GLOBAL
POSITIONING SYSTEM (GPS) - A
constellation of twenty-four satellites orbiting
around the earth that constantly beam radio
signals back. By locking onto at least four
satellites, a receiver on the Earth's surface can
locate itself to only a centimetre or two. In
volcanology, the positions of survey benchmarks
in a geodetic network are precisely located in
this way, allowing distance and height changes
between benchmarks to be determined with a
high degree of accuracy and precision. The
relative movements over time of the
benchmarks provide volcanologists with
information on the position, movement, and
volume of subsurface magma.
HYDROMAGMATIC ERUPTION - An eruption
whose explosivity is significantly enhanced by steam from non-volcanic
water (e.g., groundwater, lakewater and seawater) that has come
into contact with magma. Also termed phreatomagmatic.
IGNIMBRITE - A pumice-rich pyroclastic
flow, normally associated with, but not exclusive to, large-volume
explosive eruptions.
LAPILLI - Magmatic fragments between
2 and 64 mm across. Accretionary lapilli or pisolites are produced
in eruption clouds when coatings of ash form concentric layers
around a tiny nucleus.
LAHAR - An Indonesian term describing
mudflows of volcanic material, normally slurries of ash with
varying amounts of larger debris.
LAVA - Magma that has breached the
surface.
LAVA DOME - When viscous lava is
extruded onto a near-horizontal surface, it tends to pile-up
around the vent building a dome. The dome may grow by intrusion
of new lava into its interior (endogenous growth) or by the
overlapping of numerous small lava tongues or flows that escape
through breaches in the dome's surface (exogenous growth).
LITHIC MATERIAL - Fragments, usually
angular, of rock stripped from conduit walls
during eruption. Lithic fragments may also
include magma from the current eruption that has been chilled
against the conduit walls and later torn away.
MAGMA - Generic term describing
all molten rock. For common rock compositions, eruption temperatures
are between 900 oC and 1200 oC, lower temperatures occuring
among more-evolved magmas.
NUÉE ARDENTE - Strictly a pyroclastic flow of
poorly vesiculated magma (from the French for a "glowing
cloud"), although the term is often used loosely as an
alternative to all types of pyroclastic flow.
PHREATIC ERUPTION - An eruption driven by non-volcanic
water that has been vaporised to steam by the heat from ascending
magma. The products are fragments of pre-existing rock alone.
If new magma is also expelled, the eruption is termed phreatomagmatic
or hydromagmatic. Phreatic activity is often the first sign
that a volcano is becoming active, as rising magma comes into
contact with ground water resulting in cold explosions that
may clear the blocked vents and ease the passage of the magma
towards the surface. |
PHREATOMAGMATIC
- See hydromagmatic.
PLINIAN ERUPTION - A style of explosive
eruption producing an ascending cloud of ash and hot gas that
may rise tens of kilometres into the atmosphere before spreading
outwards. The column entrains cold surrounding air during ascent.
Cooling increases the density of the cloud, especially its outer
margins. If the cloud becomes too heavy, it collapses and falls
back to earth (column collapse) often producing pyroclastic
flows. The style of activity is named after Pliny the Younger
who described such behaviour during the AD 79 eruption of Somma
- Vesuvius.
PUMICE - Highly vesicular magma
normally produced during plinian eruptions. The high vesicularity
gives pumice its low diagnostic density of 1,000 kg m3 or less,
so that it can float on water. Normally associated with viscous
magmas of intermediate and evolved compositions.
PYROCLASTIC - Generic term describing
volcanic rock broken during eruption (from the Greek for fire(pyro)-broken(clast)).
PYROCLASTIC FLOW - Cloud of hot gas and incandescent
ash that, at temperatures of several hundred degrees Celsius,
hugs the ground and races downslope at velocities of that exceed
100 km per hour. Pyroclastic flows are commonly formed either
by the collapse of the cooler outer parts of an eruption column,
or by the disintegration of a lava dome.
PYROCLASTIC SURGE - A pyroclastic
flow consisting mostly of hot gas. Surges may occur as the dilute
outer parts of a flow or they may be generated directly at the
vent, especially during hydromagmatic eruptions when non-volcanic
steam increases the gas available for expelling the magma.
TEPHRA - Generic term for all volcanic
fragments that are explosively ejected and fall back to Earth
(from the Greek for "ashes").
TUFF - Consolidated pumice and ash
deposit, usually associated with ash fall or pyroclastic flows.
Consolidation occurs as minerals deposited by circulating groundwater
cement the volcanic fragments.
VESICLES - Gas bubbles within magma.
VOLCANIC GASES - All magmas contain
gases, the commonest of which are water, sulphur dioxide, hydrogen
sulphide, carbon dioxide, carbon monoxide, and chlorine. Other
species include fluorine, radon, and helium. The gas content
is an important factor in determining the explosivity of an
eruption, and the accumulation of gas in high viscosity magmas
is the main cause of moderate to large explosive eruptions.
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