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showing multiple flood basalt flows of the Columbia River Basalt Group . The upper basalt is Roza Member, while the lower canyon exposes Frenchmen Springs Member basalt. ]] A flood basalt or trapp 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 . Flood basalts have occurred on Continent al scales ( Large Igneous Provinces ) in Prehistory , creating great Plateau s and Mountain Range s. Flood basalts have erupted at random intervals throughout history and are clear evidence that the Earth undergoes periods of enhanced activity rather than being in a uniform steady state. One explanation for flood basalts is that they are caused by the combination of Continental Rifting and its associated decompression melting in conjunction with a mantle plume also undergoing decompression melting, producing vast quantities of a Tholeiitic Basaltic Magma . These lavas have a very low Viscosity , which is why they 'flood' rather than form taller volcanoes. The Deccan Traps of central India , the Siberian Traps and the Columbia River Plateau of the western United States are three regions covered by prehistoric flood basalts. The two largest flood basalt events in historic time have been at Eldgjá and Lakagigar , both in Iceland . The Maria on the Moon are additional, even more extensive, examples of flood basalts. Flood basalts on the ocean floor produce Oceanic Plateau s. The surface covered by one eruption can vary from around 200,000 km&2 ( Karoo ) to 1,500,000 km&2 ( Siberia ). The thickness can vary from 2000 m ( Deccan ) to 12,000 m ( Lake Superior ). These estimates are smaller than the original volumes due to erosion. Flood basalts originate at between 100 and 400 km depth in the Asthenosphere . To obtain a partial fusion as large as that of the trapps, expelling huge quantities of lava, it is necessary to have a large heat input. Such fusion can take place near a Hotspot , resulting in a mixture of magma from the depths of the hotspot with superficial magma produced by a Mantle Plume . PETROGRAPHY basalt ]] Flood basalts have tholeiite and olivine compositions (according to the classification of Yoder and Tilley). The composition of the basalts from the Paraná is fairly typical of that of flood basalts; it contains phenocrysts occupying around 25% of the volume of rock, trapped in volcanic glass. These phenocrysts are Pyroxenes ( Augite and Pigeonite ), Plagioclase s, opaque crystals such as Titanomagnetite or Ilmenite , and occasionally some olivine. Sometimes more differentiated volcanic products such Andesite s, Dacite s and Rhyodacite s have been observed, but only in small quantities at the top of former magma chambers. STRUCTURES Subaerial flood basalts can be of two kinds :
In the Massif Central in Auvergne , France , there is a good example of chaotic basalt floods, produced by eruptions from Puy De La Vache and Puy De Lassolas . At depth, flows can crystallise more slowly, producing Columnar Jointing . GEOCHEMISTRY Geochemical analysis of the major oxides reveals a composition close to that of mid-ocean ridge basalts (MORB) but also close to that of ocean island basalts (OIB). These are in fact Tholeiite s with a Silicon Dioxide percentage close to 50%. Two kinds of basaltic floods basalts can be distinguished :
The isotopic ratios Sr/Sr and Pb/Pb are different from that observed in general, which shows that the basalt flood magma was contaminated as it passed through the continental crust. It is this contamination that explains the difference between the two kinds of basalt mentioned above. The LPT type has an excess of elements from the crust such as potassium and strontium. The content in Incompatible Element s of basaltic floods is lower than that of ocean island basalts, but higher than that of mid-ocean ridge basalts. OTHER OCCURRENCES AND IMPLICATIONS
See Also: Geology of Venus#Volcanoes
LIST OF VOLCANIC FLOOD BASALTS All major continental flood basalts (also known as ''traps'') and oceanic plateaus, together forming a listing of Large Igneous Provinces , which is provided below. The listing ranges from the smallest Columbia flood basalts to the largest, although not yet well characterized remnants of a possible trap in eastern SiberiaSur l'âge des trapps basaltiques (On the ages of flood basalt events); Vincent E. Courtillota & Paul R. Renneb; Comptes Rendus Geoscience; Vol: 335 Issue: 1, January, 2003; pp: 113-140 : # The Columbia-Snake River flood basalts (see Columbia River Basalt Group ) # The Ethiopian And Yemen Traps in the Ethiopian Highlands # The Brito-Arctic Province # The Deccan Traps (India) 65 million years ago (end of Cretaceous Period) # The Caribbean Large Igneous Province # The Kerguelen Plateau # The Ontong Java–Manihiki–Hikurangi Plateau 1 Summary . # The Paraná And Etendeka Traps (Brazil-Namibia) # The Karoo And Ferrar Provinces (South Africa-Antarctica) # The Central Atlantic Magmatic Province # The Siberian Traps (Russia) 251 million years ago (end of Permian) # The Emeishan (western China) # The Viluy Traps # Pre-Devonian Traps SEE ALSO REFERENCES |
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