Harrow and Hillingdon Geological Society

Volcanic Cooling

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Currently the Earth is subject to global warming, whether man made or natural, but other factors may exacerbate or suppress it. Impacts from large meteors and volcanic eruptions can put huge quantities of rock dust and gasses into the atmosphere, thus reducing the solar radiation that reaches the Earth.

In the late 1970s and early 1980s the iridium content of the K/T boundary was found to be well above average for the Earth and it was felt that a large impact had caused this. It was believed that this resulted in climate change and ultimately the demise of the dinosaurs. However studies of effusive basaltic volcanoes have shown that where magma can tap deep Earth sources it too can emit large quantities of volatile iridium.

The Deccan Trapps, in India, were formed over millions of years. The individual lava flows have soil horizons between them showing that there was a considerable time gap between eruptions. The K/T boundary can be found between these lava flows, so the Deccan Trapps were being formed around the same time. The huge quantities of lava, erupted over a long period of time, must have had a considerable effect on the climate. It is now felt that both of these events had a bearing on the extinction of the dinosaurs, and many other species as well. The debate continues...

Explosive volcanoes emit huge quantities of rock dust right up into the stratosphere. This has an immediate effect of solar radiation but, as the dust is heavy it does fall back to Earth in a reasonably short time. The gasses emitted are water, carbon di-oxide and sulphur di-oxide. This last gas reacts with water in the presence of solar radiation to form sulphuric acid aerosols. The aerosols reflect a large amount of solar radiation and can remain up in the stratosphere for years.

Several pieces of evidence that volcanic eruptions can cause cooling of the climate can be found. In 1783 the Laki fissure opened in Iceland and basaltic magma flooded out, covering a massive area. The volcanic gasses appeared as a blue haze covering much of Europe and reaching as far as Eastern Russia and the United States. Benjamin Franklin suggested the haze was due to volcanic smog. It led to extreme weather; hot and humid with thunder storms and electric storms. There was a huge degassing of the lava, liberating HF, H2SO4 and HCl which killed vegetation. Food crops were contaminated and animals suffered from fluorosis. About 25% of the population died from famine and the eruption lasted for about 6 months.

In 1815, Tambora in Indonesia erupted. 1816 was described as the year without a summer. Mary Shelley, on holiday in the Alps, wrote Frankenstein during a very cool.

In 1991, Pinatubo in the Philippines, where the climate was warming, erupted and in the northern hemisphere there was a cooling of 0.5 of a degree C for several years.

75,000 years ago Toba erupted. When compared with Mt. Pinatubo it can be seen that this was a vast eruption and the effect must have been global.


75,000 years ago
Volume of tephra
10 km3
2,000 km3
20 mt
2,000 mt

This cooling interrupted global warming and enhanced the ice age. The world population of humans dropped to about 10,000 individuals and we nearly became extinct. Toba had a significant cooling effect for about 10 years.

These mega volcanoes are not all extinct. The next eruption of that size will probably be Yellowstone, which is even bigger than Toba; this year, next year, some time... who knows?

Ash from Pinatubo photographed in America

This photograph taken in Maine in July of 1991 shows how the ash and dust from the eruption of Mount Pinatubo had spread around the world in a month. This caused a measureable cooling of the earth for several years (Photograph by Jerome Wyckoff)

Satelite view of Yellostone This is a satellite image of the Yellowstone area of Wyoming. In the center of the image is Jackson Lake and the Snake River valley to its south-southwest bordered on the northwest by the snow-capped Teton Range. Yellowstone Lake is the large body of water in the upper right of the image. (Photograph by William Muehlberger)


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