Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 Volcano instability: a review of contemporary themes W. J. McGUIRE Department of Geography & Geology, Cheltenham and Gloucester College of Higher Education, Francis Close Hall, Swindon Road, Cheltenham GL50 4AZ and Department of Geological Sciences, University College London, Gower Street, London WCIE 6BT, UK Abstract: Active volcanoes are revealed to be dynamically evolving structures, the growth and development of which are characteristically punctuated by episodes of instability and subsequent structural failure. Edifice instability typically occurs in response to one or more of a range of agencies, including magma emplacement, the overloading or oversteepening of slopes, and peripheral erosion. Similarly, structural failure of a destabilized volcano may occur in response to a number of triggers of which seismogenic (e.g tectonic or volcanic earthquakes) or magmagenic (e.g. pore-pressure changes due to magma intrusion) are common. Edifice failure and consequent debris avalanche formation appears to occur, on average, at least four times a century, and similar behaviour is now known to have occurred at volcanoes on Mars and Venus. Realization of the potential scale of structural failures and associated eruptive activity has major implications for the development of monitoring and hazard mitigation strategies at susceptible volcanoes, which must now address the possibility of future collapse events which may be ten times greater than that which occurred at Mount St Helens in 1980. Since the spectacular landslide which triggered Labazuy this volume), Martinique (Semet & the climactic eruption of Mount St Helens during Boudon 1994), Stromboli (Kokelaar & Romag- May 1980 (Lipman & Mullineaux 1981), con- noli 1995), Augustine Island (Beg& & Kienle siderable attention has been focused upon the 1992), and the Canary Island volcanoes (Hol- unstable nature of volcanic edifices, and their comb & Searle 1991; Carracedo 1994, this tendency to experience structural failure. This volume; Weaver et al. 1994) amongst others. behaviour is now recognized as ubiquitous, with Imagery gathered using the Viking, and more evidence for edifice collapse identified both recently Magellan, spacecraft, has also revealed within the geological record 9and at many that volcano instability is not confined to the currently active volcanoes (Siebert 1984; Ui Earth, with considerable evidence supporting 1983). Francis (1994) notes, for example, that edifice failure in volcanic terrains accumulated 75% of Andean volcanic cones with heights in for both Mars (Cave et al. 1994; Robinson & excess of 2500 m have experienced collapse, while Rowland 1994; Crumpler et al. this volume; Inokuchi (1988) reports that over 100 debris Head this volume) and Venus (Guest et al. 1992; avalanche deposits have been identified around Bulmer & Guest this volume; Head this volume). Japanese Quaternary volcanoes. The potential Such phenomena are proving particularly sig- hazard presented by such behaviour is stressed nificant in permitting the effects of such factors as by Siebert (1992) who estimates that structural variations in gravity and atmospheric pressure failure of volcanic edifices has occurred four on the incidence of edifice failure and the times per century over the last 500 years. This formation and transport of debris avalanches. may in fact be an underestimate, with three Volcano instability can be defined as the major sector collapses occurring this century in condition within which a volcanic edifice has the Kurile-Kamchatka region alone (Belousov been destabilized to a degree sufficient to in- 1994), and avalanche-produced cirques evident crease the likelihood of the structural failure of on 22 Kamchatkan volcanoes (Leonov 1995). all or part of the edifice. Failure may occur in Advanced submarine imaging techniques have response to active deformation or may result also-shed light on the frequency of collapse at over a long period of time due to oversteepening, island and coastal volcanoes, with extensive overloading, or peripheral erosion. Failure sur- debris avalanche and associated deposits identi- faces and post-failure mass transport may be fied on the sea floor adjacent to the Hawaiian predominantly vertical, as in the formation of volcanoes (eg, Fornari & Campbell 1987; Moore collapse pits and calderas, or may incorporate a et al. 1994; Garcia this volume), Piton de la significant horizontal vector as in dome disin- Fournaise (R6union Island) (L6nat et al. 1989; tegration, sector collapse, or lateral edifice From McGuire, W. J., Jones, A. P. & Neuberg, J. (eds) 1996, Volcano Instability on the Earth and Other Planets, Geological Society Special Publication No. 110, pp. 1-23. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 2 W.J. McGUIRE spreading. These latter phenomena have con- the giant megaslides involving volumes in excess stituted a particular focus for recent, 'post- of 5000 km 3 generated around large ocean-island Mount St Helens' research, and a review of these volcanoes (e.g. Moore et al. 1994; Carracedo studies forms the basis of this paper, which 1994). Whereas low-volume collapse events seeks to address topical themes in the study of probably occur at one active volcano or another instability- and failure-related phenomena both every few weeks or less, the largest events have on the Earth and other bodies in the solar repeat times of tens to hundreds of thousands system. No attempt is made here to address of years (Fig. 1). The causes of instability and problems associated with coUapse-caldera for- failure are manifold (Fig. 2), with some vol- mation, which is largely driven by excess canoes having a far greater potential for failure buoyancy of the magma in large reservoirs. than others. Generally speaking, major struc- Discussion of the structure and formation of tural failure is confined to the larger edifices, collapse calderas and caldera-like structures is, with small monogenetic cones and shields only however, included in a number of papers experiencing small-scale slumping and sliding. elsewhere in this volume (De Rita et al.; Marti Massive structural instability is only charac- et al.; Crumpler et al.) Here, emphasis is placed teristic of major polygenetic volcanoes. These on the factors responsible for the development may be located on continental (e.g. Etna in of edifice instability in active volcanic terrains, Sicily, Rainier in the Cascade range, and Colima and the triggers which lead to destabilization in Mexico) or oceanic (e.g. Mauna Loa and and failure involving a significant lateral com- Kilauea on Hawaii, Piton de la Fournaise on ponent. Consideration is also given to the R6union Island, and Martinique in the Carib- hazards posed by volcano instability and col- bean) crust, and on other planetary bodies (e.g. lapse at all scales, and attention is paid to the Olympus Mons on Mars). Instability and failure problems involved both in forecasting failure appear to be frequently induced in large, basaltic events and mitigating their effects. shield volcanoes, despite low slope angles and homogeneous structure. Here rifting, associated with persistent dyke emplacement constitutes a Generating structural instability and failure major contributory factor in the progressive in volcanic terrains development of instability, with local seismicity, changes in edifice pore pressures, and environ- Growing volcanoes may become unstable and mental factors, such as large, rapid changes experience failure at any scale (Fig. 1), from in sea level, all constituting potential failure relatively minor rock falls, with volumes of a triggers. In marine settings, instability may be few hundred to a few thousand cubic metres, increased due to edifice spreading along weak occurring along caldera rims and other steep horizons of oceanic sediment (Nakamura 1980) slopes (e.g. Rowland & Munro 1992; McGuire or in response to seaward-creeping masses of et al. 1991, 1993; Munro & Rowland 1994), to olivine cumulate (Clague & Denlinger 1994). Large polygenetic edifices developed on con- tinental crust are particularly prone to failure, although the scale of the collapse events in Large st~""~ these environments rarely matches those recog- 1011 bmarine ) nized at their oceanic counterparts. Continental edifices are typically stratovolcanoes composed of mechanically unsound materials which are ~0" often superimposed in such a manner (e.g. alter- nating lava flows and weak pyroclastic layers), /'f~rger roc~ and weakened by hydrothermal alteration, so as I 0 s ~,. calclera rim & dome ) to reduce the strength of the edifice as a whole. The potential for instability and structural failure is compounded by steep slopes and high 102 precipitation rates commonly associated with elevated relief, which may contribute to changes I I 1 I I I I I in edifice pore pressures. 10 ~ 10 2 I 0 s The development of instability and the Frequency (years) potential for failure is enhanced at all types Fig. 1. Volume-frequency plot illustrating the range of of volcano by the fact that actively growing scales and repeat-times displayed by collapse events in edifices experience continuous changes in mor- active volcanic terrains. phology, with the endogenetic (by intrusion) and Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 VOLCANO INSTABILITY: A REVIEW 3 incremental displacement climaticeffects ~'J'"~ '~"'~ ~ 'Y~ due to repeateddyke i ntrusi~ \