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Basalt (International Phonetic Alphabet: ) is a common gray to black extrusive volcanic rock. It is usually fine-grained due to rapid cooling of lava on the Earth's surface. It may be porphyritic containing larger crystals in a fine Matrix (geology), or Vesicular texture, or frothy scoria. Unweathered basalt is black or gray.

Basalt magmas have formed by Igneous rock#Decompression of the Earth's mantle and by partial melting of rock in the interiors of Mars and the Earth's moon. Source rocks for the partial melts probably include both peridotite and pyroxenite (e.g., Sobolev et al., 2007). The crust (geology) portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below ocean ridges.

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 (coarse) groundmass are generally referred to as dolerite (also called diabase) or gabbro.

==Types of basalt==

• Tholeiitic basalt is relatively poor 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 Basalt Group.
• MORB (Mid Ocean Ridge Basalt), is characteristically low in incompatible elements. MORB is commonly erupted only at ocean ridges. MORB itself has been subdivided into varieties such as NMORB and EMORB (slightly more enriched in incompatible elements). See the PETDB database.

• High alumina basalt may be silica-undersaturated or -oversaturated (see normative mineralogy). It has greater than 17% alumina (Al2O3) and is intermediate in composition between tholeiite and alkali basalt; the relatively alumina-rich composition is based on rocks without phenocrysts of plagioclase.

• Alkali basalt is relatively poor in silica and rich in sodium. It is normative mineralogy and may contain feldspathoids, alkali feldspar and phlogopite.

• Boninite is a high-magnesium form of basalt or andesite that is erupted generally in back-arc basins, distinguished by its low titanium content and trace element composition.

Petrology The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene. Olivine can also be a significant constituent. Accessory minerals present in relatively minor amounts include iron oxides and iron-titanium oxides, such as magnetite, ulvospinel, and ilmenite. Because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, and paleomagnetism studies have made extensive use of basalt.

In tholeiitic basalt, pyroxene (augite and orthopyroxene or pigeonite) and calcium-rich plagioclase are common phenocryst minerals. Olivine may also be a phenocryst, and when present, may have rims of pigeonite. The groundmass contains interstitial quartz or tridymite or cristobalite. Olivine tholeiite has augite and orthopyroxene or pigeonite with abundant olivine, but olivine may have rims of pyroxene and is unlikely to be present in the matrix (geology).

In high-alumina basalts, phenocrysts of feldspar commonly are plagioclase in composition. Other common phenocryst minerals are olivine and augite; orthopyroxene is less common. Silica minerals and/or feldspar may be present in the groundmass.

Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine. Feldspar phenocrysts typically are labradorite to andesine in composition. Augite is rich in titanium compared to augite in tholeiitic basalt. Minerals such as feldspar, leucite, nepheline, sodalite, phlogopite mica, and apatite may be present in the groundmass.

Basalt has high liquidus and solidus temperatures -- values at the Earth's surface are near or above 1200 °C (liquidus) and near or below 1000 °C (solidus); these values are higher than those of other common igneous rocks.

The majority of tholeiites are formed at approximately 50-100 km depth within the mantle. Many alkali basalts may be formed at greater depths, perhaps as deep as 150-200 km. The origin of high-alumina basalt continues to be controversial, with interpretations that it is a magma and that instead it is derived from other basalt types (e.g., Ozerov, 2000).

Geochemistry Basalt compositions are rich in magnesium oxide and calcium oxide and low in silicon dioxide and sodium oxide plus potassium oxide relative to most common igneous rocks, consistent with the TAS classification.

Basalt generally has a composition of 45-55 wt% SiO2, 2-6 wt% total alkalis, 0.5-2.0 wt% titanium dioxide, 5-14 wt% iron(II) oxide and 14 wt% or more alumina. Contents of CaO are commonly near 10 wt%, those of MgO commonly in the range 5 to 12 wt%.

High alumina basalts have aluminium contents of 17-19 wt% Al2O3; boninites have magnesium contents of up to 15% MgO. Rare feldspathoid-rich mafic rocks, akin to alkali basalts, may have Na2O plus K2O contents of 12% or more.

