Abstract During the 1999 eruption of Mount Etna, Sicily, the summit of the volcano changed dramatically.Lavas erupted from a fissure on the southern flank of SECone formed a large compound flow field that buried asubstantial part of the northern wall of Valle del Boveand the ground between there and SE Cone. Hot massflow deposits formed on the eastern and western flanksof the Chasm following a period of intense Strombolianactivity. At the same time, a new vent opened on thesouthern flank of SE Cone and there was a major rock-fall from its summit. Seven weeks later, part of the outerwestern rim of Bocca Nuova crater collapsed during an-other period of intense Strombolian activity, and lavaemerged at a high effusion rate through the breached cra-ter wall. Following a subsequent collapse of the BoccaNuova crater rim, a hot avalanche flowed a few hundredmetres on top of the previous lava flow field. Similar de-posits have been described on other volcanoes, but theirimportance in the evolution of the summit of Etna hasnot previously been recognised.

Keywords Collapse · Etna volcano · Mass flows · Rockfall · Summit craters

Introduction

During many eruptions on Etna, there have been majorchanges of the summit region of the volcano. Periods ofcone growth during summit explosive or effusive activi-ty are frequently punctuated by the collapse of one ormore of the summit craters. One of the largest historical

changes on the summit of Etna took place when the Cratere del Piano (Fig. 1), a 2-km-wide caldera, col-lapsed during the Plinian event of 122 B.C. (Coltelli et al.1998). Although this was the largest change in historicaltimes, it is not unique. The largest crater collapse duringthe past millennium occurred in 1444. This, and smallerscale events in 1169, 1537 and 1669 were accompaniedby major eruptions (Chester et al. 1985). More recentcollapse events have been smaller, and most of these in-volved the collapse of part of the inner crater walls or ofthe crater floor (e.g. Guest 1973; Murray 1980a, 1980c;Bertagnini et al. 1990; Calvari et al. 1994; Calvari et al.1995; Calvari et al. 1998; Falsaperla et al. 2001). Al-though less frequent than internal crater collapse events,instabilities have also affected the outer part of summitcraters on Etna (McGetchin et al. 1974). Here we de-scribe a number of different types of mass movementevents that took place during the 1999 eruption of Etna.

The summit of Etna

The main structure of the upper part of Etna is the Cra-tere del Piano, and this currently contains three conesand four active craters. The largest cone is the CentralCone, which has a basal diameter of 1 km and containstwo craters: La Voragine, or the Chasm to the east, andBocca Nuova to the west (Fig. 1). Two satellite centres,traditionally called NE and SE Craters, are located onthe flanks of Central Cone. In this paper we refer to themas NE and SE Cones, because they have been mostlyconstructional features during the past decade.

The oldest and largest crater, the Chasm, was de-scribed 90 years ago (De Fiore 1911). NE Cone formed in1911 as a degassing vent (Guest 1973) and has undergonemany phases of cone construction and subsequent col-lapse following over-steepening of its flanks (McGetchinet al. 1974). During the 1977–1978 eruption, lava flowedthrough the upper part of NE Cone creating a majorbreach (Murray 1980b; Chester et al. 1985). In 1980, NE Cone contained the highest point on Mount Etna

Editorial responsibility: P. Mouginis-Mark

S. Calvari (✉ )Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Piazza Roma 2, 95123 Catania, Italye-mail: sonia@iiv.ct.cnr.it

H. PinkertonEnvironmental Science Department, Lancaster University, Lancaster LA1 4YQ, UK

Bull Volcanol (2002) 63:526–535DOI 10.1007/s004450100171

R E S E A R C H A RT I C L E

Sonia Calvari · Harry Pinkerton

Instabilities in the summit region of Mount Etna during the 1999 eruption

Received: 31 May 2000 / Accepted: 15 August 2001 / Published online: 21 September 2001© Springer-Verlag 2001

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(3,345 m, Murray 1980b). In 1995, NE Cone was only3,309 m high, and the summit was on the southern rim ofthe Chasm (3,329 m a.s.l.; IIV 1995, unpublished topo-graphic map).

