Types of volcanic hazard events
are volcanic hazards? A USGS fact sheet
occur where fragments of lava or rock fall back to earth after being
thrown into the air by volcanic explosions or carried upward by
an eruption column of rising gases. These air transported fragments
are termed tephra and the process forms a pyroclastic-fall deposit.
Pyroclastic-fall deposits may consist of combinations of pumice,
scoria, dense lithic material, and crystals and may range in particle
size from ash (diameter < 2 mm) through lapilli (2-64 mm), to blocks
solid and bombs molten (> 64 mm).
1. Food-producing fields of sugar cane destroyed by the pumice
fall of Mt. Pinatubo in June 1991. - V. E. Neall.
Burial and impact damage by falling fragments are the main hazards
posed to property by pyroclastic falls. Close to the volcano there
may be affects due to high temperatures, but the greatest danger
is from falling blocks and bombs (within 10 km of the volcano).
Ash may also have a serious effect on the respiratory system. Lethal
gases may also be associated with tephra close to source.
fall deposits may be heavy enough to cause the collapse of buildings
and break telephone and power lines. Crops can be killed or damaged,
and grazing livestock seriously affected, with effects ranging from
excessive tooth abrasion to death by fluorosis. Ash clouds pose
a serious hazard to all motorised traffic, particularly aircraft,
damaging and clogging engines. Ash clogs guttering and waste water
systems, and can infiltrate and damage or destroy electrical equipment.
Ash can also cause physical (turbidity) and chemical contamination
of water supplies.
falls can be potentially far more devastating if they are accompanied
by or followed by rainfall. Ash on top of buildings can soak up
large amounts of water and become much heavier - causing collapse
of buildings that may have been able to withstand an equivalent
amount of dry ash. Intensified rainfall runoff may also occur causing
excessive erosion and generation of sediment charged floods or in
some circumstances lahars.
close to the eruptive vent (generally <10 km), ejected blocks and
bombs can endanger people and property, although if low density
scoria or pumice is ejected it can be less damaging. Occasionally
pumice and scoria fragments > 10 cm in diameter can be hot enough
to start fires when they fall. This hazard is greatest within a
few kilometres of the vent. In 1973 fragments ejected from Heimaey
in Iceland smashed though windows and ignited the wooden floors
of buildings. Pumice fragments 20-30 cm in diameter started fires
on wooden roofs up to 10 km from vent during the 1707 eruption of
Fuji in Japan. Also, fires were started in forests and villages
within 8 km of Agung on Bali during a 1961 eruption.
area affected by pyroclastic fall depends on the height of the eruption
column, the volume of material ejected and the wind velocity and
direction. Hazard from ash is proportional to its thickness and
decreases downwind of the volcano.
Laterally directed blasts
events can be the most destructive of all volcanic phenomena. This
is because they can occur within a period of a few minutes with
little or no warning, can affect areas
up to hundreds of square kilometres, and can kill everything in
their path by impact, abrasion, burial and heat. People and animals
are often also killed by inhaling hot gases and ash.
directed blasts can be caused by sudden decompression of magmatic
gases or a high-pressure hydrothermal system. Decompression can
be via overpressure and explosive release or by unroofing of the
pressurised body by a landslide or debris avalanche. The resultant
blast can be directed upwards or at much lower angles, outward from
the sides of the volcano. The most hazardous are those directed
at angles approaching the horizontal or below it (i.e. down the
slopes of a volcano).
mechanisms within these blasts may include a combination of ballistic
ejection of rock fragments, pyroclastic flow, surge, and possibly
other as yet unrecognised processes. Thus the term lateral blast
denotes the overall event, even though it may involve more than
one process. Deposits of laterally directed blasts may be indicative
of one or more of the transport processes or in some cases evidence
of transport mechanisms is obscure.
