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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~ \ steepslopes ~////~J .... \ ~,~ oversteepeningdue to sunace ioaamg \,, " '~/~~agma intrusion .... \ "\ ~./'-"~l~l.~ changingsea levels eamce.. \ .,,,Y"JJ...-.II~~ (peripheralerosion & ~a__~_mg__ ~~-,,,~~ebuttressing)

mechanically i ,i, I1~ volcanic l~ unsound basementuplift seismicity tectonicseismicity structure and subsidence and fault activity

Fig. 2. Factors contributing towards the development of structural instability at active volcanoes. exogenetic (by extrusion) addition of material tens of thousands of years. Rapid-onset instabil- leading, characteristically, to oversteepening and ity is usually the result of a discrete event, such overloading at the surface. The behaviour of the as the emplacement of the dacite cryptodome at sub-volcanic basement is also important, with Mount St Helens between March and May 1980, both subsidence and uplift beneath the volcano which generated over 100m of lateral surface having the potential to decrease edifice stability. displacement in little over a month (Fig. 3a) Furthermore, the growth of a volcanic edifice (Christiansen & Peterson 1981). In contrast, on a sloping or weak (e.g. clay-rich) substrate, slow, progressive destabilization is typically strongly favours the development of lateral incremental, and often results from the cumula- spreading with all that this entails for the evo- tive effects of numerous small events. These may lution of instability both within and below the take the form of successive eruptions leading edifice. Determination of the relative impor- to the gradual loading of a steep slope by tance of the above factors, both in general terms, accumulating eruptive products (e.g. Murray & and for specific volcanoes, is crucial to any Voight this volume), or, as at Kilauea (Swanson assessment of edifice instability and the potential et al. 1976) and Etna (McGuire et al. 1990, for failure. 1991) (Fig. 3b) may be represented by persistent Although magma often has a role to play, the dyke emplacement along rift systems causing single, linking, contributory factor, in both the increased flank destabilization due to progres- development of instability and in the initiation sive lateral displacement. of structural failure, is gravity. Not only does If the appropriate conditions are maintained, this directly influence the edifice stress regime in then slow, progressive destabilization may be such a manner as to favour increasing destabi- cyclic. At Augustine volcano (Alaska) for lization as edifices grow larger through time, example, edifice failure is followed by a period and phenomena such as oversteepening and of reconstructive dome-growth which persists overloading become more prevalent, but it also until oversteepening causes another failure and provides the energy for the post-failure trans- the cycle begins again. Beg~t & Kienle (1992) port of the detached material. This applies recognize at least eleven major debris avalanches equally well to edifice-growth driven lateral at Augustine, separated by an average time collapse events, due for example to persistent interval of only 150-200 years. Such a short rifting, and to the creation of vertical collapse repeat time for edifice failure is unusual, and calderas in response to plumbing-system related emphasises the very real threat to the local instabilities such as magma-reservoir inflation, settlements around the Cook Inlet from volca- or eruption-related reservoir evacuation. nogenic tsunamis. Normally edifice-destabiliza- Destabilization of all or part of a volcanic tion cycles are of much longer duration. At edifice may be achieved over a period of weeks Colima (Mexico) for example, recurrent col- to months, or may develop over thousands or lapses appear to occur every few thousand years Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

4 W.J. McGUIRE

60 occurred in a matter of seconds in response to the seismogenic ground accelerations which led to the g 50- detachment of the northern flank and the succeeding generation of a catastrophic debris o Eo 40- avalanche. In some situations, however, deforma- E -~ 30- tion following structural failure may occur in a complex manner over a long period of time, and s 20-- involve little in the way of rapid mass-movement. tion At Etna, for example, a large sector of the eastern 10- ~ flank has become detached from the remainder of the volcano, and has been sliding slowly '1' '''i'~ 'f I ' '' '1'' ft i~l '~V seawards under gravity for at least tens of 25 I 5 10 15 20 thousands of years (e.g. Kieffer 1985; Borgia et April May al. 1992; McGuire & Saunders 1993; Stewart et al. 1993; Firth et al. Montalto et al. Rasa et aI. and Rust & Neri all this volume). '~' re~ings ~) The slow, long-term, slumping under gravity of the eastern flank of Etna, and similar phenomena, do not require a discrete, instanta- neous failure event to initiate movement, nor is any high velocity transportation of a large mass of material involved. Both phenomena do, however, occur during formation of debris avalanches, and the mechanisms involved are considered here. Once a block of volcanic terrain has become destabilised, due to one or other of 0 1~)0 200 the conditions outlined in the previous section, it Months (January 1981 = 0) becomes susceptible to failure in response to one or more of a number of 'internal' or 'external' Fig. 3. (a) Rapid-onset instability is illustrated with triggers. The former are dominantly related to reference to the growth of a dacite cryptodome the extrusion or intrusion of fresh magma. beneath the northern flank of Mount St Helens prior Magma reservoir replenishment, the filling of to structural failure and eruption on 18 May 1980. open conduits, and dyke emplacement, all have Between late April and this date, ground deformation the potential to trigger structural failure due to monitoring revealed over 30 m of lateral growth of the surface bulge which developed above the cryptodome causing changes in pore pressure (Elsworth & (from McClelland et al. 1989). (b) At Etna, a sector of Voight; Day; both this volume) or the over- the 1 km high cliff wall bordering the Valle del Bove steepening of surface slopes (Murray & Voight flank collapse is being progressively displaced by this volume), while extrusion of additional repeated dyke-induced rifting. Since 1981, four rifting material onto an already heavily loaded slope events have resulted in line-length increases totalling may also initiate failure and collapse. Other almost 5 m in a ground deformation network. internal triggers include strong volcanogenic earthquakes, and displacements associated with long-term edifice spreading. Many external triggers reflect the dynamic geological settings (Komorowski et al. 1994), while the giant flank in which volcanoes are commonly located, and collapses of the Hawaiian volcanoes have a include basement fault movement and related repeat time of the order of 25-100 Ka (Lipman tectonic seismicity. Environmental factors may et al. 1988). also be important, with both precipitation (on a short time-scale) and changing sea levels (on a longer time-scale) having the potential to initiate Triggering edifice failure structural failure by affecting pore pressures in the edifice. Structural failure- the common outcome of As previously mentioned, the role of magma edifice instability- may, like the attainment intrusion is typically paramount in triggering of instability itself, take place over a range of structural failure occurring during eruption. This time-scales, although it often takes the form may occur in response to a critical, destabilising of a near-instantaneous, catastrophic event. The change in edifice morphology, as in the Mount eventual failure at Mount St Helens, for example, St Helens case, or may result from other less Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

VOLCANO INSTABILITY: A REVIEW 5 obvious effects. In particular, Voight & Els- here over 300m of vertical displacement worth (1992) and Elsworth & Voight (this occurred prior to cessation of deformation. volume) have drawn attention to the potential The faults of this aborted collapse are well role of dykes in triggering structural failure as a exposed and are characterised by dry fault consequence of their raising pore pressures by breccias and ultra-cataclasites, rather than the means of mechanical and/or thermal straining. extremely fluidised gouge muds and mud-rich breccias encountered in exhumed collapses elsewhere in the Canary islands (Day this The role of water in structural failure volume and pers. comm.). It is possible that this lack of pressurised fluids on the faults, and There is increasing evidence that water plays a consequent lack of significant slip weakening an important role in the destabilization and (Rubey & Hubbert 1959; Rice 1992) or of mechanical failure of volcanic edifices, and in brecciation of the collapsing slump blocks by determining the manner in which the failed mass gouge dykes (Day this volume) may have behaves. Not only does hydrothermal alteration caused fault movement to cease without often play a major part in increasing suscept- debris avalanche generation. This evidence ibility to failure (Siebert et al. 1987), but the from the Canaries permits the possibility of common association of lateral collapse events reinterpreting large flank slumps, such as those with phreatic explosions (e.g. at Bandai-san, of Kilauea, as aborted collapse events which Japan, in 1888) supports a significant involve- might have generated major debris avalanches ment by hydrothermal pore fluids in the failure if conditions had been more appropriate to the process. Day (this volume) proposes that this generation of high pore pressures, either in the results from a reduction in rock strength in fault zones or within the slump blocks as a response to the generation of pore pressures whole, and thus to sustained slumping and which are a large fraction of, or even higher, slump-block disaggregation (Elsworth & Voight than contemporary confining pressures. As this volume; Day this volume). mentioned in the previous section, this situation may arise due to the heating effects associated with the intrusion of fresh bodies of magma Post-failure mass transport (Voight & Elsworth 1992; Elsworth & Voight and emplacement this volume). Additionally, Day proposes that similar conditions may result from intrusion Structural failure in volcanic terrains inevitably degassing, the discharge of pressurised fluids involves the downslope, gravity-driven, mass- from depth via clastic dykes, or by faulting- transfer of material from the source to an area associated deformation and pore collapse. of deposition. In cases of relatively minor rock- Once structural failure has been initiated, falls and slumps, the distances involved may water may also have a role to play in deter- amount to only a few tens or hundreds of mining the behaviour of the collapsing mass, metres, whereas at the other end of the scale, and in particular whether the event is aborted or gigantic debris avalanches may be transported proceeds to form a debris avalanche. The islands to distances of several hundred kilometres of La Palma and E1 Hierro (Carracedo 1994, (Stoopes & Sheridan 1992; Moore et al. 1994). this volume) in the Canary Islands, for example, Table 1 summarizes the more important para- provide excellent evidence for major, rift- meters of some of the longer, larger volume, related, flank failure, but also two examples volcanogenic debris avalanches. Depending of lateral collapse events which appear to upon a number of factors, including the have been aborted during the early stages of nature of the failed material and the underlying sliding. On the western side of the Cumbre Vieja terrain, and the precise failure mechanism, the ridge at La Palma, eruptive activity during transported mass may travel in a largely 1949 was accompanied by the opening of a coherent manner or may become totally dis- concave-downslope fracture system some 4 km rupted. Transport velocities may vary enor- in length. The vertical displacement on the mously, from over 100 m s -1 where failure has fracture amounts to only a few metres, and been catastrophic, to as little as 1-2cma -1 testifies to a seaward-sliding event which was where displacement of the detached mass aborted immediately following initial failure. involves creep-like behaviour. Structural failure On the neighbouring island of El Hierro, may be confined solely to the volcano, or may a similar event appears to have taken place, involve the underlying basement, with conse- forming the San Andres fault system (J. C. quences, in the latter case, for the composition Carracedo & S. J. Day pers. comm.) although of the deposits formed. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

6 W.J. McGUIRE

Table 1. Volumes and runout distances of selected subaerial volcanic debris avalanches

Volcano Deposit Volume Runout (km 3) (km)

Nevado di Colima 22-33 120 Socompa 17 35 Volc~in de Colima 6-12 43 Shasta 26 50 Popocatapetl 28 33 Chimborazo Riobamba 8.1 35 Mawenzi 7.1 60 Akagi Nashikizawa 4 19 Galunggung 2.9 25 Mount St Helens 1980 2.5 24 Fuji Gotenba 1.8 24 Shiveluch 1964 1.5 12 Bandai-san 1888 1.5 11 Egrnont Pungarehu 0.35 31 Unzen 1792 0.34 6.5 Asakusa Migisawa 0.04 6.5

Data from Hayashi & Self (1992), Stoopes & Sheridan (1992), and Wadge et al. (in press).