Bocca Nuova formed suddenly in 1968 as a 5-m-wide, 300-m-deep pit (Guest 1973). It collapsed and en-larged again in 1970 (Rittmann et al. 1971) and by 1975it had a diameter of 176 m. At this stage it started tobreak into the rim of the Chasm (Murray 1980a). Sincethen, the crater diameter has increased by periodicallycollapsing inwards along arcuate cracks on its westernand southern rims, and by November 1999 the septumseparating it from the Chasm had virtually disappeared.Between 1988 and 1995, Bocca Nuova was the first cra-ter in which a marked increase in the level of activitywas observed before a major flank eruption (Calvari etal. 1998).

SE Cone formed in 1971 as a collapse pit on the 1971eruptive fissure (Rittmann et al. 1971; Booth and Walker1973; Calvari et al. 1994). A small cone has since builtup around the crater, encroaching on to the south-easternslope of Central Cone (Murray 1980c). By September1998, the symmetrical cone was 50 m high, and by February 1999 it was 120 m high (measured from itssouthern base) with a basal diameter of 200–250 m.

Pit crater collapse events are common on the summitof many active volcanoes such as Vesuvius, Pu’u ‘O’o,Masaya, Lascar and Telica (Principe et al. 1987; Wolfe etal. 1988; Rymer et al. 1998; Roche et al. 2000). Al-though large-scale collapse of the flanks of volcanoes

has received considerable attention following the 1980eruption of Mount St Helens (e.g. McGuire et al. 1996),smaller scale mass movements in the summits of volca-noes have not been studied in as much detail. Examplesof the flow of newly deposited material after large explo-sive events have been reported for the 1930 eruption ofStromboli (Rittmann 1931), the 1944 eruption at Ve-suvius (Principe et al. 1987), the 1977 eruption of UsuVolcano (Yamagishi 1996), and during the growth ofPu’u ‘O’o (Wolfe et al. 1988), and the 1986 eruption ofIzu-Oshima volcano (Sumner 1998). A similar processhas been described following fountain-collapse on Me-rapi in 1998 (Voight et al. 2000). The volume of the re-sulting mass flow deposits is commonly between 70,000and 150,000 m3. Although small, these deposits cancause serious problems. For example, the hot avalanchedeposits emplaced during the 1930 eruption of Strombolikilled four people (Rittmann 1931). In this paper we de-scribe three collapse events that affected the outer flanksof Bocca Nuova, the Chasm and SE Cone during the1999 eruptive activity. We have identified potential trig-gering mechanisms for these events, most of which havenot been previously described for Etna.

The 1999 eruption: an overview

The 1999 eruption began on 4 February when a fissureopened on the south-eastern flank of SE Cone and it end-ed on 10 November following a gradual reduction in ef-fusion rate. The eruption produced two lava flow fields.The first was active during the entire eruption, andflowed eastward from the base of SE Cone (Fig. 1). Thisflow field was composed of ‘a’a lava with minor pahoe-hoe flows and a complex network of lava tubes. By Sep-tember, the flow field was 2.5 km long with an estimatedvolume of 11×106 m3. The mean effusion rate over thisperiod was 0.6 m3 s–1 (Calvari et al. 2001).

The effusive phase of the eruption changed in lateJune when Strombolian activity resumed at the summitcraters. Explosive activity gradually increased up to a

Fig. 1 Sketch map of the 1999 lava flow fields that formed at the base of SE Cone (SEC) and on the upper western wall of Valledel Bove between February and November, and of the overflowsfrom the western rim of Bocca Nuova (BN) emplaced between 18 October and 1 November. The grey areas on the map arecones; with that containing BN and VOR being the central cone.NEC NE Cone; VOR the Chasm; H1 hornitos of 4 February; H2 hornitos formed on 27 August; H3 hornitos built up since 4 September. Inset shows the distribution of scoria fall on the east-ern flank of the volcano following the paroxysmal explosive eventof 4 September (Modified after Behncke et al. 1999)

