P/Tr Volcanism

The Siberian Traps were the largest volcanic eruption in Earth history and they occurred right at the same time as the largest extinction event in Earth history. Co-incidence? It is ingrained in everyone from an early age that volcanic eruptions are dangerous to life so the Siberian Traps could indeed hold the key to explaining the Permo-Triassic extinction event.

What are they and how did they form?

The Siberian Traps are a large igneous province which were a result of a mantle plume. A mantle plume is a giant pulse of heat that rises towards the surface from the core/mantle boundary. Plumes are easily identified but not well understood and they are believed to be part of a cooling mechanism for the core. Whatever their cause a large amount of anomalously hot material rises to the surface and ponds below the earth's crust in a head which can be 1000's of km wide and 100's of km deep. This pond of basalt magma penetrates the crust through fissures pouring gigantic amounts of basalt onto the surface. Events like this are known as Flood Basalt eruptions and fortunately are very rare with only 8 have occurred in the last 250 million years.

Where are they and how big are they?

They are centred around the Siberian city of Tura and also encompass Yakutsk, Noril'sk and Irkutsk. Present coverage including associated pyroclastics is just under2 million square kilometres which is an area greater than that of Europe. Estimates of the original volume of the traps range from 1 million cubic km up to 4 million cubic km. According to P. B. Wignall in his 99 paper, "the distribution of lavas suggests that they do not constitute a single continuous province but rather the amalgamation of several subprovinces." The eruptions lasted at full intensity for about a million years which coincides with the extinction. The most accurate dating method available at the moment is Argon-Argon radiometric dating which still contains sufficient uncertainties to conclusively prove the exact timing.

Could they cause or add to a mass extinction event?

The largest eruption of the 20th century, Mt Pinatubo is tiny compared to the Siberian Traps but caused a 0.5 degree drop in global temperatures the year after it erupted. The largest eruption in historic memory occurred on Iceland in 1783-84 spewing out 12 cubic km of lava onto the island (the Siberian Traps erupted about 3 million cu km). The poisonous gases given out are recorded as killing most of the island's crops and foliage and lowering global temps by about 1 degree. If events this size can affect temperatures and large areas then the effects of a large scale flood basalt are incomprehensible.

How could the Siberian Traps cause global mass destruction?

The immediate area would be affected by such things as lava and pyroclastic flows but how does this affect the other side of the world? The real power of the Siberian Traps was the climate altering potential by the emission of ash and gases. The Siberian Traps are recognised as having a large proportion of pyroclastic deposits relative to other flood basalts. This indicates an explosive nature with much ash and gases being pumped into the atmosphere. All of this ash and gas has two main effects that, even though they are opposite to each other, act on differing time scales.

Initially sulfur aerosols and volcanic ash envelop the earth's atmosphere blocking out sunlight and sending surface temperatures plunging. Ash and sulphur aerosols can remain in the upper atmosphere for hundreds to thousands of years which would be enough to cause a significant glaciation. At the end of the Permian period the biggest ever drop in sea level in history occurred. Two scientists named Holser and Magaritz in 1987proposed that such a marine regression could be caused by a large scale glaciation.

The second major effect is the emission of greenhouse gases such as CO2, methane and also water vapour. Green house gases warm the climate by allowing sunlight to pass through, while heat reflected by the Earth itself cannot penetrate the atmosphere so is retained. Greenhouse gases stay in the atmosphere much longer so their climate changing effects can last for millions of years.

A third, minor effect is the destruction of the ozone layer caused by gas emissions. Chlorine and fluorine gases are emitted from almost all volcanic eruptions and these destroy the ozone layer. Without the ozone layer, harmful UV rays can kill organisms therefore contributing to a mass extinction.