Chapter 10 - Study Guide Unit 3: Volcanic Eruptions Chapter...

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Study Guide Unit 3: Volcanic Eruptions Chapter 10 Introduction That eruption will be explosive only if: o it is highly viscous (high Si) o it has a high content of exsolved volatiles (like H 2 O and CO 2 ), and o the “cork” or cap on the volcanic conduit (which has been so strong that it’s allowed pressure to build within) suddenly ruptures. Eruption causes (see Fig.1) High Silica o it is highly viscous (high Si) High Volatiles o it has a high content of exsolved volatiles (like H 2 O and CO 2 ) Viscous Magma
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o Very viscous Magma Cap o the “cork” or cap on the volcanic conduit (which has been so strong that it’s allowed pressure to build within) suddenly ruptures. Ruptures o Sudden release of pressure Learn progress from Fig.1 Plinean vs. Pelean style o Eruptions like the one illustrated in Figure 1 are called Plinian eruptions , named for a father/son pair that drew sketches of the eruption of Mount Vesuvius in 79 AD. Eruptions don’t always go straight out the top, of course; they blow out the weakest spot of the volcano, and sometimes that’s the side (as it was for Mt. Pelée in the Caribbean in 1902; that type is thus called a Peléan eruption ). o Plinean style goes straight out the top o Pelean style is thru the weakest point, which is on the side. A: Subduction Zone Eruption o Subduction tectonic activity characterizes the western, northern and eastern rims of the Pacific Ocean. Case Study 1: Cascade Range Ring of Fire o the rim of the Pacific is commonly referred to as “ The Ring of Fire Juan de Fuca activity; no volcanism elsewhere (see Fig.4) Mount St. Helens 1980
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o Of all the subduction volcanoes, Mount St. Helens is the best indicator that the plate process is still going on. Mount St. Helens is the most active volcano on the continent; the magma chamber, emptied in 1980 (see below) is refilling, and the volcano is very closely monitored. Hazard prediction o In 1975 three geologists working for the US Geological Survey wrote a report for Science noting that Mount St. Helens has the most extensive record of recent activity of any volcano in the continental US. They said it was the most likely volcano of the Cascades to erupt, and they predicted that an eruption might take place before the end of the century. Three years later, in 1978, two of them produced a detailed map of the areas at risk from ash and lahars ( which we discussed under ‘hazards’ in the previous chapter) . No one actually paid much attention in either 1975 or 1978, but in 1980 their hazards report was the most widely read scientific paper by emergency management people of the west coast! Evidence of magma motion Eruption event Phases o On the 21 st of March 1980 a magnitude 4.2 earthquake occurred beneath the volcano - the first sign that it was stirring to life. Seismic activity increased rapidly, indicating that magma was on the move, and o March 27 th the first little eruptions of ash and gas were noted. These apparently marked the point at which the magma contacted groundwater, producing steam. Although these were not large eruptions, they were visible for some distance, and
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This note was uploaded on 11/01/2010 for the course EARTH SCIE 2010 taught by Professor Neil during the Fall '10 term at UWO.

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Chapter 10 - Study Guide Unit 3: Volcanic Eruptions Chapter...

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