Page 21 - Fundamentals of Physical Volcanology
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                    xx  GLOSSARY



                  spreading center A boundary between two tec-  thixotropic Description of a type of non-
                    tonic plates at which new crust is being created  Newtonian fluid in which the viscosity decreases
                    by volcanic eruptions and intrusions and the  as the applied stress increases.
                    plates are moving apart.                  tholeiitic Description of a common type of basalt,
                  stoping The process whereby blocks of country  rich in the minerals plagioclase and pyroxene,
                    rock become detached from the roof or walls of a  formed at mid-ocean ridges.
                    magma reservoir and fall into the magma.  toothpaste lava A type of lava with a very rough
                  strain rate The rate at which a solid or liquid  surface texture consisting of many sharp spines
                    changes its length, expressed as a fraction of its  roughly arranged in rows.
                    original length, as a result of an applied stress.  tractional Description of a process that involves a
                  stratosphere The second layer of the Earth’s  frictional force from an overlying fluid dragging a
                    atmosphere, lying above the troposphere.    particle sideways.
                  Strombolian A style of explosive volcanic activity  triple junction A tectonically complex region
                    characterized by the intermittent arrival, at the  where the boundaries between three tectonic
                    surface of the magma in a vent, of giant gas bub-  plates meet.
                    bles that burst, throwing out the disrupted liquid  tropopause The boundary between the tropo-
                    skin of the bubble.                         sphere and the overlying stratosphere, lying at a
                  subaerial Description of any process taking place  latitude- and season-dependent height of  ∼10–
                    in an atmosphere.                           15 km.
                  subduction The process whereby some tectonic  troposphere The lowest part of the Earth’s
                    plates are forced down into the Earth’s interior  atmosphere.
                    beneath other plates.                     ultraPlinian An unusually energetic form of
                  subPlinian Description of a class of sustained  Plinian explosive eruption.
                    explosive volcanic activity producing relatively  umbrella region The uppermost part of a vol-
                    small eruption plumes in the atmosphere.    canic eruption cloud where vertical motion
                  supersaturated Description of a fluid containing  ceases and gas and pyroclasts spread sideways in
                    more of a dissolved volatile compound than it  the atmosphere.
                    should be capable of dissolving under its current  undersaturated Description of a fluid containing
                    pressure and temperature conditions. This is an  less of a dissolved volatile than it is capable of
                    unstable state leading to bubble formation by  dissolving under its current pressure and tem-
                    exsolving gas.                              perature conditions.
                  surface tension A molecular attraction force act-  vein A relatively small and narrow fracture in a
                    ing parallel to any interface between two fluids  rock containing material that is different from the
                    and tending to reduce the area of contact.  host rock. Commonly used in volcanology to
                  tensile strength The strength of a material when  describe a small off-shoot from a dike.
                    subject to a tensional force.             vesicles The holes in volcanic rocks showing
                  tephra General term for relatively fine grained  where gas bubbles were present as magma rose
                    fragmented volcanic rock.                   to the surface.
                  tephra jet Alternative name for a cock’s tail  viscosity The property of a fluid describing its
                    plume.                                      resistance to deforming and flowing when a
                  terminal velocity The steady speed reached by  stress is applied to it.
                    an object falling through a fluid in a gravitational  volatile Generally, the description of any chemi-
                    field when the upward drag force exerted on it  cal compound with a low boiling point. Used
                    by the fluid is just equal to the downward gravi-  in volcanology to refer to the gases commonly
                    tational force.                             dissolved in magma at depth in the planet.
                  tessera  (plural:  tesserae) A type of terrain on  Volcanic Explosivity Index A single number be-
                    Venus characterized by extensive faulting in  tween 0 and 8 giving a combined measure of the
                    more than one direction.                    magnitude and intensity of a volcanic eruption.
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