birth, growth and morphologic evolution of the ‘laghetto’ cinder cone during the 2001 etna...

Birth, growth and morphologic evolution of the ‘Laghetto’cinder cone during the 2001 Etna eruption

Sonia Calvari a;�, Harry Pinkerton b

a Istituto Nazionale di Geo¢sica e Vulcanologia ^ Sezione di Catania, Piazza Roma 2, 95123 Catania, Italyb Environmental Science Department, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster LA1 4YQ, UK

Received 30 January 2002; received in revised form 12 February 2003; accepted 15 July 2003

Abstract

We have undertaken detailed observations of the formation of the ‘Laghetto’ cinder cone, a new cone thatformed during a 2-week period of intense activity in Piano del Lago, on the upper slopes of Mount Etna in summer2001. We describe the events leading to the formation of a small graben, the formation of pit craters on the base ofthe graben, the onset of phreatomagmatic activity, a transition to intense Strombolian activity, and a return tophreatomagmatic activity as the eruption came to an end. We discuss the reasons for these transitions, and describethe morphological development of the cone during these events. Arcuate cracks on the southern part of the cone wererelated to withdrawal of magma at the end of the eruption. Other slope instabilities that developed during theeruption include the formation of small radial grain flows on the outer flanks of the cone and the collapse into thecrater of part of the crater rim. Some of the failure planes we observed were first identified using a FLIR TM 695thermal infrared camera. This is the first time that infrared thermography has been used to detect instability ofvolcanic structures. Results obtained during this test case demonstrate that thermal cameras are a very useful tool forstudies of volcanic instability.4 2003 Elsevier B.V. All rights reserved.

Keywords: Etna volcano; cinder cone; volcano instability; thermal images; phreatomagmatic activity

1. Introduction

Cinder cones are found on most basaltic volca-noes and several approaches have been used toimprove our knowledge of their formation. Theseinclude detailed stratigraphic and sedimentologi-cal studies of extinct and eroded cones (Houghton

and Schmincke, 1989; Brown et al., 1994;Houghton et al., 1999), and these have led tocomprehensive descriptions of the internal struc-tures of cinder cones and to models of their for-mation (Hooper and Sheridan, 1998). A combinedapproach, involving laboratory modelling, fol-lowed by a comparison of modelled cones withcinder cone morphology in various parts of theworld, has led to the suggestion that cinder coneshape is controlled by tectonic structures inheritedfrom the basement (Tibaldi, 1995). However,there are remarkably few ¢eld-based accounts of

0377-0273 / 03 / $ ^ see front matter 4 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0377-0273(03)00347-0

* Corresponding author. Tel. : +39-095-448084;Fax: +39-095-435801.

E-mail addresses: [email protected] (S. Calvari),[email protected] (H. Pinkerton).

VOLGEO 3008 16-3-04 Cyaan Magenta Geel Zwart

Journal of Volcanology and Geothermal Research 132 (2004) 225^239

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cinder cone growth. Two of the most comprehen-sive include a description of the birth of Paricutin,in Mexico (Luhr and Simkin, 1995), and an ac-count by Murray (1980) of the evolution of theNE Cone on Etna between 1976 and 1978. In thepresent contribution, we describe the main eventsthat took place during the birth and growth of anew cone that formed during the July^August2001 eruption of Etna. Our visual observationsand measurements are augmented by data fromthermal images collected at the end of the erup-tion.

2. The 2001 Etna £ank eruption

On 12 July 2001, a seismic swarm in the uppersouthern £ank of Etna signalled the start of a new£ank eruption (Calvari et al., 2001). On 13 July, a¢eld of fractures, oriented roughly N^S, openedsouth of the SE Cone between Belvedere (2700 ma.s.l.) and La Montagnola (2550 m a.s.l.). Be-tween 15 and 16 July, the ¢eld of cracks propa-gated southward down to 2400 m a.s.l., wherethey split and continued to propagate aroundboth sides of La Montagnola (Fig. 1). At that

time, the ¢eld of fractures was 800 m wide, ap-proximately 2 km long and ran parallel to thewestern margin of the Valle del Bove. The ¢eldwas composed of normal faults with a measureddownthrow on individual faults of up to 0.5 mand lateral extension up to 2 m. In the centralpart of the fault system, a central graben formed(Calvari et al., 2001).Additional ¢ssures and fractures opened during

