veld, welt •fan belt • seat belt • lifebelt • sunbelt •rust belt • Copperbelt • heartfelt •underfelt • backveld • bushveld •Roosevelt •atilt, built, gilt, guilt, hilt, jilt, kilt, lilt, quilt, silt, spilt, stilt, tilt, upbuilt, wilt •Vanderbilt • volte •assault, Balt, exalt, fault, halt, malt, salt, smalt, vault •cobalt • stringhalt • basalt •somersault • polevault •bolt, colt, dolt, holt, jolt, moult (US molt), poult, smolt, volt •deadbolt • Humboldt • thunderbolt •megavolt • spoilt • Iseult •consult, cult, exult, indult, insult, penult, result, ult •adult • occult • tumult • catapult •difficult • Hasselt

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