ANDESITE - A type of pale volcanic
rock, very common in the Caribbean. ASH - Particles of magma
less than 2 mm across. Volcanic ash is produced during explosive
eruptions. It may be formed either from a disrupted spray of
liquid magma ejected from a volcanic vent, or from the pulverisation
of preexisting rock that may have blocked the vent during a
period of quiescence.
BOMB - Rounded volcanic fragment
larger than 64 mm across ejected from a volcano during an explosion.
During flight through the air, bombs may develop distinctive
torpedo or spindle shapes.
BOULDER - Generic term for rounded
rock fragments, not necessarily volcanic, more than 256 mm across.
CALDERA - A giant volcanic crater
(notionally larger than 1 km across) formed by collapse or explosion,
collapse being more important among larger calderas. The name
comes from the Caldera Taburiente on La Palma in the Canary
Islands.
CINDER - Generic term for coarse
volcanic ash and lapilli.
COLUMN COLLAPSE - During moderate
to large explosive eruptions an enormous amount of ash and coarser
debris is ejected from the vent to form an eruption column that
can rise to tens of kilometres. When the mass of the debris
is too heavy it begins to fall back to Earth - a condition known
as column collapse. Commonly the collapsed material pours off
down the flanks in the form of pyroclastic flows.
CONE - Conical constructs built
up by the accumulation of material around a vent. They may consist
of tephra or a mixture of tephra and lava flows. Cones may be
the result of a single eruption or the product of many eruptive
episodes.
CRATER - A pit or depression, typically
located around a vent. Craters may be formed during construction
of an enclosing cone, by the excavation of rock during volcanic
explosions, or by the collapse of ground left without support
after magma has been erupted. Craters wider than about 1 km
are |
normally termed calderas.
DACITE - Type of volcanic rock,
typical of volcanic domes like those of Monserrat. Very common
throughout the Caribbean.
DYKE - Vertical fractures filled
with solidified magma. When magma stops flowing through a fissure
on the ground it solidifies to form a 'wall' of volcanic rock
that may be exposed by future erosion. Similar features lying
almost horizontally are termed sills.
ELECTRONIC DISTANCE METER (EDM)
- A surveying instrument that is used in volcanology to measure
very precisely the distances between benchmarks in a ground
deformation monitoring network. A beam of infrared, laser, or
microwave radiation generated by the instrument - mounted on
a tripod positioned over one benchmark - is bounced back by
a reflector mounted on a second tripod mounted above another
benchmark, and the distance determined to within a centimetre
or two over distances of several kilometres.
ERUPTION COLUMN - During explosive
eruptions, ash and debris ejected from the vent forms a vertical
jet that may reach tens of kilometres in height. Because it
is hotter than the surrounding atmosphere, an eruption column
rises due to buoyancy.
FISSURE - A surface fracture. Often
the surface expressions of dykes, fssures may also open near
the rims of unstable slopes, including craters.
FORECAST - A statement describing
the expected behaviour of a volcano.
FUMAROLE - Fissure or vent in the
surface formed by the escape of volcanic gases and heated groundwater.
FUMAROLE FIELD - Fumarole fields
are formed in active volcanic areas where the crust is hot at
very shallow depths and where there is a ready supply of water
in the form of precipitation. Rainwater or groundwater, is heated
underground and changed to steam by the hot rock beneath, and
makes its way back to the surface through cracks and fissures.
This process give the name to many of the volcanoes of the Antilles
as fumarole fields are called "soufriere" in french.
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