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paroxysmal event on 4 September, which involvedBocca Nuova, the Chasm and, to a lesser extent, SECone. During this event, bombs up to 2 m in diameterwere ejected over 1 km, and a heavily cratered bombfield formed on the southern, western and northernflanks of Central Cone. Following this activity, complexhot mass flows were observed on the rim and outerslopes of the Chasm (Fig. 2), and there were major rock-falls on the southern flank of SE Cone. Other episodes ofintense Strombolian activity during October and Novem-ber were accompanied by rockfalls, partial collapse ofSE Cone, and massive outward slumping of the rim ofBocca Nuova. Each of these was accompanied by chang-es in effusive activity. Following the 4 September event,lava flows discharged from the upper part of the 4 Febru-ary fissure and from a new vent that opened at the east-ern base of SE Cone (west of H3, Fig. 3A, B).

The second lava flow field formed between 17 Octo-ber and 5 November during a series of overflows fromthe western rim of Bocca Nuova. This flow field wastypically composed of simple ‘a’a lava flows emplacedat high effusion rates (>15 m3/s; Calvari et al. 2001) thatspread towards the west from two spillways of the west-ern crater rim of Bocca Nuova (Figs. 1 and 4). Betweenthen and the end of the eruption, most of the intenseStrombolian activity within Bocca Nuova was accompa-nied by more gas-rich magma flowing from the base ofSE Cone and by the formation of a number of hornitos.A new channelled ‘a’a lava flow field formed on thewestern flank of Bocca Nuova during October by a com-

Fig. 2 Sketch map of the mass flow deposits (MFA, MFB andMFC) from the Chasm and summit collapse of SE Cone. (1) Massflow deposits. The area covered by ballistic ejecta produced by (2)Bocca Nuova, (3) SE Cone and the (4) Chasm after the event of 4 September 1999 are also shown. (5) Inner parts of the summitcraters. (6) Pit crater depressions. Black dots are active vents

Fig. 3 A Sketch map of fractures around the base of SE Cone af-ter 4 September. Flow 49 was emplaced from the 4 February fis-sure on 4 September, and flow 48 came out from the new vent thatopened on 4 September at the east base of SE Cone. H1, H2 andH3 are the three hornitos formed on 4 February, 27 August and 4 September, respectively. B Photo showing the south-east base ofSE Cone, with hornitos H1, H2 and H3 and tumuli shown insketch A (Photo courtesy of Daniele Andronico)

Fig. 4 View from a helicopter of the summit craters of MountEtna taken from the south-west on 17 October 1999. In foregroundis Bocca Nuova crater filled with lava, the Chasm is the degassingcrater in the middle, and NE Cone is in the background. Note thetwo depressions at the lower left end of the photograph, which be-came the two main channels for lava overflows from this craterduring the effusive activity in October (Photograph courtesy ofDaniele Andronico)

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bination of overflows, spatter-fed flows, and flowthrough an ephemeral vent 6 m below the western rim ofBocca Nuova (Fig. 5). Massive outward slumping of partof the outer rim of Bocca Nuova on 25 October was fol-lowed by a large increase in discharge rates (>15 m3 s–1;Calvari et al. 2001) and advance rates of lava flowsdown the steep western flank of the volcano. Eruption oflavas from Bocca Nuova ceased on 3rd November. Effusion rates from the base of SE Cone decreased on 7 November, and by 10 November only very low-effu-sion-rate lavas were erupted from the top of a hornito.This was the last day of the 1999 eruption.

The 4 September 1999 event

At 18:00 on 4 September, Etna guides observed a gradu-al increase in Strombolian activity from Bocca Nuovaand the Chasm. They left as large pieces of scoria andbombs began to fall at the southern base of SE Cone,1 km from the crater rim. Strombolian activity increased,and a few hours later a sustained eruption column rose 1–2 km above the crater rims (Behncke et al. 1999).Scoria and ash layers a few centimetres thick fell on Milo, Fornazzo, S. Alfio and neighbouring villages tothe east of the summit. Explosive activity from BoccaNuova and the Chasm stopped after a few hours. Be-tween 1 and 5×106 m3 of tephra were ejected during thisperiod (Behncke et al. 1999).

