JOHN MEEHAN / SCIENCE PHOTO LIBRARY. JOHN MEEHAN / SCIENCE PHOTO LIBRARY.
First eruption of Mt St Helens volcano, on 18th May 1980 at 8.32am, Washington State, USA. Mt St Helens forms part of the Cascade range, an almost unbroken chain of geologically young volcanoes running from southern British Columbia, Canada, to northern California, USA. The range has built up as a result of plate tectonics; the submergence of the Pacific Ocean plate under the North American continent. The initial blast was horizontal, followed by a vertical explosion, which surged upward for 12 miles into the atmosphere. The vertical expulsion of material, called a Plinian column (see here), consists mainly of ash, gas & pulverised rock.
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