2. On Windy Ridge, Mount St. Helens showing fallen logs from
former forest and stumps in lee of ridge (to left in distance),
destroyed by the lateral blast on 18 May 1980. Note how the blast,
travelling from right to left, removed all the vegetation and soil
from the exposed right-hand side of the ridge. Half way down the
ridge is a person for scale. - V.E. Neall
Clasts within laterally directed blasts travel at speeds much greater
than those expected from normal gravitational acceleration. At Mt.
St. Helens the lateral blast cloud travelled initially at 600 km/hr
and was still travelling at 100 km/hr 25 km from source. The deposits
of these events range from being cold to hot enough to start fires.
Parts of the Mt. St. Helens deposit remained at 277 oC
several weeks after it was deposited, while the remaining deposit
was around 150 oC a few days after deposition.
directed blasts can affect areas ranging between 1 to 100s
of square kilometres. In 1915, a lateral blast at Lassen Peak (U.S.)
affected 10 km2, and one at Mt. St. Helens 1150 yrs B.P.
affected around 40 km2. In contrast, the 1956 Bezymianny
(Russia) blast involved 1 km3 of rock debris and devastated
an area of 500 km2, and the 1980 St. Helens blast (with
c. 0.2 km3 of rock debris) affected a 600 km2
area. The larger blasts were accompanied by partial destruction
of the volcano, forming large horse-shoe shaped craters, open in
the direction of blast.
are defined here as hot, dry, masses of particulate volcanic material
and gas (mostly heated air) moving in contact with the ground surface.
The term Pyroclastic flow includes a spectrum of volcanic events
including: nuées ardentes or glowing avalanches, block and ash flows,
and pumice flows. Pyroclastic flows contain hot volcanic clasts
in contrast to avalanches of cold rock debris and other gravitational
processes on the flanks of volcanoes.
flows generally comprise three main components: a relatively dense
basal layer, one or more accompanying pyroclastic surges, and rising
clouds of ash above the other two components. Basal flows are concentrated
mixtures of ash to block- or bomb-sized fragments and gas whose
movement is controlled by gravity. Hence the basal flows are controlled
by topography, moving along depressions and valleys. Clouds of ash
elutriated from the top of the other parts of the pyroclastic flow
are dispersed by winds and deposited cool. Maximum temperatures
in pyroclastic flow deposits are often between 350 and 550 oC.
With greater distance from source, incorporation of air into the
flow can reduce the temperature of flows.
associated with pyroclastic flows include: asphyxiation, burial,
incineration, and impact damage by the large fragments within the
Pumiceous pyroclastic flows
flows contain predominantly hot to incandescent pumice, with subordinate
lithic fragments, crystals and gases. Other names for these events
are ashflows and pumice flows, and their deposits are often termed
ignimbrites or ashflow tuffs. Many pumiceous pyroclastic flows are
generated by the collapse of gas-rich vertical eruption columns
or clouds, whilst others are formed by ejection and flow of an emulsion
of frothy magma, accompanied by explosive activity. Pumiceous pyroclastic
flows and deposits can range from 0.001-1000 km3 in volume, 200
km in length, and 20 000 km2 in area.
pumiceous pyroclastic flows (Pumiceous pyroclastic flows of moderate
volume (1-10 km3) can extend for tens of kilometres from
volcanoes. At Mt. Mazama (U.S.) 7000 year-old flows extended 60
km down valley and one moved 213 m up a slope to cross a drainage
divide at 17 km from source. A minimum velocity of 234 km/hr is
required for the flow to climb this slope (assuming no friction,
otherwise required velocity is around 350 km/hr). Other flows have
crossed even higher barriers. The Ito pumiceous pyroclastic flow
(Japan) extended 70 km from the vent and crossed 720 m-high mountain
passes. The Campanian pumiceous pyroclastic flow (Italy) travelled
65 km and crossed many mountain passes up to 500 m high, and is
found 1000 m higher than source at 50 km distance.
very large pumiceous pyroclastic flows have occurred in the late
Pleistocene and Holocene, none have occurred during historic time.
