Large Igneous provinces (LIP) are
extremely large accumulations of igneous rocks, either or both intrusive and extrusive, found in the earths crust. The
term 'LIP' was originally proposed by Coffin and Eldholm (1992) to
refer to a variety of mafic
igneous provinces extending over areas greater than 100,000 square
kilometers (slightly larger than the area of Portugal), erupted
over extremely short geological time
intervals of a few million years or less, that originated by
processes not associated with 'normal' plate tectonics and seafloor
spreading[1].
The definition of 'LIP' has been expanded and refined, and is still a work in progress. 'LIP' is now frequently used to also describe voluminous areas of, not just mafic, but all types of igneous rocks. Sub-categorization of LIP's into Large Volcanic Provinces (LVP) and Large Plutonic Provinces (LPP), and including rocks produced by 'normal' plate tectonic processes, has been proposed.[2].
Mantle plumes, a recently apprehended and still poorly-understood and controversial geological process, are thought to be the source of many or all of the LIP's that are not associated with 'normal plate tectonics.'
Some LIP's are now intact, i.e. the basaltic Deccan Traps in India; others have been dismembered by plate tectonic motion, i.e. the basaltic Central Atlantic Magmatic Province (CAMP) parts of which are found in Brazil, the eastern United States and Canada, and northwestern Africa.
As originally defined LIPs include continental flood basalts, oceanic plateaus, large dike swarms (the eroded roots of a volcanic province), and volcanic rifted margins. Most of these LIPs consist of basalt, but some contain large volumes of associated rhyolite (i.e. the Columbia River Basalt Group in the western United States; the rhyolite is typically very dry compared to island arc rhyolites, with much higher eruption temperatures (850°C to 1000°C) than normal rhyolites. Some new definitions of the term 'LIP' include large granitic provinces such as those found in the Andes Mountains of South America and in western North America.
When created, LIPs often have an areal extent of a few million km² and volumes on the order of 1 million km3. In most cases, the majority of a basaltic LIP's volume is emplaced in less than 1 million years. One of the conundra of such LIPs origins is to understand how enormous volumes of basaltic magma are formed and erupted over such short time scales, with effusion rates up to an order of magnitude greater than mid-ocean ridge basalts.
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Large igneous provinces are often linked to active hotspots by linear chains of volcanic islands or volcanoes, leading to models that connect their origins to mantle plumes. In this hypothesis, mantle plumes consist of a bulbous head and a thin tail that feeds hot mantle into the head. When the rising plume head encounters the lithosphere, it spreads out and melts catastrophically to form large volumes of basalt magma in 1-2 million years. Subsequent volcanism originates with the plume tail. The movement of lithosphere across the surface of the Earth in response to plate tectonics causes the plume tail volcanics to form linear island chains. The impact of the plume on the base of continental lithosphere may cause rifting and breakup of the continent, creating conjugate LIPs on opposite sides of an ocean basin (e.g., the Parana-Etendeka pair of South America-Africa).
Alternate theories include delamination of eclogitic lower crust, edge effects of thick lithosphere, and meteorite impact (see Mantle plumes for more complete discussion of alternate models).
Eruptions or emplacements of LIP's appear to have, in some cases, occurred simultaneously with oceanic anoxic events and extinction events. The most important examples are the Deccan Traps (Cretaceous–Tertiary extinction event), the Karoo-Ferrar (Pliensbachian-Toarcian extinction), the Central Atlantic Magmatic Province (Triassic-Jurassic extinction event), and the Siberian traps (Permian-Triassic extinction event).
Several mechanism's are proposed to explain the association of LIP's with extinction events. The eruption of basaltic LIP's onto the earth's surface releases large volumes of sulfate gas, which forms sulfuric acid in the atmosphere; this absorbs heat and causes substantial cooling (e.g., the Laki eruption in Iceland, 1783). Oceanic LIP's can reduce oxygen in seawater by either direct oxidation reactions with metals in hydrothermal fluids or by causing algal blooms that consume large amounts of oxygen (Kerr, 2005).
These are well documented large igneous provinces in geological research.
Continental Flood basalts
Oceanic Plateaux
Volcanic Rifted Margins
Dike Swarms
Sills
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