the period 13^20 July between the Valle del Leoneand La Montagnola, and lava £ows emerged froma number of vents along the fracture system. On18 July, a new eruptive ¢ssure opened between2150 and 2100 m a.s.l. on the southern £ank ofthe volcano, near Rifugio Sapienza. There wasintense Strombolian activity and ¢re fountainingfrom this ¢ssure, and a new lava £ow emergedand £owed south towards the village of Nicolosi(Fig. 2).On 19 July at 19:00, two pit craters opened

close to the central axis of the ¢eld of fracturesat 2550 m a.s.l. on the northern £ank of La Mon-tagnola, at a location called Piano del Lago. Thisis Italian for the ‘plane of the lake’, and there is asuggestion that a water-¢lled depression existed atthis location 50 years ago (O. Consoli, personal

Fig. 1. Aerial view of Piano del Lago from the southwest on 17 July 2001. La Montagnola cone is on the right of the photo, theValle del Bove is at the top, and north is on the upper left corner of the photograph. La Cisternazza pit crater on the upper leftis 150 m wide. Note the two parallel fractures north of La Montagnola. These are located on the area subsequently covered bythe Laghetto cone.


S. Calvari, H. Pinkerton / Journal of Volcanology and Geothermal Research 132 (2004) 225^239226

communication). Phreatomagmatic explosionsproduced a thick, dark grey ash cloud on July19 (Taddeucci et al., 2002). There was an increasein phreatomagmatic activity on July 20, and thiscontinued until July 22 when 100 m high ¢refountains formed on the lower part of the 2100m ¢ssure (Taddeucci et al., 2002). At the sametime, there was increasing explosive activity atthe top of the 2100 m ¢ssure, where metre-sizedblocks were ejected to heights of up to 200 m.During the afternoon of July 22, the lava £owfrom the 2100 m vent reached 1040 m a.s.l.,southeast of Mt. Rinazzi (Fig. 2).On 23 July, the ¢ssure on the northeast £ank of

the SE Cone, which had been active between Jan-uary and early July 2001, re-activated and therewas Strombolian activity from a vent on this ¢s-sure. Meanwhile, fractures opened between thenorthern base of the SE Cone and the Valle delLeone, and a new fracture opened between thesouthern rim of the SE Cone and the 2900 mvent (Fig. 2). The two pit craters at 2550 m wid-ened during the day, and explosive activity in-creased and fed thick ash columns. The falloutmaterial from the ash cloud fell on the east coastof Sicily. The thickness of ash erupted during thistime was su⁄cient to close the airport at Cataniafor several days.On 25 July, the amount of juvenile material

from these vents increased, and activity becamemagmatic (Taddeucci et al., 2002), with ¢re foun-tains several hundreds of metres high. Tephra ac-cumulated around the pits and formed a cindercone 200 m north of the summit of La Montag-nola. Lava continued to be erupted from the 2100m vent and it attained its maximum length of 6.5km on July 25. The e¡usion rate from the 2100 mvent was 15^20 m3 s31 at that time. This valuewas obtained from surface £ow velocity measure-ments along the main £ow channel, channel widthmeasured with a laser binocular, and £ow thick-ness obtained from levees heigth above the SP 92road.Just after midnight on 26 July, lava over£owed

from the western rim of the new cinder cone at2550 m. A new, fast lava £ow travelled south andreached 1950 m a.s.l., close to Rifugio Sapienza.Lava stopped £owing from the upper part of the¢ssure at the end of July, and there was a short-lived increase in the e¡usion rate from the 2100 mvent at that time. Lava continued to dischargefrom the lowest part of this ¢ssure, forming over-£ows from the previous channel. These £ows wid-ened and thickened the £ow ¢eld but did notpropagate further downslope from the most ad-vanced fronts. The e¡usion rate from this vent,calculated on the basis of daily measurementsalong the same £ow channel, decreased signi¢-cantly to 4 m3 s31 on 4 August. The drainage ofthe eruptive ¢ssure system was marked by contin-uous emission of lithic ash from the upper part ofthe 2100 m ¢ssure, the cinder cone at Piano del

Fig. 2. Map of the lava £ow ¢eld and ¢eld of fractures thatdeveloped during the 2001 £ank eruption. Modi¢ed afterCalvari et al. (2001).


