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Mount St. Helens vented a new column of steam
October 10, a lazy plume that
rose out of the crater of the snow-dusted volcano.
The billow of steam 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. 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
of the years in the 1980s during dome-building and a few
years after when the system 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 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.
The steam emission followed an increase in earthquake activity over the
previous two days, with quakes of magnitude 2.4 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," 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.
(Agencies) |
10月10日，一股蒸汽從圣海倫斯火山口噴發出來，這縷煙從白雪覆蓋的火山口緩緩升起。
火山口的圓頂狀巖石已經逐漸上涌或凸起，翻滾的蒸汽從這里升起。這座8364英尺高的火山上方升起高達幾百英尺的煙柱，微風把它緩緩吹向南方和東南方向。
這次噴射使科學家想起了圣海倫斯火山20年前的那次大爆發，自1980年那場災難性的火山爆發后，圣海倫斯火山口形成了現在的圓頂狀。
美國地質調查局的地質學家威利·斯科特說：“這很容易讓人回憶起上世紀80年代的情景，火山噴發后形成了圓頂狀火山口，在這之前的幾年中，整座火山都是滾燙的，水被加熱后不斷升起水蒸氣，形成云。這個過程持續了幾年，然后火山爆發了。”
科學家說，煙柱看上去主要是由水蒸汽組成的，其中含有的火山灰很可能是80年代火山爆發的殘余物。
斯科特把這次噴發形容為“一束緩緩升起的濕熱空氣”，不像前幾周的噴發那么有力。前幾周火山噴發時，從火山口噴射出大量的火山灰，巨大的巖石和冰川的冰塊。
在過去的兩天里，地震變得頻繁起來，隨之而來的就是水蒸氣的噴發。每隔兩分鐘就發生一次里氏2.4級的地震，直到周日，地震變得更頻繁了，但震級已降至1級或1級以下。
華盛頓大學西雅圖地震研究室的地震學家托尼·卡馬爾說：“這些地震的特別之處在于它們在規模上一致，數量上卻不成比例。”
這預示著火山內部的壓力非常均勻，這也就意味著巖漿在不斷上涌。卡馬爾說：“壓力在不斷增大，巖石開始迸裂，然后就發生地震了。”
地質學家不希望發生任何與1980年5月18日那場火山爆發同樣的情形。那次火山爆發導致57人死亡，整個圣海倫斯火山的高度降低了1300英尺。
（中國日報網站譯） |
Updated: 2004-10-12 10:24