Inevitably, the downslope removal of material accompanying volcanic activity, typically pro- during the failure process leaves voids in the duces deposits made up of more or less disrupted source area. Where large-scale, catastrophic volcanic material with or without a basement failure has take place, these commonly take the contribution. Where fresh magma is involved, form of near parallel-sided amphitheatre-like either in triggering the collapse or due to post- depressions which open downslope (Siebert failure unroofing and decompression of a 1984) and which are typically surrounded, or shallow reservoir, juvenile material may be partly bounded, by steep walls which may be in present in the form of chilled lava fragments excess of a kilometre in height, although more which may show evidence of a plastic nature complex forms also occur. Less spectacular during transport. Such material is reported by events of an effectively instantaneous nature Wadge et al. (in press) in the 7000 years BP may leave small collapse scars, for example Socompa debris avalanche deposit (Chile), along the margins of actively growing domes or where the occurrence of fresh, glassy lava unstable caldera rims. Where failure-related blocks of dacitic composition is interpreted in transport is a slower, longer-term process, the terms of the extrusion of lava during the collapse area between the mobile, detached block and the event. Where structural failure triggers a major remainder of the edifice is typically marked by decompressive eruption, as during the May 1980 incipient or open fractures or by active fault event at Mount St Helens (Fig. 4), base-surge systems. Although the more impressive morpho- and pyroclastic flow formation closely follows logical features associated with catastrophic the generation of the debris avalanche, and failure may remain extant and easily recognisa- deposits of these magma-rich events may be ble for a considerable period of time, those mixed with or overlie the more lithic-dominated associated with less-rapidly operating mechan- avalanche material. isms may be difficult to discern, and the recog- A volcanic 'dry' avalanche deposit has been nition and nature of the displacement may defined by Ui (1983, p. 135) as a 'volcaniclastic require the use of ground deformation monitor- deposit formed as a result of large-scale sector ing techniques. collapse of a volcanic cone associated with some Because catastrophic failure events are com- form of volcanic activity'. This is, however, an monly associated with eruptive or intrusive unnecessarily restrictive definition as it implies activity, the deposits they produce often reveal that deposits formed by similar mass-removal a wide range of lithological and sedimentologi- events at volcanoes which are not associated cal characteristics (Ui 1989). The generation of with volcanic activity are somehow different. 'dry' avalanches (Ui 1983), in the absence of A more appropriate definition may be that Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

VOLCANO INSTABILITY: A REVIEW 7

Fig. 4. The spectrum of deposits associated with the climactic 18 May eruption at Mount St Helens reveals how a major structural failure event can trigger a wide range of potentially hazardous eruption-related phenomena including pyroclastic flows, lahars, and atmospheric blast (after Lipman & Mullineaux, 1981). proposed by McGuire (1995, p. 88), which topographic lows coincide with the more defines volcanic 'dry' avalanche deposits as disrupted and finer-grained material, and suc- 'having been formed by the large-scale collapse cumb rapidly to post-depositional modification of a volcanic edifice, or part thereof, in the in which slumping and water channelling have a absence of significant amounts of water'. tendancy to fill-in the lows and produce a planar In many cases, however, the avalanches surface from which the megablocks protrude. formed during catastrophic edifice failure are Marginal levees are a common morphological far from 'dry'. Available water from saturated feature, and have been described from the 1792 volcaniclastic sequences, from snow and ice avalanche deposits at Unzen volcano, Japan (Ui fields, or from surface water bodies commonly 1983), from Mount St Helens (Voight et al. becomes entrained into the avalanching mass 1981, 1983), and from Volc~m Socompa in Chile causing a progressive transition from dry debris (Wadge et al. in press). avalanche to debris flow or debris-laden flood Internally, dry, volcanic debris avalanche (broadly termed lahars when generated in deposits typically have a bimodal fabric consist- volcanic environments) (Fig. 4). Large volumes ing of block and matrix facies (Crandell et al. of surface water are not, however, essential to 1984). Block dimensions may range from metres transform a debris avalanche into a debris flow. to hundreds of metres, and represent fragments According to Fairchild (1987), the water source of volcanic material together with entrained for the North Fork lahars at Mount St Helens debris. Blocks are characteristically strongly was finely comminuted ice incorporated within fractured and broken, but individual pieces are the avalanche deposit during collapse. Lahar often only in close proximity, resulting in the formation here is attributed to liquefaction of formation of an interlocking 'jig-saw' texture. the water saturated debris avalanche due to This fabric argues against turbulent flow during harmonic tremor associated with the post- emplacement, and for the transport of giant collapse eruption. megablocks in a coherent manner. As might be Large debris avalanche deposits formed by expected, both the frequency and size of mega- structural failure display a number of common blocks fall with increasing distance from the morphological and fabric-related features source, corresponding to a complementary rise (Fig. 5). The surfaces of such deposits are in the matrix proportion (Mimura & Kawachi typically hummocky in form, reflecting the 1981). At both Mount St Helens (Glicken 1991) underlying presence of relatively undisturbed and Mount Shasta (Crandell 1989) in the United megablocks and megablock 'nests' which may States, and at Nevado de Colima in Mexico have dimensions in excess of a kilometre. The (Stoopes & Sheridan 1992), debris-avalanche Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

8 W.J. McGUIRE

Fig. 5. Anatomy of a typical volcanic debris avalanche and source amphitheatre. Characteristic features include a region of relatively coherent blocks emplaced by toppling or sliding in the vicinity of the collapse source; a bimodal megablock and matrix facies; hummocky terrain commonly bounded by marginal lev6es; and the development of secondary lahars. Source amphitheatres may be in excess of 10km across, while associated subaerial volcanogenic debris avalanche deposits may be over 100 km long. Megablocks may be over 1 km across and marginal lev6es may reach heights of over 100 m.

blocks are rare in distal locations, and the The role of persistent rifting deposits are dominated by the poorly-sorted, pulverised matrix of angular clasts. Commonly, Siebert (1984), in his global review of the these clasts consist of centimetre- to decimetre- occurrence of volcanogenic debris avalanches, sized, lithic fragments of volcanic material, drew attention to the fact that structural collapse although soil and other components may have was more common at edifices characterised by been entrained during transport. Evidence from the existence of parallel dyke swarms. This link, the Socompa debris avalanche deposit in Chile which supports the role of persistent, dyke- (Francis & Self 1987), and from other volcanoes induced rifting as one of the major causes of indicates that the spatial distribution of clast edifice destabilization and failure, is clearly types reflects the original stratigraphy of the illustrated by the orientations of the long axes collapsing mass. This supports observations of flank collapse scars relative to adjacent dyke which reveal that debris avalanche emplace- zones. At Stromboli (Tibaldi et al. 1994; Tibaldi ment involves laminar rather than turbulent this volume) and La Palma (Canary Islands) flow. (Carracedo 1994, this volume) for example, As previously mentioned, where water is these are oriented normal to a single dominant present, debris avalanche deposits often grade dyke zone (Fig. 6a and b), whereas at Etna downslope into debris flows, the deposits of (Fig. 6c) (McGuire & Pullen 1989; McGuire et which may be distinguished on the basis of al. 1993), Piton de la Fournaise (R6union Island) sedimentological differences. Such a transition (Duffield et al. 1982), and E1 Hierro (Canary is observed both at Mount St Helens and at Islands) (Fig. 6d) (Carracedo 1994, and this Mount Shasta (Crandell 1989). Typically, volume), sector collapses bisect the angles megablocks are absent in debris flow deposits formed by two intersecting zones of persistent derived from debris avalanches, and the fabric dyking. Where repeated dyke emplacement is similar to that of the matrix facies of the along a preferential path has taken place over latter. Reverse grading may be present, while a long period of time (e.g. 104 years or more), stratification and cross-bedding may reflect accumulated erupted and intruded products hyperconcentrated flow during the waning typically result in the growth of a pronounced stages of emplacement. topographic ridge overlying the rift zone. Due to Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

VOLCANO INSTABILITY: A REVIEW 9

Fig. 6. At Stromboli in the Aeolian islands (a), and at La Palma in the Canaries (b), sector collapse has occurred in a direction normal to a single, principal zone of dyke emplacement. Contrastingly, at Etna in Sicily (e), and at El Hierro in the Canary Islands (d) sectors of flank mobility and collapse are bounded by two intersecting rift zones. In both scenarios, instability develops over a long period of time in response to incremental dyke-induced lateral displacements, and stresses are eventually relieved by edifice failure. Double-ended arrows show directions of dyke-related extension; larger single-ended arrows indicate directions of collapse. oversteepening and loading effects, this structure & Voight this volume). Pore-pressures generated tends to become less stable over time, and more in this way may reduce frictional resistence to susceptible to failure in response to single failure by lowering the effective stresses on destabilizing events such as an earthquake available basal failure planes, permitting failure (volcanogenic or otherwise), or dyking event. and collapse even on the low angle flanks of As previously mentioned, in the latter case a shield volcanoes. slope at the limit of its stability may be induced Due to the manner in which dyke orientations to slide by the mechanically, and especially, the at shallow depths are controlled by a gravita- thermally generated increases in pore-fluid tional stress regime which is a reflection of the pressure which accompany the emplacement of edifice morphology (McGuire & Pullen 1989), the magma (Voight & Elsworth 1992; Elsworth the existence of large sector collapse structures, Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

