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Flood Basalt
The result of a very large volume eruption of basalt lava
Moses Coulee in the US 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
The formation and effects of a flood basalt depend on a range of factors, such as continental configuration, latitude, volume, rate, duration of eruption, style and setting (continental vs. oceanic), the preexisting climate, and the biota resilience to change.[4]
One proposed 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 basalticmagma. These have a very low viscosity, which is why they 'flood' rather than form taller volcanoes. Another explanation is that they result from the release, over a short period, of melt that has accumulated in the mantle over a long period.[5]
The surface covered by one eruption can vary from around 200,000 km² (Karoo) to 1,500,000 km² (Siberian Traps). The thickness can vary from 2000 metres (Deccan Traps) to 12,000 m[] (Lake Superior). These are smaller than the original volumes due to erosion.
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 in a fine-grained matrix. These phenocrysts are pyroxenes (augite and pigeonite), plagioclases, opaque crystals such as titanium rich magnetite or ilmenite, and occasionally some olivine. Sometimes more differentiated volcanic products such as andesites, dacites and rhyodacites have been observed, but only in small quantities at the top of former magma chambers.
Structures
Subaerial flood basalts can be of two kinds :
with a smooth or twisted surface (P?hoehoe) : very compact surface; vesicles (gas bubbles) are rare. Degassing was easy (magma maintained at a high temperature and more fluid in a chamber of a size such that confining pressures did not confine gases to the melt before expulsion). Such lava flows may form underground rivers; when degassing fractures and conduits are present, very large flows may reach the surface.
with a chaotic surface (?A) : the basalt flood is very rich in bubbles of gas, with an irregular, fragmental surface. Degassing was difficult (less fluid magma expelled from a rift with no chance of progressive expansion in a hot chamber; the degassing took place closer to the surface where the flow forms a crust which cracks under the pressure of the gases in the flow itself and during more rapid cooling).
Large Igneous Provinces (LIPs) were originally defined as including voluminous outpourings, predominantly of basalt, over geologically very short durations. This definition did not specify minimum size, duration, petrogenesis, or setting. A new attempt to refine classification focuses on size and setting. LIPs characteristically cover large areas and the great bulk of the magmatisim occurs in about less than 1 Ma. Principal LIPs in the ocean basins include Oceanic Volcanic Plateaus (OPs) and Volcanic Passive Continental Margins. Oceanic flood basalts are LIPs distinguished from oceanic plateaus by some investigators because they do not form morphologic plateaus, being neither flat-topped nor elevated more than 200 m above the seafloor. Examples include the Caribbean, Nauru, East Mariana, and Pigafetta provinces. Continental flood basalts (CFBs) or plateau basalts are continental manifestations, or traps referencing the step-like geomorphology of eroded flow layers. (e.g. Deccan Traps and Siberian Traps)[6]
those rich in P2O5 and in TiO2, called high phosphorus and titanium
The isotopic ratios87Sr/86Sr and 206Pb/204Pb 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 low phosphorus and titanium type has an excess of elements from the crust such as potassium and strontium.
The content in incompatible elements of flood basalts is lower than that of ocean island basalts, but higher than that of mid-ocean ridge basalts.
Basalt floods on the planet Venus are larger than those on Earth.
List of flood basalts
Representative continental flood basalts (also known as traps) and oceanic plateaus, arranged by chronological order, together forming a listing of large igneous provinces:[7]
^Neal, C.; Mahoney, J.; Kroenke, L. (1997). "The Ontong Java Plateau"(PDF). Large Igneous Provinces: Continental, Oceanic, and Planetary Flood Volcanism, Geophysical Monograph 100. Archived from the original(PDF) on 2017-01-01.
^Negi, J. G.; Agrawal, P. K.; Pandey, O. P.; Singh, A. P. (1993). "A possible K-T boundary bolide impact site offshore near Bombay and triggering of rapid Deccan volcanism". Physics of the Earth and Planetary Interiors. 76 (3-4): 189. Bibcode:1993PEPI...76..189N. doi:10.1016/0031-9201(93)90011-W.
^Winter, John (2010). Principles of Igneous and Metamorphic Petrology (2nd ed.). New York: Prentice Hall. pp. 301-302. ISBN9780321592576.
^Sur 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
^M.A. Richards, R.A. Duncan, V.E. Courtillot; Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails; SCIENCE, VOL. 246 (1989) 103-108
^Barbara P. Nash, Michael E. Perkins, John N. Christensen, Der-Chuen Lee, & A.N. Halliday; The Yellowstone hotspot in space and time: Nd and Hf isotopes in silicic magmas; Earth and Planetary Science Letters 247 (2006) 143-156
^Wallace, P. J.; Frey, F. A.; Weis, D.; Coffin, M. F. (2002). "Origin and Evolution of the Kerguelen Plateau, Broken Ridge and Kerguelen Archipelago: Editorial". Journal of Petrology. 43 (7): 1105-1108. Bibcode:2002JPet...43.1105W. doi:10.1093/petrology/43.7.1105.
^J, Ricci; et al. (2013). "New 40Ar/39Ar and K-Ar ages of the Viluy traps (Eastern Siberia): Further evidence for a relationship with the Frasnian-Famennian mass extinction". Palaeogeography, Palaeoclimatology, Palaeoecology. 386: 531-540. doi:10.1016/j.palaeo.2013.06.020.
^Brueseke, Matthew E.; Hobbs, Jasper M.; Bulen, Casey L.; Mertzman, Stanley A.; Puckett, Robert E.; Walker, J. Douglas; Feldman, Josh (2016-09-01). "Cambrian intermediate-mafic magmatism along the Laurentian margin: Evidence for flood basalt volcanism from well cuttings in the Southern Oklahoma Aulacogen (U.S.A.)". Lithos. 260: 164-177. Bibcode:2016Litho.260..164B. doi:10.1016/j.lithos.2016.05.016.
White, R.S.; McKenzie, D.P. (1989). "Magmatism at rift zones: The generation of volcanic continental margins and flood basalts". J. Geophys. Res. 94: 7685-7729. Bibcode:1989JGR....94.7685W. doi:10.1029/jb094ib06p07685.