Flood basalts have covered areas as large as a continent in prehistory, creating great plateaus and mountain ranges. Flood basalts have erupted at random intervals throughout Earth 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 melting, in conjunction with a mantle plume also undergoing decompression melting, producing vast quantities of a basaltic magma. These 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 western North America 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 largest and best-preserved continental flood basalt terrain on Earth is part of the Mackenzie Large Igneous Province in Canada. The maria on the Moon are additional, even more extensive, flood basalts. Flood basalts on the ocean floor produce oceanic plateaus.
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 originate at between 100 and 400 km depth, in the asthenosphere. To get a partial melting as large as that of the traps, expelling huge quantities of lava, it is necessary to have a large heat input. Such melting 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.
References[change | edit source]
- Mahoney JJ and Coffin MF. Large Igneous Provinces: continental, oceanic, and planetary flood volcanism. Geophysical Monograph 100. Washington, DC: American Geophysical Union. http://myweb.uiowa.edu/dpeate/downloads/Parana-review.pdf.
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