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                    xviii  GLOSSARY



                  maria  (singular:  mare) The latin name for the  p waves Primary waves, compressive waves
                    dark areas on the Moon, consisting of floods of  spreading out from an earthquake focus into the
                    basaltic lava filling very large impact craters.  surrounding rocks. These waves travel faster
                  mass extinction A biological event in which   than any other type of seismic waves.
                    large numbers of species die out in a geologically  pahoehoe A type of lava where the surface is
                    relatively short space of time.             smoothly folded into a series of ripples called
                  mass flux The mass of magma passing though a   ropes.
                    volcanic system every second.             pahoehoe toe A small lava flow unit consisting
                  meteoric Generally, description of any phe-   entirely of pahoehoe lava extending for a short
                    nomenon associated with the atmosphere. In   distance from a larger flow unit.
                    volcanology, applied to near-surface water that  partial melting The process in which part of a
                    has collected as a result of rain or snow.  mass of rock melts, the liquid still containing the
                  mid-ocean ridge The ridge formed along a con-  mineral grains that have not yet melted.
                    structive tectonic plate margin by the accumula-  perched lava pond A lava flow that has spread
                    tion of lavas erupted onto the ocean floor.  out sideways so that it has a similar width to
                  mush column A zone within the crust where     its length. The shape is sometimes controlled
                    magma passes through so often that the host  by pre-existing topography but can be self-
                    rocks are partly molten.                    generated by a suitable combination of eruption
                  negatively buoyant Description of material that  rate and very shallow ground slope.
                    is denser than the material surrounding it, so that  peridotite A type of rock rich in olivine, found in
                    it will tend to sink through its surroundings.  the Earth’s mantle.
                  neutral buoyancy level Any location above or  petrology The general term for the study of all
                    below ground where volcanic materials have the  aspects of rocks.
                    same density as their surroundings.       phoenix cloud Alternative term for a co-
                  Newtonian Description of a fluid with the      ignimbrite cloud.
                    property that any change in applied stress pro-  phreatomagmatic Description of an eruption
                    duces a directly proportional change in rate of  involving interaction between magma and sur-
                    deformation.                                face, near-surface or ground water.
                  nonNewtonian Description of a fluid in which  phreato-Plinian Description of a sustained ex-
                    the rate of deformation is not directly propor-  plosive eruption in which magma interacts with
                    tional to a change in the applied stress.    surface or ground water, generally resulting in
                  normally graded Description of a pyroclastic  more fragmentation of the magma than in a
                    deposit in which the grain size increases down-  Plinian eruption.
                    ward in the deposit.                      pillow A lava flow lobe that is approximately as
                  nova (plural: novae) A type of tectonic structure  wide as it is thick, produced by a low lava extru-
                    on Venus in which fractures radiate out from a  sion rate under water.
                    central zone.                             pillow lava A lava flow consisting of a pile of
                  nuclear winter A prolonged period of cooling  pillows.
                    when heat reaching the surface from the Sun   plastic Description of a fluid that is capable of de-
                    is greatly reduced by dust in the atmosphere  forming smoothly in response to an applied stress.
                    thrown up from the explosion of a large number  plastic viscosity The property of a non-
                    of nuclear weapons.                         Newtonian fluid expressing the ratio between a
                  nuée ardente French for “burning cloud,” one of  change in applied stress and a corresponding
                    the possible names for a pyroclastic surge.  change in rate of deformation.
                  ophiolite A body of rock consisting of the subsur-  plate One of a series of sections of the Earth’s
                    face part of an old spreading center now uplifted  lithosphere behaving as a rigid solid.
                    and exposed at the Earth’s surface by tectonic  plate tectonics The term applied to our current
                    forces.                                     understanding of the structure of the Earth’s
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