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Factors Contributing to Explosivity, Structural Flank Instabilities, Mass Edifice Failures and Debris Avanlanches of Volcanoes - Potential for Tsunami Generation


George Pararas-Carayannis


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.

INTRODUCTION

Earthquakes, volcanic 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.

Active geo-dynamic 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.

Radial fractures, escarpements and circular type of failure on the flank of the Piton de La Fournaise volcano on Reunion Island in the Indian Ocean.

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, 2002).

Destructive tsunamis 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.

Factors Contributing to Explosivity, Structural Flank Instabilities, Mass Edifice Failures and Debris Avanlanches of Volcanoes - Potential for Tsunami Generation

Geochemical Factors

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.

(Right) Explosive 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.

Growth and 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, 2004).

Volcanic Explosivity Factors

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, 2003).

Blast Geometry Factors

Additionally, the 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.

CITED REFERENCES AND OTHER PERTINENT BIBLIOGRAPHY

AAPG International Meeting,, 2003.Caribbean Plate Origin. Caribbean Tectonics, Barcelona, Spain, September 21-24.

Anderson, T., 1903. Recent volcanic eruptions in the West Indies. The Geographical Journal.

Anderson, T., 1908. Report on the eruptions of the Soufrière in St. Vincent in 1902, and on a visit to Montagne Pelèe in Martinique - The changes in the district and the subsequent history of the volcanoes. Philosophical Transactions of the Royal Society Series A, 208 (Part II):275-352.

Anderson, T., and Flett S. J.,1903. Report on the eruption of the Soufrière of St. Vincent in 1902 and on a visit to Montagne Pelèe in Martinique. Part I. Royal Society Philosophical Transactions Series A-200:353-553.

Aspinall, W.P., 1973. Eruption of the Soufrière volcano on St. Vincent island, 1971-1972. Science 181:117-124.

Aspinall, W.P., H. Sigurdsson, and Shepherd. J.B.,1973. Eruption of the Soufriere volcano on St. Vincent island, 1971-19721. Science 181:117-124.

Aspinall, W.P., Sigurdsson, H., Shepherd, J.B., Almorales, H. and P.E. Baker. 1972. Eruption of the Soufrière Volcano on St. Vincent island, 1971-72. In Smithsonian Institute for Short-Lived Phenomena.

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Brown, G.M., Holland, J.G., Sigurdsson, H., Tomblin, J. F. and R.J. Arculus. 1977. Geochemistry of the Lesser Antilles volcanic island arc. Geochimica et Cosmochimica Acta 41:785-801.

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