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basalt(bəsôlt`, băs`ôlt), fine-grained rockrock,
aggregation of solid matter composed of one or more of the minerals forming the earth's crust. The scientific study of rocks is called petrology. Rocks are commonly divided, according to their origin, into three major classes—igneous, sedimentary, and metamorphic.
..... Click the link for more information. 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 asthenosphereasthenosphere
, region in the upper mantle of the earth's interior, characterized by low-density, semiplastic (or partially molten) rock material chemically similar to the overlying lithosphere.
..... Click the link for more information. which reaches the mid-ocean ridges, such as the Mid-Atlantic Ridge, and forms the new oceanic crust, the uppermost layer of the lithospherelithosphere
, brittle uppermost shell of the earth, broken into a number of tectonic plates. The lithosphere consists of the heavy oceanic and lighter continental crusts, and the uppermost portion of the mantle.
..... Click the link for more information. . 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 ProjectDeep Sea Drilling Project,
U.S. program designed to investigate the evolution of ocean basins by core drilling of ocean sediments and underlying oceanic crust. Funded by the National Science Foundation, the project was directed by the Joint Oceanographic Institution for Deep
..... Click the link for more information. and the now defunct Project Mohole (see Mohole, ProjectMohole, Project,
program proposed in 1957 to drill a hole down to the boundary between the crust and the mantle, known as the Mohorovičić discontinuity at about 4 to 43 mi (7 to 70 km) below the earth's surface.
..... Click the link for more information. ). 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 porphyryporphyry
, igneous rock composed of large, conspicuous crystals (phenocrysts) and a groundmass in which the phenocrysts are embedded. Some authorities consider the expression "porphyritic rock" better usage than porphyry, since the term refers only to the texture of the
..... Click the link for more information. ). Phenocrysts are usually formed in the molten lava before eruption and are often composed of the minerals olivineolivine
, an iron-magnesium silicate mineral, (Mg,Fe)2SiO4, crystallizing in the orthorhombic system. It is a common constituent of magnesium-rich, silica-poor igneous rocks; metamorphism of some high magnesium sediments also can form olivine.
..... Click the link for more information. and pyroxenepyroxene
, name given to members of a group of widely distributed rock minerals called metasilicates in which magnesium, iron, and calcium, often with aluminum, sodium, lithium, manganese, or zinc occur as X in the chemical formula XSiO3.
..... Click the link for more information. . 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 schistsschist
, metamorphic rock having a foliated, or plated, structure called schistosity in which the component flaky minerals are visible to the naked eye. Schists are distinguished from the other foliated rocks, slates and gneisses, by the size of their mineral crystals; these are
..... Click the link for more information. 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.
a magmatic rock. Basalt consists predominantly of plagioclase (labradorite); also present are pyroxenes, olivine, magnetite, sphene, apatite, and other minerals. The chemical composition of basalt is close to gabbro which is its analogue in depth. Almost completely crystalline basalts are called dolerites. Basalts which are greatly decomposed and altered by secondary processes are known as diabases and basalt-porphyry. Basalts which have erupted from large tectonic fissures over enormous areas of hundreds and thousands of square kilometers are called plateau-basalts or traps; after setting and cooling, their flows crack and form pillars separating closely adjoining pentagonal and hexagonal columns. Basalts are common among volcanic beds of all ages. Modern volcanoes also throw up basaltic lavas (on Kamchatka, the Hawaiian Islands, Iceland, New Zealand, and in many other regions). The origin of basaltic magma is one of the basic problems of geology. In the USSR, basalt is quarried in the Caucasus and the Ukraine (near Rovno). Basalt is a good building material and can be used for making gravel and block rubble, as well as for facing buildings. Basalt is durable and polishes well. For this reason, it has long been used for sculpture (ancient Egypt, Assyria, late Rome, Byzantium, and contemporary Armenia). It melts easily. Fused basalt is used in manufacturing acid-resistant chemical vessels, pipes, and strong current electric insulators.
REFERENCESLevinson-Lessing, F. Iu. Petrografiia: Izbr. trudy, vol. 4. Moscow, 1955.
Zavaritskii, A. N. Izverzhennye gornye porody. Moscow, 1955.
Ioder, H. S., and C. E. Tilley. Proiskhozhdenie bazal’tovykh magm. Moscow, 1965. (Translated from English.)
V. P. PETROV