S. Calvari, H. Pinkerton / Journal of Volcanology and Geothermal Research 132 (2004) 225^239 227

Lago, and the SE Cone. The eruption ended on9 August 2001.

3. Growth of the ‘Laghetto’ cinder cone

The growth of the cinder cone that developedon Piano del Lago (Fig. 2) was monitored closelyduring daily helicopter surveys of the eruption.The Etna Guides proposed that this new cindercone be called ‘Laghetto’ (little lake), and thisname is now widely accepted by volcanologistsworking on Etna. The formation of a ¢eld offractures just north of La Montagnola cone on16 July marked the early stages of emplacementof this vent. The fractures widened on 17 July(Fig. 1) and propagated around the eastern andwestern sides of La Montagnola cone. On 18 July,a lava £ow from the upper ¢ssure system invadedthe western portion of Piano del Lago, and on 19

July, following the widening of the fracture ¢eld,two pit craters formed and coalesced (Fig. 3), anda large amount of lithic ash was erupted (Tad-deucci et al., 2002). Ballistically emplaced bombsand blocks formed a rampart on the western sideof the pits, while ¢ne ash was transported by windto the east (Fig. 3). Phreatomagmatic activityfrom the pits was very intense during the follow-ing days, and three major vents aligned N^S wereobserved on 22 July (Fig. 4). Fire fountains wereclearly visible from at least two of the three ventson 23 July, but phreatomagmatic and magmaticactivity was observed from adjacent vents duringmuch of the eruption.It is worth noting that there was a similar sit-

uation along the 2100 m ¢ssure. When observedat 11:55 on July 30, the upper vent on the 2100 m¢ssure was the most explosive, and incandescentblocks were being erupted every few minutes.Brown ash was erupted from the middle vent,

Fig. 3. View from the northwest of the eruption site on 20 July. The degassing vent on the right of the photograph is the 2100m vent on the southern £ank of the volcano. The building in the centre of the photograph is the cable car station, which wassurrounded by lava at the end of July. The black smoking £ow in the lower part of the photograph is the lava £ow from the2700 m vent which £owed south (right of the photograph). The pits smoking in the middle of the photograph are the two de-pressions with arcuate western rims that formed at 2550 m a.s.l.. This eventually formed the Laghetto cone. The ash plume risingfrom the pits is about 700 m high.



and the lower vent erupted lava from the southernside of a Strombolian cone. Grey columns, possi-bly of water vapour, were erupted from a vent inbetween the lower and the middle, ash-laden vent.Di¡erent kinds of contemporaneous activity fromclosely spaced vents have also been observed dur-ing the growth of the Ukinrek maars in 1977 inAlaska (Kienle et al., 1980).Dominantly phreatomagmatic activity at the

Laghetto vents became increasingly magmatic on24 July (Fig. 5) when the ash plume was com-posed mostly of black, juvenile ash (Taddeucciet al., 2002). On 25 July, the northern vent activ-ity was characterised by ¢re fountains, and adome-like structure was visible on the southernpit (Fig. 6). Our observations suggest that thedome-like structure on the southern pit formedduring the eastward collapse, within the crater,of a large portion of the western scoria cone.The magmatic phase resulted in rapid growth ofthe scoria cone, and only small amounts of ¢ne

ash were erupted from the Laghetto vents at thistime (Fig. 6). On 26 July, the Laghetto cone was20^30 m high, and there was explosive magmaticactivity from at least three active vents (Fig. 7).At this time, a lava £ow was observed beingerupted at high £ux rates from the southernbase of the cinder cone. This £ow advanced rap-idly towards the upper cable car station (Fig. 7)and then down towards the Rifugio Sapienza.Magmatic activity continued on 27 July, by whichtime a 50 m high cinder cone had formed. Thiscontained three active vents (Fig. 8), from whichthere were alternating Strombolian and ¢re foun-taining activity. This stage of cone growth wasessentially dominated by ballistic scoria falloutaround the active vents.During the magmatic phase, which lasted from

