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                                                                                       GLOSSARY   xix


                   lithosphere, with rigid plates sliding as discrete  rootless lava flow A lava flow formed by the
                   structures on top of the plastic mantle and inter-  accumulation and coalescence of hot pyroclasts
                   acting at their edges.                       falling from a lava fountain.
                 Plinian Description of a sustained explosive dis-  rootless vent The site of a volcanic explosion that
                   charge of volcanic gas and pyroclasts forming a  is not directly underlain by a volcanic vent, for
                   large eruption cloud in the atmosphere.      example a place where an explosion occurs in a
                 Poisson’s ratio An elastic property of a solid,  lava flow advancing over waterlogged ground.
                   specifically the ratio of transverse to longitudinal  s waves Secondary waves, shear waves spreading
                   strain (i.e., fractional deformation) when a ten-  out from an earthquake focus into the surround-
                   sional force is applied.                     ing rocks. These waves can propagate in solids
                 potential energy Generally, a form of energy   but not in liquids.
                   associated with the position of an object in a  saltate To bounce over the ground, as when parti-
                   force field. In volcanology this is the planetary  cles are almost suspended in a strong wind.
                   gravitational field.                        saturated Description of a fluid containing the
                  pressure-balanced Description of a volcanic   maximum amount of volatiles allowed by the
                   fluid emerging from a vent in such a way that the  current pressure and temperature.
                   pressure within the fluid is equal to the atmo-  sedimentation The settling of particles from a
                   spheric pressure at the level of the vent.   fluid to form a layer at the base of the fluid.
                  pressure-release melting An alternative term for  seismic gap A subsurface region within which
                   decompression melting.                       no sources of seismic waves occur because the
                  pumice A piece of volcanic rock containing    region is occupied by magma.
                   vesicles.                                  seismic velocity The speed of a seismic wave, a
                  pyroclastic density current A mixture of gas  sound wave in rock generated by an earthquake.
                   and suspended or entrained solids released in   shear modulus The elastic property of a solid
                   a sustained explosive eruption and forming a  expressing the fractional amount by which it
                   dense fluid that moves along the ground at high  deforms in response to a shearing stress.
                   speed.                                     sheet flow A lava flow that is very wide compared
                  pyroclast General term for any fragment of    with its thickness.
                   volcanic material produced in an explosive   sill A sheet-like body of magma, often approxim-
                   eruption.                                    ately horizontal, intruded at some depth below

                  pyroclastic fountain A mixture of gas and     the surface along the interface between two pre-
                   pyroclasts erupted explosively though a vent,   existing rock layers.
                   traveling upward, and then falling back to the  sinuous rille A meandering type of channel found
                   surface.                                     on the Moon and Mars where hot turbulent lava
                  pyroclastic surge A relatively short-lived form of  has eroded the surface over which it has flowed.
                   pyroclastic density current.               slug A body of gas rising through a volcanic dike
                  regolith A fragmental layer on the surface of a  or conduit where the vertical extent of the gas
                   planet. If a biological component is present, as  is much greater than the width of the dike or
                   on Earth, the regolith is called soil.       conduit.
                  residence time The time that particles or aerosols  solidus The temperature below which a magma is
                   spend in the atmosphere before settling to the  completely solid.
                   ground. More generally the time taken for any  soliton A solitary wave, i.e., a wave that travels
                   particles suspended in a fluid to settle out.  without changing its size or shape.
                  rheology The study of the way fluids deform in  solubility law The law specifying how much of a
                   response to applied stresses.                given volatile compound can be dissolved in a
                  rhyolite A rock type with a high silica content.  magma at a given pressure and temperature.
                  Ring of Fire The regions around the rim of the  spatter rampart A ridge parallel to a fissure vent
                   Pacific Ocean dominated by volcanic activity.  consisting of pyroclasts ejected from the fissure.
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