Basalt is a common extrusive igneous (volcanic) rock formed from the rapid cooling of basaltic lava exposed at or very near the surface of a planet or moon. Flood basalts describes the formation in a series of lava basalt flows.
By definition, basalts is an aphanitic (fine-grained) igneous rock with generally 45-55% silica (SiO2) and less than 10% feldspathoid by volume, and where at least 65% of the rock is feldspar in the form of plagioclase. It is the most common volcanic rock type on Earth, being a key component of oceanic crust as well as the principal volcanic rock in many mid-oceanic islands, including Iceland, the Faroe Islands, Réunion and the islands of Hawaii. Basalts commonly features a very fine-grained or glassy matrix interspersed with visible mineral grains. The average density is 3.0 gm/cm3.
Basalts is defined by its mineral content and texture, and physical descriptions without mineralogical context may be unreliable in some circumstances. Basalt is usually grey to black in colour, but rapidly weathers to brown or rust-red due to oxidation of its mafic (iron-rich) minerals into hematite and other iron oxides and hydroxides. Although usually characterized as “dark”, basaltic rocks exhibit a wide range of shading due to regional geochemical processes. Due to weathering or high concentrations of plagioclase, some basalts can be quite light-coloured, superficially resembling andesite to untrained eyes. Basalts has a fine-grained mineral texture due to the molten rock cooling too quickly for large mineral crystals to grow; it is often porphyritic, containing larger crystals (phenocrysts) formed prior to the extrusion that brought the magma to the surface, embedded in a finer-grained matrix. These phenocrysts usually are of olivine or a calcium-rich plagioclase, which have the highest melting temperatures of the typical minerals that can crystallize from the melt.
Basalt with a vesicular texture is called vesicular basalts, when the bulk of the rock is mostly solid; when the vesicles are over half the volume of a specimen, it is called scoria. This texture forms when dissolved gases come out of solution and form bubbles as the magma decompresses as it reaches the surface, yet are trapped as the erupted lava hardens before the gases can escape.
- Tholeiitic basalts is relatively rich in silica and poor in sodium. Included in this category are most basalts of the ocean floor, most large oceanic islands, and continental flood basalts such as the Columbia River Plateau.
- High and low titanium basalts. Basalts rocks are in some cases classified after their titanium (Ti) content in High-Ti and Low-Ti varieties. High-Ti and Low-Ti basalts have been distinguished in the Paraná and Etendeka traps and the Emeishan Traps.
- Mid-ocean ridge basalts (MORB) is a tholeiitic basalt commonly erupted only at ocean ridges and is characteristically low in incompatible elements.
- E-MORB, enriched MORB
- N-MORB, normal MORB
- D-MORB, depleted MORB
- High-alumina basalts may be silica-undersaturated or -oversaturated (see normative mineralogy). It has greater than 17% alumina (Al2O3) and is intermediate in composition between tholeiitic basalt and alkali basalt; the relatively alumina-rich composition is based on rocks without phenocrysts of plagioclase.
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