VOLCANIC ERUPTIONS  

A.  WHAT CAUSES ERUPTIONS?

        1.  RISING MAGMA

             Temperatures and pressures within the mantle are high enough
             to melt rock, producing what is called magma.  Because magma
             is less dense than the overlying solid rock, it will rise toward the
             surface.  As magma approaches the surface, pressures from the
             overlying rock are much reduced, so 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 ("awake") or that could
             erupt in the future ("dormant").  An extinct volcano will not erupt
             in the future because it  has been cut off from its 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.  VOLCANIC PRODUCTS

              Lava can be extruded from any type of volcano, but there
              are different  types of lava.   Only low viscosity lava tends
              to flow very far from the crater.

              Flows can sometimes be diverted from population centers.   

         2.  TEPHRA

              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.

         3.  VOLCANIC GASES

              The gases that propel volcanic eruptions are also emitted
              from volcanoes, either during an eruption of lava and ash,
              or as a separate eruption of gas.
 

D.  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 the differences between volcanoes within the U.S.

       1.  HAWAII vs WESTERN U.S.

            Hawaii :  the primary hazards are lava flows and "VOG". 
           

            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.  (Mt. Rainier: America's Most Dangerous Volcano).
 

            What is the difference?  Hawaii is a shield volcano created by
            a mantle hot spot; the western U.S. has both stratovolcanoes 
            and lava domes created by a subduction zone boundary.
                
                  These various categories of volcanoes differ with respect
                  to lava viscosity, which is directly related to SiO₂ content.
                  Viscous lava does not flow readily, and its gases cannot
                  escape.  These factors lead to more explosive eruptions.

       2.  THE CASCADE RANGE

            Although volcanoes that make up the Cascade Range have
            the same origin, they do not pose equal threats in terms of
            their potential to cause damage during an eruption.   Why?

            One important way in which these volcanoes differ is in their
            eruption periodicities (see Cascade Range eruptive history).
            Studies suggest that eruption periodicity is also an important
            factor in explosivity.  

               Example:  Mount Saint Helens 1980 eruption  (Mt. St. Helens Eruption)
                                  (Map view)