Monday, March 21, 2011
The Tambora eruption is estimated to have rated 7 (or super-colossal) on thevolc anic explosivity index (VEI), which runs from 0 to 8. By comparison, the 1981 Mt St. Helens eruption rated 5, and the Krakatoa eruption rated 6. Montgomery (p 110) quotes an estimate that the Krakatoa explosion was the equivalent of 100 million tons of dynamite. The Tambora eruption did not occur as a single event, but rather as a series of events which climaxed between April 10 and April 15. Most accounts give April 12 as the day the mountain blew its top off. The Tombora eruption was comfortably the largest explosion in reliably recorded history. It also threw more debris into the atmosphere than any other eruption in history. Tambora put 25 times more debris into the atmosphere than the 1980 Mount St. Helens eruption, and about 5 times as much as the 1991 Mount Pinatubo eruption. The eruption column reached a height of 28 miles.
EFFECTS ON THE IMMEDIATE AREA
Before Mount Tambora erupted, it is estimated to have been about 4,000m high. The eruption reduced it to 2851m, and left behind a caldera 7 km across. We must rely on contemporary reporting to chronicle most of the effects of the explosion. I have summarized the effects below. The concussions produced by the explosions are said to have been felt for 1,000 miles all around, and the sound was reported to be heard 1,700 miles distant from the volcano. In European settlements 200 and 750 miles away the sounds of the eruption were thought to be cannon fire, and ships and soldiers were despatched to investigate what were assumed o be attacks on nearby outposts. In Java, some 300 miles west the day was darkened by the ash, which fell several inches deep. Closer to ground zero, the rooves of houses 40 miles from the eruption collapsed under the weight of ash. All vegetation on several nearby islands died, significant masses of pumice were reported in the ocean off the western coasts of Sumatra, and the immediate area death toll was around 92,000. 10,000 died from the direct effect of the explosion, lava flows, falling debris and pyroclastic flows, with another 82,000 dying subsequently through starvation and disease. The eruption generated an hour long and extremely destructive "whirlwind", and the sea rose 12 feet above its high tide mark, causing widespread flood damage. It was 5 years before new vegetative growth returned to the area within 250 miles from the eruption.
CLIMATIC EFFECTS OF LARGE VOLCANIC ERUPTIONS
Large, explosive volvanic eruptions can punch huge amounts of gasses and dust high into the atmosphere. The gasses have a long term effect on atmospheric composition (and subsequent climate variations). However, volcanic dust is another story in the short term. Volcanic dust takes months to circulate the globe in the upper atmosphere, and years to settle out. While the dust is in the upper atmosphere it can have a profound effect on the short term global climate. The dust takes some months to circle the globe, and while it is in the upper atmosphere it is postulated that it filters out or blocks a portion of the incoming solar radiation. This in turn lowers the temperatures in the lower atmosphere (including that prevailing at the surface). Meanwhile, a temporary spike in sulfur rich gasses combines with water vapour to form more sulfuric acid droplets in the upper atmosphere. These droplets both absorb solar radiation, and scatter it back to space. Recently, better understanding of these processes, allied to better instrumentation, has meant that some direct correlations have been made between major explosive volcanic events and subsequent short term climatic "blips". To use a recent and well studied example, the 1991 eruption of Mt. Pinatubo is thought to have been responsible for a global temperature lowering of 0.5 deg C the following year. The delay in measureable effect is explained by the time it takes for such gasses and dust to spread around the globe. 0.5 deg C might not seem much, but in marginal agricultural environments it is enough to delay the arrival of spring thaws, enough to project killing frosts into the growing season, and enough to shorten the growing window.
VolcanoeGeo
6:14 AM
