|MadSci Network: Earth Sciences|
Mount St. Helens is a stratovolcano, and these volcanoes usually erupt explosively. A brief summary of their characteristics can be found here: http://volcanoes.usgs.gov/Products/Pglossary/stratovolcano.html That pamphlet, from the US Geological Survey, mentions that some stratovolcanoes erupt basalt and that these tend not to erupt explosively. However, your question was specifically about high-silica volcanoes and those are not basaltic. Here is another summary of stratovolcano characteristics: http://volcano.und.nodak.edu/vwdocs/vwlessons/volcano_types/strato.htm That page explains that stratovolcanoes tend to be composed of about half lava and half pyroclastic debris. This means that in terms of the volume produced, about half of the eruptions are not explosive. In other words, this kind of volcano can erupt relatively calmly, but any such volcano is going to erupt explosively some of the time. The composition of the magma in a given stratovolcano can vary quite a lot: http://www.geology.sdsu.edu/how_volcanoes_work/stratovolc_page.html ...and this is the cause of variation in the kind of eruption of a single volcano. The calmer eruptions from stratovolcanoes probably occur when the silica content of the magma is lower. An Internet search using google or another search engine will yield many other sites with information about this kind of volcano. David C. Kopaska-Merkel Geological Survey of Alabama box 869999 Tuscaloosa, AL 35486 205-247-3695 firstname.lastname@example.org
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