10 W. J. McGUIRE 6 ai :i:i:i:i:i:i:i:i:i:i:i:i:il,ai 6 :::::::::::::::::::::="":::::::::::::::::::::::::::::::::::::::::: VALLE , o.5 krn, iiiiiiiiiiiiiiiiiiiiiii:iii~iii:i:ii:iiiiiiii~iiiiii....i:i:!:iven!s:i:i:i:i:}~s....i.iii E:ICrater, DEE TyrrhenianSea ~ i:!:!:i:!:i:! ::~i:i:~~ace"exp~es~~shall~ dY~!il ...~ BOVE /S~l'a~a~ il iilii !!!!i i!ii!i!!!i!iiii!i!ii!i:

...... Fig. 7. Dyke orientations at shallow depths are controlled by a gravitational stress regime which is a reflection of the edifice morphology. The existence of large sector-collapse structures, which occurred in response to dyke-induced rifting, may in turn, therefore, control the disposition of post-collapse zones of persistent rifting. This behaviour is illustrated by the orientation of post-lateral collapse dykes at both Etna (a) and Stromboli (b) (after Tibaldi this volume). At Etna, dyke paths on the upper flanks are constrained to parallel the 1 km high western rim of the Valle del Bove sector collapse. This has the dual effect of channeling magma towards the major tourist base on the volcano, while at the same time laterally displacing (by almost 5 m in 12 years) a 2 km long block of the Valle del Bove rim.

which occurred in response to dyke-induced Lateral edifice growth rifting, may in turn control the disposition of post-collapse zones of persistent rifting, and Recent papers have highlighted a number of therefore the locations and directions of sub- different ways in which volcanic edifices can sequent collapses. This behaviour is illustrated become destabilized and experience failure by the orientation of post-lateral collapse dykes during lateral edifice growth. Two contrasting at both Etna (Fig. 7a) (McGuire et al. 1990, mechanisms involve (i) relatively deep gravita- 1991) and Stromboli (Fig. 7b) (Tibaldi et al. tional spreading along basal thrusts, due to their 1994; Tibaldi this volume). In the former case, increasing mass, of volcanic structures such as dyke paths on the upper southern and eastern those constituting the Hawaiian Islands (e.g. flanks of the volcano are constrained to follow a Nakamura 1980; Dieterich 1"988; Borgia 1994; route which parallels the 1 km high western rim Clague & Denlinger 1994), and the Concepcidn of the Valle del Bove sector collapse (Fig. 7a). and Maderas volcanoes in Nicaragua (van Wyk Not only is this effect channeling magma de Vries & Borgia, this volume), and (ii) shallow towards the major tourist base on the volcano, gravitational sliding of sectors of volcanoes due but four dyke-induced rifting events over the to oversteepening, peripheral erosion, basement past 12 years have led to the 5m lateral dis- slope or tilting, or a combination of these and placement of a 2 km long block of the Valle del other factors (e.g. Kieffer 1985; Lo Giudice & Bove rim (Fig. 3b), and a consequent increase in Rasa 1992; Stewart et al. 1993; Ancochea et al. slope instability in this part of the volcano. 1994; Carracedo 1994; Tibaldi et al. 1994; In addition to essentially vertical dykes, Carracedo; Day; Firth et al.; Montalto et al.; subhorizontal sheet intrusions (sills) have also Rasa et al.; Rust & Neri; Tibaldi; all this been suggested to have a destabilising role. In volume). Both mechanisms lead to lateral particular, Adushkin et al. (1995) and Delemen edifice growth which may contribute towards (1995) have proposed that the emplacement of greater edifice instability. The manner in which such a magma body at the unstable Klyuchevs- the terms gravitational spreading and gravita- koi volcano in Kamchatka may be sufficient to tional sliding are often used synonymously when trigger future edifice failure resulting in a debris applied to volcanic edifices, illustrates that the avalanche with a volume of 4-8 km 3. differences between the two mechanisms have Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

VOLCANO INSTABILITY: A REVIEW 11 not been clearly defined. Although, as the due to the loading effect of the edifice itself. terminology indicates, gravity has a major role Manifestations of this type of behaviour are to play in both types of behaviour, this role is well summarized in Borgia (1994) and in van not identical and does not lead to the formation Wyk de Vries & Borgia (this volume), and of the same phenomena. include (Fig. 8a) peripheral compressive struc- Here, it is proposed that the term volcanic tures such as thrust-fault propagation folds, spreading should not be used in a genetic within the volcano and/or its substrate, together manner, and should be confined simply to with tension-related features on the upper describing the phenomenon of lateral edifice flanks. Characteristically, the substrate is de- enlargement. Mechanisms by which spreading pressed due to the volcanic load, and spreading is accomplished can then be summarized as is accomplished at least partly by means of gravitational sliding, gravitational thrusting, or outward and upward displacements along thrust edifice collapse. Each mechanism may further- faults. Effective gravitational thrusting requires more be described as being radial, where the either edifice growth directly on incompetent, entire circumferance of the edifice is involved, or deformable, sediments such as clays or evapor- sector, where spreading involves only part of the ites, or at least that a similar weak layer be edifice. Using this classification, the form of present within the sub-volcanic sequences spreading proposed for Kilauea would be de- although not immediately underlying the volca- scribed as sector thrusting, and that for Etna, as nic pile (van Wyk de Vries & Borgia this sector sliding. Other forms of spreading include volume). Where such a substrate is not present, radial thrusting at Mombacho (Nicaragua), thrust-related deformation is either minimal or radial collapse at Augustine Island (Alaska), absent. Recently it has been suggested (Dieterich and sector collapse at numerous volcanoes. 1988) that for the Hawaiian volcanoes even a Within the proposed classification scheme, combination of a considerable volcanic load and gravitational thrusting is reserved for the a weak substrate is insufficient to maintain the process whereby a volcanic edifice, and/or its observed thrusting behaviour and its associated substrate, spreads along basal thrusts entirely geophysical phenomena. Clague & Denlinger

Fig. 8. Structural features which may be associated with the gravitational thrusting (a) and gravitational sliding (b) of volcanic edifices. The former occurs on an initially horizontal substrate and is characterized by basement depression and the formation of upward- and outward-facing thrusts. The latter requires a tilted basement and typically results in the detachment of a mobile sector of the edifice on the downslope side. Both phenomena are characterized by tensional conditions in the summit region and by peripheral deformation. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

12 W.J. McGUIRE

(1994) have, in response, proposed a new model Instability and failure at island and in which thrusting is enforced by the seaward coastal volcanoes movement of dense masses of olivine cumulate material. This model highlights the potential Many of the largest landslides resulting from importance of mineral rheology in generating instability and structural failure are located and maintaining instability at active volcanoes. adjacent to the margins of island and coastal In addition to a role for olivine, weak, easily volcanoes. All the Hawaiian volcanoes, for deformable layers of plagioclase-rich material example, are surrounded by submarine aprons may also potentially play a part in edifice of allochthonous volcanic material emplaced by spreading at some volcanoes by providing sliding or slumping (Moore et al. 1989, 1994), surfaces along which detachment and move- which may grade into volcanic turbidites (Garcia ment may occur. & Hull 1994; Garcia this volume). Similar Contrastingly, although the load of the deposits have been recognized around many volcanic edifice plays a part in the initiation marine volcanoes, using techniques such as sea- of gravitational sliding, it is the form of the beam bathymetry and high-resolution side-scan underlying substrate which provides the appro- sonar imaging, including Piton des Neiges and priate conditions. While the load-driven spread- Piton de la Fournaise (Rrunion Island) (Rousset ing mechanism is initiated on a fiat substrate, et al. 1987; Lenat et al. 1989; Labazuy this which thereafter becomes downwarped, gravity- volume), Piton du Carbet (Martinique) (Semet controlled sliding requires the development of an & Boudon 1994), the Marquesas volcanoes asymmetrical sloping surface beneath the edifice, (Barsczus et al. 1992; Filmer et al. 1992), Tristan which may, as at Etna, result from differential de Cunha (Holcomb & Searle 1991), the Galapa- uplift beneath the volcanic pile (Fig. 8b). As with gos Islands (Chadwick et al. 1992), the Canary the thrusting mechanism, the sliding of large Islands (Holcomb & Searle t991; Weaver et al. sectors of volcanic edifices along a sloping 1994), Stromboli and Alicudi (Aeolian Islands) basement typically takes place at centimetric (Romagnoli & Tibaldi 1994), and at Augustine annual rates, but displacements occur along a Island (Alaska) (Beg& & Kienle 1992). The sizes decollement (or series thereof) which slopes of deposits are enormously variable although downwards. Further similarities with gravita- Holcomb & Searle (1991) report that many single tional thrusting involve the requirement for a landslides affecting oceanic volcanoes may have ductile sub-volcanic horizon, the development of been sufficiently large as to involve the transport tensional conditions in the upper levels of the of up to 20% of the edifice volume. Some of the edifice and the potential for generating periph- Hawaiian landslides have volumes greater than eral compressional features. In gravitational 5000 km 3 and lengths in excess of 200 km making sliding, the long axis of the mobile sector of them the largest such structures recorded on the volcano is typically oriented parallel to the Earth (Moore et al. 1994). slope of the underlying substrate, and is The common occurrence of aprons of desta- separated from the remainder of the edifice by bilised material around marine volcanoes is to faults or fault zones characterized by significant be expected for a number of reasons. Most strike-slip components. significantly, the seaward-facing flank of any While convincing cases for the operation of volcano located at the land-sea interface is gravitational thrusting have been made for the inevitably the least buttressed. This applies Hawaiian volcanoes, and a number of other both to coastal volcanoes such as Etna, where smaller edifices (van Wyk de Vries & Borgia this the topography becomes increasingly elevated volume), the mechanism has also been proposed, inland, and to island volcanoes such as Hawaii less credibly, to attempt to explain the unstable where younger centres (such as Kilauea) are and mobile nature of the eastern flank of the Etna buttressed on the landward side by older edifices volcano (Borgia et al. 1992; Borgia 1994). Here, (e.g. Mauna Loa). The morphological asymme- however, a combination of generally shallow try resulting from this effect leads to the seismicity (Lo Giudice & Rasa 1992; Montalto et preferential release of accumulated intra-edifice al. this volume) and the common occurrence of stresses, due for example to surface-overloading creep-related surface ruptures (Rasa et al. this or to repeated dyke-emplacement, in a seaward volume), argue strongly for gravitational sliding direction (Fig. 9). This stress release may take of the eastern sector of the edifice over a clay- the form of the slow displacement of large smeared substrate which is downfaulted seaward sectors of the edifice in the form of giant slumps, and continues to be uplifted at an average annual of co-seismic downfaulting, or of the episodic rate of 0.8 to 1.4mma -1 (Stewart et al. 1993; production of debris avalanches, or a combina- Firth et al. this volume). tion of all three. The relatively unstable nature Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