Strong Strombolian activity within Bocca Nuova wasobserved by the authors on 5 September. During someexplosions blocks and bombs were ejected up to 50 mfrom the crater rim. Reddish-brown plumes of mainlylithic ash rose from the crater every 30 min. Our surveyrevealed that ejecta from Bocca Nuova, the Chasm andSE Cone was distributed in three directions (Fig. 2).Mass flow deposits were emplaced on the eastern andnorth-western outer slopes of the Chasm at this time, andthere was a large rockfall following partial collapse ofthe summit of SE Cone.

Complex hot mass flows from the Chasm

Following the intense Strombolian event on 4 Septem-ber, active cinder cones in the Chasm disappeared, andthe floor deepened by over 50 m. Complex mass flowdeposits with a cumulative volume of about 110,000 m3

and a total area of 74,000 m2 were emplaced around theChasm (Fig. 2). Deposits MFA and MFB formed on thenorth-west flank of the Chasm. Their surface was hotwhen first observed on 5 September. At the front ofMFA, the lower unit, a fresh ‘a’a lava lobe had been ex-truded. On both the inner and outer sides of the craterrim, the MFB deposits were split by a large number of1–2-m-wide arcuate crevasses, with typical depths be-tween 1 and 5 m. The uppermost part of MFB was cov-ered with spatter, but this was not found inside the cracks(Fig. 6). This strongly suggests that the crevasses devel-oped after the spatter was emplaced and once the upper5 m had cooled below the brittle–ductile transition tem-perature. Both MFA and MFB are heterogeneous andcontain welded spatter, bombs, massive lava blocks, andaltered red and yellow ejecta similar to that found aroundthe crater rim before this event (Fig. 7).

MFC was emplaced on the eastern flank of the Chasm(Fig. 2 and 8). This has two main flow units with lengthsof 600 and 750 m and a thickness of 1.5–2.0 m and twoshorter lateral branches. The middle flows have well-developed 2-m-wide levées less than 0.5 m higher thanthe flow surface, and all four flow units were hot whenobserved on 15 and 24 September. They are composi-tionally similar to MFA and MFB, and are composed ofblocks of agglutinated scoria, bombs, lava and spatterbetween 0.1 and 1 m in diameter. Approximately 30% isloose, light orange, weathered scoria with a mean diame-ter of a few tens of centimetres. The remaining 70% isdarker in colour and welded with individual lava blocksup to 1 m in size. Our observations confirm that, during

Fig. 5 Ephemeral vent on the northern wall of Bocca Nuova on 20 October 1999. The channel is 3 m wide, and the rim of BoccaNuova is on the upper part of the photograph

Fig. 6 View from the north of the cracks on the upper part of themass flow deposit (MFB) emplaced on the north-west flank of theChasm. The black carpet on top of the blocks is composed of spat-ter of the 4 September paroxysmal event. Photo taken on 10 Sep-tember

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emplacement, lava flowed in a highly plastic state, andsome was extruded through the rest of the flow materialat a late stage. On the lower frontal zone the deposits arecovered with lapilli and fresh scoria. The northern levéeof the southerly MFC flow was composed entirely of‘a’a clinker. In common with MFB, the proximal part ofMFC contains large arcuate cracks.

Rockfall deposits and evidence of instability of SE Cone

The southern rim of SE Cone collapsed both internallyand on its southern flank on 4 February. This followedthe opening of an eruptive fissure on its southern flank.Subsequent minor collapse events took place at varioustimes during this eruption. On 4 September, a new ventformed at the base of SE Cone (west of H3, Fig. 3A) andthis fed a new lava flow (flow 48, Fig. 3A) with a maxi-mum surface velocity of 3 ms–1. The calculated dis-charge rate of 12 to 18 m3 s–1 was 20 times the mean out-put rate estimated during the eruption of the eastern flow

field (Calvari et al. 2001). The eruption of flow 49 fromthe upper part of the 4 February 1999 fissure was accom-panied by weak explosive activity that formed threesmall hornitos in the upper part of the fissure (Fig. 3A).The formation of this new vent coincided with a majorrockfall, and the resulting deposit was composed of an-gular and sub-rounded lithic blocks <2 m diameter thatwere coated with fresh frothy lava. The deposit originat-ed at a rectangular notch that formed in the south-easternpart of the summit region (Fig. 9). It was mostly con-fined to the flanks of SE Cone, although some blocks upto 4 m in size rolled 1 km towards the rim of Valle delBove (Figs. 2 and 9). The deposit was approximately150 m wide and 150 m long with an estimated meanthickness of 0.5 m and a total volume of about11,250 m3.

