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Since the 1980's, volcanism has been the main alternative hypothesis for the K/T extinction event. Through the early 90's proponents of the volcanic hypothesis argued that all the evidence for an impact (iridium anomaly, shocked quartz, etc.) could more easily be explained by massive volcanic eruptions for which there was direct evidence in the form of flood basalts in India (Deccan Traps).
The discovery of the Chicxulub Impact Structure, and the determination of both its very large size and geologic age clearly ended the argument over whether there had been an impact at the end of the Cretaceous. The legacy of the volcanists is, however, a very strong one, with intense debate now centered on the contribution of the massive volcanic eruptions to not just the K/T extinction, but to many major extinction events, including the greatest biologic crisis in the history of the earth - the Permo-Triassic extinction event.
FLOOD BASALT ERUPTIONS
Flood Basalts are formed by a series of fissure eruptions (where lava is extruded from long fractures in the earth's crust as opposed to volcanoes) which occur over a long period of time. Theoretically, the massive amounts of basaltic magma for these eruptions are the result of plume of magma rising from a destabilized "D" layer found at the core/mantle boundary deep inside the earth (see figure below).
The magma rises towards the surface in the form of a plume (see figure below) which forms a "hot spot" on the earth's surface. Depending on the size of the plume, the hot spot can result in tremendous upward stress on the base of the crust forming long fractures which result in fissure eruptions, or can produce large single volcanoes such as the volcanoes which form the Hawaiian Island chain.
The end product of fissure eruptions over long periods of time is a lava plateau, a large flat area in which all topography has been eliminated because the low viscosity basaltic magma fills all the pre-existing valleys and depressions. In Washington State the Columbia River Plateau (15 million years old) is such a structure, formed by about sixty separate flows which total up to 2 kilometers thick in some areas.
As mentioned above, the Deccan flood basalts have been pointed to as an alternative candidate to the impact for the cause of the K/T extinction. Why? Because they possibly represent a forcing mechanism for dramatic climatic degradation, and also date around the K/T boundary (see figure below).
The Deccan Traps in India consist of over 2,000,000 cubic kilometers of basaltic lava flows, in some areas reaching a thickness of over 2,400 meters. Estimates of annual basalt production range from 2 to 8 km.3 per year. The amount of lava of and by itself is not the feature that scientists are interested in. It is the amounts of various gases given off by these eruptions that are of interest. The current estimate is that during the formation of the Deccan Traps, the following amounts of gas were given off:
33 trillion tons of CO2
6.6 trillion tons of SO2
66 billion tons of fluorine and chlorine
The next question is the amount of time. If old estimates that the Deccan Traps were erupted over 5 million years or so are correct, then the effects of these gases is minimized. However, recent paleo-magnetic work on the Deccan Traps suggests that they were erupted over the space of only 500,000 years. If this were the case, then the amount of greenhouse gases added to the atmosphere in the form of CO2 and sulfur aerosols, plus the amount of acid rain generated would have had severe impacts on the environment. One model predicts that as the gases built up there would be a drop in average global temperature of about 3 to 5 oC, followed by a long term global warming of roughly 5oC. This variation in climate over a relatively short period of time, in conjunction with acid rain, would have a major environmental impact.
But isn't this all moot since we've found the impact crater a Chicxulub? Maybe not. There may be a correlation between the occurrence of mass extinctions and the presence of flood basalts (see figure below).
It is important to note that while there is no confirmed evidence for an impact at the Permo-Triassic boundary (although there are some Wannabes), there is the largest flood basalt accumulation in history, know as the Siberian Traps. These flood basalts total over 3,000,000 km.3 of basalt, easily 33% more than the Deccan Traps. The exact length of time it took for the Siberian Traps to be extruded is still debated, but it did occur in conjunction with the great mass extinction at the end of the Permian.
The correlation of flood basalts versus mass extinctions through time would suggest that major environmental changes caused by what is in effect massive natural air pollution caused, or at least strongly contributed to the extinctions.
If we take the 33 trillion tons of CO2 which has been suggested as the total given off during the extrusion of the Deccan Flood Basalts, and average it out over 500,000 years, we come up with about 66million tons/year. This is equivalent to the amount of CO2 being produced yearly in the 1920's. A recent EPA estimate for the amount of CO2 currently being added to the atmosphere is 7 Gigatons. Think about it.