Page 16 - Fundamentals of Physical Volcanology

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GLOSSARY xv

diffusion The process whereby the atoms or exsolution level The depth at which gas dis-
molecules of a volatile compound migrate by solved in a rising magma first starts to come out
moving between the atoms or molecules of a of solution and form gas bubbles in response to
host material. the decreasing pressure.
dike A fracture filled with volcanic material cut- exsolve Release a gas from solution in a liquid.
ting through earlier-emplaced host rocks. The opposite of dissolve.
dilatant Description of a type of nonNewtonian fiamme Pumice clasts in the interior of an
fluid in which the viscosity increases as the ignimbrite deposit that have become stretched
applied stress increases. sideways while still hot as the deposit is com-
divergent margin A boundary between two pressed by the weight of overlying material.
tectonic plates, almost always on the ocean floor, filter-pressing The process whereby magma is
at which new crustal material is being supplied squeezed out of its partly molten source rocks
by volcanic eruptions or intrusions as the plates when stress causes compaction of the unmelted
move apart. material.
dome A deposit of (commonly viscous) lava fissure A fracture in rock, more particularly an
where the width and maximum thickness are of elongate surface vent from which magma is
the same order because the lava has not spread erupted.
far from the vent. flood basalt eruption A rare kind of basaltic
dusty gas A gas containing solid particles so small eruption in which a very large volume (thou-
that frictional drag forces effectively force the sands of cubic kilometers) of basalt is erupted in
particles to travel at the same speed as the gas. a geologically short space of time.
ejecta Any material thrown out from a vent (rather flow unit A lava flow which is the product of a
than flowing away from it) during volcanic single eruptive event from a single vent.
activity. focus The location beneath the surface where an
elastic Description of any material that changes its earthquake takes place.
shape when a stress is applied to it but recovers fractional crystallization The formation of crys-
its original shape when the stress is removed. tals in a cooling liquid. One or more types of
energy equation An equation describing the law crystal may form at any one time, but each forms
that says that the total energy of a system can- over its own characteristic temperature range.
not be destroyed, only redistributed among its fragment A piece of material broken from a larger
parts. piece of material.
entrainment The process whereby the flow of fragmentation The process of breaking a mater-
one material through or past another surround- ial into smaller pieces. The material may be a
ing material causes some of the surrounding solid or a liquid.
material to be mixed into the flowing material. fragmentation level The depth below the sur-
equivalent diameter A geometric property of a face at which shearing stresses tear a magma
flowing fluid in a channel equal to four times containing gas bubbles apart into clots of liquid
the cross-sectional area at right angles to the carried along by the gas released from the bub-
direction of flow divided by the length of the bles broken by the tearing process.
perimeter in contact with the floor and walls. friction A force that opposes the motion of any
exit velocity The speed at which volcanic materi- two materials in contact and sliding past one
als emerge through a surface vent. another.
expansion wave A moving zone within a fluid frost ring A layer inside the trunk of a tree, form-
across which the pressure decreases ing parallel to the bark and marking a time when
significantly. the growth of the tree was inhibited by unusually
explosive Description applied to any process that cold weather.
takes place suddenly, or that involves a very large fumaroles Places where volatile compounds
pressure change. being released from the interior of a volcanic