In addition to the rockfall, a 100-m-wide field ofcracks and crevasses formed at the SE base of SE Conebetween H1 and H3 (Fig. 3A, B). These were first ob-served on 8 September, and they had a range of orienta-tions and widths. Those at the southern base of H1 wereoriented E–W, and were 10 to 20 cm wide. An ‘a’a lava

Fig. 7 View from the north of the upper mass flow deposit (MFB)emplaced on the north-west flank of the Chasm. The small hill inthe middle rim of the crater is the remains of the septum dividingBocca Nuova (right) from the Chasm (left). Note the black carpetof scoria fallout on the left of the photograph

Fig. 8 View from the east of the mass flow deposits that formedon the eastern flank of the Chasm (MFC in the text) after 4 Sep-tember. Note the development of lateral levées on the two longerflows, and the large fracture on the upper north flank of SE Cone(Photo courtesy of Romolo Romano)

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flow that emerged from the upper part of the eruptive fis-sure on 4 February had a high concentration of crackswith a similar orientation. Between H1 and H2 parallelcracks up to 50 cm wide were oriented N70°E. However,the same ‘a’a lava flow behind H2 (i.e. east of SE Cone)contained two systems of fractures: one oriented N15°Ewith a mean width of 1 m (Fig. 3A, B), the other orient-ed N140°E with a mean width of 2 m. Crack widthsslowly increased until November 1999, when the erup-tion stopped.

Collapse of the western rim of Bocca Nuova

During our 6 September survey, the crater floor insideBocca Nuova was approximately 80 m below the craterrim (Fig. 10), and two pits had formed at the positionspreviously occupied by active cinder cones (Fig. 2). Ex-plosive activity from Bocca Nuova increased in intensitybetween 10 and 12 September, and was accompanied bythe growth of hornito H3 at the base of SE Cone(Fig. 3A).

During early October, Strombolian activity fromBocca Nuova increased in intensity. The low ejection an-gle of many bombs (Fig. 11) suggested that magma wasat a high level in the crater. This was confirmed when alarge lava flow emerged from the western crater rim on18 October. At this time, Strombolian activity was con-structing five cinder cones along a NW–SE trend insidethe crater. The most westerly cone had a basal diameterof ~50 m, and its western edge was only a few metersfrom the rim of Bocca Nuova. At that time, there werenumerous short-lived overflows from the two lowestpoints on the western rim (Fig. 4). Shortly after this, anephemeral vent opened on the outer crater wall approxi-mately 6 m below the lowest part of the crater rim(Fig. 5), a region that had previously acted as a naturalspillway for lava erupted within Bocca Nuova. The flowthat emerged from this vent was approximately 3 m wide

when observed on 20 October, and the calculated dis-charge rate was 2 m3 s–1 (Fig. 12).

During intense Strombolian activity on October 22, alarge spatter-fed lava flow cascaded down the western

Fig. 9 View from the east of the flank of SE Cone and of the newvent that opened at its base on 4 September. Note the boxed out-line of the summit, and the rockfall deposit formed from its col-lapse on the outer flank of the cone. Photo taken on 6 October

Fig. 10 The inner north-west wall of Bocca Nuova crater on 6 September. Note the boundary between the upper part, made ofthin lava flows and spatter, and the lower part, formed by a thicklava flow or lava pond. The total height of the wall is over 100 m

Fig. 11 View from the south-west of Bocca Nuova north-westernflank on 26 October. The hummocky block on the left side hascollapsed a number of times towards the left (north-west) duringOctober. The dark flow on the lower left of the photo is the south-ern boundary of the western flow field emplaced during October.Note bombs above the crater rim; lines show the ejection angle.The white arrow on the photograph shows the position of the ventthat opened at the base of the outer slope of Bocca Nuova

