A flood basalt or trap 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 continental scales (large igneous provinces) in prehistory, creating great plateaus and mountain ranges. Flood basalts have erupted at random intervals throughout geological 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 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 maria on the Moon are additional, even more extensive, examples of 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 obtain a partial fusion as large as that of the traps, 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.
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), plagioclases, opaque crystals such as titanomagnetite or ilmenite, and occasionally some olivine. Sometimes more differentiated volcanic products such andesites, dacites and rhyodacites have been observed, but only in small quantities at the top of former magma chambers.
At depth, flows can crystallise more slowly, producing columnar jointing.
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 tholeiites with a silicon dioxide percentage close to 50%.
Two kinds of basaltic floods basalts can be distinguished :
The isotopic ratios 87Sr/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 LPT type has an excess of elements from the crust such as potassium and strontium.
The content in incompatible elements of basaltic floods is lower than that of ocean island basalts, but higher than that of mid-ocean ridge basalts.