圣海伦斯火山18年后再次喷发

Mount St. Helens shoots out more steam 

        美国西北部、西雅图市以南大约150公里处的圣海伦斯火山平静了18年以后10月1日再次爆发。

        爆发前，地质勘探局连续几天监测到数千次地震活动。

        爆发时，火山喷射出白色蒸汽和火山灰，在空中形成庞大的烟柱。蒸汽升腾处，是火山口内305米高熔岩喷口的南侧边缘。

        这次爆发持续了大约20分钟。随后，山体复归平静，烟柱逐渐消散。事先听闻火山即将爆发，一些人赶到当地观看。

该火山杀伤力最强的一次爆发发生在1980年。当时，圣海伦斯火山如今相距海平面2549米的顶峰喷射出熔岩，致使57人丧生，火山灰散播到了400多公里外。

美国国家气象局向航空部门通报了这次火山活动，以便民航飞机改道绕行。

Mount St. Helens vented a new column of steam October 10, a lazy plume(烟柱) that rose out of the crater of the snow-dusted volcano.

The billow of steam(翻滚的蒸汽) rose from an area where a large upwelling or bulge of rock has been growing on the dome-shaped formation of rock in the crater(火山口的圆顶状岩石). The plume rose several hundred feet above the 8,364-foot volcano, and light wind slowly blew it toward the south and southeast.

The venting reminded scientists of the volcano's activity 20 years ago, when it built the dome following its catastrophic 1980 eruption.

"It's a view very, very reminiscent(令人回想的) of the years in the 1980s during dome-building and a few years after when the system(整座火山) was hot and water was being heated and vapor was rising and steam clouds were forming," said Willie Scott, a geologist with the U.S. Geological Survey.

The plume appeared to be mostly steam, and scientists said any volcanic ash that was included was probably from past eruptions during the 1980s.

Scott described the emission as a "very lazy conductive rise(缓缓升起) of this warm, moist air," unlike previous weeks' bursts characterized by more vigorous jetting that threw up ash, large pieces of rock and glacier ice(冰川的冰块).

The steam emission followed an increase in earthquake activity over the previous two days, with quakes of magnitude 2.4(里氏2.4级) occurring every two minutes until Sunday, when the vibrations were more frequent but weakened to magnitude 1 or less.

"What has been peculiar about these earthquakes is that there seems to be a disproportionate number of them that are uniform in size(这些地震的特别之处在于它们在规模上一致，数量上却不成比例)," said seismologist Tony Qamar at the University of Washington's seismic lab in Seattle.

It indicates that pressure in the system is very uniform, which may suggest magma is constantly moving upward, he said. "The pressure will build up, the rock will break, and then you'll get an earthquake," Qamar said.

Geologists do not anticipate anything similar to the May 18, 1980, blast that killed 57 people, blew 1,300 feet off the top of the peak.