VOLCANIC ERUPTIONS  

A.  WHAT CAUSES ERUPTIONS?

        1.  RISING MAGMA

             Temperatures and pressures within the mantle are high enough to melt
             rock, producing what is known as magma.  Because magma is less dense
             than the overlying solid rock, it will rise toward the surface.  As magma
             nears the surface, where pressures are greatly reduced, magmatic gases 
             expand to generate the localized pressures within a magma chamber that
             cause an eruption.  (Picture of an erupting lava fountain).

        2.  EXTINCT VOLCANOES

             An active  volcano is one that is erupting (i.e., "awake") or that could erupt
             sometime in the future (i.e., "dormant").  An extinct  volcano will not erupt
             in the future because it  has been cut off from its former magma source.
                      
  

B.  WHERE ERUPTIONS OCCUR  (Map,  By Region,  Current)

        1.  OVER SUBDUCTION ZONES  ("Ring of Fire")

         2.  OVER MANTLE HOT SPOTS   (Map)

              Abnormally hot spots within the asthenosphere produce
              rising plumes of magma that can reach the surface and
              form volcanoes (e.g., the Hawaiian Islands and Iceland). 

         3.  OVER DIVERGENT BOUNDARIES

             Divergent plate boundaries (also called rift zones) create
             a pathway for magma to rise up from the asthenosphere.
             However, although these boundaries can produce discrete
             volcanoes, they often lead to fissure eruptions.

C.  TYPES OF VOLCANOES

        Not all volcanoes are created equal, especially when it comes
        to the hazards they pose to humans and the environment.  We
        shall consider three broad categories of volcanoes:

        1.  SHIELD VOLCANOES

             a.  Shield volcanoes are the largest volcanoes in terms of
                  surface area and have broad, gentle slopes.   Shield
                  volcanoes are composed mostly of lava flows.
       
     
             b.  Shield volcanoes extrude basaltic magma, which has  
                  a relatively low SiO₂ content (less than 52%).
              

        2.  STRATOVOLCANOES

              a.  Stratovolcanoes (also known as composite volcanoes) 
                   are tall and steep-sided, what most people envision 
                   when they think of volcanoes.  Stratovolcanoes are
                   composed of alternating layers of cinders, ash, and
                   lava.
             
     
             b.  Stratovolcanoes extrude andesitic magma, which has  
                  an intermediate SiO₂ content (between 52 and 63%).

        3.  LAVA DOMES

             a.  Lava domes are dome-shaped features that have steep
                  slopes. They can form as separate volcanoes but often
                  form within existing stratovolcanoes.
             
     
             b.  Lava domes extrude rhyolitic magma, which has  
                  a relatively high SiO₂ content (above 68%).

              Lava can be extruded from any type of volcano, but only
              low viscosity lava tends to flow very far from the crater.
              Flows can sometimes be diverted from population centers.

         2.  ASH FALLS

              The term tephra (also called pyroclastic debris) refers to
              fragments of volcanic rock and lava that are blasted into
              the air during eruptions. Tephra includes a wide range in
              particle sizes, which are given different names.

              The largest sized tephra (blocks and bombs) fall closest
              to a volcano, but ash is blasted high into the sky and can
              travel long distances before settling to the ground.  Ash is
              dangerous to breathe and can cause damage to engines or
              machines, including aircraft.

         3.  ASH FLOWS

              Ash flows (also known as pyroclastic flows or the French
              term nuees ardentes) are fast-moving clouds of volcanic
              gas and pyroclastic debris.  These intensely hot and very
              poisonous clouds destroy everything in their paths. 

              The most famous ash flow occurred in 1902, when Mount
              Pelee in Martinique erupted, killing 30,000 people.

        4.  DEBRIS FLOWS 

              Debris flows (also known as lahars, an Indonesian word)
              are fast-moving flows composed of water and pyroclastic
              debris.  These flows are initiated when rain or snowmelt
              saturates pyroclastic debris ejected during an eruption. 

              Lahars were prominent during the 1980 eruption of Mt.
              Saint Helens, the 1985 eruption of Nevado del Ruiz, and
              the 1991 eruption of Mt. Pinatubo.  Lahars are also one
              of the concerns facing residents near Mt. Rainier (map).

         5.  TSUNAMIS

               Volcanic eruptions, like earthquakes, can generate giant
               sea waves ("tidal waves") if they occur in the ocean. One
               of the more destructive tsunamis in human history (which
               killed 36,000 people) was caused by the 1883 eruption of
               Krakatoa in Indonesia.

         6.  COLD WEATHER

              Large eruption clouds can send ash high into the strato-
              sphere, the upper atmosphere where particles can remain
              suspended for long periods of time.  The presence of ash
              in the stratosphere causes a reduction in solar radiation
              reaching the earth's surface, which then results in lower
              temperatures.

              When Mount Tambora in Indonesia erupted in 1815, the
              amount of ash reaching the stratosphere resulted in three
              years of abnormally low temperatures in northern Europe
              and the eastern United States, causing the "year without
              a summer" in 1816. 

         7.  VOLCANIC GASES

              Besides propelling volcanic eruptions, volcanic gases have
              other harmful effects, including the formation of sulfuric
              acid ("VOG") the destruction of atmospheric ozone, and 
              the death of surface vegetation.

E.  DISTRIBUTION OF HAZARDS

       The distribution of hazards near volcanoes is not the same for 
       all volcanoes.   Why should this be?   To answer this question, 
       consider differences between volcanoes within the U.S. in terms
       of their volcanic hazards.

       1.  HAWAII vs WESTERN U.S.

            a.  Hawaii:  the primary hazards are lava flows and "VOG". 
            (Example: Mauna Loa eruption of 1984).

            b.  Western U.S.:  here there is a wider range of hazards. 
            A close-up look at Mt. Rainier reveals that specific hazards
            include lava flows, pyroclastic (or ash) flows, ashfall, and
            lahars (or mudflows).

            Why the difference?  Hawaii is a shield volcano created by
            a mantle hot spot; the western U.S. has stratovolcanoes and 
            lava domes created by a subduction zone boundary.
            
            The various differences between these types of volcanoes
            include lava viscosity (a measure of  its resistance to flow),
            which is directly related to its SiO₂ content.  Viscous lava
            does not flow readily, and its gases cannot escape.  These
            factors lead to more explosive eruptions.

       2.  MT. BAKER vs. NEWBERRY

            The hazard zone surrounding Newberry volcano in Oregon
             is much larger than that surrounding Mt. Baker in northern
            Washington.  Both are the same type of volcano, so how do
            they differ?   Eruption periodicity (the time period between
            eruptions) is also an important factor in explosivity.  

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