VOLCANO INSTABILITY: A REVIEW 13

Fig. 9. Repeated dyke emplacement along the Southern and East Rift Zones of Kilauea leads to preferential and progressive seaward movement of the southern flank of the volcano. Horizontal surface displacements between 1958 and 1970 (after Swanson et al. 1976) total over 2m at some locations.

of the seaward-facing flanks of any volcano is mass-wasting processes at all volcanoes located further enforced by the dynamic nature of the in marine environments. The two mechanisms land-sea contact. Not only does marine erosion are not mutually exclusive, with debris ava- provide a constant destabilizing agent, but large lanches often forming from the disaggregation changes in global sea levels of up to 130m, of oversteepened or overpressured slumps, or occurring over periods as short as 18 000 years, from injection of pore fluids or gouge muds with catastrophic rises recently identified of from the lower regions of slumps into the upper ll.5m in <160 + 50 years (Blanchon & Shaw layers, which may then disintegrate (Day this 1995), offer the means of modifying internal volume). stress regimes and water pore pressures in Moore et al. (1994) draw attention to both the favour of edifice destabilization. different characteristics and the alternative The Hawaiian Ridge, extending from near mechanisms responsible for the emplacement of Midway Island to Hawaii, provides by far the the Hawaiian slumps and debris avalanches. most impressive evidence for volcano instability While the former are typically both wide (some- and collapse in the marine environment. Follow- times over 100km) and thick (up to 10km), the ing a cooperative submarine survey by the latter are long (up to 230 km) and relatively thin United States Geological Survey and the UK (0.5-2 km). The authors explain that the slumps Institute of Oceanographic Sciences, using the are deeply rooted in the edifice and may be GLORIA side-scan sonar system, 68 landslides bounded by rift zones on their landward side, and with lengths in excess of 20km have been by the edifice-substrate interface at their base. identified. In a comprehensive review of land- Movement is typically slow and creep-like, slide generation along the Hawaiian Ridge, although evidence for major co-seismic displace- Moore et al. (1994) highlight a number of ments are recorded on the active Hilina slump of characteristic features which are likely to be Kilauea (Lipman et al. 1985). In contrast, the generally applicable to the destabilization and debris avalanche features reported by Moore et collapse of marine volcanoes. Although occur- al. (1994) include well-defined amphitheatres in ring throughout the lifetimes of the volcanoes, their source regions, hummocky terrain with the largest landslides occurred when the centres megablocks up to 2 km across, and evidence for were young and unstable, were close to their uphill transport on the Hawaiian Arch submarine maximum size, and when seismic activity was at ridge, implying high emplacement velocities. a high level. The authors differentiate slumps From the range of features described from the from debris avalanches and report the existence Hawaiian Ridge, it becomes apparent that the of intermediate forms. Slumping and avalanch- term 'landslide', which has common usage in ing can therefore be viewed as end-members of a describing the products of seaward mass-move- continuous sequence of emplacement mechan- ment at coastal and island volcanoes, is not isms which is probably applicable to large-scale wholly appropriate. Strictly-speaking this term is Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

14 W.J. McGUIRE best confined to describing the rapidly emplaced R6union Island in the Indian Ocean, in parti- debris avalanche deposits, rather than the cular adjacent to the active Piton de la Fournaise relatively slow-moving slump blocks. volcano (Rousset et al. 1987) (Fig. 10). As at the The results of the extensive submarine surveys Hawaiian volcanoes, both slumping and debris conducted around the Hawaiian volcanoes have avalanche production appear to have been highlighted the important role of large-scale important in the seaward transport of large edifice destabilization and collapse in constrain- masses of volcanic material. Large, arcuate fault ing the morphological and structural evolution scarps have been interpreted (Duffield et al. of marine volcanoes. Any chosen point in the 1982) as the back wails of giant seaward- lifecycle of such an edifice represents a 'snap- slumping blocks analogous to the Hilina slump shot' of a continuing conflict between construc- at Kilauea, while the cliff-bounded Grand Brfil6 tive forces represented by endogenous and exo- flank depression is evidence of the more rapid genous growth during, respectively, intrusive transport of material seaward. Lenat et al. and extrusive activity, and destructive influences (1989) report that this structure can be traced dominated by mass-wasting due to slumping, offshore for over 20 km and probably resulted avalanching, and other erosive mechanisms. As partly due to landsliding and partly from suggested by Fornari & Campbell (1987), these seaward slumping of a relatively undisturbed latter phenomena may actually act in concert in block. Labazuy (this volume) reports that the the long term to restabilize the edifice by total volume of allochthonous material derived widening its base. from Piton de la Fournaise amounts to around Although it is now apparent that many, if not 500 km 3, an order of magnitude smaller than the all, coastal and island volcanoes are character- largest single slide on the Hawaiian Ridge but ized by adjacent allochthonous materials depos- nevertheless evidence for the importance of ited in the submarine environment, the source- gravity collapse at this relatively young volcano. region of the transported material often remains Comparisons of the terminologies used to enigmatic. This is particularly the case in very define collapse structures and deposits at Piton active volcanic terrains or in areas of high de la Fournaise and the Hawaiian Ridge reveal sedimentation where collapse scars may be discrepancies which need resolving in order to rapidly obscured. Determination of the source prevent confusion in the literature. In particular, of collapse is important in order to resolve large masses of allochthonous material trans- whether failure has occurred below sea level or ported for distances of tens of kilometres off- subaerially. At Kilauea, the back-walls of at shore are referred to as slump deposits (Rousset least some of the slump blocks are subaerial, as et al. 1987; Labazuy this volume). These features evidenced by the active fault scarps which mark the landward termination of the Hilina slump on the south flank of Kilauea (Swanson et al. 1976 Similarly, many of the Hawaiian debris ava- lanches appear to have at least a partly subaerial source, with morphological evidence supporting the existence of subaerial landslide amphithea- tres associated with both the East Ka Lae (Moore & Clague 1992) and Alika (Moore & Mark 1992) debris avalanches. It is likely that in many cases, the scars left by such landslides extend below sea level, and many appear to be initiated immediately below sea level where some of the steepest slopes on the Hawaiian volcanoes (up to 19 ~ are encountered (Mark & Moore 1987). The apparent predominance of collapse events at the land-sea interface, for which there is also evidence from Stromboli hints once again at a role for changing sea levels in destabilising Fig. 10. At Piton de la Fournaise (R6union Island), volcanic edifices, particularly when, as pre- collapse of the eastern flank is related to repeated dyke viously mentioned, these occur catastrophically emplacement along the NE and SE Rift Zones. On (Blanchon & Shaw 1995). land, the Grand Brul6 represents the collapse scar, Many of the features representative of volcano while offshore surveys have revealed the existence of destabilization and collapse on the Hawaiian large volume (totalling 500 km3) debris avalanche Ridge are mirrored around the shores of deposits (after Rousset et al. 1987). Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