Some prehistoric flows covered areas of 10 000 km2 with
volumes exceeding 1000 km3. These flows travelled up
to 150 km radially from source over topographic barriers >1000
m in topographic relief. Such a large flow would be an extreme hazard
to a large region and economically crippling to many countries.
Huge areas would need to be evacuated with even topographic highs
being vulnerable, if not from the flow itself, then from associated
surges and gases.
3. The pumiceous pyroclastic flow deposits of the Taupo Ignimbrite,
dated about 1800 years before present, forming upper half of a forestry
road cutting, east of Lake Taupo. Here the pyroclastic flow removed
the top of the pre-existing ridge and dragged the underlying tephra
beds into a huge overturned fold to the right of the spade. Follow
the white pumice layer above the head of the man with the red jersey
and note how it swings back on itself. The grey layer
between has been folded on top of itself rather like a folded newspaper.
Note also the drag thrust faulting in the beds to the
left of the cutting. - V.E. Neall.
Other widely distributed pumiceous pyroclastic flow deposits are
deceptively thin, with a very low ratio of thickness to lateral
extent (1:100 000 compared to normal flows with ratios of 1:500
- 1:1000). These are termed low-aspect ratio ignimbrites. Low-aspect
ratio pumiceous pyroclastic flows travel at exceptionally high,
and perhaps supersonic, velocities. One of these events, depositing
the 1800 year old Taupo Ignimbrite (N.Z.), surmounted Mt Tongariro,
1400 m higher than source. High velocities are also indicated by
their ability to knock down trees in directions radial from the
vent and transverse to the grain of local topography. The deposits
of these events are almost circular and centred on the vent, irrespective
of topography. The thin deposits of these types of flows are often
unpreserved, overlooked, or their significance unrecognised. The
Koya flow deposit in Japan extends 100 km from source but is mostly
<1 m thick. Many low-aspect flows travel distances up to 200 km,
the Taupo Ignimbrite is found>200 km from its vent.
volcano in Papua New Guinea has produced 3 small (<10 km3)
low-aspect type flows in the last 4000 years and Taupo volcano has
produced one with a volume of 30 km3 less than 2000 years
ago. Pumiceous pyroclastic flows of low aspect ratio-type constitute
an exceptional hazard because of their ability to surmount an entire
landscape adjacent to a volcano, leaving little potential for refuge.
Pyroclastic flows of lithic or scoriaceous rock debris
are termed glowing avalanches, nuées ardentes, hot avalanches, or
block and ash flows. These flows originate from; avalanches of hot
material from a volcanic dome, vertical expulsion of material from
a crater followed by fallback and flowage down the volcano flanks,
and laterally directed explosions at the base of a dome. Pyroclastic
flows formed in these ways are termed Merapi, St. Vincent and Peléean
4. Exposures of block-and-ash flow deposits in Pyramid Stream,
Egmont National Park. The lower two-thirds of the cliff exposure
comprises deposits emplaced at high temperature about 9000 years
ago. Above is 3 m of yellow-brown ash above which are units of the
Maero formation deposited by hot block-and-ash flows over the last
500 years. The lowermost units of the Maero formation contain numerous
charred logs from the pre-existing forest, whilst upper units contain
mainly twigs and blades of tussock grasses dated at younger than
1750 A.D. Person in white shorts at top of scree to left for scale.
Merapi- and Peléean-type flows generally affect a relatively narrow
sector of the volcano, often only a single valley, although wind-borne
ash clouds associated with the flows can affect a broader area.
St. Vincent type pyroclastic flows can simultaneously descend a
large sector or all flanks of a volcano.
are turbulent, low density mixtures of rock debris, gases and sometimes
steam or water, that move rapidly above the ground surface. Differing
types of surges have been termed glowing clouds, ash hurricanes,
base surges, ground surges and ash-cloud surges. Pyroclastic surges
typically hug the ground and the controlling effects of topography
depend on the surge velocity and density as well as the topographic
relief. Surges originating on a volcanic cone can attain high speed
on their descent of its flanks. Pyroclastic surges are subdivided
into those which contain hot rock debris and those containing only
cool or cold rock debris.