23 to 28 July, sustained ¢re fountains were severalhundred metres high, and ejecta comprised juve-nile bombs up to 2 m diameter, lapilli, ash, and alarge number of sedimentary xenoliths (mainly

Fig. 4. Helicopter view from the southwest of the pit craters at 2550 m a.s.l. on 22 July. Three closely spaced depressions arevisible in the centre of the photograph, and lithic blocks are accumulating below the ash plume. The hill on the right is La Mon-tagnola cone, and the white box on its £anks is the shelter that housed the INGV web camera. This was destroyed during theeruption.



quartzite) from the basement with no evident re-action rims. The deposition of coarse-grainedtephra was not signi¢cantly in£uenced by windduring emplacement, resulting in symmetricalgrowth of the cinder cone during this stage. Re-mobilisation of the fallout material on the La-ghetto cinder cone was associated with tremorsduring the most vigorous explosions (Fig. 9). On29 July, two lava £ows poured out from both thenorthern and southern base of the cone (Fig. 9).At that time, the ash plume rising up from thecone contained a mixture of juvenile (black) andlithic (light brown) ash, suggesting collapse of theinner walls of the feeder conduit. This may havearisen following drainage and partial collapse ofthe upper ¢ssure system, which gradually ceaseddischarging lava by the end of July (Calvari et al.,

2001). Lava continued to £ow from the two ventsat the base of the cone on 30 July, when onlyminor amounts of lithic ash were erupted fromthe summit crater of the Laghetto cone. Eruptiveactivity gradually decreased, and on 31 July (Fig.10) vertical ash plumes up to a few tens of metresin height were erupted from the summit of theLaghetto cone.

4. Morphology of the Laghetto cinder cone

At the end of the eruption, we performed asurvey of the newly formed Laghetto cindercone. This fairly symmetrical structure (Fig. 11)was 62 m high, with a 300 m diameter base, and a90 m diameter, 50 m deep summit crater. Thecrater rim was approximately 10 m higher onthe eastern side. The surface of the £anks wassmooth, but with small-scale radial £ow-like fea-tures with lobate fronts. These £ow deposits weretens of centimetres wide, a few centimetres highand tens of metres in length. The base of the conewas surrounded by 1^2 m wide angular to sub-rounded lava bombs and blocks. At the southernbase of the cone, there were two e¡usive vents.One of these was 100 m south of the cone andwas aligned along a continuation of the centre ofthe original graben. This vent, which fed a lava£ow that extended into the Valle del Bove, wascharacterised by late-stage viscous squeeze-ups(Fig. 11). Between this vent and the base of theLaghetto cone there were two minor craters (Fig.11). These probably represent the remains of thevents that were active during early stages of con-struction of the eruptive structure (see Fig. 7 forcomparison). The other e¡usive vents are partlyobscured beneath the cone, but the drained chan-nels that they supplied can be seen clearly in Fig.11 on the SW and NE base of the cone.A trench excavated V200 m from the eastern

base of the cone revealed over 3 m of new tephrabetween the cone and the western rim of the Valledel Bove. A small arcuate crevasse intersected theupper western rim of the Laghetto cone (Fig. 11).This feature appears to be located above the pre-vious, larger arcuate depression formed betweenthe outer rim and the collapsed portion within the

Fig. 5. View from the north-northwest of the growing coneat 2550 m a.s.l. and the 600^700 m high ash plume on 24July. La Montagnola cone is in the background, and thelava £ow from the 2700 m vent is beneath the ash plume.The black portion of the plume contained substantially morejuvenile ash than the brown, lithic-rich plume. Ash fallout ispredominantly east of the plume (left of the photograph).



crater observed on 25 July during cone growth(compare Figs. 11 and 6). Fig. 11 also revealsevidence of an instability on the western side ofthe upper part of the cone. It shows clearly thatthe eastern part of the inner crater walls is steepand, unlike the western side, it is not covered inash. We suggest that this asymmetry within thecrater, together with the already noted di¡erencein crater height, can be explained by the collapseof 3^10 m of the western rim of the Laghettocone. Observation with the naked eye did not re-veal any other evidence of instabilities, eitherwithin the crater or on the £anks of the cone.However, thermal images showed that some fail-ure planes had formed since the end of the erup-tive activity. These thermal measurements are de-scribed in the next section.

