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The Effect of Volcanism:

 

One possible cause of the end Triassic mass extinction is an enormous rifting event. This was, perhaps, the largest igneous event in Earth history. At the time, many of the present day continents were still joined as one continent, known as Pangea, Across this continent  large rift zones were developing. The 6000 km diameter Central Atlantic Magmatic Province had began to form and could be seen stretching from east to west coasts through what is now known as northwest Africa, eastern America (both north and south) and parts of Europe. The volcanism would have erupted massive floods of basaltic lavas and caused the release of large amounts of gas.

Location of the Central Atlantic Magmatic Province

 (adapted from Olsen 1999)

 

 

Volcanoes release a range of gases into the stratosphere of which SO2 and CO2 are volumetrically the most important (Devine, J.D. Sigurdsson, H. Davis, A.N. (1984)). Sulphur dioxide (SO2) produces an initial greenhouse warming effect but reacts rapidly with water in the atmosphere to produce sulphate aerosols that backscatter and absorb the sun’s radiation (Devine, J.D. Sigurdsson, H. Davis, A.N. (1984)). The other climatic consequence of SO2 eruptions is acid rain which may have led to poisoning in certain cases. We have seen that the end Triassic marks the beginning of a warm period for the Earth and hence, SO2 is unlikely to have contributed to  the extinction. Especially as the effects only normally last a couple of years.

The other main gas is carbon dioxide (CO2), which cannot be removed so rapidly.

Carbon dioxide is also a greenhouse gas and so, can produce large increases in climate temperatures by the trapping of Ultra Violet radiation (UV), which enters the earths atmosphere as short wave radiation, and leaves as a long wave form, which is reflected by CO2 in the upper atmosphere.  This process stops heat escaping from the earth and into space in the same way as a greenhouse traps heat under the glass.

Graph showing the release of CO through the different geological periods.

(Effects of these can be seen in ‘Climate’ section.)

 

            Single volcanic events are unlikely to have emitted enough CO2 to really change the atmosphere in a way that may have caused an extinction. However, due to the long residence time of CO2 in the surficial system, the cumulative effects of successive large eruptions, typical of those encountered in Flood Basalt Provinces, could be climatically significant (Wignall, 2001). And so, we must look at Large Igneous Provinces (LIP) as a possible cause of extinction.

 

            At the End of the Triassic/ Early Jurassic a major rifting event, maybe the largest on Earth ever, was beginning as the continent of Pangea slowly began to break up. One of the largest LIPs’ of all time, the 6000 km diameter Central American Magmatic Province (CAMP) had began to erupt huge amounts (volume of at least 2*106 (2,000,000) cubic kilometers) of basalt lavas. Scattered outcrops of the rocks can now be found located around the periphery of the central Atlantic region, for example the flood basalts of the Newark Basin, in the NE of the United States. These enormous volcanic provinces are thought to be the result of decompressive melting associated with rifting.

 

The Carbon dioxide released through this volcanicity would have yet been insufficient to alter the atmosphere in a way that would have caused an extinction. It is now believed that the warming effects of the CO2 release may have triggered the dissociation of massive amounts of methane hydrates, and thereby produced a d13 C excursion and exacerbated the warming trend (Wignall, 2001)

 

            Two effects of warming, the decline of the equator-to-pole temperature gradient and consequent decrease in oceanic circulation, together with the solubility of oxygen in warmer waters, may have been responsible for marine anoxia (Wignall and Twitchett, 1996).

 

            Curiously, the onset of eruptions during end-Triassic extinction/Central Atlantic volcanism slightly post-dates the main phase of extinction. Violence of the eruptions also seems not to play a part in the extent of extinction. Six out of 11 provinces coincide with episodes of global warming and marine anoxia/dysoxia, a relationship that suggests that volcanic CO2 emissions may indeed play an important role in this extinction.

Extinction rates shown from 'shelly' marine invertebrate fossils (based on Sepkoski (1997), related to continental flood basalt events.

 

            The only substantial example of a basalt fissure eruption in historical times occurred at Laki in Iceland between 1783 and 1784. The local effects of this were devastating yet this fissure was tiny in comparison to the CAMP event.

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