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The estimated dates of the younger continental flood basalts compiled from recent sources are shown in Table 1. Several lines of evidence suggest that in most instances the greatest number of individual eruptions and the largest volumes of lava probably occurred within a million years or less. 

Large igneous provinces are thought to be caused by the arrival of a mantle plume in the Earth's outermost layer, the lithosphere. The plumes are proposed to be richer in lighter elements and hotter than the surrounding mantle. As they rise, magma (liquid rock) is generated by partial melting of the plume material. The magma is injected into the lithosphere and erupted onto the Earth's surface to form huge basalt lava flows. The first few million years of a newly arrived mantle plume seem to be the most fertile in terms of magma production and flood basalts are therefore formed in a very short period of geological time.The surface manifestations of mantle plumes are often called hotspots. 

Plumes are thought to originate very deep in the Earth - perhaps at the core-mantle boundary for the larger ones and at a depth of about 600 km deep for the smaller ones - but they seem to be related to the breakup of continents (rifting), so there is some influence from global plate tectonic processes. (A numerical simulation of the development of a mantle plume has been developed by Paul Tackley of UCLA). 

A 'snapshot' view of a theoretical situation involving several plumes in varying stages of development distributed around the globe is shown in the figure (left), kindly provided by Paul Tackley of UCLA. This may represent a typical pattern of plume distribution at any period of geological time.

The estimated dates of the younger continental flood basalts compiled from recent sources are shown in Table 1. Several lines of evidence suggest that in most instances the greatest number of individual eruptions and the largest volumes of lava probably occurred within a million years or less.

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Flood Basalts & Mass Extinctions

Every now and again in geology, as in any other science, evidence is obtained and presented that cannot easily be explained in terms of familiar processes or accepted ideas. Such a case was continental drift, proposed by Wegener in 1912, which languished as a theory for about 45 years because there was no logical explanation of HOW continents could move. The mechanism of seafloor spreading proposed in the late 1950's led to the development of the modern theory of plate tectonics, which provides an explanation for continental drift.

The time relationship between flood basalt province formation and mass extinctions of organisms is another example of a scientific "hard nut to crack."

Extinction events are increasingly seen as important factors in the history of life on Earth, and recent studies suggest catastrophic causes for at least some mass extinctions. Two catastrophic processes that have been invoked are (1) impacts of asteroids or comets and (2) large volcanic eruptions. The end-Cretaceous (Cretaceous/Tertiary or K/T boundary) mass extinction has been convincingly correlated with the impact of a 10-km diameter asteroid with the Earth about 65 million years ago. Evidence of similar impacts has been found at the times of several other extinction events.

Table 2 compares the LIP ages given in Table 1 with the estimated ages of stratigraphic boundaries involving significant biotic changes, dated according to the most recent geological time scale. In at least three cases (the Deccan, Newark, and Siberian flood basalts), a direct measure of correlation with major extinction events is possible. The probability that three major volcanic events that typically last ~1 Myr should occur within 1 Myr of major extinction events during the last 250 Myr (of which there are ~12) is about 10-4. Thinking about the ways in which these two types of global event might be causally linked is a worthy scientific challenge.