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flank of Bocca Nuova. This eroded the crater rim andtransported blocks up to 3 m in diameter as far as 100 mfrom the rim. During a rapid traverse of this flow 2 h after it was emplaced, we noted that many of the large blocks had surface temperatures well in excess of100 °C. Most of the blocks were covered in a thin veneerof lava, suggesting that they had rolled during transport.However, large ‘skid marks; on the surface of the flowsabove some blocks confirmed that they moved after theflow was emplaced, but while the surface of the lava wasplastic.

During another period of intense Strombolian activityin Bocca Nuova on 25 October, there was a spectacularwestward failure of part of the western rim of this crater.The sector is inferred to have failed along a basal discon-tinuity, clearly visible in early September, between thelower portion of the northern crater wall, composed of aover 100-m-thick lava flow or lava pond and a 10-m se-quence of thin (~1 m) lava flows alternating with pyro-clastic deposits (Fig. 10). The failure mechanism is in-ferred to be similar to that of NE Cone early in 1978(Murray 1980b; Chester et al. 1985). The sector ofBocca Nuova that collapsed on October 1999 wasbounded to the east by a large cinder cone that hadgrown inside the crater (Fig. 4) and to the south by theephemeral vent formed between 18 and 20 October(Fig. 5). The crater was the site of strong Strombolianactivity from at least three active vents just before theemplacement of the hot avalanche (Fig. 11). During thisphase, a 4-m-thick layer of spatter accumulated on theouter crater rim, and tephra ejected to heights of 500 mabove the crater rim fell on the summit of NE Cone,500 m away. A few minutes before the collapse, observ-

ers estimated that part of the crater rim was uplifted. Wesuggest that this took place as magma intruded the craterwall, possibly along the discontinuity seen in Fig. 10.Debris from the resulting instability covered the 18–20October ephemeral vent (Fig. 5), and a new channelformed on the northern margin of the collapse area. In-tense radiative heat prevented observers from approach-ing this channel until 27 October. It had a minimumwidth of 7 m and a minimum estimated discharge rate of15 m3 s–1 and was one of the most spectacular we haveseen on Etna (Fig. 13). Boulders several metres in diam-eter were transported in this seething mass of lava anddebris. Following this event, the lava flows on the steepwestern flanks lengthened (Fig. 1).

Discussion

The processes that took place in late 1999 reveal an inti-mate relationship between explosive, effusive and massmovement activity in the summit region of Etna. Themass flow deposits on the eastern and north-western out-er flanks of the Chasm were clearly triggered by rapidaccumulation of spatter. The arcuate crevasses that cutthrough this welded spatter on the crater rim (Fig. 6) areevidence of continued movement of the deposits after theupper few metres of spatter had cooled below the brit-tle–ductile transition temperature. It is also important tonote that scoria on the upper surface of the northern sideof the Chasm were very unstable. Any slight perturba-tion resulted in whole scale collapse of the upper fewtens of centimetres of the deposit. If large amounts ofspatter accumulate on a surface in this state, collapse ofthis material is inevitable. It is also relevant to note thatthe main dispersal axis of tephra on 4 September was to-

Fig. 12 View from the west on 20 October showing the 3-m-widelava channel that emerged through the ephemeral vent on the outerwall of Bocca Nuova crater (see Fig. 11). Figure in the upper partof the channel for scale

Fig. 13 Channel with a minimum width of 7 m and a minimumdischarge rate of 15 m3 s–1 on the northern flank of Bocca Nuova,27 October 1999. Boulders up to 4 m in size can be seen in thecentre of the active channel. These were being transported fromthe collapsed part of the crater rim (upper right part of the photo-graph), and they were still moving at a rate of approximately20 mm s–1 when observed on 27 October. NE Cone is the featurein the background and to the right of the large blocks

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wards the east (Fig. 1, inset). The recent accumulation ofunconsolidated tephra could account for the highly un-stable nature of the eastern side of the Chasm. The pres-ence of well developed levées on the mass flow deposits(Figs. 7 and 8), together with the lobate nature of theflow fronts is compatible with emplacement as a highparticle concentration mass flow deposit.