5. Dominantly surge deposits from Tepexitl tuff ring in the
Mexican Highlands, 170 km east of Mexico City. Note the coarse sandy
pinching deposits below, which were eroded by subsequent
finer-grained surges, with a layer of accretionary lapilli marked
by the arrow. - V.E. Neall.
Hot pyroclastic surges
can be generated in several ways. Explosion of a volcanic dome by
escaping gases under high pressure, or its disruption by a flank
collapse can both generate hot surges which generally affect a narrow
sector of a volcano. Also, the collapse of a vertical eruption column
may be followed by radiating-outward pyroclastic surges. The spill
over of an inflated mixture of ash and gases from a crater can also
create surges down one or more flanks of a volcano. Hot surges can
also be generated by a laterally directed blast; these can include
both hot and cold rock and are contained within a 180 degree sector.
pyroclastic surges can occur alone or can accompany a pyroclastic
flow of which they are a dilute, low-density facies. Such surges
are more independent of topography than the basal flows. They generally
move along the same general direction as the denser basal portion
of the pyroclastic flow, but in some instances the surges can detach
from the basal flows to travel on differing paths. Surges may also
outrun and jet forward of a pyroclastic flow.
hazards of hot pyroclastic surges include damage of vegetation and
structures, severe abrasion, impact damage by rock fragments and
burial by ash and debris. People can be killed by asphyxiation,
heat, presence of noxious gases and lack of oxygen. Only a strong
windowless, fireproof structure could protect people in the path
of a surge. The speed of surges can be so great that escape is impossible
once the surge has been generated. Hence, zones at risk from these
events should be evacuated before the start of an eruption that
may generate pyroclastic surges.
the margins of pyroclastic flows, seared zones mark the extent of
hot pyroclastic surges associated with them. The climactic 1951
eruption of Mt Lamington (Papua New Guinea) had seared zones up
to 4 km wide surrounding the limits of the pyroclastic flows. All
animals and most plants were killed within these zones.
Cold pyroclastic surges
are also generally known as base surges and are generated by phreatic
and phreatomagmatic explosions. The early stages of vertical explosions
can produce a surge that radiates laterally from the vent unconstrained
by topography. These surges have high velocities caused by the force
of the explosion. Collapse of material from a vertical eruption
column can also produce secondary pyroclastic surges, their velocity
is due to gravitational acceleration as the material falls to earth
and descends the volcano slopes. Cold pyroclastic surges contain
varying amounts of water, particularly in their distal parts indicating
a temperature <100 oC. The velocity of these flows ranges
between 50-300 km/hr, but they tend to lose their energy quickly
and typically stop within 10 km from source.
are modelled as being initially highly inflated, and cold surges
probably fluidised by steam and air entrained during the collapse
of an eruption column. As the surge cloud moves from the vent it
deflates as air escapes and steam condenses. The energy of the cloud
is dissipated by turbulence as well as friction with the ground
and air, causing the solid components to settle.
hazards of cold pyroclastic surges are include destruction of structures
and vegetation, severe abrasion, impact by rock fragments and burial
by debris. As in the case of hot surges, their velocities are too
high to permit escape. Hence, the only possible mitigation measures
possible are to evacuate people from the zones likely to be affected
prior to a potential surge-generating eruption and to avoid such
areas for construction and occupation.
Lahars and floods
and floods often accompany eruptions at snow-covered volcanoes and
those with crater lakes. The largest floods resulting from volcanic
activity are those in Iceland, where eruptions beneath glaciers
cause melting and the generation of huge floods, termed jökulhlaups.
The peak discharges of these enormous jökulhlaups can be at least
100 000 m3/s and reach 3-4 times this value. The most
recent of these events occurred in early 1997.
is a rapidly flowing mixture of rock debris and water (other than
normal streamflow) from a volcano. The term is of Indonesian origin.
