Editing Flood basalt (section)

==Description==
[[Image:Ethiopian highlands 01 mod.jpg|thumb|[[Ethiopian Highlands]] basalt]]
[[File:Ages of flood basalt events 1.png|thumb|Ages of flood basalt events and oceanic plateaus.<ref>[[Vincent Courtillot]], [[Paul Renne]]: ''[https://www.sciencedirect.com/science/article/pii/S1631071303000063?via%3Dihub On the ages of flood basalt events]''</ref>]]

Flood basalts are the most voluminous of all [[Extrusive rock|extrusive igneous rocks]],<ref>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |edition=2nd |page=52}}</ref> forming enormous deposits of [[basaltic]] rock<ref name=Jackson1997>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=plateau basalt}}</ref><ref name=Allaby2013>{{cite book |last1=Allaby |first1=Michael |title=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |location=Oxford |isbn=9780199653065 |edition=Fourth |chapter=flood basalt}}</ref> found throughout the geologic record.<ref name=Jackson1997/>{{sfn|Philpotts|Ague|2009|p=380}} They are a highly distinctive form of [[intraplate volcanism]],<ref>{{cite book |last1=Schmincke |first1=Hans-Ulrich |title=Volcanism |date=2003 |publisher=Springer |location=Berlin |isbn=978-3-540-43650-8 |page=107}}</ref> set apart from all other forms of volcanism by the huge volumes of lava erupted in geologically short time intervals. A single flood basalt province may contain hundreds of thousands of cubic kilometers of basalt erupted over less than a million years, with individual events each erupting hundreds of cubic kilometers of basalt.{{sfn|Philpotts|Ague|2009|p=380}} This highly fluid basalt lava can spread laterally for hundreds of kilometers from its source vents,{{sfn|Philpotts|Ague|2009|p=53}} covering areas of tens of thousands of square kilometers.{{sfn|Schmincke|2003|p=107}} Successive eruptions form thick accumulations of nearly horizontal flows, erupted in rapid succession over vast areas, flooding the Earth's surface with lava on a regional scale.<ref name=Jackson1997/>{{sfn|Philpotts|Ague|2009|p=52}}

These vast accumulations of flood basalt constitute [[large igneous province]]s. These are characterized by plateau landforms, so that flood basalts are also described as ''plateau basalts''. Canyons cut into the flood basalts by erosion display stair-like slopes, with the lower parts of flows forming cliffs and the upper part of flows or [[interbedded]] layers of sediments forming slopes. These are known in Dutch as ''trap'' or in Swedish as ''trappa'', which has come into English as ''trap rock'', a term particularly used in the quarry industry.{{sfn|Philpotts|Ague|2009|p=52}}{{sfn|Schmincke|2003|p=108}}

The great thickness of the basalt accumulations, often in excess of {{convert|1000|m|sigfig=1|sp=us}},{{sfn|Schmincke|2003|p=108}} usually reflects a very large number of thin flows, varying in thickness from meters to tens of meters, or more rarely to {{convert|100|m||sp=us}}. There are occasionally very thick individual flows. The world's thickest basalt flow may be the Greenstone flow of the [[Keweenaw Peninsula]] of [[Michigan]], US, which is {{convert|600|m||sp=us}} thick. This flow may have been part of a lava lake the size of [[Lake Superior]].{{sfn|Philpotts|Ague|2009|p=53}}

Deep erosion of flood basalts exposes vast numbers of parallel dikes that fed the eruptions.{{sfn|Philpotts|Ague|2009|p=57}} Some individual dikes in the [[Columbia River Plateau]] are over {{convert|100|km|sigfig=1|sp=us}} long.{{sfn|Schmincke|2003|p=108}} In some cases, erosion exposes radial sets of dikes with diameters of several thousand kilometers.{{sfn|Philpotts|Ague|2009|p=380}} Sills may also be present beneath flood basalts, such as the [[Palisades Sill]] of [[New Jersey]], US. The sheet intrusions (dikes and sills) beneath flood basalts are typically [[diabase]] that closely matches the composition of the overlying flood basalts. In some cases, the chemical signature allows individual dikes to be connected with individual flows.{{sfn|Philpotts|Ague|2009|pp=381-382}}

===Smaller-scale features===
Flood basalt commonly displays [[columnar jointing]], formed as the rock cooled and contracted after solidifying from the lava. The rock fractures into columns, typically with five to six sides, parallel to the direction of heat flow out of the rock. This is generally perpendicular to the upper and lower surfaces, but rainwater infiltrating the rock unevenly can produce "cold fingers" of distorted columns. Because heat flow out of the base of the flow is slower than from its upper surface, the columns are more regular and larger in the bottom third of the flow. The greater hydrostatic pressure, due to the weight of overlying rock, also contributes to making the lower columns larger. By analogy with Greek temple architecture, the more regular lower columns are described as the ''colonnade'' and the more irregular upper fractures as the ''entablature'' of the individual flow. Columns tend to be larger in thicker flows, with columns of the very thick Greenstone flow, mentioned earlier, being around {{convert|10|m|sigfig=1|sp=us}} thick.{{sfn|Philpotts|Ague|2009|p=55}}