VOLCANO INSTABILITY: A REVIEW 15 clearly do not have the same morphological and of landslides recognized are also certain to structural characteristics as the 'slumps' increase, as imagery is obtained for as yet described by Moore et al. (1994) and others, relatively poorly studied volcanic island chains from Hawaii, nor is the emplacement mechanism such as the Canaries and the Azores. Already, likely to be the same. The Piton de la Fournaise however, there is a sufficient body of data to deposits appear to be rootless, and as such are indicate that repeated flank collapse is a likely to represent landslides in which the ubiquitous occurrence in the normal lifecycle disruption of the collapsing mass has been of marine volcanoes. minimal. The term 'slump' should be limited to masses of allochthonous material which satisfy all the following criteria; emplacement is slow Evidence of volcanic edifice failure on other and progressive, the structure is coherent and planetary bodies relatively undisturbed, and part of the mobile mass remains rooted in the source region. High resolution imagery from the Moon, Mars, Structures such as the Hilina slump satisfy these and Venus, has permitted comparisons to be criteria; at Piton de la Fournaise, however, made about the nature of volcanic edifice failure these features are probably best exhibited by on a range of diverse planetary bodies with the subaerial, arcuate scarp-bounded blocks differing geological structures, gravitational described by Duffield et al. (1982), rather than fields and atmospheric pressures (Head this by the submarine deposits. volume). Broadly speaking, both the Earth and Many collapse scars such as the Grand Brfil6 Mars are characterized by the existence of large on Piton de la Fournaise, the Valle del Bove on edifices which often show evidence of flank Etna, and the Sciara del Fuoca on Stromboli failure and sector collapse. In contrast on the were formed during the Holocene, and are only Moon, where large volcanic structures are absent, visible due to their youth. Older structures are, and on Venus where volcano heights are lower, however, rapidly buried or covered, and may evidence for very large-scale collapse is absent. only be recognizable by unexplained varia- Martian shield volcanoes display many of the tions in edifice morphology. At Mauna Loa, for features shown by those of Earth, including example, anomalously steep slopes developed lithospheric loading effects, rift-zone develop- along the entire length of the west flank have ment, summit caldera formation, and flank been tentatively interpreted (Moore et al. 1994) failure (Crumpler et al. this volume). As these in terms of young lava flows filling a sequence of edifices tend, however, to be volumetrically three older collapse amphitheatres. Similar anoma- orders of magnitude greater than their largest lously steep slopes on the flanks of the Cumbre terrestrial counterparts the scale of edifice failure Vieja ridge on La Palma (Canaries), and other is equally increased. The gigantic sizes of the marine volcanoes may conceal structures gener- martian volcanoes are a reflection of a number ated by older collapse events, evidence for which of different factors, including the stability and may only be found in the submarine record. thickness of the underlying lithosphere, which The transport of volcanically derived debris permits the accumulation of a huge volume of into the marine environment due to catastrophic material at one site, and minimizes the degree of landsliding may be enhanced by the triggering of load-related height diminution due to litho- large-scale turbidite formation at the distal ends spheric depression (Head this volume). As at of the avalanches. Garcia & Hull (1994) and Hawaii (Moore et al. 1989, 1994), both slumps Garcia (this volume), report turbidite currents and debris avalanches are observed on the large related to Hawaiian landslides, which travelled martian shield volcanoes. The latter, in the case over 1000 km and flowed over sea-bed obstruc- of Olympus Mons, extend nearly a thousand tions 500 m high. Similarly, Weaver et al. (1994), kilometres beyond the basal scarp of the vol- present evidence for turbidity currents over cano, and have volumes of up to 170 x 103 km 3 600 km in length which appear to be related to (Head this volume); an order of magnitude slope failure on the flanks of the westernmost greater than debris avalanches associated with Canary Islands of La Palma and Hierro around the Hawaiian volcanoes. On an even larger 18 ka BP. scale, Crumpler et al. (this volume), report the Further submarine surveys, using increasingly possible existence of gravity sliding at Olympus advanced imaging techniques, can be expected, Mons of the type evident at Etna, but with the over the next decade, to improve our knowledge entire edifice sliding down the regional slope of of the morphologies and structures of volcanic the substrate on a detachment horizon lubri- landslides and associated deposits emplaced cated by water ice. Cave et al. (1994) draw in the submarine environment. The numbers attention to the link between water and Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

16 W. J. McGUIRE instability and collapse in volcanic terrains on eventually occur and provision must be made for Mars. In particular, they point to the fact that mitigating its impact. many of the large fluvial outflow channels Edifice failure at Mount St Helens was originate in areas of collapsed terrain and have particularly important, from the hazard mitiga- a close association with volcanic centres such as tion point of view, because it demonstrated in the Tharsis and Elysium regions. clearly that volcanic landslides occurring in the In contrast to their martian counterparts, presence of a shallow magma body can generate volcanic edifices on Venus have geometries char- a whole spectrum of destructive phenomena in acterized by greater height to width ratios, and addition to the landslide itself, including lateral reveal little evidence for the type of large-scale blast, lahars, pyroclastic flows, and extensive flank failure shown by the Tharsis volcanoes. ash-fall. Furthermore, the speed with which these The broader, flatter forms of large (c. 100km) phenomena were unleashed following landslide volcanic constructs on Venus are less susceptible initiation indicated that no mitigation procedures to failure due to a number of factors outlined in could be implemented once the sequence of Head (this volume). These include slower rates events had started. The lesson being that such of vertical migration of magma, and the absence measures must be in place well in advance of the of liquid-lubricated slip zones due to the high expected event. surface temperatures. Smaller (c. tens of kilo- Assessing the hazard posed by structural metres) volcanic constructs on Venus, parti- instability at a particular volcano should ideally cularly those of dome-like form, do, however, adopt a two-pronged approach based upon provide evidence for ubiquitous flank failure (eg, mapping and surveillance. The former provides Guest et al. 1992; Bulmer & Guest this volume; information on the nature, extent, and frequency Head this volume). Both collapse scars and of past collapse events, and identifies areas likely associated debris avalanche deposits are visible, to fail at some point in the future, while the the latter of which tend to have longer runout latter concentrates on detecting and monitoring distances, for a given drop, than those on Earth the onset and development of instability using or Mars. electronic distance measurement (EDM) and related techniques. Because edifice failure and debris avalanche formation is often an episodic event which constitutes part of the normal life- The hazard implications of cycle of a volcano, considerable information volcano instability about the behaviour of a future landslide can be gleaned from examining older deposits asso- As shown by collapse of the northern flank of ciated with similar events. Determining the Mount St Helens in 1980, the consequences of extent of such deposits is particularly important edifice failure can be both dramatic and in hazard zonation map preparation, as it catastrophically destructive. Failure to forecast provides an estimate of the area which might such an event and/or to initiate the appropriate be affected by a future debris avalanche. The mitigation procedure- which can only involve size-estimates and shapes of hazard zones evacuation of the entirepopulation of the area determined on this basis must always, however, at risk- will result in major loss of life. With be regarded only as a guide to future behaviour. velocities in excess of 100ms -1 (360kmh-l), A volcano may not, for any one of a number of and momentum sufficient to mount topographic reasons, precisely replicate past activity, and barriers hundreds of metres high, no man-made destructive phenomena may affect larger or structures can survive the impact of a major different parts of the surrounding terrain debris avalanche. The area affected is also likely during a future collapse event. Furthermore, to be large; the relatively small-scale debris not all the effects associated with edifice failure avalanche at Mount St Helens travelled over may be sufficiently well-preserved in the geolo- 24km from its source, but this is minimal gical record. At Mount St Helens, for example, compared with the 120 km long late Pleistocene the hazard zonation maps of Crandell & debris avalanche at Nevado di Colima (Stoopes Mullineaux (1978) proved to be highly accurate & Sheriden 1992), which covers an area of for all phenomena except the lateral blast which around 2200 km 2. A volcanic landslide on this accompanied the May 18 landslide. This scale has not been observed during historic extended three times further, and covered an times, and the problems involved in forecasting area up to fifteen times greater than pre-dicted and mitigating the effects of such an event, (Miller et al. 1981), probably reflecting the low particularly in a densely populated area, would preservation potential of the products of earlier, be enormous. Nevertheless, such a scenario will similar events. In assessing the potential for Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

VOLCANO INSTABILITY: A REVIEW 17 future instability and failure, detailed structural accomplished by means of (i) a judicious con- mapping also has an important role to struction policy which avoids susceptible areas, play, particularly in identifying features which (ii) an effective monitoring system, based on may provide clues as to the nature and locations alarmed trip-wires and seismometers, which is of future collapse. These may include creeping/ able to provide sufficient time for evacuation of episodically moving faults, peripheral thrust the area, and (iii) having in place an effective plan faults and folds, old collapses, active rift zones, for rapid evacuation. Damage to property can be and zones of alteration. Recognizing changes minimised by the upstream installation of a with time of fault geometries or dyke-zone sequence of sediment dams and baffles designed orientations may also be important in terms of to reduce the sediment and boulder load thereby reflecting stress regime modifications prior to reducing the destructive power of the lahar. edifice destabilization. The preference for destabilization and col- As mentioned earlier, debris avalanche pro- lapse on the seaward-facing flanks of volcanic duction in active volcanic terrains may be edifices has already been discussed in terms of accompanied by other destructive phenomena. the buttressing effect of adjacent terrain on the A particular threat lies in the transition, during landward side. From the hazard point of view, transport, from debris avalanche to debris flow this effect has major implications, favouring as it (lahar), due to the melting of snow and ice fields does the formation of volcanogenic tsunamis. or to the entrainment of surface water during the Some 5% of all tsunamis are estimated to have collapse event. Such behaviour effectively been formed by volcanic activity, and at least increases the length of the runout, compared to one fifth of these result from volcanic landslides dry avalanches, thereby enlarging the extent of (Smith & Shepherd this volume). One of the damage and destruction. Debris avalanches may most recent occurred at Harimkotan (Severgina) also provide conditions favouring lahar forma- volcano (Kurile Islands) during 1933, when a tion by damming water catchments and forming small (0.5 km 3) debris avalanche entered the sea new lakes which may drain catastrophically and generated a 9m high tsunami (Belousov (Costa & Shuster 1988). Additionally, lahars 1994). As revealed by the Mount Unzen (Japan) and mud-laden floods may be generated due to collapse in 1792, even small landslides can dewatering of the avalanche material (Janda generate highly destructive waves if they enter et al. 1981), or may form by means of a large body of water. At Unzen, a collapsing remobilization caused by heavy rainfall. In volume of only about 0.34 km 3 (Hayashi & Self these ways, a major collapse event may provide 1992), which was not connected with volcanic a sufficient source of debris to feed numerous activity, entered Ariake Bay and triggered a precipitation-related lahar/flood events over a tsunami which caused 14500 deaths. Similar period of years or decades. This is an extended- 'cold' collapses are probably relatively common hazard problem which has become familiar to events, particularly on steep-sided volcanoes the inhabitants of towns in the vicinity of the in the marine environment. Kick 'era Jenny Pinatubo volcano (Philippines) since the erup- volcano in the Lesser Antilles, for example tion in June 1991 (Pierson 1992). Although here (Smith & Shepherd this volume), must be the source of the lahars is a thick mantle of viewed as a volcanogenic tsunamis source. This pyroclastic flow, rather than debris avalanche, submarine volcano has erupted 12 times in the material, a similar scenario could be expected to last 53 years and possesses steep flanks bearing follow emplacement of a major volcanic land- signs of previous collapse events. The steep- slide in an area of very high rainfall. sided Stromboli volcano in the Aeolian Islands The Nevado del Ruiz (Colombia) catastrophe (Tibaldi et al. 1994; Kokelaar & Romagnoli in 1985 (Herd et al. 1986; Voight 1990), in which 1995; Tibaldi this volume) might also be close to 25000 lives were lost, demonstrated considered a prime candidate for flank collapse graphically the devastating potential of lahars. sufficient to generate a tsunamis capable of Ironically, considering this was one of the worst affecting the coastal regions of northern Sicily volcanic disasters of the century, the detrimental and western Calabria. Beg6t & Kienle (1992) effects of these phenomena are some of the also highlight the tsunami risk at Mount St easiest to mitigate. This is largely due to their Augustine volcano (Alaska). They report the being more topographically constrained than formation of a 20m high tsunami due to the debris avalanches, tending to follow river valleys emplacement of a small debris avalanche into and pond in areas of low relief. It should be the Cook Inlet during the 1883 eruption, and relatively easy, therefore, given the funding and expect a similar event to occur sometime during political will, to effectively mitigate the lahar the next century. Some of the largest volcano- problem at any particular volcano. This may be genic tsunamis were undoubtedly associated Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

