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 (2,012–2,192°F; 1,100–1,250°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

basalt ‘Basalt’ is a term widely used and abused. It has been employed both as a specific rock name, designating a narrow range of compositions (46–50 per cent silicon dioxide, (SiO₂)), and as a general term for almost any dark, fine-grained igneous rock, more particularly for those with the minerals plagioclase feldspar and pyroxene. Not all rocks of basaltic composition are, however, dark and fine-grained, nor are all dark, fine-grained, igneous rocks of basaltic composition. While the term ‘basalt’ remains a useful field name, it should be borne in mind that whole books have been written on the finer details of the sub-classification of rocks of basaltic composition. In practice many basalts also contain easily visible crystals (phenocrysts) of feldspar, pyroxene, olivine, or amphibole.

Basalts are the most common type of rock on Earth; they are also abundant on the moon and on other planets in the solar system. Almost the entire ocean floor is made of basalt, which was, and still is, erupted at mid-ocean ridges. These mid-ocean ridge basalts (MORB for short), in common with some other types of basalt, are erupted as a result of the divergence of tectonic plates. As two plates pull apart, the Earth's mantle rises to fill the gap, is decompressed, and melts to form magma. As the melt reaches the surface of the Earth it cools to form basalt.

Most MORBs are of what is known as tholeiitic composition; i.e. they have free silica and contain calcium-poor pyroxene. The basalts of oceanic islands (OIBs) are also tholeiitic, but vary widely in composition. Of special interest are the komatiites, which are all more than 2.5 billion years old. They are rich in magnesium and contain large olivine crystals. Komatiites were evidently erupted at higher temperatures than other volcanic rocks, and it is likely that they were generated by plumes in the mantle.

Extensive flows of basalt are present on the Moon, where they form the dark areas known as the maria. These lunar basalts are in general similar to those found on Earth, although they are much older (all the available samples have of ages of more than 3000 million years), but their chemical composition is more variable and they contain some minerals not known on Earth. A notable feature of the lunar basalts is the complete absence of water.

Judith M. Bunbury

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.

basalt. A hard igneous rock that is very durable and takes a fine polish. It is widely distributed and has many varieties, differing in consistency and ranging in colour from black, brown, or dark green to pale blue. It is almost as difficult to work as granite. Black basalt was much used by the ancient Egyptians for portrait sculpture and was also used in the Middle East. In the context of Neoclassicism the potter Josiah Wedgwood experimented with pottery materials simulating the stones favoured for sculpture in antiquity. Basalt-ware, a hard black stoneware, was the first of these materials to be successfully produced, in 1766.

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.

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.’

basalt A dark-coloured, fine-grained igneous rock low in silica, and composed essentially of calcium-rich plagioclase feldspar (over 50%) and pyroxene. Basalt is the most common extrusive igneous rock, solidified from molten magma or volcanic fragments erupted on to the surface of the Earth, Moon, and other planets. The lunar mare regions are covered with basaltic rocks.

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

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