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Tuesday, 14th October 2003 Global impact of Volcanism on the Environment by Dr Steven Edwards Notes by R Coleman. |
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The Society extended a warm welcome to Dr Steven Edwards, senior lecturer, Earth and Environmental Sciences at Greenwich University. This was the first ever visit by Steve to our club and at least there were one or two he knew, so at least he wasn't amongst a total bunch of strangers. Steve stated that he was limiting his talk to the impact of volcanism to the Earth's atmosphere and was going to concentrate on the interaction of eruptions with the Troposphere and Stratosphere and put to one side the issues of sea and land. Firstly, Steve reviewed the varying types of volcanic magmas, their relative viscosity, and the varying types of eruptions that these cause. Rhyolites at the top of the scale being the most "sticky", were responsible for the most explosive eruptions whilst basalts being at the low viscosity end produced the more tame events. However, he reminded us that Basalt eruptions like those that produced the Decan Traps, which lasted several million years, may be considered to have had a major influence on the change of climate and the K/T mass extinction event. Sticky magmas don't allow gases to expand and dissolve, hence the gases remain at high pressure within the magma. When melt eventually breaks surface, the rapid expansion of gases is responsible for explosive events which give ejecta (including gases) sufficient velocity to reach into the stratosphere. Of the numerous volcanic events over the ages mentioned by Steve, we were shown a few slides of the Pinatubo eruption. These explosive events were described as looking similar to the nuclear mushroom cloud, and at the top, flattening and spreading as the gases balance in density with the atmosphere. Here in the stratosphere they are spread globally, absorbing energy from the Sun that would otherwise heat the Troposphere, the result being global cooling. Two gases mentioned were Sulphur Dioxide and Carbon Dioxide. One slide, taken from satellite, or perhaps space shuttle, highlighted the troposphere boundary, tropopause, and stratosphere. In the stratosphere there were two discernible rings of pollution said to be of sulphur dioxide. It should be said that reactions in the stratosphere and above are due to radiation energy from the sun, basically temperature increases with height within the Stratosphere. Troposphere "circulations" have different interactions fuelled by heating through "convection" from the Earth's surface, which in turn is heated by the Sun. It certainly started me thinking of what physics was going on to produce these boundaries and "rings or shells", something for later study. Click here for a picture on another website. This deals with the spread of Sulphur Dioxide and the Mt St Helens event, with a few pictures which might explain my uninformed comments above.. For the alternative case study, Steve described the recorded events of the eruption of Laki on Iceland, a basaltic eruption that went on for several months. Steve believes the unusually hot, humid summer in England for that year and the violent thunderstorms were an attribute to the amount of Sulphur Dioxide in the Troposphere. An effect caused by the absorption of the Sun's energy by the gases, similar to before, but with the gas being convected within the Troposphere. Strangely, the following nine years showed a trend of lower temperatures. Which everyone seems to attribute to this eruption but then how does the gas get into the stratosphere to cause this effect? Please bear in mind that the volumes of basalt and gases were enormous, which might indicate how relatively little needs to get into the Stratosphere to promote global cooling. Locally, to Laki, the ecosystem suffered from acid rain and other pollutants and a whole community was wiped out because it could not survive these effects. Click here for another website with more informed background on Laki.
It was a stimulating talk and much more was covered in detail than noted above. Steve did make mention of Professor Bill McGuire, who also spoke to us sometime ago, (Global Natural Catastrophes).
With reference to my comments above: Those in normal type are based on what I heard at the talk, those in italics are my own comments. My only hope is that I haven't misrepresented Steve's talk. Roger Coleman Member's Finds on Display. Sorry folks, no pictures as I forgot my camera.
All text copyright R.Coleman 2003 |