Stratovolcanoes:  explosive or Andean-type, andesitic, composite volcanoes
     subduction zone volcanoes that form a magmatic arc (crescent shaped array)
     typically erupt intermediate magma (andesitic to dacitic), semi-tacky
     composite/stratovolcano made of both lava flows and pyroclastic debris
          reflects physical behavior of intermediate magma
      composite volcanoes very unstable, easily eroded, don't last long (1 Ma max.)
     created by subduction/recycling of oceanic crust (inter. magma can form other ways)
         old model:  simple model, ocean crust melted at 100 km = 1000 ° C
              BUT ocean crust converts to eclogite on way down, eclogite doesn't melt
         new model: wet/hydrated oceanic crustal rocks dehydrate at 100 km depth
              water/carbon dioxide/etc. ("volatiles") spritz the overlying mantle wedge
              partial melting of wet mantle produces andesitic magma (dry mantle --- basalt)
              reproduced successfully in laboratory, explains some other dirty details

Cascade Volcanic Arc:  not to be confused with the Cascade Mountains underneath
     reflects the subduction of the JDF plate underneath NAM, part of "Ring of Fire"
          shallower angle of subduction of JDF means that arc is farther from trench
     volcanoes extend from southern BC to northern CA, all active (quakes, eruptions)
       recent activity of some selected Cascade volcanoes:
          Lassen (CA)—major eruptions in 1914 and 1915, well documented
          McLoughlin (OR)—cored by a giant unstable cinder cone and capped with flows
          Mazama (OR)—monster eruption 6800 years ago created Crater Lake caldera
               debris from now-gone Mazama scattered all over PNW into Canada
          Hood (OR)—a little steam and ash in 1976, big threat to Portland
          Baker (WA)---a little steam and ash in 1977
          Garibaldi (CAN)—erupted under and on top of glaciers about 5,000 years ago
 
Mt. St. Helens:  Loowit:  one of youngest volcanoes in Cascade Volcanic Arc
     oldest lavas barely 20,000 years b.p., most of volcano built in last 2000 years
     very unusual volcano: shows apparent cyclic eruptive behavior
          first noticed by Crandall and Mullineux, erupts every 150 years or so
          cyclic behavior allowed them to forecast 1980 eruption back in 1960's
          mechanism for apparent cyclic behavior unknown
     Sunday 18 May 1980 8:28 a.m. eruption culminated 2 months of activity
          magma moved up into volcano, caused severe unstable bulge on N side
          magma movement triggered 5.0 quake on 18 May, caused giant slump of bulge
          landslide popped cork, lateral blast of magma came out sideways
           glowing cloud/stone wind scoured countryside
          two more giant landslides followed, created vertical blast.  eruption lasted a day.
           landslides went 10 km downvalley, lahars extended 50-70 km

Mt. Rainier:  one of oldest volcanoes in Cascade Volcanic Arc
     oldest lavas around 1 million years b.p., no major eruption in last 10,000 years
     very uniform chemical composition, just a few flavors of andesite
     giant landslide and resulting mudflow about 5800 years b.p.:  Osceola lahar
          Rainier's upper 500 meters slid off N side down White River valleys
          scooped up everything in path, spread out over Eunumclaw, Auburn, Renton...
          would kill 150,000 people if same lahar happened today
     smaller mudflow about 300 years ago made it all the way to Boeing Field in Seattle
 

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