18 W.J. McGUIRE with emplacement of the Hawaiian Island debris structural failure and landslide inititation avalanches. A wave-train associated with a was triggered by a magnitude 5 earthquake collapse on the flanks of Lanai volcano around (McClelland et at. 1989). It is a matter for 105kaBP appears not only to have reached an conjecture whether or not an acceleration in the elevation of around 375 m on the island itself growth rate of the bulge would have been (Moore & Moore 1984), but also to have crossed observed had it not been prematurely detached the Pacific and impinged energetically upon the from the northern flank by seismogenic ground coast of New South Wales in Australia. Here, accelerations. The situation does, however, Young & Bryant (1992) report the results of illustrate an important point, which is that catastrophic wave erosion related to the tsu- external events- in this case seismic activ- nami, at heights of at least 15 m above present ity- may accelerate the onset of instability sea level. Both forecasting the onset of such a caused by a separate, although in this case collapse and mitigating its effects are beyond related, phenomena (e.g. magma intrusion), and current capabilities, and likely to remain so for may initiate failure sooner than might otherwise some time. be expected. It is worth noting that not all instability related Monitoring edifice instability has also been hazards are associated with rapid-onset events shown to be a useful tool in predicting future such as debris avalanche formation. Slower- eruptive activity at Etna, where geodetic moni- moving slumps, for example, can result in exten- toring has revealed a relationship between the sive damage to man-made structures due, as at rate of downslope creep on the upper eastern Etna (Rasa et al. this volume), to continuous flanks of the volcano and the timing of future aseimic creep, or, as in the case of the 1975 eruptions. Murray & Voight (this volume) Kilauea quake (Denlinger & Okubo, in press), propose that accelerating rates of downslope to lower periodicity seismic activity related to creep occur in response to increasing magma episodic, but larger scale, downslope slump pressures. The creep behaviour eventually re- movement. duces the effective tensile strength of the rock to a level at which it is exceeded by magma pressure, thereby permitting eruption. On the basis of inverse-rate analysis of geodetic data Monitoring and forecasting volcano accumulated during the 1980s, the authors also instability and failure believe that this method permits good erup- tion predictions more than three months in Successfully forecasting collapse in active volca- advance. nic terrains remains strongly dependent upon A range of low- and high-tech ground the monitoring of ground deformation and deformation monitoring techniques are now in displacement using geodetic and related meth- use for detecting and observing the surface ods. Such techniques are particularly important displacements at active volcanoes which may in identifying sites of increasing destabilization signal increasing instability and presage forth- on the flanks of newly reactivated volcanoes, coming failure. At the low-tech end of the and in monitoring the development of instability spectrum, simple steel tapes have been effec- with a view to defining progressively smaller tively used to measure displacements across predictive 'windows' in order to attempt to basal thrust faults reflecting the progressive forecast the timing of eventual structural failure. growth of the previously mentioned post- May Defining such windows relies upon observing an 1980 lava dome at Mount St Helens, contribut- increasing acceleration in the rate of deforma- ing invaluable data towards successful forecasts tion or displacement, with failure becoming of future episodes of dome destruction (Swanson increasingly likely as the rate of deformation et al. 1983, 1985). More commonly used, less becomes greater. risky, but more expensive techniques involve This technique proved particularly effective, using either infra-red or laser electro-optical when combined with data gathered from other distance meters (EDMs), which are capable of monitoring methods, in forecasting dome- monitoring horizontal distance changes of only destroying eruptions at Mount St Helens a few centimetres over distances of uF to tens during the early 1980s (Swanson et al. 1983, of kilometres (e.g. McGuire et al. 1990; 1991; 1985). Unfortunately, it was not so successful in Iwatsubo & Swanson 1992; Murray et al. 1995). predicting the major landslide event of May Displacement data gathered in this way may 1980. In this case, cumulative growth of the also usefully be supplemented by tiltmeters and cryptodome-induced bulge demonstrated a con- precise-levelling surveys (Dzurisin 1992; Murray stant displacement rate right up to the time when et al. 1995; Toutain et al. 1995) to build a more Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

VOLCANO INSTABILITY: A REVIEW 19 comprehensive picture of the pattern of defor- References mation associated with a destabilization event. Airborne (Garvin this volume), and space-based ADUSHKIN, V. V., ZYKOV, N. YU. & FEDOTOV, S. A. systems are also becoming more important, and 1995. Mechanism of volcanic slope failure. the increasingly accessible and cost-effective Assessment of potential collapse and debris global positioning system (GPS), in particular, avalanches at Klyuchevskoi volcano. Volcanology can be viewed as an effective new tool in the and Seismology, 16, 667-684 ANCOCHEA, E., Hernan, F., Cendrero, A., CANTA- armoury of geodetic hardware (Shimada et al. GREL, J. M., FUSTER, J. M., IBARROLA, E. & 1990; Nunnari & Puglisi 1995). Radar inter- COELLO, J. 1994. Constructive and destructive ferometry techniques using successive synthetic episodes in the building of a young oceanic island, aperture radar (SAR) images from satellite La Palma, Canary Islands, and genesis of the platforms have also been shown recently Caldera de Taburiente. Journal of Volcanology (Massonet et al. 1993) to have enormous and Geothermal Research, 60, 243-262. potential in measuring centimetric surface BARSCZUS, H. G., FILMER, P. E. & DESONIE, D. displacements associated with co-seismic defor- 1992. Cataclysmic collapses and mass masting mation fields around active faults. This techni- processes in the Marquesas. Los, Transactions of the American Geophysical Union, 73, 313 que is now being applied in volcanology (abstract). (Massonet et al. 1995) and offers the possibility BEGIRT, J. E. & KIENLE, J. 1992. Cyclic formation of of monitoring very precisely, and at one go, debris avalanches at Mount St Augustine changes in the overall surface deformation volcano. Nature, 356, 701-704. pattern at a chosen volcano. On the ground, BELOUSOV, A. B. 1994. Large-scale sector collapses at better prediction of the timing of landslide Kurile-Kamchatka volcanoes in the 20th century. initiation may come from the seismic monitor- In: Proceedings of International Conference on ing of seismogenic faults which have the Volcano Instability on the Earth and Other potential to act as slide surfaces, the application Planets. The Geological Society of London, 1994. BLANCHON, P. & SHAW, J. 1995. Reef drowning during of acoustic emission techniques, which may the last glaciation: evidence for catastrophic sea- permit detection of very small-scale micro- level rise and ice-sheet collapse. Geology, 23, 4-8. fracturing preceding and presaging major struc- BORGIA, A. 1994. Dynamic basis of volcanic spread- tural failure, and ground penetrating radar ing. Journal of Geophysical Research, 99, B9, (GPR) which may be suitable for mapping 17791-17804. fracture patterns in unstable areas. , FERRARI, L., & PASQUARE, G. 1992. Impor- tance of gravitational spreading in the tectonic and volcanic evolution of Mount Etna. Nature, 357, 231-235. Summary BULMER, M. H. & GUEST, J. E. 1996. Modified volcanic domes and associated debris aprons on The development of instability and the condi- Venus. This volume. tions which favour structural failure of volcanic CARRACEDO, J. C. 1994. The Canary Islands: an example of structural control on the growth of edifices will continue to form important foci of large oceanic-island volcanoes. Journal of Volca- study into the next century. Accumulating nology and Geothermal Research, 60, 225-241. evidence for the ubiquity of these phenomena --1996. A simple model for the genesis of large has played an important role in redefining the gravitational landslide hazards in the Canary frequency and scale of associated hazards, and Islands. This volume. has drawn attention to the potentially devastat- CAVE, J., GUEST, J. E. & BULMER, M. 1994. Slope ing consequences of a major structural failure instability on Elysium Mons & other martian event occurring at a volcano in a densely volcanoes. In: Proceedings of International Con- populated area. Both extant monitoring strate- ference on Volcano Instability on the Earth and Other Planets. The Geological Society of London, gies and hazard mitigation blueprints at suscep- 1994 tible volcanoes must now be modified in order to CHADWICK, W. W., MOORE, J. G., FOX, C. O. & plan for such a scenario. CHRISTIE, D. M. 1992. Morphologic similarities of submarine slope failures: south flank of Kilauea, Hawaii, and the southern Galapagos Results presented in this paper relating to work on the platform. Los, Transactions of the American Etna, Stromboli, and Canary Island volcanoes would Geophysical Union, 73, 507 (abstract). not have been acquired without the support of the CHRISTIANSEN, R. I. & PETERSON, D. W. 1981 Commission of the European Communities, DG XII, Chronology of the 1980 eruptive activity. In: Environment Programme, Climatology and Natural LIPMAN, P. W. & MULLINEAUX, D. R. (eds) The Hazards Unit, in the framework of the contract EV5V- 1980 eruptions of Mount St Helens. US Geological CT92-0170. Survey Professional Paper 1250, 17-30. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