5. Thermal data and failure planes

Apparent temperatures recorded by infrared

cameras are in£uenced by the temperature of theobject being measured, its emissivity, and atmo-spheric attenuation. This last in£uence is a func-tion of the distance of the object from the cam-era; humidity; atmospheric temperature; dust,gas, snow, ice and aerosol concentrations in theair between the object and the camera; and re£ec-tion of solar radiation. Since all thermal measure-ments during this study were made duringsummer and during similar and stable weatherconditions, we have used the constant values foratmospheric humidity (56%), atmospheric temper-ature (20‡C) and atmospheric transmission (0.88).For emissivity we use here 0.99, following Flynnet al. (1993) and Pinkerton et al. (2002). Fieldmeasurements made on Etna in August 2001 re-vealed that solar radiation increased apparenttemperature by up to 20‡C, and this needs to betaken into account when comparing temperaturescollected at di¡erent times. However, in this paperwe are dealing with relative temperatures acrossindividual images. Consequently, we can ignore

Fig. 6. Aerial view from the northwest of the Laghetto cone on 25 July. La Montagnola cone is on the right, and the Valle delBove is on the upper part of the photo. Note the two craters, with Strombolian activity taking place from the northern (left)vent and a dome-like feature within the southern (right) crater. The shelter (1.5 m high) of the destroyed INGV web camera ison the right of the photograph for scale.



Fig. 7. This photograph, taken from west on 26 July, shows three closely spaced vents within the Laghetto cone, and lava £ow-ing from the southern margin of the cone towards the cable car station (white building on the lower right just above the clouds).The summit of La Montagnola on the right is 130 m above the Piano del Lago.

Fig. 8. Aerial view on 27 July showing the three to four vents that were active within the cone. At that time, activity was mostlymagmatic. Note the lava £ow being erupted from the vent at the southern base of the Laghetto cone and other £ows from the2700 m elevation vent.



the e¡ects of solar radiation in the present study.For similar reasons, we can ignore the e¡ects ofviewing distance on apparent temperatures,though we recognise that temperatures of individ-ual pixels are mean values averaged over the totalsize of a pixel. In the present survey we used a

lens with a ¢eld of view of 24‡U18‡, giving indi-vidual images of 320U280 pixels. At a typicalviewing distance of 300 m, this results in a pixelsize of 42 cm, which can therefore lead to a re-duction in apparent peak temperatures.We collected the ¢rst thermal images using a

Fig. 9. Photo taken on 29 July from the west showing the Laghetto cone towards the end of the main construction period. Twolava £ows are fed from vents on the northern (left) and the southern (right) base of the cone. Note the yellow-brownish ash ris-ing from the £anks of the cone associated with tremors from the most powerful explosions of the Laghetto cone.

Fig. 10. Photo taken on 31 July from the west showing weak ash emission from the summit of the Laghetto cone. The coneheight is about 60 m.



FLIR TM 695 thermal camera on 9 August, theday that the eruption ended. Thermal images re-corded on 10 August revealed arcuate cracks onthe southern £ank of the Laghetto cone (Fig. 12).The apparent temperature of the surface of thecone was between 40 and 50‡C on 10 August.By comparison, the apparent temperature of theinactive vent at the eastern base of the cone (notshown in the ¢gure) was 106‡C, the maximumapparent temperature recorded at the vent onthe southern base of the cone was 240‡C, andthe maximum surface apparent temperature ofthe lava that descended into the Valle del Bovefrom the western vent, measured on isolatedspots, was up to 260‡C. Arcuate cracks on thesouthern £ank of the cone had an apparent tem-perature between 60 and 124‡C (Fig. 12), consid-erably greater than the mean surface apparenttemperature of the cone. Using Leica Vector1500 DAE/DAES laser binoculars, the distancebetween the cone and the thermal camera was244 m, at which distance pixel size was approxi-mately 34 cm. Consequently, the maximum ap-

parent temperature in Fig. 12 is a mean valueaveraged over a surface 34 cm wide. The areacontaining cracks was hotter than the surround-ings, probably as a result of greater permeabilityto hot gases released from the ¢ssure system be-low the Laghetto cone.One image recorded on 28 August from the

WSW during a helicopter £ight revealed verticalfaults intersecting the ENE portion of the cone(Fig. 13), and a plane at about 40 m below the

Fig. 11. Helicopter view of the Laghetto cone on 9 August. The Valle del Bove rim is on the right upper corner of the photo-graph, north is on the upper left. Note the two vents on the southern base of the cone, one with a very viscous lava that had£owed east into the Valle del Bove (not active at the time when the photo was taken), and another forming a nearly-circularvent. La Montagnola is on the low right corner of the photograph.

