In addition to clouds of ash that rise above a volcano, some explosive eruptions produce avalanches(雪崩)
of hot ash, rock and gas that move at high speeds down the slopes of a volcano. Ash flows are the most destructive type of volcanic activity. They are extremely hot and can move across the ground as fast as several hundred kilometres per hour. Ash flows knock down and burn everything in their path. Few living things can survive an ash flow. The eruption of Mount St. Helens in 1980 generated(产生)an extremely large ash flow that completely devastated(毁坏)an area of 600 square kilometres.
Close to Mount St. Helens forests were striped(剥去,除掉)completely from hills. Trees two metres in diameter(直径)were mowed down like blades(草叶)of grass twenty five kilometres from the volcano. Logging equipment was tossed(扔,掷)about like toys. Everything in the path of the ash flow was destroyed. Cars not mangled(损坏)by the ash flow were buried with rock
debris(碎片). Ash flows are difficult to observe because they are extremely dangerous and move quickly. An ash flow cannot be outrun. As an ash flow moves down the side of a volcano, it usually pours into canyons and river valleys, but at the bottom of the volcano it can spread out across the land into populated areas. (International Association of Volcanology and Chemistry of Earth's Interior)
The slopes of Smith Creek valley, east of Mount St. Helens, show trees blown down by the lateral(横的)blast(爆炸的冲击波). Two U. S. Geological Survey scientists (lower right) give
scale(测量). The direction of the blast, shown here from left to right, is apparent(明显的) in the alignment(排列成行)of the downed trees. Over four billion board feet of usable
timber(木材), enough to build 150,000 homes, was damaged or destroyed. (September 24, 1980, by Lyn Topinka, USGS/CVO)
Mount St. Helens from Spirit Lake, NE shore
Logs(园木)and branches, thrown into Spirit Lake by the 1980 blast, are still afloat, much as they were right after the eruption. In May 1985, a tunnel was opened connecting Spirit Lake and South Coldwater Creek in order to lower the lake level and to prevent flooding downriver. I had great fun taking the photos for this
panorama(全景): I managed to set up the tripod(三脚架)on a huge, floating log
which, fortunately did not roll over. Walking on the floating logs, however, is not generally recommended ;-)
On May 18, 1980, at 8:32 a.m. Pacific Daylight Time, a magnitude 5.1 earthquake shook Mount St. Helens. The bulge(凸出)and surrounding area slid away in a gigantic(巨大的)rockslide(岩滑动)and debris
avalanche(崩), releasing pressure, and triggering(触发)a major pumice(浮石)and ash eruption of the volcano. Thirteen-hundred feet (400 meters) of the peak(山顶)collapsed(崩溃)or blew outwards. As a result, 24 square miles (62 square kilometers) of valley was filled by a debris avalanche, 250 square miles (650 square kilometers) of
recreation(消遣), timber, and private lands were damaged by a lateral blast, and an estimated 200 million cubic(立方)yards (150 million cubic meters) of material was deposited(堆积)directly by lahars (volcanic mudflows) into the river channels. Fifty-seven people were killed or are still missing. (May 18, 1980, by Austin Post,
USGS)