20 W.J. MCGUIRE

CLAGUE, D. A. & DENLINGER, R. P. 1994. Role of FIRTH, C., STEWART, I., MCGUIRE, W. J., KERSHAW, olivine cumulates in destabilizing the flanks of S. & VITA-FINZI, C. 1996. Coastal elevation Hawaiian volcanoes. Bulletin of Volcanology, 56, changes in eastern Sicily: implications for vol- 425-434. cano instability at Mount Etna. This volume. COSTA, J. E., SCHUSTER, R. L. 1988. The formation FORNARI, D. J. & CAMPBELL, J. F. 1987. Sub- and failure of natural dams. Geological Society of marine topography around the Hawaiian Islands. American Bulletin, 100, 1054-1068. In: DECKER. R. W., WRIGHT, T. L. & CRANDELL, D. R. 1989. Gigantic debris-avalanche STAUFFER, P. H. (eds) Volcanism in Hawaii. US of Pleistocene age from ancestral Mount Shasta Geological Survey Professional Paper, 1350, 109- volcano, California, and debris avalanche hazard 124. zonation. Bulletin of the US Geological Survey, FRANCIS, P. W. 1994. Large volcanic debris ava- 1861. lanches in the central Andes. In: Proceedings of -- & MULLINEAUX, D. R. 1978. Potential hazards International Conference on Volcano Instability on from future eruptions of Mount St Helens, the Earth and Other Planets. The Geological Washington. Bulletin of the US Geological Society of London, 1994. Survey, 1353-C. -- & SELF, S. 1987. Collapsing volcanoes. Scientific , MILLER, C. O., GLICKEN, H. X., CHRIST1ANSEN, American, 255(6), 90-97. R. L., NEWHALL, C. G. 1984. Catastrophic debris GARCIA, M. O. 1996. Turbidites from slope failure on avalanche from ancestral Mount Shasta volcano, Hawaiian volcanoes. This volume. California. Geology, 12, 143-146. -- & HULL, D. M. 1994. Turbidites from giant CRUMPLER, L. S., HEAD, J. W. & AUBELE, J. C. Hawaiian landslides: results from Ocean Drilling 1996. Calderas on Mars: characteristics, Program Site 842. Geology, 22, 159-162. structure, and associated flank deformation. This GARVIN, J. B. 1996. Topographic characterization and volume. monitoring of volcanoes via airborne laser alti- DAY, S. J. 1996. Hydrothermal pore-fluid pressure & metry. This volume. the stability of porous, permeable volcanoes. This GLICKEN, H. 1991. Rockslide-debris avalanche of volume. May 18th, 1980, Mount St Helens volcano, DELEMEN, I. F. 1995 Gravitational instability Washington. US Geological Survey Professional mechanisms in volcanic cones (with reference to Paper, 1488. Klyuchevskoi volcano). Volcanology and Seis- GUEST, J. E., BULMER, M. H., BERATAN, K., mology, 16, 649-666. MICHAELS, G. & SAUNDERS, S. 1992. Gravita- DENLINGER, R. P. & OKUBO, P. A huge landslide tional collapse of the margins of volcanic domes structure on Kilauea volcano, Hawaii. Journal of on Venus. Lunar and Planetary Sciences, 23, Geophysical Research, in press. 461-462 (abstract). DE RITA, D., DI FIL~PPO, M. & ROSA, C. 1996. HAYASHI, J. N. & SELF, S. 1992. A comparison of Structural evolution of the Bracciano volcano- pyroclastic flow and debris avalanche mobility. tectonic depression within the Sabatini volcanic Journal of Geophysical Research, 97, 9063-9071. district, Italy. This volume. HEAD, J. W. 1996. Volcano instability development: a DIETERICH, J. n. 1988 Growth and persistence of planetary perspective. This volume. Hawaiian volcanic rift zones. Journal of Geophy- HERD, D. G. & THE COMITE DE ESTUD1OS sical Research, 93, 4258-4270. VULCANOLOGICOS. 1986. The 1985 Ruiz volcano DUFFIELD, W. A., ST1ELTJES, L., & VARET, J. 1982 disaster. Los, 67, 457-460. Huge landslide blocks in the growth of Piton HOLCOMB, R. T. & SEARLE, R. C. 1991. Large de la Fournaise, La Rrunion, & Kilauea vol- landslides from oceanic volcanoes. Marine Geo- cano, Hawaii. Journal of Volcanology Geothermal technology, 10, 19-32. Research, 12, 147-160. INOKUCHI, T. 1988 Gigantic landslides and debris DZURISIN, D. 1992. Electronic tiltmeters for volcano avalanches on volcanoes in Japan. In: Proceedings monitoring: lessons from Mount St Helens. of the Kagoshima International Conference on Vol- In: EWERT, J. W. & SWANSON, D. A. (eds) canoes. National Institute for Research Adminis- Monitoring Volcanoes: Techniques and Strategies tration, Japan, 456-459 used by the staff of the Cascades Volcano IWATSUBO, E. Y. & SWANSON, D. A. 1992 Trilatera- Observatory, 1980-90. USGS Bulletin, 1966, tion and distance-measuring techniques used at 69 -84. Cascades and other volcanoes. In: EWERT, J. W. ELSWORTH, D. & VOIGHT, B. 1996. Evaluation of & SWANSON, D. A. (eds) Monitoring Volcanoes: volcano flank instability triggered by dyke Techniques and Strategies used by the staff of intrusion. This volume. the Casacades Volcano Observatory, 1980-90. FAIRCHILD, L. H. 1987. The importance of lahar Bulletin of the US Geological Survey, 1966, initiation processes. Reviews in Engineering Geol- 95-102. ogy, 7, 51-61. JANDA, R. J., SCOTI~, K. M., NOLAN, R. M. & FILMER, P. E., MCNUTT, M. K., WEBB, H. & DIXON, MARTINSON, H. A. 1981. Lahar movement, D. J. 1992. Volcanism and archipelagic aprons: a effects, and deposits. In: LIPMAN, P. W. & comparison of the Marquesan and Hawaiian MULLINEAUX, D. R. (eds) The 1980 eruptions of islands. Los, Transactions of the American Geo- Mount St Helens, Washington. US Geological physical Union, 73, 313 (abstract) Survey Professional Paper, 1250, 461-478. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

VOLCANO INSTABILITY: A REVIEW 21

KIEFFER, G. 1985. Evolution structurale et dynamique --, PULLEN, A. D. & SAUNDERS, S. J. 1990. Recent d'un grand volcan polygOnetique: stades d'~difica- dyke-induced large-scale block movement at tion et activitO actuelle de l'Etna. Th6se Doct. Sci.; Mount Etna and potential slope failure. Nature, Univ. Clermont-Ferand. 343, 357-359. KOKELAAR, P. & ROMAGNOLI, C. 1995 Sector --, MURRAY, J. B., PULLEN, A. D. & SAUNDERS, collapse, sedimentation, and clast population S. J. 1991. Ground deformation monitoring at evolution at an active island-arc volcano: Strom- Mount Etna; evidence for dyke emplacement and boli, Italy. Bulletin of Volcanology, 57(4), 240-262. slope instability. Journal of the Geological Society, KOMOROWSKI, J. C., NAVARRO, C., CORTES, A. & London, 148, 577-583. SIEBE, C. 1994. The unique tendency for recurrent MARK, R. K. & MOORE, J. G. 1987. Slopes of the collapse of Colima volcanoes (Mexico): the Hawaiian Ridge. In: DECKER, R. W., WRIGHT, challenge of reconciling geologic evidence with T. L. & STAUFFER, P. H. (eds) Volcanism in C 14 chronology, implications for hazard assess- Hawaii. US Geological Survey Professional ment. In: Proceedings of International Conference Paper, 1350, 101-107. on Volcano Instability on the Earth and Other MARTI, J., VILA, J. & REY, J. 1996. Deception Island Planets. The Geological Society of London, 1994 (Bransfield Strait, Antarctica): an example of LABAZUY, P. 1996. Recurrent landsliding events on volcanic caldera developed by extensional tec- the submarine flank of Piton de la Fournaise tonics. This volume. volcano (R~union Island). This volume. MASSONET, D., ROSSI, M., CARMONA, C., ADRAGNA, LENAT, J. F., VINCENT, P., & BACHELERY, P. 1989 F., PELTZER, G., FEIGL, K. & RABAUTE, T. 1993. The off-shore continuation of an active basal- The displacement field of the Landers earthquake tic volcano: Piton de la Fournaise (Rrunion mapped by radar interferometry. Nature, 364, Island, Indian Ocean); structural and geomorph- 138-142. ological interpretation from sea beam mapping. --, BRIOLE, P. & ARNAUD, A. 1995. Deflation of Journal of Volcanology and Geothermal Research, Mount Etna monitored by spaceborne radar 36, 1-36. interferometry. Nature, 375, 567-570. LEONOV, V. L. 1995. Lineaments, tectonic fractures, MILLER, C. D., MULLINEAUX, D. R. & CRANDELL, and mechanical behaviour of Klyuchevskoi vol- D. R. 1981. Hazards assessments at Mount St cano. Volcanology and Seismology, 16, 627-648. Helens. In: L1PMAN, P. W. & MULLINEAUX, D. LIPMAN, P. W. & MULLINEAUX, D. (EDS). 1981. The R. (eds) The 1980 eruptions of Mount St Helens, 1980 eruptions of Mount St Helens. US Geological Washington. US Geological Survey Professional Survey Professional Paper, 1250. Paper, 1250, 789-802. , LOCKWOOD, J. P., OKAMURA, R. T., SWANSON, MIMURA, K. & KAWACHI, S. 1981. Nirasaki debris D. A. & YAMASHITA, K. M. 1985. Ground avalanche, a catastrophic event at the Yatsugatake deformation associated with the 1975magnitude volcanic chain, central Japan (abstract). Pro- 7.2 earthquake and resulting changes in activity of ceedings 114 VCEI Symposium. Tokyo and Kilauea volcano, Hawaii. U.S. Geological Survey Hakone, 237. Professional Paper, 1276. MONTALTO, A., VINCIGUERRA, S., MENZA, S. & , NORMARK, W. R., MOORE, J. G., WILSON, J. B. PATANE, G. 1996. Recent seismicity of Mount & GUTMACHER, C. E. 1988. The giant submarine Etna: implications for flank instability. This Alika debris slide, Mauna Loa, Hawaii. Journal of volume. Geophysical Research, 93, 4279-4299. MOORE, J. G. & CLAGUE, D. A. 1992. Volcano growth LO GIUDICE, E & RASA, R. 1992. Very shallow and evolution of the island of Hawaii. Geological earthquakes and brittle deformation in active Society of America Bulletin, 104, 1471-1484

volcanic areas: the Etnean region as an example. -- & MARK, R. K. 1992. Morphology of the island Tectonophysics, 202, 257-268. of Hawaii. GSA Today, 2, 257-259

MCCLELLAND, L., SIMKIN, T., SUMMERS, M., -- & MOORE, G. W. 1984 Deposit from a giant NIELSON, E. & STEIN, T. C. 1989. Global wave on the island of Lanai, Hawaii. Science, 226, Volcanism 1975-1985. Prentice Hall, New Jersey. 1312-1315. McGUIRE, W. J. 1995. Volcanic landslides and related --, NORMARK, W. R. & HOLCOMB, R. T. 1994. phenomena. In: Landslides Hazard Mitigation - Giant Hawaiian Landslides. Annual Reviews of with particular reference to developing countries. Earth and Planetary Science, 22, 119-144 The Royal Academy of Engineering, London, --, CLAGUE, D. A., HOLCOMB, R. T., LIPMAN, pp. 83-95. P. W., NORMARK, W. R., & TORRESAN, M. E.