Fig. 12. Thermal image taken on the southern £ank of theLaghetto cone on 10 August 2001. Note the arcuate cracksin the middle, and the brightly coloured zone in between,suggesting a higher permeability of the tephra beneath thefractured upper surface. The maximum apparent temperatureof the cracks was 95‡C. The yellow bright low cone at thelow right end of the image is the small circular vent at thesouthern base of the cone which can be seen in Fig. 11.



crater rim that could be a possible failure surface.Where the vertical faults intersect the crater rim,there is a notch. This appears to be the surfaceexpression of a small graben bounded by faultsthat can be seen clearly in the crater wall. Anotherthermal image collected on 28 August includes theentire fractured area of the southern £ank of theLaghetto cone. The apparent temperature of thecracks is between 50 and 153‡C, whereas the sur-face apparent temperature of the cone is between13 and 30‡C (Fig. 14). The precision of the read-ings, for the thermal camera FLIR TM 695 weused, is R 2%.Thermal images taken from a helicopter survey

on 5 September 2001, showed that the tempera-ture of the cracks was consistently 20‡C higherthan the surrounding surface of the cone. Thethermal image in Fig. 15, recorded at an estimatedviewing distance of 660 m (pixel size of 92 cm),reveals a maximum apparent temperature of190‡C on the northern crater wall. The apparenttemperature of the fractures on the south £ank ofthe Laghetto cone was between 30 and 52‡C,whereas mean temperatures of the cone wereslightly lower (between 30 and 42‡C). This imagereveals that the fractures on the southern £ank ofthe cone do not appear to be wider than in theprevious image of 28 August. It also reveals agrain-£ow deposit of the SE £ank of the cone.This was observed during ¢eld surveys, but wasnot evident in previous images. Finally, two frac-tures on the crater rim above it appear wider thanin any previous image. These are parallel (Fig.15), oriented SSW, and bound a wedge that

may trigger further larger instabilities of this £ankof the cone.

6. Discussion

Phreatomagmatic activity on Etna volcano israther unusual. A small number of examples arerecorded from previous eruptive centres along theinner southwest walls of the Valle del Bove (Cal-vari et al., 1994), during the Ellittico caldera col-lapse (De Rita et al., 1991), and during a ¢ssureeruption 18.7 ka ago on the lower eastern £ank(Andronico et al., 2001). Initial phreatomagmaticactivity at the Laghetto vent is con¢rmed by de-tailed morphometric analyses of emitted ash (Tad-deucci et al., 2002). Phreatomagmatic activitytook place when ascending magma interactedthermally with rocks with a high initial water con-tent and a high permeability. Evidence in supportof a high water content of rocks in the upper partof the conduit includes the presence of a lake inthis region 50 years ago (O. Consoli, personalcommunication) which we infer formed abovethe upper pyroclastics of the Cuvigghiuni eruptivecentre (Calvari et al., 1994). Additionally, a sec-tion through the adjacent Cisternazza pit craterreveals sub-horizontal, highly ¢ssured lavas withoccasional rubbly tops and bases and thin ashhorizons. This would result in both high ¢ssureand high intergranular permeabilities of rocks inthe upper part of the Piano del Lago. Alterna-tively, phreatomagmatic activity at the Laghettosite may have been triggered by interaction be-tween rising magma and an aquifer in the base-ment. This activity has been suggested as a possi-bility at other locations on the southern £ank ofEtna where the basement is at high levels beneaththe volcano (Andronico et al., 2001). Indirect evi-dence in support of this mechanism is the pres-ence of extensive basement xenoliths in lavas andpyroclastics erupted from that part of the July2001 ¢ssure system which lies between the Laghet-to cone and the 2100 m ¢ssure. If a deep aquiferwere responsible for phreatomagmatic activity, itwould help to explain the presence of xenoliths.They could have been ripped o¡ the wall rocksadjacent to the ¢ssure system during deep-seated

