Flood basalt
A flood basalt is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Many flood basalts have been attributed to the onset of a hotspot reaching the surface of the Earth via a mantle plume. Flood basalt provinces such as the Deccan Traps of India are often called traps, after the Swedish word trappa, due to the characteristic stairstep geomorphology of many associated landscapes.
Michael R. Rampino and Richard Stothers cited eleven distinct flood basalt episodes occurring in the past 250 million years, creating large igneous provinces, lava plateaus, and mountain ranges. However, more have been recognized such as the large Ontong Java Plateau, and the Chilcotin Group, though the latter may be linked to the Columbia River Basalt Group.
Large igneous provinces have been connected to five mass extinction events, and may be associated with bolide impacts.
Description
Flood basalts are the most voluminous of all extrusive igneous rocks, forming enormous deposits of basaltic rock found throughout the geologic record. They are a highly distinctive form of intraplate volcanism, 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. This highly fluid basalt lava can spread laterally for hundreds of kilometers from its source vents, covering areas of tens of thousands of square kilometers. 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.These vast accumulations of flood basalt constitute large igneous provinces. 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.
The great thickness of the basalt accumulations, often in excess of, usually reflects a very large number of thin flows, varying in thickness from meters to tens of meters, or more rarely to. 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 thick. This flow may have been part of a lava lake the size of Lake Superior.
Deep erosion of flood basalts exposes vast numbers of parallel dikes that fed the eruptions. Some individual dikes in the Columbia River Plateau are over long. In some cases, erosion exposes radial sets of dikes with diameters of several thousand kilometers. Sills may also be present beneath flood basalts, such as the Palisades Sill of New Jersey, US. The sheet intrusions 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.
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 thick.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 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.
Petrology
At still smaller scales, the texture of flood basalts is aphanitic, consisting of tiny interlocking crystals. These interlocking crystals give trap rock its tremendous toughness and durability. 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. 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 dikes that fed lava to the surface.Flood basalts are most often quartz tholeiites. Olivine tholeiite occurs less commonly, and there are rare cases of alkali basalts. Regardless of composition, the flows are very homogeneous and rarely contain xenoliths, fragments of the surrounding 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. Occasionally, flood basalts are associated with very small volumes of dacite or rhyolite, which forms late in the development of a large igneous province and marks a shift to more centralized volcanism.
Geochemistry
Flood basalts show a considerable degree of chemical uniformity across geologic time, being mostly iron-rich tholeiitic basalts. Their major element chemistry is similar to mid-ocean ridge basalts, while their trace element chemistry, particularly of the rare earth elements, resembles that of ocean island basalt. They typically have a silica content of around 52%. The magnesium number is around 55, versus 60 for a typical MORB. The rare earth elements show abundance patterns suggesting that the original 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. In other words, the flood basalt is moderately evolved. However, only small amounts of plagioclase appear to have crystallized out of the melt.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 and a high phosphorus and titanium group. The difference has been attributed to inhomogeneity in the upper mantle, but strontium isotope ratios suggest the difference may arise from the LPT magma being contaminated with a greater amount of melted crust.
Formation
Theories of the formation of flood basalts must explain how such vast amounts of magma could be generated and erupted as lava in such short intervals of time. They must also explain the similar compositions and tectonic settings of flood basalts erupted across geologic time and the ability of flood basalt lava to travel such great distances from the eruptive fissures before solidifying.Generation of melt
A tremendous amount of heat is required for so much magma to be generated in so short a time. This is widely believed to have been supplied by a mantle plume impinging on the base of the Earth's lithosphere, its rigid outermost shell. The plume consists of unusually hot mantle rock of the asthenosphere, the ductile layer just below the lithosphere, that creeps upwards from deeper in the Earth's interior. The hot asthenosphere rifts the lithosphere above the plume, allowing magma produced by decompressional melting of the plume head to find pathways to the surface.The swarms of parallel dikes exposed by deep erosion of flood basalts show that considerable crustal extension has taken place. The dike swarms of west Scotland and Iceland show extension of up to 5%. Many flood basalts are associated with rift valleys, are located on passive continental plate margins, or extend into aulacogens Flood basalts on continents are often aligned with hotspot volcanism in ocean basins. The Paraná and Etendeka traps, located in South America and Africa on opposite sides of the Atlantic Ocean, formed around 125 million years ago as the South Atlantic opened, while a second set of smaller flood basalts formed near the Triassic-Jurassic boundary in eastern North America as the North Atlantic opened. However, the North Atlantic flood basalts are not connected with any hot spot traces, but seem to have been evenly distributed along the entire divergent boundary.
Flood basalts are often interbedded with sediments, typically red beds. The deposition of sediments begins before the first flood basalt eruptions, so that subsidence and crustal thinning are precursors to flood basalt activity. The surface continues to subside as basalt erupt, so that the older beds are often found below sea level. Basalt strata at depth have been found by reflection seismology along passive continental margins.