Factors Contributing to Explosivity, Structural Flank Instabilities, Mass Edifice Failures and Debris Avanlanches of Volcanoes - Potential for Tsunami Generation
Excerpts from presentation at the 2004 National Science Foundation Tsunami Workshop in San Juan, Puerto Rico , from other papers published in SCIENCE OF TSUNAMI HAZARDS, http://www.TSUNAMISOCIETY.ORG and from other publications and review of the literature.
eruptions, volcanic island flank failures and sub aerial and
underwater landslides have generated numerous destructive tsunamis
in the world's oceans and seas. Convergent, compressional and
collisional tectonic activity is responsible for zones of subduction,
the formation of island arcs and the evolution of particular
volcanic centers on the overlying plates. Inter-plate tectonic
interaction and deformation along marginal tectonic boundaries
results in seismic and volcanic events that can generate tsunamis.
processes in mid-ocean or along contintental boundaries create
arcs of island and continental volcanoes characterized by both
effusive and explosive activity. Contributing tectonic factors
to volcanic flank instability include arc volcanism that overlies
a subduction zone and which can result in the most catastrophic
types of eruptions. The volcanic eruption mechanisms are complex
and often anomalous. The factors that contribute to flank instability
may be different for each type of volcano. For example, lava
dome collapses often precede major eruptions of volcanoes in
the Eastern Caribbean Region and these eruptions may vary in
intensity from Strombolian to Plinian. Their style of eruptive
activity contributes to the development of unstable flanks.
escarpements and circular type of failure on the flank of the
Piton de La Fournaise volcano on Reunion Island in the Indian
Many additional specific
factors determine the eruption style, higher explosivity, the
generation of pyroclastic flows, the structural flank instabilities,
the slope failures and the debris avalanches that characterize
volcanoes that are mainly andesitic . Shield volcanoes, which
are mailny basaltic, have different styles of eruption - thus
the factors which contribute to mass edifice flank failures differ
from those of volcanoes along continental boundaries. However
all types of volcanoes can undergo large scale flank failures.
Mass edifice collapses of island volcanoes can generate tsunamis (Pararas-Carayannis,
may be generated from both aerial and submarine volcanic edifice
mass edifice flank failures, which may be triggered by volcanic
episodes, caldera and lava dome collapses, or simply by gravitational
instabilities. Locally catastrophic, short-period tsunami-like
waves can also be generated directly by lateral, direct or channelized
volcanic blast episodes, or in combination with collateral air
pressure perturbations, nuess ardentes, pyroclastic flows, lahars,
or cascading debris avalanches. Submarine volcanic caldera collapses
can also generate local destructive tsunami waves (Pararas-Carayannis, 2004).
The following is a
brief overview of some of the major factors which contribute
to the explosivity, structural flank instabilities, mass edifice
failures and debris avanlanches of all basaltic/dacitic/andesitic
volcanoes, and thus, to tsunami generation.
Contributing to Explosivity, Structural Flank Instabilities,
Mass Edifice Failures and Debris Avanlanches of Volcanoes - Potential
for Tsunami Generation
Whether a volcano
will have effusive eruptive activity or explosive type of bursts
will depend primarily on geochemical factors. The build up of
pressure of volatile gases within the magmatic chambers determines
a volcano's eruption style, explosivity, flank instability and
potential for subsequent tsunami generation.
Variations in the chemical composition
of volcanic eruptive effluents result from differences in the
mineralogy and the bulk composition of mantle material, differences
in depth at which melting occurs, differences in percentages
of melting at the source, and alterations in composition as the
magma rises to the surface (Wright
and Helz, 1987). The chemical composition of molten rocks in the
magmatic chambers of a volcano and the different abundances of
elements, particularly silica, determine the viscosity of effluents
that can rise to the surface.
(Left) Flow of low
viscosity pahoehoe lava of a mainly basaltic stratovolcano.
eruption (1976) of the Soufriere volcano on St. Vincent in the
Lesser Antilles islands of the Caribbean
Magmas, which are
low in silica and rich in iron and magnesium, like the basalts
of Hawaiian types of volcanoes, are very fluid. Basalt contains
anywhere from 45% to 54% silica, and generally is rich in iron
and magnesium. Dissolved gases escape prior to an eruption, thus
resulting in a subsequent effusive eruptive activity associated
with relatively gentle lava flows. Because of the low viscosity,
lava flows travel great distances from the eruptive vents, and
produce broad, shield-shaped volcanoes. Kilauea, in Hawaii, is
an example of such a volcano.
By contrast, the magmatic
chambers of stratovolcanoes along continental margins contain
magma which is composed primarily of andesite or dacite, that
are relatively high in silica and low or moderate in iron and
magnesium. Andesite may contain anywhere from 54 to 62 percent
silica. Thus magma material tends to be very viscous. Gases are
trapped and cannot escape until the magma enters the volcanic
conduits leading to the surface. The reduction in pressure, allows
the gas bubbles in the magma material to nucleate and expand.
When the outward pressure exerted by the gas bubbles exceeds
the strength of the lava material above it, the volcano erupts
violently. Because of the high pressure of the expanding volatiles,
the lava is fragmented to produce volcanic ash and pyroclastics
that are ejected out of the volcanic vent at high velocity. Most
of the ash travels far but pyroclastics accumulate near the vent,
thus producing a steep-sided stratovolcano with very unstable
flanks. An example of such a volcano is Mount St. Helens in the
State of Washington.