Fig. 13. Thermal image taken inside the Laghetto crater on28 August from the southwest. In this case the image showsdi¡erences related to emissivity within the cone’s layers, un-conformities due to vertical fault displacement (middle of the¢gure), and heat concentrated along failure planes (inclined).Note the notch on the cone’s rim, which is directly abovethe vent that opened at the base of the cone during the erup-tion.



phreatomagmatic interaction. This is a more ef-fective method of producing xenoliths than pas-sive stopping of basement rocks by ascendingmagma. Previous £ank eruptions close to LaMontagnola and other parts of the south upper£ank of the volcano have also produced lavasenriched in basement xenoliths (Clocchiatti andMetrich, 1984). This suggests that the basementis closer to the surface here than in other parts ofthe upper slopes of the volcano.We infer that the transition to magmatic activ-

ity during the growth of the Laghetto cone was aconsequence of (i) reduced availability of waterfollowing the initial highly explosive interactionwith saturated rocks adjacent to the ¢ssure sys-tem; (ii) cooling of magma at the dike margins,leading to decreased permeability of the conduitwalls ; and (iii) generation of high water vapourpressures as water is boiled on approach to thehot conduit. Before the conduit walls are cooled,vapour pressure may be dissipated by vapour re-lease into the magma. However, once the conduitwalls are coated with solidi¢ed magma, water va-pour pressure will increase, and this could preventwater from approaching the conduit.A return to phreatomagmatic activity could

arise through a combination of (i) accidentaldamage of the lining of part of the conduit duringhigh-intensity explosive activity allowing watervapour, followed by water, to interact with as-cending magma; (ii) increased water pressuresarising from continued £ow of water from adja-cent parts of the aquifer exceeding localised watervapour pressures, resulting in thermal cracking ofthe lining on the conduit wall and ingress of waterinto the conduit ; or (iii) decreased magma pres-sure within the dike due to magma drainage, al-lowing aquifer water to enter the system.E¡usive vents at the northern and southern

base of the Laghetto cone formed by one of twomechanisms. Either they were located on the main¢ssure at the edge of the cone or they formed bytunnelling through of magma from the main vent.Since all active e¡usive vents were located on the¢ssure system, and since the vent at the southernbase was located on or close to the most southerlyexplosive vent, we favour development along the¢ssure system. This is supported by the simulta-

neous eruption of lithic brown ash from the mainvent in the Laghetto crater on July 29 and simul-taneous eruption of lava from the northern andsouthern e¡usive vents. The emission of lithic ashfrom the main vent suggests magma withdrawal,causing lack of support for the inner walls of thevent, and collapse of the walls inside the feederdike, causing lithic ash emission. The contrast ineruptive style from adjacent e¡usive and explosivevents can most readily be explained if ascendingmagma degassed from the main vent, allowingdegassed magma to be erupted from the adjacente¡usive vents.It is worth noting that the shape of the Laghet-

to cone at the end of the eruption concealed thecomplex processes involved in its construction. Inparticular, the morphological evidence of a ¢s-sure-fed source is not immediately obvious fromthe symmetrical nature of the ¢nal cone. Closeexamination of Fig. 15 reveals a ghostly outlineat the southern base of the cone, and suggests thatthere was explosive activity from more than onevent. In addition, asymmetric failure planes sug-gest that the cone is not axially symmetrical, andthat some of these formed as a consequence of amore complex earlier elongate cone system.The formation of arcuate incipient failure

cracks could arise because of instability of therapidly growing cone, especially if the cone wasundermined by sill-like intrusions of magma.However, the location of arcuate fractures onthe southern part of the cone directly above thecentre of the ¢ssure system, together with the ob-servation that cracks formed up to 10 days afterthe end of the eruption, strongly suggests that

Fig. 14. Thermal image on 28 August of a small portion ofthe southern £ank of the Laghetto cone showing the arcuatecracks as in Fig. 12. Compared with Fig. 12 the high-temper-ature zone is concentrated around the fractures, and themaximum apparent temperature is higher (115‡C comparedto 95‡C).