Lahars can carry both hot and/or cold debris. Lahars that carry
more than c. 60% by volume sediment tend to flow in a laminar fashion
and are termed debris flows. Those with sediment concentrations
in the range of c. 50-20% by volume have varying degrees of turbulent
flow behaviour and are termed hyperconcentrated streamflows. Increasing
sediment concentration in a lahar slurry and particularly large
amounts of silt and clay-sized sediment enables lahars to attain
very high yield strengths and cohesiveness. Hence, lahars can often
carry very large masses of rock and sweep or carry away large man-made
can be generated in many ways both during and some time after an
eruption. These generation mechanisms include: - Sudden drainage
of a crater lake, either by explosion or collapse of a crater wall.
- Entrainment and melting of ice and snow by passing pyroclastic
surges and pyroclastic flows or by bombardment and fall of hot pyroclasts.
- Entry and mixing of a pyroclastic flow into a river. - Movement
of a lava flow onto snow and ice. - Avalanching of water-saturated
rock debris from a volcano. - Fall of torrential rain on unconsolidated
or fragmental deposits (particularly air fall tephra and pyroclastic
flow deposits following an eruption).
largest lahar formed during the 1980 eruption of Mt. St. Helens
was generated by remobilisation of a 2.8 km3 landslide
deposit which had been emplaced for a few hours to at least 25 km
from source. The lahar generated from the landslide deposit travelled
for another 70 km downvalley.
lahars were formed by melting and entrainment of snow and glacier
ice by pyroclastic flows on Nevado del Ruiz in Colombia in 1985.
These lahars claimed the lives of more than 23 000 people in towns
more than 70 km downstream of the volcano.
New Zealand, failure of a portion of Ruapehus Crater Lake
wall caused a lahar on Christmas eve 1953 which partially destroyed
a railway bridge at 42 km from source. A passenger train fell into
the lahar and 151 lives were lost.
the 1995-1996 eruptions of Ruapehu over 35 lahars were generated,
26 by explosive ejection of Crater lake water that entrained snow,
ice and rock debris to form lahars. The remainder were formed by
heavy rainfall on freshly deposited pyroclastic fall on the snow-covered
volcano slopes. These rainfall-induced lahars are sometimes called
secondary lahars or rain lahars in Indonesia.
6. The Kiosk building on the Whakapapa Skifield at Mt. Ruapehu
following the eruption of 22 June 1969. Note the large size of the
boulders picked up by the flowing sand and mud (lahar) that smashed
the left wall, which collapsed, and later phases of the flow lapped
up on the fallen roof. - V.E. Neall.
The distance reached by a lahar depends on its volume, water content,
and the gradient of its path. Many lahars can travel beyond 100
km from source. Broad and low gradient gullies tend to allow a lahar
to spread, slow and stop within a shorter distance than within a
narrow steep gully.
of lahars are greatest where the slope is steepest (i.e. on the
flanks of the volcano) and decrease with distance from source. Factors
such as channel width, solids:water ratio, and volume also affect
lahar velocity. An avalanche of rock debris travelling at 180km/hr
on Tokachidake volcano in Japan generated a lahar that travelled
its first 24 km at an average velocity of 58 km/hr. Lahars caused
by an eruption of Kelut (Indonesia) travelled their first 16 km
at 65 km/hr. An extremely long lahar generated during an eruption
at Cotopaxi volcano (Equador) travelled at an estimated 27 km/hr
for 300 km. The 1980 St. Helens lahars were estimated to be travelling
at speeds of >165 km/hr on the upper volcano slopes, however,
downstream of the volcano average velocities were generally <25
km/hr. During the 1995-1996 Ruapehu eruptions, lahars on the volcano
flanks were estimated to be travelling at speeds of>100 km/hr, but
downstream of the volcano they travelled between 10 and 30 km/hr.
