Flood basalt
Many flood basalts have been attributed to the onset of a hotspot reaching the surface of the Earth via a mantle plume.
Michael R. Rampino and Richard Stothers (1988) cited eleven distinct flood basalt episodes occurring in the past 250 million years, creating large igneous provinces, lava plateaus, and mountain ranges.
[13] Deep erosion of flood basalts exposes vast numbers of parallel dikes that fed the eruptions.
[18] Flood basalt commonly displays columnar jointing, formed as the rock cooled and contracted after solidifying from the lava.
This is generally perpendicular to the upper and lower surfaces, but rainwater infiltrating the rock unevenly can produce "cold fingers" of distorted columns.
The greater hydrostatic pressure, due to the weight of overlying rock, also contributes to making the lower columns larger.
By analogy with Greek temple architecture, the more regular lower columns are described as the colonnade and the more irregular upper fractures as the entablature of the individual flow.
These have a distinctive appearance likened to a clay tobacco pipe stem, particularly as the vesicle is usually subsequently filled with calcite or other light-colored minerals that contrast with the surrounding dark basalt.
[20] At still smaller scales, the texture of flood basalts is aphanitic, consisting of tiny interlocking crystals.
Olivine tholeiite (the characteristic rock of mid-ocean ridges[22]) occurs less commonly, and there are rare cases of alkali basalts.
[28] Theories of the formation of flood basalts must explain how such vast amounts of magma could be generated and erupted as lava in such short intervals of time.
[11] This is widely believed to have been supplied by a mantle plume impinging on the base of the Earth's lithosphere, its rigid outermost shell.
[29][30][15] The plume consists of unusually hot mantle rock of the asthenosphere, the ductile layer just below the lithosphere, that creeps upwards from deeper in the Earth's interior.
[32][17] The swarms of parallel dikes exposed by deep erosion of flood basalts show that considerable crustal extension has taken place.
[15][16] However, the North Atlantic flood basalts are not connected with any hot spot traces, but seem to have been evenly distributed along the entire divergent boundary.
[11] The surface continues to subside as basalt erupt, so that the older beds are often found below sea level.
[17] Basalt strata at depth (dipping reflectors) have been found by reflection seismology along passive continental margins.
The original melt formed in the upper mantle (the primitive melt) cannot have the composition of quartz tholeiite, the most common and typically least evolved volcanic rock of flood basalts, because quartz tholeiites are too rich in iron relative to magnesium to have formed in equilibrium with typical mantle rock.
One possibility is that a primitive melt stagnates when it reaches the mantle-crust boundary, where it is not buoyant enough to penetrate the lower-density crust rock.
This restores buoyancy and permits the magma to complete its journey to the surface, and also explains why flood basalts are predominantly quartz tholeiites.
Over half the original magma remains in the lower crust as cumulates in a system of dikes and sills.
The resorption (dissolution back into the melt) of a mixture of solid olivine, augite, and plagioclase—the high-temperature minerals likely to form as phenocrysts—may also tend to drive the composition closer to quartz tholeiite and help maintain buoyancy.
However, the lateral extent of individual flood basalt flows is astonishing even for so fluid a lava in such quantities.
[36] Studies of the Ginkgo flow of the Columbia River Plateau, which is 30 to 70 meters (98 to 230 ft) thick, show that the temperature of the lava dropped by just 20 °C (68 °F) over a distance of 500 kilometers (310 mi).
[26] Large Igneous Provinces (LIPs) were originally defined as voluminous outpourings, predominantly of basalt, over geologically very short durations.
The carbon dioxide produced extreme greenhouse conditions, with global average sea water temperatures peaking at 38 °C (100 °F), the highest ever seen in the geologic record.
[63] Individual eruptive episodes were likely similar in volume to flood basalts of Earth, but were separated by much longer quiescent intervals and were likely produced by different mechanisms.
