Big, explosive eruptions can send huge quantities of gas and fine debris into the atmosphere. The bigger ones have sent debris high into the stratosphere where it spreads around the globe and might remain for months or even years. The debris has the ability to filter out a portion of the sun's energy and to lower the earth's temperature. The most notable cold spell linked to a volcano was the one that followed the eruption of Mount Tambora in Indonesia in 1815. Many scientists attribute the abnormally cold spring and summer of 1816 to the clouds of volcanic debris put into the atmosphere by Mount Tambora. There were other less dramatic effects from Kakatoa in 1883 and Mount Agung in 1963. However, it wasn't until the eruptions of Mount St. Helens in the state of Washington in 1980 and El Chichon in Mexico in 1982 that scientists really got to study the volcanic blasts with the use of sophisticated satellites and remote sensing instruments. Prior to 1980, Mount St. Helens, Washington was a steep conical volcanic peak that rose 9,680 feet, had a snow-capped summit and even a few small glaciers. But on the morning of May 18th, 1980, the entire north side of the summit came down - about half a cubic mile of rock and ice. An instant later and enormous explosion of expanding steam and volcanic gas rocked the countryside. The gases formed a ground-hugging black cloud filled with hot, dense debris that raced over four major ridges and valleys up to 17 miles from the volcanic summit. For the first couple of miles from the summit, everything was obliterated. All that could be seen was a blanket of ash. A horseshoe-shaped crater 1.2 miles across and roughly 2460 feet deep replaced the peak. But the most impressive thing was the blowdown zone where huge virgin Douglas firs were snapped like matchsticks and lay on their sides, covered with ash. The U.S. Forest Service estimated that 10 million trees were felled by the blast. When Mount St. Helens Erupted, there was instant speculation that it could have major effects on climate. For a time the large volume of volcanic ash that it emitted had significant effects both locally and regionally. But worldwide cooling was less than 0.2 degrees Fahrenheit. However, studies following the eruption of El Chichon showed a worldwide cooling effect on the order of .5 to .9 degrees Fahrenheit. Why, if it was less explosive than Mount St. Helens, did it have a greater impact on global temperatures? The reason is that the material emitted by Mount St. Helens was fine ash that settled out relatively quickly. El Chichon, on the other hand, spewed an estimated 40 times more sulfur-rich gases than Mount St. Helens. These clouds combined with moisture in the stratosphere to produce dense clouds of sulfur acid droplets, which both absorbed and reflected solar radiation. Explosiveness alone is a poor instigator of climatic change. For volcanism to impact the climate, you would have to have a whole bunch of volcanoes going off over a relatively short period of time. Such an idea was once floated as the reason for the ice ages, but scientists have other ideas today. |
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