affect population, agriculture and constructions on valley floors
at great distances from a volcano. Even if people are aware of an
eruption occurring they often consider themselves at no risk, being
so far away from the volcano, particularly if ash clouds are blown
in another direction. Due to their high density, lahars can displace
and carry large and heavy objects, including cars, buildings and
bridges. Burial of roads and destruction of bridges can often cut
evacuation routes from affected areas. Build up of rock debris in
river channels by lahars, reduces their capacity and often exacerbates
the effects of later floods or lahars which are able to spread more
widely and affect a larger area of valley floor. Lahars can also
cause changes in the course of river channels, block tributary streams
and create dammed lakes.
levees and other lahar retention structures can be built along river
channels to mitigate the effects of lahars but these structures
may be rendered ineffective by aggradation of the river channel,
particularly in a series of lahars. Following the 1991 eruption
of Pinatubo (Philippines) over 9 km3 of pumiceous pyroclastic
flow deposits were emplaced surrounding the volcano. Remobilisation
of these during the rainy seasons in the years since 1991 have produced
hundreds of lahars which have built up and buried large areas of
land including all retention structures built to contain them. Following
lahars of the 1951 eruption of Kelut volcano the Indonesian government
built an extensive system of diversion dams and lahar traps along
the valleys draining the volcano. These structures are designed
to limit the spread of future lahars across valley floors and to
bring them to rest without causing damage. The costs of these types
of constructions must be weighed up with the potential losses due
to lahar inundation and passage.
reservoirs can be effective traps for lahars, but if they are full
when a lahar enters, they can magnify potential hazards. On the
basis of past lahars generated by Mt. St. Helens, it was recommended
that 125 million m3 of storage capacity be made available
if an eruption began. This did occur during the 1980 eruption and
the dam was lowered by 7 m, a lahar of 13.5 million m3
entered the reservoir and caused its level to rise around 0.75 m.
rarely threaten human life because of their relatively slow motion
and the degree to which their paths are controlled by local topography.
If rates of lava emission and movement are determined, the flow
arrival time at various localities can be predicted. Some lavas
do flow rapidly, but these are normally confined to lava tubes or
channels. Higher flow rates are associated with steeper slopes,
greater emission rates and particularly fluid lava compositions.
Extremely fluid basaltic flows at Nyiragongo volcano in Zaire in
1977 killed around 300 people living on or near it. This was caused
by the sudden drainage of a lava lake in the crater that spread
lava over a 20 km2 area in less than an hour. The flow
had an extremely low viscosity with average depth being <1 m and
velocities of 30-100 km/hr.
7. The church of San Juan Parangaricutiro, destroyed by a lava
flow in June 1944, from the newly-formed Parícutin volcano in Central
Mexico. The church lies at around 4.5 km from the vent of Parícutin.
The lava flow travelled right to left in this picture, destroying
the side walls of the church and leaving the front and rear walls
standing. The remaining buildings in this town were poorly constructed
from wood and were completely destroyed by the advancing aa lava
flow travelling at c. 5 m per hour. - S. J. Cronin.
The distance that lava flows is determined by its volume, as well
as the topography and slope over which it flows. Basaltic lavas
may flow >50 km, but andesitic flows rarely extend beyond 20
km. One of the largest lava flows of historic times was from the
1783 eruption of Lakagigar in Iceland. Over 12.3 km3
of lava was extruded from a fissure and spread over a 565 km3
area, flowing for up to 65 km from source. Some prehistoric flood
basalt lavas have covered areas of 1000->10 000 km3,
although these highly destructive events are extremely rare in the
geologic record. Dacitic and rhyolitic lava extrusions tend to form
domes or very short stubby flows due to their very low rate of magma
flows cause total destruction in their paths by burning, crushing
and burial. Burial of land by lavas almost always results in a very
long term (near permanent in human terms) loss of agricultural production
of an area. Lava flows that melt snow and ice also have the potential
to produce destructive lahars and floods. Lava flows can also potentially
start fires in heavily vegetated areas.