these formed during drainage of the underlyingdike.Radial £ow features that characterise the £anks

of the Laghetto cone could have formed by one oftwo processes : (i) gravitational collapse of part ofthe eruption column; or (ii) gravitational instabil-ity of crater walls, with decreased stability duringperiods of increased tremor or explosivity.The restricted size range of large (1^2 m diam-

eter) bombs and blocks at the base of the conemay be explained by the maintenance of high ex-

cess pressures in the vent for su⁄cient time toallow bombs of similar size to be erupted. Alter-natively, they might be limited by the diameter ofthe vents within the Laghetto cone. At the 2100 mvents, bombs of similar sizes are abundant aroundthe vents, and they are similar in size to the openvents on this ¢ssure, suggesting that, at least here,vent size is a limiting factor.Small landslides from the crater rim took place

within the cone during its growth, probably trig-gered by changes in magma level within the erup-

Fig. 15. Thermal image taken on 5 September from south. Note the zone with arcuate cracks on the southern £ank of the cone,and the grain £ow on the east £ank. This appears to be related to the fractures on the crater’s rim.



tive ¢ssure or by widening of the ¢ssure itself.Inner collapse into the crater during cone growthhas also been described by Houghton andSchmincke (1989).The notch on the northeast rim of the Laghetto

cone lies just above one of the e¡usive ventsopened at the base of the cone, and is boundedby normal faults. This suggests that the openingof a vent at the base of the Laghetto cone mayhave created a tunnel that collapsed because ofthe weight of the cone itself creating the notchon the crater rim, a process observed during the1999 eruption at the SE Cone after the opening ofthe e¡usive vent on 4 September 1999 (Calvariand Pinkerton, 2002). Thermal images collectedafter the eruption showed that zones of activefaulting and movement have thermal anomalies.These higher-temperature zones may be explainedby increased surface permeability due to fractur-ing. Along these planes hot gases from the stillcooling ¢ssure system can concentrate, revealingincipient failure planes.

7. Conclusions

We have described the formation and post-em-placement morphological and structural changesof the Laghetto cone. The ¢nal symmetrical shapeof the newly formed cinder cone conceals theelongated ¢eld of ¢ssures that opened on thesite during the 2001 Etna eruption. In contrastwith observations on other volcanoes (Tibaldi,1995), tectonic structures inherited from the base-ment have not controlled the external morphologyof the Laghetto cone. Instability features, such asfaults and notches along the crater rim, arcuatecracks, and grain £ows, developed on the cone1 week after the end of the eruption. These wereinitially detected using an infrared thermal cameraFLIR TM 695, and were probably highlighted bygreater permeability of the fractured zone to hotgases released from the ¢ssure below the cone.Instability features were clearly related to the ini-tial structure of the cone. Collapse within theeruptive ¢ssure system at the base of the coneoccurred during early stages of activity. They pro-duced unconformities in the inner cone structure

that controlled the distribution of some of theinstability features that developed subsequently.The formation of a notch above a small grabenon the crater rim of the Laghetto cone formedduring withdrawal of magma from this region.During the 1999 eruption from the SE Cone, anotch and associated rock fall formed followingdrainage of a sill-like tube system (Calvari andPinkerton, 2002). A similar mechanism mayhave been responsible for the formation of thenotch and small graben on the Laghetto cone.By contrast, the arcuate fractures on the southernside of the Laghetto cone formed by subsidence ofthe cone into the upper part of the ¢ssure follow-ing drainage. Our analysis using the thermalimaging system has con¢rmed that high-resolu-tion thermal imaging cameras have considerablepotential for revealing processes on volcanoes thatcannot be detected visually.

Acknowledgements

The authors gratefully acknowledge fundingfrom the EC Framework IV Environment Pro-gramme, Contract ENV4-CT97-0713, and fromINGV^Sezione di Catania. Thanks are due tothe Civil Protection for allowing daily helicoptersurveys, to the pilots for their courage and exper-tise in approaching the eruption site, and to Prof.Enzo Boschi for his support. The authors thankD. Dingwell and M. Polacci for their helpful com-ments on an earlier draft of this paper.

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birth, growth and morphologic evolution of the ‘laghetto’ cinder cone during the 2001 etna...

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