The environmental impact of the eruption of large volumes of basalt can be severe. Huge volumes of sulfur dioxide, carbon dioxide, chlorine, and fluorine are released during large basaltic eruptions. Much of this gas may get injected into the upper troposphere and lower stratosphere during the eruption process, being released from eruption columns that reach two to eight miles (3-13 km) in height. Carbon dioxide is a greenhouse gas, and can cause global warming, whereas sulfur dioxide and hydrogen sulfate have the opposite effect and can cause short-term cooling. Many of the episodes of volcanism preserved in these large igneous provinces were rapid, repeatedly releasing enormous quantities of gases over periods of less than one million years, and released enough gas to significantly change the climate more rapidly than organisms could adapt to these changes. For instance, one eruption of the Colombia River basalts is estimated to have released 9 billion tons of sulfur dioxide, and thousands of millions of tons of other gases, compared to the eruption of Mount Pinatubo in 1991, which released about 20 million tons of sulfur dioxide.
The Colombia River basalts of the Pacific Northwest are instructive about how flood basalts can influence climate. These lavas continued erupting for years at a time, for approximately a million years. During this time the gases released would be equivalent to that of Mount Pinatubo every week over periods maintained for decades to thousands of years at a time. The atmospheric consequences are sobering. Sulfu-ric acid aerosols and acid from the fluorine and chlorine would form extensive poisonous acid rain, destroying habitats and making waters uninhabitable for some organisms. At the very least, the environmental consequences would be such that organisms were stressed to the point that they would not be able to handle an additional environmental stress, such as a global volcanic winter and subsequent warming caused by a giant impact.
Mass extinctions have been correlated with the eruption of the Dec-can flood basalts at the Cretaceous-Tertiary (K/T) boundary and with the Siberian flood basalts at the Permian-Triassic boundary. There is still considerable debate about the relative significance of flood basalt volcanism and impacts of meteorites for extinction events, particularly at the Cretaceous-Tertiary boundary. However, most scientists would now agree that global environment was stressed shortly before the K/T boundary by volcanic-induced climate change, and then a huge meteorite hit the Yucatan Peninsula, forming the Chicxulub impact crater, causing the massive K/T boundary extinction and the death of the dinosaurs.
The Siberian flood basalts cover a large area of the Central Siberian Plateau northwest of Lake Baikal. They are more than half a mile thick over an area of 210,000 square miles (543,900 km2) but have been significantly eroded from an estimated volume of 1,240,000 cubic miles (3,211,600 km3). They were erupted over an extraordinarily short period of less than 1 million years 250 million years ago, at the Permian-Triassic boundary. They are remarkably coincident in time with the major Permian-Triassic extinction, implying a causal link. The Permian-Triassic boundary at 250 million years ago marks the greatest extinction in Earth history, where 90 percent of marine species and 70 percent of terrestrial vertebrates became extinct. It has been postulated that the rapid volcanism and degassing released enough sulfur dioxide to cause a rapid global cooling, inducing a short ice age with associated rapid fall of sea level. Soon after the ice age took hold the effects of the carbon dioxide took over and the atmosphere heated, resulting in a global warming. The rapidly fluctuating climate postulated to have been caused by the volcanic gases is thought to have killed off many organisms, which were simply unable to cope with the wildly fluctuating climate extremes.
The close relationship between massive volcanism and changes in climate that have led to mass extinctions shows how quickly life on Earth can change. The effects of massive global volcanism are much larger than any changes so far caused by humans and operate faster than other plate tectonic and supercontinent related changes.
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