A flood basalt is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Many flood basalts have been attributed to the onset of a hotspot reaching the surface of the earth via a mantle plume. Flood basalt provinces such as the Deccan Traps of India are often called traps, after the Swedish word trappa (meaning "stairs"), due to the characteristic stairstep geomorphology of many associated landscapes. Michael R. Rampino and Richard Stothers (1988) cited eleven distinct flood basalt episodes occurring in the past 250 million years, creating large volcanic provinces, lava plateaux, and mountain ranges. However, more have been recognized such as the large Ontong Java Plateau, and the Chilcotin Group, though the latter may be linked to the Columbia River Basalt Group. Large igneous provinces have been connected to five mass extinction events, and may be associated with bolide impacts.
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. Continental flood basalt provinces typically form on a timescale of 1 to 3 million years.
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 basaltic magma. 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.
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 Mesoproterozoic Mackenzie Large Igneous Province in Canada contains the Coppermine River flood basalts related to the Muskox layered intrusion. 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 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.
Subaerial flood basalts can be of two kinds :
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)
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 flood 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 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.
|Name||Initial or peak activity
(in thousands of km2)
|Columbia River Basalt Group||17||160||174,300||Yellowstone Hotspot|
|Ethiopia-Yemen Continental Flood Basalts||31||600||350,000|
|North Atlantic Igneous Province (NAIP)||56 (phase 2)||1300||6,600,000||Paleocene-Eocene Thermal Maximum|
|Deccan Traps||66||1500||3,000,000||Cretaceous-Paleogene extinction event|
|Caribbean large igneous province||95 (main phase)||2000||4,000,000||Cenomanian-Turonian boundary event (OAE 2)|
|Kerguelen Plateau||119||1200||Aptian extinction|
|Ontong-Java Plateau||120 (phase 1)||2000||80,000,000||Selli event (OAE 1a)|
|High Arctic Large Igneous Province (HALIP)||120-130||1000||Selli event (OAE 1a)|
|Paraná and Etendeka Traps||132||1500||2,300,000|
|Karoo and Ferrar Provinces||183||3000||2,500,000||Toarcian turnover |
|Central Atlantic Magmatic Province||201||11000||~2-3 × 106||Triassic-Jurassic extinction event|
|Siberian Traps||251||7000||4,000,000||Permian-Triassic extinction event|
|Emeishan Traps||265||250||300,000||End-Capitanian extinction event|
|Viluy Traps||373||320||Late Devonian extinction|
|Southern Oklahoma Aulacogen||540||40||250,000||End-Ediacaran event|
|Mackenzie Large Igneous Province||1270||2700||500,000|
In historic times, Eldgjá in 939 was in one of the largest (18 km3 on 800 km2)