page_title: Basalt
section_title: Definition
hierarchical_section_title: Basalt / Definition
caption_attribution_description: Basalt-organs in the Mount Szent György
context_page_description: Basalt is a mafic extrusive igneous rock formed from the rapid cooling of lava rich in magnesium and iron exposed at or very near the surface of a terrestrial planet or a moon. More than 90% of all volcanic rock on Earth is basalt, and the eruption of basalt lava is observed by geologists at about 20 volcanoes per year.
  Molten basalt lava has a low viscosity due to its relatively low silica content, resulting in rapidly moving lava flows that can spread over great areas before cooling and solidifying. Flood basalts are thick sequences of many such flows that can cover hundreds of thousands of square kilometers and constitute the most voluminous of all volcanic formations.
  Basaltic magmas are thought to have their origin in the Earth's upper mantle. The chemistry of basalts thus provides clues to conditions deep in the Earth's interior.
  Basalt is also an important rock type on other planetary bodies in the Solar System; for example, the lunar maria are plains of flood basaltic lava flows, and basalt is a common rock on the surface of Mars.
context_section_description: Basalt is an aphanitic (fine-grained) igneous rock that is relatively low in silica and alkali metals. It has less than 10% feldspathoid by volume, with at least 65% of the rock consisting of feldspar in the form of plagioclase. This places basalt in the basalt/andesite field of the QAPF diagram. Basalt is further distinguished from andesite by its silica content of less than 52%. However, it is often not possible to determine the mineral composition of volcanic rocks, due to their very fine grain size, and basalt is then defined chemically as volcanic rock with a content of 45% to 52% silica and not more than 5% alkali metal oxides. This places basalt in the B field of the TAS classification.  The average density of basalt is 2.9 g/cm³.
  Basalt is usually dark grey to black in colour, due to a high content of augite or other pyroxene minerals, but can exhibit a wide range of shading. Some basalts are quite light-coloured due to a high content of plagioclase, and these are sometimes described as leucobasalts. Lighter basalt can be difficult to distinguish from andesite, but a common rule of thumb, used away from the laboratory, is that basalt has a color index of 35 or greater. 
  Basalt 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 augite, olivine, or a calcium-rich plagioclase, which have the highest melting temperatures of the typical minerals that can crystallize from the melt and are therefore the first to form solid crystals.
  Basalt often contains vesicles, formed when dissolved gases bubble out of the magma as it decompresses during its approach to the surface, and the erupted lava then solidifies before the gases can escape. When vesicles make up a substantial fraction of the volume of the rock, the rock is described as scoria.
  The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarser) groundmass are more properly referred to as diabase (also called dolerite) or, when more coarse-grained (crystals over 2 mm across), as gabbro. Diabase and gabbro are thus the hypabyssal and plutonic equivalents of basalt.
  In the Hadean, Archean, and early Proterozoic eons of Earth's history, the chemistry of erupted magmas was significantly different from today's, due to immature crustal and asthenosphere differentiation.  These ultramafic volcanic rocks, with silica (SiO₂) contents below 45% are usually classified as komatiites.
caption: Columnar basalt at Szent György Hill, Hungary
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