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    Monday, March 17, 2008

     

    She Pours Out Her Essence to Create land masses - Large Igneous Provinces

    Large igneous province

    From Wikipedia, the free encyclopedia

    http://en.wikipedia.org/wiki/Large_Igneous_Province

    Large Igneous provinces (LIPS) were originally defined by Coffin and Eldholm (1992) as areas of Earth's surface that contain very large volumes of magmatic rocks (typically basalt but including rhyolites) erupted over extremely short geological time intervals of a few million years or less. These provinces are not associated with normal plate tectonic magmatism, ie, mid-ocean ridges and island arcs. LIPS include continental flood basalts, oceanic plateaus, large dike swarms (the eroded roots of a volcanic province), and volcanic rifted margins, recognized by the presence of dike swarms and "seaward dipping reflectors" -- seismically-imaged tabular features buried deep beneath sediment that lie parallel to a passive continental margin along the continental slope, and interpreted to represent lava flows or sills that formed during rifting of the continent). Most LIPS consist of basalt, but some contain large volumes of associated rhyolite; the rhyolite is typically very dry compared to island arc rhyolites, with much higher eruption temperatures (850º to 1000º C) than normal rhyolites.

    When created, LIPS often occupy a few million km² and have volumes on the order of 1 million km3. In most cases, the majority of a LIP's volume is emplaced in less than 1 million years. One of the conundra of 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.


    Theories of formation

    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).

    Relationship to extinction events

    Because a LIP may in several cases have occurred simultaneously with oceanic anoxic events and extinction events, it has been proposed that the volcanic byproducts of LIP formation may have had a profound and deleterious effect on the global environment, perhaps contributing to 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).

    LIPS have two impacts on environment that can cause extinction: first, they release large volumes of sulfate gas that forms sulfuric acid in the atmosphere; this absorbs heat and causes substantial cooling (e.g., the Laki eruption in Iceland, 1783). Second, oceanic LIPS 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).

    Examples of LIPs

    These are well documented large igneous provinces in geological research.

    Continental Flood basalts

    Oceanic Plateaux

    Volcanic Rifted Margins

    Dike Swarms

    See also

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