The volcanoes of the
Lesser Antilles in the Caribbean region are mainly andesitic
stratovolcanoes, characterized by both effusive and explosive
activity (Brown et al. 1977). However, even effusive
activity may culminate into explosive activity at later stages
of an eruption, as the more viscous magma material reaches the
surface. Explosive eruptions of mainly andesitic volcanoes, such
as those in the Eastern Caribbean, may last for hours and will
result in greater volcanic cone instabilities. Intensities and
types of explosive episodes may range from Strombolian, Vulcanian
to even Vulcanian/Plinian. During such eruptions, destructive
sea waves may be generated directly by pyroclastic flows, lahars
and nuées ardentes reaching the sea, and by atmospheric
pressure waves of blast events. Debris avalanches and massive
volcanic flank failures may be triggered which can also generate
destructive waves during an eruption or subsequently. The 1883
eruption of Krakatau provides the best understanding of the tsunamigenic
potential from mass failures and collapses of island volcanoes.
Collapses of Lava Domes
As a result of the geochemistry
and the higher viscosity of mainly andesitic magma, volcanic
activity often results in the formation and growth of lava domes
near a volcano's summit or along its flanks (Voight 2000). Rapid lava dome growth, after
a quiet volcanic activity period, is indicative that pressure
is building up within a volcano. Subsequent collapses of lava
domes often trigger a flurry of volcanic eruptions which will
vary in intensity and may be associated with catastrophic pyroclastic
flows, lahars, debris avalanches and large scale flank failures
which can generate directly destructive local tsunamis.
USGS Graphic of Mount
St. Helens Lava Dome Development, 1980-1983
In fact the collapse
of a growing lava dome has the potential of generating directly
destructive pyroclastic flows, lahars or debris avalanches, which,
upon reaching the sea, can generate destructive local tsunamis.
Indirectly, such volcanic processes may weaken the flanks of
the volcano, thus flank failures may be retrogressive and time-dependent.
Thus, lava dome growth, chemical and geometric factors, and the
mechanisms of weakening that result in eventual lava dome collapses,
are important factors in assessing a volcano's overall instability
and in forecasting a major eruption or even the generation of
a tsunami along a particular coastal area (Pararas-Carayannis,
As already indicated,
the sudden releases of gases are responsible for a volcano's
explosivity and flank instability and thus trigger flank failures
which can generate destructive tsunami waves. Additionally, the
direct explosive outflux of volcanic volatiles can create sudden
atmospheric pressure disturbances which, coupled with the sea
surface, can also generate destructive waves.
Significant atmospheric pressure perturbations that are propagated
by buoyancy forces during violent volcanic eruptions can have
periods greater than 270 seconds (Beer, 1974). The periods of the atmospheric
waves is dependent on the volumes of volatiles and the duration
of volcanic bursts. For example, spasmodic volcanic bursts of
Caribbean volcanoes during a violent eruption may produce extremely
large movements of air parcels similar to those produced by the
1980 Mount St. Helens eruption (Mikumo and Bolt, 1985; Tilling
1984; Tilling, et al 1990) or to
the 1992 Pinatubo eruption (Tahira
et al., 1996). Eruptive phases consisting of numerous eruptive
pulses may last from a few hours to days. Explosive pulses may
last from a few seconds to minutes and may vary in intensity
depending on the impulsivity of the degassing source (Newhall
& Self, 1982). Such volcanically generated atmospheric pressure
waves can generate several destructive tsunami-like waves of
longer periods with heights, which will not attenuate as rapidly
as those from other wave generation mechanisms.
For example, the fourth
paroxysmal eruption/explosion of the Krakatau volcano in Indonesia
on August 27, 1883, blew away the northern two-thirds of Rakata
Island. Almost instantaneously, the atmospheric pressure shock
waves coupled with the sea surface and, together with the massive
collapse of the volcano, were responsible for the generation
of tsunamis locally in the Sunda Strait. The waves that were
observed or recorded at distant locations, were generated by
the atmospheric shock waves from the paroxysmal explosions of
the volcano (Pararas-Carayannis,
geometry of eruption blasts is a factor that will also determine
a volcano's flank instability and its potential for massive failures,
avalanches or subareal or submarine landslides and subsequent
tsunami generation. Generally, volcanic blasts mechanisms can
be vertical, lateral-direct or channelized. If the blasts affect
directly a body of water, then more destructive local tsunamis
can be expected.
Vertical Blast Mechanisms
- More typical for
most volcanoes are the vertical type of blast mechanisms, which
will deposit unconsolidated pyroclastic debris and build up cones
with unstable flanks. Gravity forces may eventually lead to cone
collapse or other failures, which in turn may trigger lahars,
larger flank landslides or debris avalanches, usually on the
steep-sided stratovolcanoes. As previously discussed, strong
blasts can also generate atmospheric pressure perturbations which,
depending on duration and intensity, may couple with the sea
surface to form additional destructive waves of varying periods.
Distribution of deposits
from the May 18, 1980 lateral and channelized blast of Mt. St.
Helens (after Tilling, 1984)
Lateral and Channelized
Blast Mechanisms: Lateral and channelized blasting effects can
be far reaching if they occur near a body of water. For example,
large lateral blast from an erupting Caribbean island volcano
could be extremely destructive and could also generate much more
destructive local tsunami waves. Recently, the Soufriere Hills
volcano on the island of Montserrat had several lateral blasts
that made the city of Plynouth uninhabitable and resulted in
massive flank failures and the generation of local tsunamis.
Also, on May 8, 1902, a lateral blast of Mt. Pelée on
the island of Martinique destroyed the city of St. Pierre. The
violent volcanic eruption followed the collapse of a large lava
dome. Cascading lahars and nuées ardentes generated destructive
local tsunamis (Pararas-Carayannis, 2004).
Finally , the distribution
of pyroclastic deposits on the western flank of the underwater
volcano near the Caribbean island of Grenada known as Kick'em
Jenny, indicates that similar lateral blasting has occurred there
in the past. There is also a major escarpment that suggests other
types of massive slope failures underwater.
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