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Eruption
Phenomena and their Potential Hazards
Volcanic
phenomena directly associated with eruption
•
Lava flow, dome growth
• Pyroclastic flow, pyroclastic surge, lateral blast
• Tephra fall - ash fall, volcanic bomb
• Volcanic gas
Volcanic
phenomena indirectly associated with eruption
•
Lahar, flooding
• Debris avalanche, landslide
• Tsunami, seiche
• Subsidence, fissuring
• Secondary/hydrothermal explosion
• Secondary pyroclastic flow
Types
of hazards posed by an active volcano
LAVA
FLOW
Lava
flow is a
higly elongated mass of molten rock materials cascading
downslope
from
an erupting vent.
The lava flow being extruded has low silica and low
water contents.
-
Rate
of flow: 3
km/day (slightly high viscosity) or 45 km/hour (low viscosity)
-
Speed
and geometry of lava flows depend on local
topography.
-
Steep
slopes encourage
faster and longer flows than gentle slopes or terrain
DOME
GROWTH
Lava
dome is a
pile or mound of lava that grew on the floor of an active
crater, on the side
slopes
via a feeder vent that breached through the surface of
the edifice, or
inside
the volcanic edifice.
Types:
PYROCLASTIC
FLOW
Pyroclastic
flow refers
to hot dry masses of fragmented volcanic materials
that
move along the slope and in contact with ground surface.
This includes:
- pumice flow
- ash flow
- block-and-ash flow
- nuee ardente
- glowing avalanche
Mechanism...
Pyroclastic
flow mechanism:
Nuee
ardente is a
glowing eruption cloud characterized by:
-
extreme
heat (about 500 °C or higher)
-
high
gas content
-
rapid
flow down the slope of an erupting volcano
-
enormous
amounts of ash and other fragmental volcanic materials
A
nuee ardente may originate directly from an active crater or
from a collapse of a
growing
lava dome.
PYROCLASTIC
SURGE
Pyroclastic
surges are turbulent low-concentration density currents of
gases, rock debris
and
in some cases, water, that move above the ground
surface at high velocities.
Types:
- Ground surge
- Ash-cloud surge
- Base surge
HOT
BLASTS
-
Hot blasts arise when pent-up gases facilitate their way
out through the impermeable overlying materials and cause a very
rapid escape into the atmosphere. Blasts that are directed obliquely often do much damage
and could exact a high toll in human lives.
-
Lateral blasts are combination of pyroclastic flows and
pyroclastic surges with an especially strong initial
laterally-directed thrust. They have an initial velocity of 600 kph and slow down to
about 100 kph near its margin 25 km from the volcano.
TEPHRA
FALLS
Tephra
falls may consist of pumice, scoria, dense lithic materials or
crystals
or
combination of the four.
Particle
size:
- less than 2 mm diameter (ash)
- 2-64 mm diameter (lapilli)
- more than 64 mm diameter (blocks and bombs)
VOLCANIC
GAS
- Volcanic gas is one of the basic components of a magma or lava.
- Active and inactive volcanoes
may release to the atmosphere gases in the form of:
- water vapor
- hydrogen sulfide
- sulfur dioxide
- carbon monoxide
- hydrogen chloride
-
hydrogen fluoride
·
Aside from the major constituents, minor amounts of
nitrogen, methane, argon and
helium
may be also present in volcanic gases. The proportion of
these components
changes
with changing temperature.
LAHAR
-
Primary
or hot lahar - associated directly with volcanic eruption
-
Secondary
or cold lahar - caused by heavy rainfall
•
Lahar distribute and redistribute volcanic ash and debris
deposited around the volcano after the materials has cooled and
has become water logged.
Lahar in tropical areas can be produced by:
-
sudden draining of a crater lake, caused by either an explosive eruption or collapse of a crater fall (e.g.
Agua, Kelut, Ruapehu)
- movement of a pyroclastic flow into a river or lake, displacing and mixing with water
- avalanche of water-sustained rock debris, where water can be from heavy rain, hydrothermal activity or other sources
- torrential rainfall on unconsolidated deposits on slope
of a volcano (e.g. Pinatubo)
- collapse of a temporary dam, where recent volcanic
deposits have blocked a steam channel (e.g. Asama, Pinatubo
TSUNAMI
- Tsunami
are long-period sea waves or wave trains that are generated
by the
sudden
displacement of water.
They travel at high speed water as low
broad
waves and build to great heights as they approach shores.
- Most
tsunamis are caused by fault displacements on the sea floor
and of volcanic
origin
including volcanic or volcano-tectonic earthquakes, explosions,
collapse
or subsidence, landslides, lahars, pyroclastic flows or
debris avalanches
entering
bodies of water, and atmospheric
waves
that couple with the sea.
OTHER
ERUPTION PHENOMENA
Debris
avalanche -
fast downhill movement of soil and rock
·
speed: 70 km/hr (due to high water content and steep
slopes)
·
caused by slope failure on the cones of stratovolcanoes
Hydrothermal
explosions -
explosions from instantaneous flashing of steam
upon
contact
with hot rocks
Secondary
explosions
are caused by the contact of water with hot pyroclastic flow
deposits.
Subsidence
is a ground deformation resulting from the downward adjustment
of surface
materials
to the voids caused by volcanic activity.
This
may result also from mine workings or geothermal water or oil
extraction.
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