Page 15 - Fundamentals of Physical Volcanology

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xiv GLOSSARY

buoyancy The phenomenon whereby a low- conservation of momentum The physical law
density body surrounded by a higher-density that asserts that the momentum of a system
fluid in a gravitational field experiences an (the product of mass and velocity) cannot be
upward force. destroyed, only redistributed among the com-
caldera A steep-walled depression, commonly ponents of the system.
found at the summit of a volcano, formed when a continental arc An arcuate chain of volcanoes at
large volume of magma is removed quickly from the margin of a continent.
an underlying magma reservoir and the overlying convective region Part of the interior of a fluid or
rocks slide down along faults to fill the vacated plastic solid within which convection (relative
space. movement of different parts of the fluid due to
canali Long (many hundreds of kilometers), density differences) is taking place.
narrow (a few kilometers wide) channels seen on convergent margin A location where the edges
the surface of Venus. of two tectonic plates are being driven into
carbonatite A rare type of magma consisting collision.
mainly of liquid carbonates rather than liquid cooling unit One or more layers of pyroclastic
silicates, produced in the mantle beneath some particles (or lava) emplaced so soon after one
continental areas. another that they cool as though they had been
channelized lava flow A lava flow in which emplaced at the same time.
liquid lava moves in a central region bordered on cooling-limited Description of a lava flow that
either side by a bank (called a levée) of stationary ceases to move because the front of the flow has
lava. cooled to the point of effectively being a solid.
choked flow The flow of a fluid under conditions corona (plural: coronae) One of a number of
such that the speed of the fluid is equal to the large (many hundreds of kilometers) roughly
speed of sound within the fluid. This is the circular regions on Venus where tectonic forces
maximum speed that can be reached by the fluid have fractured and folded the surface rocks.
unless special conditions apply. cryovolcanism A volcanic process in which
coalescence The joining together of two separate liquid water rather than liquid rock is the moving
gas bubbles within a liquid (or of two droplets of fluid.
liquid in a gas). dacite A type of magma with intermediate to high
cock’s tail plume The distinctive “feather-edged” silica content.

jet of ash and steam formed in an explosion when dark halo deposit A roughly circular region on
a large amount of water gains access to a vent. the Moon where dark pyroclasts are deposited
co-ignimbrite ash fall deposit A fine-grained around an explosive vent.
deposit of pyroclasts settling out on the ground de Lavalle nozzle Part of a volcanic conduit or
from a co-ignimbrite cloud. dike where the shape changes from converging
co-ignimbrite cloud An eruption cloud of gas upward to diverging upward, thus allowing
and small pyroclasts formed as gas rising through magma flowing through the conduit to acceler-
an ignimbrite carries small particles upward with ate from subsonic to supersonic speeds.
it as it escapes. decompression The expansion of a material,
column collapse The condition in which an especially a gas, when the pressure acting on it
eruption column fails to be positively buoyant decreases.
in the atmosphere, so that a lower fountain of decompression melting The process whereby
gas and entrained particles forms over the vent solid rock begins to melt when the pressure act-
instead. ing on it decreases even if the temperature does
compound lava flow field A region containing not change.
many lava flow units, most of which have formed diapir A body of plastic material that rises buoy-
by new flows breaking out from the margins of antly as a coherent mass within a larger body of
earlier-emplaced flows. plastic material.