Page 17 - Fundamentals of Physical Volcanology

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xvi GLOSSARY

deposit reach the surface and settle to form homogeneous nucleation The process of the
deposits on the ground as they cool. formation of gas bubbles in a liquid supersatu-
Gas Laws The laws describing the way the pres- rated in a dissolved volatile compound when
sure, temperature, density, and internal energy the bubbles have no solid surfaces on which to
of gases are related. nucleate and so appear at random within the
gas-thrust region The lowest part of an eruption liquid.
column, where the inertia of the erupted materi- hot spot Place where there is an unusually large
als, which has been determined by the expansion upward flow of heat from the mantle toward the
of volcanic gases beneath the surface, is the main surface. Generally a location of significant vol-
control on their motion. canic activity.
giant dike swarm A group of dikes radiating for hyaloclastite A type of fragmental and chemically
great distances (at least many hundreds of kilo- altered rock produced when erupting lava inter-
meters) away from some region where a very acts strongly, generally explosively, with surface
large magma reservoir has existed at some time. water.
graben A trench-like depression formed in an area hyaloclastite ridge A ridge composed of frag-
of extensional forces. The crust is forced apart mental and chemically altered rock produced
and breaks along two parallel normal faults when a fissure eruption occurs in shallow water,
dipping toward one another, with the ground most commonly beneath a glacier.
between the faults moving downward. hydromagmatic Description of any eruption
grading The variation of the average grain size of process in which magma or lava interacts with
a deposit with vertical position within it. In nor- external water.
mal grading the mean size increases downward, hydrothermal Description of any process involv-
whereas in reverse (or inverse) grading it in- ing the circulation of water at shallow depths in
creases upward in the deposit. the crust as a result of heat supplied by intruded
granular flow The flow of a body of material magma.
consisting of discrete solid clasts in which only ignimbrite A large body of rock formed from the
the interaction between clasts controls the deposition of pyroclasts that have traveled from a
motion – any gas or liquid between the clasts has vent as a pyroclastic density current.
no important effects. ignimbrite-forming A type of explosive erup-
Hawaiian Description applied to eruptions like tion that produces large volumes of pyroclasts

those common in Hawai’I, where basaltic lava is emplaced as pyroclastic density currents.
erupted, commonly explosively. inertial region An alternative description (see
heat pipes Regions where heat is transported “gas-thrust region”) of the lowest part of an erup-
upward through the crust mainly by the fre- tion column where the inertia of the erupted
quent passage of magma through the surround- material dominates the motion.
ing rocks. inflation The word has two uses in volcanology:
heterogeneous nucleation The process of the (i) the enlargement of a magma chamber as new
formation of gas bubbles in a liquid supersatu- magma is added to it from the mantle; (ii) the pro-
rated in a dissolved volatile compound when the cess whereby a lava flow gets thicker after it has
bubbles nucleate on crystals in the liquid or irreg- been emplaced as a result of additional magma
ularities in the boundary between the liquid and being forced into its interior.
its solid surroundings. intraplate Any process that occurs within, i.e.,
hindered settling The settling of solid particles in well away from the boundaries of, a tectonic
a fluid where the particles are so close together plate.
that they either collide with one another or inter- inversely graded Description of a deposit of
fere with the smooth flow of the fluid around pyroclasts in which the average grain size
them. increases upward in the deposit.