Great Volcano Eruptions in Earth History

Environmental Effects of Flood Basalt Eruptions

If there is a causal link between flood basalt events and mass extinctions, it may lie in the environmental impact of the gases released, because basalt eruptions are not particularly explosive. Several kinds of environmental effects have been suggested, including climatic cooling from sulphuric acid aerosols, greenhouse warming from CO2 and SO2 gases, and acid rain. Basaltic magmas are often very rich in dissolved sulphur, and sulphuric acid aerosols formed from sulphur volatiles (largely SO2) are injected into the stratosphere by convective plumes rising above volcanic vents and fissures.

Indirect environmental effects include changes in ocean chemistry, circulation, and oxygenation, especially from basaltic volcanism associated with large submarine oceanic plateaus that may represent flood basalt eruptions in an oceanic environment.

A major uncertainty is the nature and severity of the environmental effects of the eruptions and their potential impact on life. Although the correlation between some flood basalt episodes and extinctions may implicate volcanism in the extinctions, it is also possible that other factors lead to an apparent association. Flood basalt episodes have been attributed to mantle plume activity, and thus may represent one facet of a host of related global geological factors (eg, changes in sea-floor spreading rates, rifting events, increased tectonism and volcanism, sea-level variations) that tend to be correlated, and may be associated with unusual climatic and environmental fluctuations that could lead to significant faunal changes. It has also been suggested that a coincidence of both a large impact and a flood basalt eruption might be necessary in causing severe mass extinctions.

USGS/Hawaiian Volcano Observatory Hawaii's eruptions are tiny compared to ancient flood basalts.

Missing Pieces of the Puzzle

How often do flood basalt lavas occur? It would be desirable to be able to obtain age dates on individual lavas in a flood basalt pile to determine how often these huge eruptions occurred, but the dating methods available at present do not have sufficent accuracy to pinpoint individual flows in time - virtually a whole flood basalt field fits within the errors on some of the dates.

Were ancient flood basalts associated with major outgassing? Due to their age and altered state, the amount and signature of the minor volatile elements in these old lavas will be difficult to obtain. It is possible to show whether the eruptions degassed vast amounts of sulphur and carbon dioxide if these species can be measured, as they can in some of the younger lavas such as those in the Columbia River province. More accurate modelling of dense atmospheric aerosol clouds, and their effects on atmospheric dynamics and chemistry, is needed before the likely climatic impact of events such as flood basalts can be properly estimated.