MORB basalts and their intrusive equivalents, gabbros, are the characteristic igneous rocks formed at mid-ocean ridges. They are tholeiites particularly low in total alkalis and in incompatible trace elements, and they have relatively flat rare earth elements patterns normalised to mantle or chondrite values. In contrast, alkali basalts have normalized patterns highly enriched in the light REE, and with greater abundances of the REE and of other incompatible elements. Because MORB basalt is considered a key to understanding plate tectonics, its compositions have been much studied. Although MORB compositions are distinctive relative to average compositions of basalts erupted in other environments, they are not uniform. For instance, compositions change with position along the Mid-Atlantic ridge, and the compositions also define different ranges in different ocean basins (Hofmann, 2003).

Isotope ratios of chemical elements such as strontium, neodymium, lead, hafnium, and osmium in basalts have been much-studied, so as to learn about evolution of the Earth's mantle. Isotopic ratios of noble gases, such as 3Helium/4He, are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiite (normalized to atmospheric values), but to 15-24+ for ocean island basalts thought to be derived from mantle plumes.

Morphology and textures The shape, structure and rock microstructure of a basalt is diagnostic of how and where it erupted - whether into the sea, in an explosive cinder eruption or as creeping pahoehoe lava flows, the classical image of Hawaiian basalt eruptions. basalt in Turkey

Subaerial eruptions Basalt which erupts under open air (that is, subaerially) forms three distinct types of lava or volcanic deposits: scoria, volcanic ash or cinder; breccia and lava flows.

Basalt in the tops of subaerial lava flows and cinder cones will often be highly Vesicular texture, imparting a lightweight "frothy" texture to the rock. Basaltic cinders are often red, coloured by oxidised iron from weathered iron-rich minerals such as pyroxene.

‘A‘a types of blocky, cinder and breccia flows of thick, viscous basaltic lava are common in Hawaii. Pahoehoe is a highly fluid, hot form of basalt which tends to form thin aprons of molten lava which fill up hollows and sometimes forms lava lakes. Lava tubes are common features of pahoehoe eruptions.

Basaltic tuff or pyroclastic rocks are rare but not unknown. Usually basalt is too hot and fluid to build up sufficient pressure to form explosive lava eruptions but occasionally this will happen by trapping of the lava within the volcanic throat and build up of volcanic gases. Hawaii's Mauna Loa volcano erupted in this way in the 19th century, as did Mount Tarawera, New Zealand in its violent 1886 eruption. Another example is the 0.2 Mega-annum Diamond Head, Hawaii, Hawaii. in the basalt of the Giant's Causeway in Northern IrelandMaar volcanoes are typical of small basalt tuffs, formed by explosive eruption of basalt through the crust, forming an apron of mixed basalt and wall rock breccia and a fan of basalt tuff further out from the volcano.

Amygdaloidal structure is common in relict vesicle (geology)s and beautifully crystallized species of zeolites, quartz or calcite are frequently found.

Columnar basalt During the cooling of a thick lava flow, contractional Joint (geology) or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form. The extensive fracture network that develops results in the formation of columns. The topology of the lateral shapes of these columns can broadly be classed as a random cellular network. These structures are often erroneously described as being predominantly hexagonal. In reality, the mean number of sides of all the columns in such a structure is indeed six (by geometrical definition), but polygons with three to twelve or more sides can be observedD. Weaire and N. Rivier. Contemporary Physics 25 1 (1984), pp. 55–99. Note that the size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (

Basalt (International Phonetic Alphabet: ) is a common gray to black extrusive volcanic rock. It is usually fine-grained due to rapid cooling of lava on the Earth's surface. It may be porphyritic containing larger crystals in a fine Matrix (geology), or Vesicular texture, or frothy scoria. Unweathered basalt is black or gray.

Basalt magmas have formed by Igneous rock#Decompression of the Earth's mantle and by partial melting of rock in the interiors of Mars and the Earth's moon. Source rocks for the partial melts probably include both peridotite and pyroxenite (e.g., Sobolev et al., 2007). The crust (geology) portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below ocean ridges.