The colour and degree of fumarolic alteration ofmany fragments within the mass flow deposits (Fig. 7)suggests that some of these were derived from the pre-vious crater rim, although we cannot discount post-emplacement fumarolic alteration of some material.However, because the source region is the crater rim, itis clear that at least some of the fumarolic alteration islikely to be primary. The presence of a flow front at thedistal end of MFA that closely resembles an ‘a’a flowfront, coupled with the presence of an ‘a’a levée on thenorthern side of the southernmost MFC flow, supportsour view that at least some parts of these depositsformed when spatter accumulated sufficiently rapidlythat it was able to flow once the basal shear stresses ex-ceeded the yield strength of the spatter. This resulted inthe formation of spatter-fed lava flows. In other places,movement took place within the unstable substrate be-neath the spatter. This resulted in a complex mixture ofolder debris and re-mobilised spatter. The underminingof the slope by this process of mechanical erosion in turntriggered additional instabilities, which in places over-ran the initial flows. The intrusion of very viscous lavathrough parts of the mass flow are compatible with thismodel, and textures closely resemble tension gashes, ripple and striation features described on Izu-Oshimavolcano (Sumner 1998; Fig. 8). The formation of cre-vasse structures on the upper part of these flows, coupledwith our observation that these formed once explosiveactivity ceased, suggests that there was continued creepof the flows once they had been emplaced.

Our observations suggest that the 4 September rock-fall on SE Cone was triggered by subsidence above anelongate cavity at the base of SE Cone (Figs. 8 and 9).We infer that this cavity was a drained conduit that de-veloped in response to the high levels of lava within SECone on 3 September. At that time, the lava within SECone had a minimum height of 50 m above the south-east base of the cone. There is unequivocal evidence forthis because flow 49 (Fig. 3A) was erupted from themiddle of the fissure on SE Cone on 4 September. As-suming a cone height of 120 m and magma density of2,500 kg m–3, the resulting magma pressure at the baseof SE Cone during this period is calculated to be1.5 MPa. This is similar to the vertical stress exerted bythe mass of the cone close to its centre. Once the magmapressure exceeds the vertical stress within the relativelylow density, poorly cemented, permeable cone, we arguethat magma can readily migrate through the scoria andemerge through one or more conduits. Evidence of thisprocess could be seen in the highest vents that formed atthe base of SE Cone (Fig. 3A, B). Here, fresh lava wasseen to have invaded the pre-existing joints within the la-

va as dykelets and randomly oriented fine stringers. Per-vasive penetration of lava through much of the tephra isadditional evidence of ‘fingering’ on a sub-millimetre tocentimetre scale. Vents at the base of cinder cones arenot uncommon, and they can lead to cone collapse on alarge scale. See, for example, the spectacular collapse ofthe main cone during the 1973 eruption of Heimaey, inthe Westmann Islands, southern Iceland (Pinkerton1987), or the distinct asymmetry of NE Cone in 1983following partial collapse during the 1977–1978 eruption(Chester et al. 1985). Similar situations are described byGutmann (1979) in the Pinacate Volcanic Field.

The open fractures that developed in the southern andeastern side of SE Cone (see Fig. 3A) could be formedby one of three mechanisms: deep-seated whole-scalegravitational creep of the edifice; downhill creep of theunderlying lavas and pyroclastics caused by loading ofthe slope by new lava; or continued flow of lava afteremplacement. The location (close to each of the eruptionsites), orientation (south, south-east and east), and timing(formation took place after 3 September when lava wasat a high level within SE Cone) are compatible with eachof these mechanisms.