Another common small-scale feature of flood basalts is ''pipe-stem vesicles''. Flood basalt lava cools quite slowly, so that dissolved gases in the lava have time to come out of solution as bubbles (vesicles) that float to the top of the flow. Most of the rest of the flow is massive and free of vesicles. However, the more rapidly cooling lava close to the base of the flow forms a thin [[chilled margin]] of glassy rock, and the more rapidly crystallized rock just above the glassy margin contains vesicles trapped as the rock was rapidly crystallizing. These have a distinctive appearance likened to a clay [[tobacco pipe]] stem, particularly as the vesicle is usually subsequently filled with [[calcite]] or other light-colored minerals that contrast with the surrounding dark basalt.{{sfn|Philpotts|Ague|2009|p=58}}

=== Petrology ===
At still smaller scales, the [[Texture (geology)|texture]] of flood basalts is [[aphanitic]], consisting of tiny interlocking crystals. These interlocking crystals give trap rock its tremendous toughness and durability.{{sfn|Philpotts|Ague|2009|p=55}} Crystals of [[plagioclase]] are embedded in or wrapped around crystals of [[pyroxene]] and are randomly oriented. This indicates rapid emplacement so that the lava is no longer flowing rapidly when it begins to crystallize.{{sfn|Philpotts|Ague|2009|p=53}} Flood basalts are almost devoid of large [[phenocrysts]], larger crystals present in the lava prior to its being erupted to the surface, which are often present in other extrusive igneous rocks. Phenocrysts are more abundant in the [[Dike (geology)|dikes]] that fed lava to the surface.{{sfn|Philpotts|Ague|2009|p=383}}

Flood basalts are most often [[quartz]] [[tholeiite]]s. [[Olivine]] tholeiite (the characteristic rock of [[mid-ocean ridges]]{{sfn|Philpotts|Ague|2009|p=366}}) occurs less commonly, and there are rare cases of [[alkali basalt]]s. Regardless of composition, the flows are very homogeneous and rarely contain [[xenoliths]], fragments of the surrounding rock ([[country rock (geology)|country rock]]) that have been entrained in the lava. Because the lavas are low in dissolved gases, [[pyroclastic rock]] is extremely rare. Except where the flows entered lakes and became [[pillow lava]], the flows are massive (featureless). Occasionally, flood basalts are associated with very small volumes of [[dacite]] or [[rhyolite]] (much more silica-rich volcanic rock), which forms late in the development of a large igneous province and marks a shift to more centralized volcanism.{{sfn|Philpotts|Ague|2009|p=381}}

=== Geochemistry ===
[[File:Parana traps.JPG|thumb|Parana traps]]
Flood basalts show a considerable degree of chemical uniformity across geologic time,{{sfn|Philpotts|Ague|2009|p=380}} being mostly iron-rich tholeiitic basalts. Their major element chemistry is similar to mid-ocean ridge basalts (MORBs), while their trace element chemistry, particularly of the [[rare earth elements]], resembles that of [[ocean island basalt]].<ref name=Wilson2007>{{cite book |last1=Wilson |first1=Marjorie |title=Igneous Petrogenesis |chapter=Continental tholeiitic flood basalt provinces |date=2007 |pages=287–323 |doi=10.1007/978-94-010-9388-0_10|isbn=978-0-412-75080-9 }}</ref> They typically have a silica content of around 52%. The magnesium number (the [[mol%]] of magnesium out of the total iron and magnesium content) is around 55,{{sfn|Philpotts|Ague|2009|p=383}} versus 60 for a typical MORB.{{sfn|Philpotts|Ague|2009|p=367}} The [[rare earth elements]] show abundance patterns suggesting that the original (primitive) magma formed from rock of the [[Earth's mantle]] that was nearly ''undepleted''; that is, it was mantle rock rich in [[garnet]] and from which little magma had previously been extracted. The chemistry of plagioclase and olivine in flood basalts suggests that the magma was only slightly contaminated with melted rock of the [[Earth's crust]], but some high-temperature minerals had already crystallized out of the rock before it reached the surface.{{sfn|Philpotts|Ague|2009|p=382}} In other words, the flood basalt is moderately [[Magma differentiation|evolved]].<ref name=Wilson2007/> However, only small amounts of plagioclase appear to have crystallized out of the melt.{{sfn|Philpotts|Ague|2009|p=382}}

Though regarded as forming a chemically homogeneous group, flood basalts sometimes show significant chemical diversity even with in a single province. For example, the flood basalts of the [[Parana Basin]] can be divided into a low phosphorus and titanium group (LPT) and a high phosphorus and titanium group (HPT). The difference has been attributed to inhomogeneity in the upper mantle,<ref>{{cite journal |last1=Hawkesworth |first1=C. J. |last2=Mantovani |first2=M. S. M. |last3=Taylor |first3=P. N. |last4=Palacz |first4=Z. |title=Evidence from the Parana of south Brazil for a continental contribution to Dupal basalts |journal=Nature |date=July 1986 |volume=322 |issue=6077 |pages=356–359 |doi=10.1038/322356a0|bibcode=1986Natur.322..356H |s2cid=4261508 }}</ref> but [[strontium isotope]] ratios suggest the difference may arise from the LPT magma being contaminated with a greater amount of melted crust.<ref>{{cite journal |last1=Mantovani |first1=M. S. M. |last2=Marques |first2=L. S. |last3=De Sousa |first3=M. A. |last4=Civetta |first4=L. |last5=Atalla |first5=L. |last6=Innocenti |first6=F. |title=Trace Element and Strontium Isotope Constraints on the Origin and Evolution of Paran Continental Flood Basalts of Santa Catarina State (Southern Brazil) |journal=Journal of Petrology |date=1 February 1985 |volume=26 |issue=1 |pages=187–209 |doi=10.1093/petrology/26.1.187}}</ref>