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 (coarse) groundmass are generally referred to as dolerite (also called diabase) or gabbro.

==Types of basalt==

• Tholeiitic basalt is relatively poor 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 Basalt Group.
• MORB (Mid Ocean Ridge Basalt), is characteristically low in incompatible elements. MORB is commonly erupted only at ocean ridges. MORB itself has been subdivided into varieties such as NMORB and EMORB (slightly more enriched in incompatible elements). See the PETDB database.

• High alumina basalt may be silica-undersaturated or -oversaturated (see normative mineralogy). It has greater than 17% alumina (Al2O3) and is intermediate in composition between tholeiite and alkali basalt; the relatively alumina-rich composition is based on rocks without phenocrysts of plagioclase.

• Alkali basalt is relatively poor in silica and rich in sodium. It is normative mineralogy and may contain feldspathoids, alkali feldspar and phlogopite.

• Boninite is a high-magnesium form of basalt or andesite that is erupted generally in back-arc basins, distinguished by its low titanium content and trace element composition.

Petrology The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene. Olivine can also be a significant constituent. Accessory minerals present in relatively minor amounts include iron oxides and iron-titanium oxides, such as magnetite, ulvospinel, and ilmenite. Because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, and paleomagnetism studies have made extensive use of basalt.

In tholeiitic basalt, pyroxene (augite and orthopyroxene or pigeonite) and calcium-rich plagioclase are common phenocryst minerals. Olivine may also be a phenocryst, and when present, may have rims of pigeonite. The groundmass contains interstitial quartz or tridymite or cristobalite. Olivine tholeiite has augite and orthopyroxene or pigeonite with abundant olivine, but olivine may have rims of pyroxene and is unlikely to be present in the matrix (geology).

In high-alumina basalts, phenocrysts of feldspar commonly are plagioclase in composition. Other common phenocryst minerals are olivine and augite; orthopyroxene is less common. Silica minerals and/or feldspar may be present in the groundmass.

Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine. Feldspar phenocrysts typically are labradorite to andesine in composition. Augite is rich in titanium compared to augite in tholeiitic basalt. Minerals such as feldspar, leucite, nepheline, sodalite, phlogopite mica, and apatite may be present in the groundmass.

Basalt has high liquidus and solidus temperatures -- values at the Earth's surface are near or above 1200 °C (liquidus) and near or below 1000 °C (solidus); these values are higher than those of other common igneous rocks.

The majority of tholeiites are formed at approximately 50-100 km depth within the mantle. Many alkali basalts may be formed at greater depths, perhaps as deep as 150-200 km. The origin of high-alumina basalt continues to be controversial, with interpretations that it is a magma and that instead it is derived from other basalt types (e.g., Ozerov, 2000).

Geochemistry Basalt compositions are rich in magnesium oxide and calcium oxide and low in silicon dioxide and sodium oxide plus potassium oxide relative to most common igneous rocks, consistent with the TAS classification.

Basalt generally has a composition of 45-55 wt% SiO2, 2-6 wt% total alkalis, 0.5-2.0 wt% titanium dioxide, 5-14 wt% iron(II) oxide and 14 wt% or more alumina. Contents of CaO are commonly near 10 wt%, those of MgO commonly in the range 5 to 12 wt%.

High alumina basalts have aluminium contents of 17-19 wt% Al2O3; boninites have magnesium contents of up to 15% MgO. Rare feldspathoid-rich mafic rocks, akin to alkali basalts, may have Na2O plus K2O contents of 12% or more.

MORB basalts and their intrusive equivalents, gabbros, are the characteristic igneous rocks formed at mid-ocean ridges. They are tholeiites particularly low in total alkalis and in incompatible trace elements, and they have relatively flat rare earth elements patterns normalised to mantle or chondrite values. In contrast, alkali basalts have normalized patterns highly enriched in the light REE, and with greater abundances of the REE and of other incompatible elements. Because MORB basalt is considered a key to understanding plate tectonics, its compositions have been much studied. Although MORB compositions are distinctive relative to average compositions of basalts erupted in other environments, they are not uniform. For instance, compositions change with position along the Mid-Atlantic ridge, and the compositions also define different ranges in different ocean basins (Hofmann, 2003).