8. Altar of the church inundated by lava at San Juan Parangaricutiro.
This church housed The Cross of Miracles which attracted
pilgrims from the surrounding areas within Central Mexico. When
destruction of the church seemed inevitable, the cross was removed
to be eventually installed in a new church constructed in the relocated
town, San Juan Nuevo. The altar stands c. 4 m high. - S. J. Cronin.
Volcanic domes and cryptodomes
dome is formed by the extrusion of highly viscous lava, which cannot
flow more than a few hundred metres from the vent. Lava is generally
extruded upward causing a mass of lava to inflate directly above
the vent. As the dome inflates and when it reaches a maximum height
of a few hundred metres it becomes increasingly unstable. Avalanches
and rock falls frequently occur down its sides, some caused by gas
release explosions and others by movement of the dome. Merapi-type
pyroclastic flows may be generated from the flanks of growing domes
and impact up to 10 km from source.
can grow near the summit of a volcano, anywhere on its flanks, or
even in areas surrounding it. Domes grow rapidly, many reach their
maximum size and become dormant within a year. Following the climactic
eruption of Mt. Lamington (Papua New Guinea) in 1951, a dome began
to form, initially growing up to 30 m per day. Pyroclastic flow-generating
explosions decreased its size and alternated with periods of further
slow dome growth until 1955. A dome was also extruded following
the 1980 St. Helens eruption and continued to grow and partially
collapse until 1983. During the 1990-1995 eruption of Mt Unzen (Japan)
a dome sporadically grew. Between 1991 and 1994 over 9000 Merapi-type
pyroclastic flows were generated by collapse and explosions from
the dome, causing the deaths of 44 people.
that form on or beyond the base of a volcano can damage and destroy
man-made structures, but their principal hazard is due to pyroclastic
flows and ballistic rock fragments thrown out by explosions.
are those that grow beneath the ground surface, and these pose less
hazards than many other volcanic events. However, a sudden unroofing
of a cryptodome on the flank of Mt. St. Helens precipitated an explosive
and destructive eruption. Unroofing of cryptodomes may result in
the eruption of pyroclastic material or lava.
composite cone of Usu volcano (Japan) has several cryptodomes on
its flanks in addition to one in its crater. A cryptodome grew during
1944 and after the ground surface rose 50 m a fissure eruption began
on top of the newly formed mound, the dome rose to 150 m high and
was later followed by extrusion of a 100-120 m high lava dome on
top in 1945. Cryptodomes associated with Mt. Misery on St Kitts
(West Indies) uplifted coral reefs up to 100 m above sea level.
Near Bazman volcano (Iran) three cryptodomes formed by basaltic
intrusions have uplifted Quaternary gravels by 60-100 m in an area
of monogenetic volcanism.
the mapping of volcanoes it may be possible to delineate areas where
domes and cryptodomes are likely to occur. The potential hazard
zones associated with these features needs to take into account
the range of laterally directed blasts, ballistics and pyroclastic
flows derived from a future growing dome.
explosions are due to the superheating of water or steam rather
than resulting from magmatic eruptions. Phreatic explosions are
common at composite volcanoes which are by nature composed of alternating
layers of permeable (and thus water holding) and impermeable strata.
Within these volcanoes, water movement can be restricted and if
heat is applied, water can be superheated until the vapour pressure
rises enough to exceed the confining lithostatic load. This results
in a steam explosion which is generally followed by a progressive
release of super-heated steam which is accompanied by rock debris
from the walls of the explosion conduit.
explosions may ballistically eject large rock fragments and produce
ash clouds. A series of phreatic explosions at Kilauea volcano (Hawaii)
in 1924 ejected around 800 000 m3 of rock debris. These
explosions were probably the result of groundwater entering the
conduit of the volcano along fractures caused by a sudden subsidence
of the crater.
associated with ascent of magma may cause rock fracturing and the
rise of superheated waters from depth, increasing vapour pressures
and causing phreatic explosions, even if no magma is ever erupted.
