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.