Isotope ratios of chemical elements such as strontium, neodymium, lead, hafnium, and osmium in basalts have been much-studied, so as to learn about evolution of the Earth's mantle. Isotopic ratios of noble gases, such as 3Helium/4He, are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiite (normalized to atmospheric values), but to 15-24+ for ocean island basalts thought to be derived from mantle plumes.

Morphology and textures The shape, structure and rock microstructure of a basalt is diagnostic of how and where it erupted - whether into the sea, in an explosive cinder eruption or as creeping pahoehoe lava flows, the classical image of Hawaiian basalt eruptions. basalt in Turkey

Subaerial eruptions Basalt which erupts under open air (that is, subaerially) forms three distinct types of lava or volcanic deposits: scoria, volcanic ash or cinder; breccia and lava flows.

Basalt in the tops of subaerial lava flows and cinder cones will often be highly Vesicular texture, imparting a lightweight "frothy" texture to the rock. Basaltic cinders are often red, coloured by oxidised iron from weathered iron-rich minerals such as pyroxene.

‘A‘a types of blocky, cinder and breccia flows of thick, viscous basaltic lava are common in Hawaii. Pahoehoe is a highly fluid, hot form of basalt which tends to form thin aprons of molten lava which fill up hollows and sometimes forms lava lakes. Lava tubes are common features of pahoehoe eruptions.

Basaltic tuff or pyroclastic rocks are rare but not unknown. Usually basalt is too hot and fluid to build up sufficient pressure to form explosive lava eruptions but occasionally this will happen by trapping of the lava within the volcanic throat and build up of volcanic gases. Hawaii's Mauna Loa volcano erupted in this way in the 19th century, as did Mount Tarawera, New Zealand in its violent 1886 eruption. Another example is the 0.2 Mega-annum Diamond Head, Hawaii, Hawaii. in the basalt of the Giant's Causeway in Northern IrelandMaar volcanoes are typical of small basalt tuffs, formed by explosive eruption of basalt through the crust, forming an apron of mixed basalt and wall rock breccia and a fan of basalt tuff further out from the volcano.

Amygdaloidal structure is common in relict vesicle (geology)s and beautifully crystallized species of zeolites, quartz or calcite are frequently found.

Columnar basalt During the cooling of a thick lava flow, contractional Joint (geology) or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form. The extensive fracture network that develops results in the formation of columns. The topology of the lateral shapes of these columns can broadly be classed as a random cellular network. These structures are often erroneously described as being predominantly hexagonal. In reality, the mean number of sides of all the columns in such a structure is indeed six (by geometrical definition), but polygons with three to twelve or more sides can be observedD. Weaire and N. Rivier. Contemporary Physics 25 1 (1984), pp. 55–99. Note that the size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (

Kappa Software - Basalt
Extends BASIC with new keywords. List of features, manual, and downloads.

Basalt
Basalt is one of the most common igneous rocks and is major constituent of the upper layer of the ocean floors (usually as pillow lava), and hot spot volcanoes (such as the ...

Basalt - Wikipedia, the free encyclopedia
Basalt (pronounced /ˈbæsɒlt, bəˈsɔːlt/) is a common mafic extrusive volcanic rock. It is usually gray to black and fine-grained due to rapid cooling of lava at the surface ...

Flood basalt - Wikipedia, the free encyclopedia
A flood basalt or trap basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava.

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Illustrates and describes the volcano-related term ... B asalt Basalt is a hard, black volcanic rock with less than about 52 weight percent silica (SiO 2).

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Definition: basalt from Online Medical Dictionary
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Wainwright Basalt Aggregate Quarry
Suppliers of basalt aggregate in the UK.

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Category:Basalt - Wikimedia Commons
Media in category "Basalt" The following 60 files are in this category, out of 60 total.