This occurred at Soufrière de Guadeloupe (West Indies) in 1975-1976
where seismic activity built up to 19 phreatic explosions which
ejected a mixture of steam at around 100oC, and old rock
magma rises into a volcano, it may cause heating of water within
the cone and generate phreatic explosions. Before the cataclysmic
1902 eruption of Mont Pelée (Martinique) weeks of phreatic explosions
occurred. Phreatic activity also preceded the 1980 St. Helens
some volcanoes a constantly high heat flow causes intense hydrothermal
alteration of rocks as well as phreatic explosions. The hydrothermally
altered rocks form an impermeable cap causing the build-up of vapour
pressure and producing the conditions necessary for phreatic explosions.
phreatic eruptions grade into those which involve varying amounts
of magma, these are termed phreatomagmatic eruptions. Some eruptions
along a fissure, such as the 1886 Tarawera eruption (New Zealand),
are magmatic at points where the water content of rocks was low
(e.g. on the Tarawera massif itself) and phreatic in topographically
lower areas (e.g. at Lake Rotomahana).
contain varying amounts of gases dissolved within them, these are
released during the ascent and eruption of the magma. Volcanic gases
mostly comprise water vapour, CO2, CO, and variable amounts
of S, Cl, F, H and N. Carbon monoxide is toxic, and even though
CO2 is not, it can commonly dilute or displace oxygen
and cause asphyxiation. Both of these gases are heavier than air
and odourless, they tend to flow down slope and collect in depressions
killing unsuspecting people and animals. In Java in 1979, phreatic
eruptions in the Dieng Mountains released gases, including CO2
and H2S, that moved downslope to kill 145 residents and
4 rescue workers trying to evacuate by foot. In 1986 over 1700 people
died during a sulphurous CO2 emission from the Nyos crater
and SO3 are common toxic volcanic gases and can be detected
easily by their strong odour; SO2 rapidly oxidises in
air to form sulphuric acid. Hydrogen sulphide (H2S) is
also detectable by its odour at low concentrations but at high concentrations
when it can be an irritant or cause asphyxiation it becomes odourless.
Nitrogen combines with hydrogen to form ammonia, which although
toxic, tends to form less harmful compounds. Volcanic gas compounds
are emitted and transported as acid aerosols, films on tephra particles,
and microscopic salt particles, in addition to their gaseous form.
gases can affect health and life as well as damaging crops and property.
Acidic components of gases affect eyes, skin and the respiratory
systems of animals and people. Some gases (e.g. SO2)
kill vegetation although the effects are highly dependant on gas
concentrations. Sulphurous gases emitted from Masaya volcano (Nicaragua)
at times seriously damage coffee and other crops up to 40 km from
the volcano. Acid rains caused by the mixture of raindrops and aerosols
and gases can cause skin irritation and fabric damage as far as
2000 km from source.
gases are rapidly diluted in air to sub-toxic concentrations, usually
within 10 km of the volcano. In rare cases of extreme gas release
over an extended period very serious regional effects can occur.
In 1783, Lakagigar volcano (Iceland) erupted large amounts of HCl,
SO2, CO2, and F with ash fall. This caused
a haze that persisted for months, killing and damaging vegetation
over the entire island, and contributing to the death of 50-70%
of the livestock. More than 10 000 human deaths occurred in the
resultant famine. A haze of fine ash and gas affected Europe and
other parts of the Northern Hemisphere, causing a detectable effect
most tsunamis are generated by large sea floor earthquakes, some
types of volcanic events can trigger them as well. At Unzen volcano
(Japan), volcanic earthquakes in 1792 triggered a huge landslide
into a shallow bay. This generated a tsunami with wave heights up
to 10 m that killed >14 500 people. During the violently explosive
eruptions of Krakatoa (Indonesia) in 1883, most of the 36 000 fatalities
were caused by huge tsunamis with waves over 30 m high. The tsunamis
were likely to have been generated by collapse of part or all of
the volcano, local subsidence of the sea floor, and pyroclastic
flows entering the sea. Other cases of volcanically triggered tsunamis
are discussed in the Tsunami
study guide for this course.