-- & PULLEN, A. D. 1989. Location and orientation 1989. Prodigious submarine landslides on the of eruptive fissures and feeder-dykes at Mount Hawaiian ridge. Journal of Geophysical Research, Etna; influence of gravitational and regional 94, 17465-17484. tectonic stress regimes. Journal of Volcanology MUNRO, D. C. & ROWLAND, S. K. 1994. Implications and Geothermal Research, 38, 325-344. of patterns of caldera instability in the western

-- & SAUNDERS, S. J. 1993. Recent earth move- Galapagos Islands for mechanisms of caldera ments at active volcanoes: a review. Quaternary formation on the terrestrial planets. In: Proceed- Proceedings, 3, 33-46. ings of International Conference on Volcano

-- & PULLEN, A. D. 1993. Rifting at Mount Instability on the Earth and Other Planets. The Etna. WOVO News, 4, 16-18. Geological Society of London, 1994 Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

22 W. J. McGUIRE

MURRAY, J. B. & VOIGHT, B. 1996. Slope stability and SHIMADA, S., FUJINAWA, Y., SEKIGUCHI, S., OHMI, eruption prediction on the eastern flank of Mount S., EGUCHI, T. & OKADA, Y. 1990. Detection of a Etna. This volume. volcanic fracture opening in Japan using Global , PULLEN, A. D. & SAUNDERS, S. J. 1995. Positioning System measurments. Nature, 343, Ground deformation surveying of active volca- 631-633. noes. In: McGUIRE, W. J., KILBURN, C. R. J. & SIEBERT, L. 1984. Large volcanic debris avalanches: MURRAY, J. B. (eds) Monitoring Active Volca- characteristics of source areas, deposits, and noes: strategies, procedures, and techniques. UCL associated eruptions. Journal of Volcanology and Press, London, 113-150. Geothermal Research, 22, 163-197. NAKAMURA, K. 1980. Why do long rift zones develop 1992. Threats from debris avalanches. Nature, in Hawaiian volcanoes? - a possible role of thick 356, 658-659. oceanic sediments. Bulletin of the Volcanological --, GLICKEN, H. X. & Ui, T. 1987. Volcanic hazards Society of Japan, 25, 255-269 from Bezymianny- and Bandai-type eruptions. NUNNARI, G. & PUGLISI, G. 1995. GPS - Monitoring Bulletin of Volcanology, 49, 435-459. volcanic deformation from space. In: McGUIRE, SMITH, M. S. & SHEPHERD, J. B. 1996. Tsunamigenic W. J., KILBURN, C. R. J. & MURRAY, J. B. landslides at Kick 'em Jenny. This volume. (eds) Monitoring Active Volcanoes: strategies, STOOPES, G. R. & SHERIDAN, M. F. 1992. Giant procedures, and techniques. UCL Press, London, debris avalanches from the Colima Volcanic 151-183. Complex, Mexico: implications for long runout PIERSON, T. C. 1992. Rainfall-triggered lahars at Mt landslides (>100 km) and hazard assessment. Pinatubo, Philippines, following the June 1991 Geology, 20, 299-302. eruption. Landslide News, 6, 6-9. SWANSON, D. m., DUFFIELD, W. A., & FISKE, R. S. RASA, m., AZZARO, R. & LEONARDI, O. 1996. 1976. Displacement of the south flank of Kilauea Aseismic creep on faults and flank instability at volcano: the result of forceful intrusion of magma Mount Etna volcano, Sicily. This volume. into the rift zones. US Geological Survey Profes- RICE, J. R. 1992. Fault stress states, pore pressure sional Paper, 963. distributions, and the weakness of the San --, CASADEVALL, T. J., DZURISIN, D., MALONE, Andreas Fault. In: EVANS, B. & WONG, T-F. S. D., NEWHALL, C. G. & WEAVER, C. S. 1983. (eds) Fault mechanics and transport properties of Predicting eruptions at Mount St Helens, June rocks. Academic Press, London, pp. 475-504. 1980 through December 1982. Science, 221, ROBINSON, M. & ROWLAND, S. 1994. Evidence for 1369-1376. large scale sector collapse at Tharsis Tholus, --, HOLCOMB, R. T., NEWHALL, C. G., Mars. In: Proceedings of International Conference MALONE, S. D. & WEAVER, C. S. 1985. Forecasts on Volcano Instability on the Earth and and predictions of eruptive activity at Mount St Other Planets. The Geological Society of London, Helens, USA: 1975-1984. Journal of Geody- 1994 namics, 3, 397-423. ROMAGNOLI, C. & TIBALDI, A. 1994. Volcanic STEWART, I., McGUIRE, W. J., VITA-FINZI, C., collapse in different tectonic settings: an example FIRTH, C., HOLMES, R. & SAUNDERS, S. 1993. from the Aeolian Arc, Italy. In: Proceedings of Active faulting and neotectonic deformation on International Conference on Volcano Instability on the eastern flank of Mount Etna, Sicily. the Earth and Other Planets. The Geological Z. Geomorph., 94, 73-94. Society of London, 1994 TIBALDI, m. 1996. Mutual influence of dyking ROUSSET, D., BONNEVILLE, A., & LENAT, J-F. 1987. and collapses, Stromboli volcano, Italy. This Detailed gravity study of the offshore structure of volume. Piton de la Fournaise volcano, Rrunion Island. PASQUARE, G., FRANCALANCI, L. & Bulletin of Volcanology, 49, 713-722. GARDUNO, V. H. 1994. Collapse type and ROWLAND, S. K. & MUNRO, D. C. 1992. The caldera recurrence at Stromboli volcano, associated of Volcan Fernandina: a remote sensing study of volcanc activity, and sea-level changes. Atti dei its structure & recent activity. Bulletin of Convegni Lincei, Roma, 112, 143-151. Volcanology, 55, 97-109. TOUTAIN, J. P., BACHELERY, P., BLUM, P. A., RUBEY, W. M. & HUBBERT, M. K. 1959. Role of fluid DELORME, H. & KOWALSKI, P. 1995. Real-time pressure in the mechanics of overthrust faulting ground deformation monitoring. In: McGUIRE, I: mechanics of fluid-filled porous solids and its W. J., KILBURN, C. R. J. & MURRAY, J. B. (eds) application to overthrust faulting. Geological Monitoring Active Volcanoes: strategies, proce- Society of America Bulletin, 70, 115-166 dures, and techniques. UCL Press, London, RUST, D. & NERI, M. 1996. The boundaries of large- 93-112. scale collapse on the flanks of Mount Etna, Sicily. UI, T. 1983. Volcanic dry avalanche deposits- This volume. identification and comparison with non-volcanic SEMET, M. P. & BOUDON, G. 1994. Large scale debris stream deposits. Journal of Volcanology and collapse structure at Piton du Carbet, Martinique: Geothermal Research, 18, 135-150.

geological and petrological constraints. In: Pro- -- 1989. Discrimination between debris avalanches ceedings of International Conference on Volcano and other volcaniclastic deposits. In: LATTER, Instability on the Earth & Other Planets, The J. H. (ed.) Volcanic Hazards. Springer-Verlag, Geological Society of London, 1994 Berlin, 201-209. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

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VOIGHT, B. 1990. The 1985 Nevado del Ruiz volcano --, JANDA, R. J., GLICKEN, H. & DOUGLASS, P. M. catastrophe - anatomy and retrospection. Journal 1983. Nature and mechanics of the Mount St of Volcanology and Geothermal Research, 42, Helens rockslide-avalanche of 18 May 1980. 151-188. Geotechnique, 33, 224-273.

-- & ELSWORTH, D. 1992. Resolution of mech- VAN WYK DE VRIES, B. & BORGIA, A. 1996. The role anics problems for prodigious Hawaiian land- of basement in volcano deformation. This volume. slides: magmatic intrusions simultaneously WADGE, G., FRANCIS, P. W., & RAMIREZ, C. F. 1995. increase driving force and reduce driving resis- The Socompa collapse and avalanche event. tance by fluid pressure enhancement. Eos, Journal of Volcanology and Geothermal Research, Transactions of the American Geophysical Union, 66, 309-336. 73, 506 (abstract). WEAVER, P. P. E., MASSON, D. G., & KIDD, R. B. --, GLICKEN, H., JANDA, R. J., & DOUGLASS, 1994. Slumps, slides, and turbidity currents- P. M. 1981. Catastrophic rockslide avalanche of sealevel change and sedimentation in the Canary May 18. In: LIPMAN, P. W. MULLINEAUX, D. R. Basin. Geoscientist, 4(1), 14-16. (eds) The 1980 eruption of Mount St Helens, YOUNG, R. W. & BRYANT, E. A. 1992. Catastrophic Washington. US Geological Survey Professional wave erosion on the south-eastern coast of Aus- Paper, 1250, 347-377. tralia: impact of the Lanai tsunami ca. 105 KA? Geology, 20, 199-202.