The gravitational creep mechanism requires deep-seated movement at the lower part of the cone; shallowmovement in this region would have resulted in com-pressional features developing at the base of the cone, aswas observed by one of us (HP) immediately prior to thecollapse of the growing cone on Heimaey, south Icelandin March 1973. There are some similarities between theinferred situation at SE Cone on Etna and the sector col-lapse of Casita Volcano (van Wyk de Vries et al. 2000),although there are major differences in scale. The lavaloading mechanism is similar to that published by Murray (1988) whose measurements suggest that loadingof slopes by lavas on Etna can result in slope movement.The final mechanism is similar to the lava loading mech-anism, but differs in one important respect. The lavaflow mechanism involves continued downhill movementof lava once it has cooled below the brittle–ductile tran-sition. In this case, the tensional stress is generated bydifferential movement between more distal parts of theflows and those at the base of SE Cone whereas, in thetwo earlier mechanisms, the tensional stress is generatedby differential movement in the substrate in these two re-gions.

It is of interest to note the morphological similaritybetween the crevasse structures formed at the base ofSE Cone and those that formed on of the MFB depositson the crest and on the outer and inner upper parts of therim of the Chasm. There is no evidence that these areassociated with gravitational creep of the cone; the simi-larity between crevasses on the outer and inner rimssuggests that this can be discounted here. However,there is strong evidence of movement of the hot massflow deposits in which the crevasses form, eitherthrough movement of the deposits themselves or the un-derlying ash. The subsequent burial of these featuresprecludes any additional work on them, but there is

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scope for additional field observations involving post-emplacement movement of older lavas and hot massflow deposits.

Failure of part of the western crater rim of Bocca Nuova on 25 October was influenced by a number offactors. The discontinuity between the underlying thicklava and the thinner lava and relatively porous pyroclas-tics on the rim of Bocca Nuova created the potential forthe development of a pathway for lava at this interfaceonce lava level increased within the crater. The forma-tion of the ephemeral vent on the outer crater rim on18–20 October was clear evidence that the magma pres-sure within Bocca Nuova was sufficient to allow lava topenetrate through its upper rim. We suggest that thecombined effects of high lateral stresses and increased‘pore magma pressure’ along the basal interface led tothe dramatic uplift and failure of the western wall ofBocca Nuova.

Conclusion

The processes that took place on Etna in late 1999 revealintimate interactions between explosive, effusive andmass movement activity in the summit region. We notethat failure of one of the summit cones on Etna has takenplace before; it is likely that the failure of NE Cone dur-ing the 1977–1978 eruption took place with little warn-ing. This has implications for the safety of the largenumber of tourists who visit the volcano.

Some of the critical evidence of mass movement andhot mass flows on Etna was partially, or in some casescompletely, buried by pyroclastics a few weeks afterthey formed. Thus, although the features described inthis paper have not previously been observed on Etna,events of this type may not be unusual. Related pro-cesses have been noted on Stromboli (Rittmann 1931), Vesuvius (Principe et al. 1987), Pu’u ‘O’o (Wolfe et al.1988), Usu (Yamagishi 1996), Izu-Oshima (Sumner1998) and Merapi (Voight et al. 2000). It is clear thatsudden failure of the outer flanks of summit cones canoccur during strong explosive activity and when the levelof lava within the craters is high. Therefore, there is aneed for rapid remote surveys of the upper, potentiallyunstable parts of volcanic edifices during these periods.Recent tests using a new laser scanning system on Etna(Hunter et al. 2001) show that this is potentially a power-ful and effective technique in these situations.

Acknowledgements We wish to thank all of the Etna guides, andBoris Benchke, Orazio Consoli, Alain De Chambrier, Ulrich Kueppers, Franco Emi, Marco Fulle and Marco Ponte, who sharedtheir observations of the eruptive activity with us. Orazio Consoli,Mike Kelly, Russell Kent, Gill Pinkerton and Marco Ponte alsohelped during fieldwork. Daniele Andronico and Romolo Romanokindly allowed us to use their precious photos. Kathy Cashmanand Massimo Pompilio willingly shared their ideas with us. Com-ments from John Murray, Benjamin van Wyk de Vries, GeoffWadge and Bruce Houghton greatly improved an earlier version ofthe manuscript. This paper was undertaken with the assistance of agrant from the commission of the European Communities under

the Fourth Framework Programme, Environment and Climate,Contract ENV4-CT97-0713, and is a contribution to projectNo. 455 of the International Geological Correlation Programme.

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