Lava tube morphology on Etna and evidence for lava

Journal of Volcanology and Geothermal Research 90 Ž1999. 263–280 www.elsevier.comrlocatervolgeores

Lava tube morphology on Etna and evidence for lava flow emplacement mechanisms Sonia Calvari b

a,b,)

, Harry Pinkerton

b

a Istituto Internazionale di Vulcanologia, Piazza Roma 2, 95123 Catania, Italy EnÕironmental Science Department, Lancaster UniÕersity, Lancaster LA1 4YQ, UK

Received 19 December 1998; accepted 29 January 1999

Abstract Lava tubes play a pivotal role in the formation of many lava flow fields. A detailed examination of several compound ‘a‘a lava flow fields on Etna confirmed that a complex network of tubes forms at successively higher levels within the flow field, and that tubes generally advance by processes that include flow inflation and tube coalescence. Flow inflation is commonly followed by the formation of major, first-order ephemeral vents which, in turn, form an arterial tube network. Tube coalescence occurs when lava breaks through the roof or wall of an older lava tube; this can result in the unexpected appearance of vents several kilometers downstream. A close examination of underground features allowed us to distinguish between ephemeral vent formation and tube coalescence, both of which are responsible for abrupt changes in level or flow direction of lava within tubes on Etna. Ephemeral vent formation on the surface is frequently recorded underground by a marked increase in size of the tube immediately upstream of these vents. When the lining of an inflated tube has collapsed, ‘a‘a clinker is commonly seen in the roof and walls of the tube, and this is used to infer that inflation has taken place in the distal part of an ‘a‘a lava flow. Tube coalescence is recognised either from the compound shape of tube sections, or from breached levees, lava falls, inclined grooves or other structures on the walls and roof. Our observations confirm the importance of lava tubes in the evolution of extensive pahoehoe and ‘a‘a flow fields on Etna. q 1999 Elsevier Science B.V. All rights reserved. Keywords: lava flow; lava tube; tube coalescence; flow inflation

1. Introduction There is a widespread misconception that lava tubes form only in pahoehoe flows and at very low effusion rates ŽGreeley, 1971, 1987; Peterson and )

Corresponding author. Istituto Internazionale di Vulcanologia, Consiglio Nazionale delle Richerche, Piazza Roma 2, 95123 C atan ia, Italy . F ax : q 0 0 3 9 -0 9 5 -4 3 5 8 0 1 ; E -m ail: [email protected]

Swanson, 1974; Peterson et al., 1994; Hallworth et al., 1987.. This is disputed by Calvari and Pinkerton Ž1998. who describe a complex system of lava tubes on the 1991–1993 ‘a‘a lava flow field on Etna where tubes formed at effusion rates spanning more than two orders of magnitude ŽCalvari et al., 1994a.. This eruption was not unique. Many descriptions of lava tubes on ‘a‘a flows on Etna have been published during the past three centuries ŽAnonymous, 1669; Recupero, 1815; Lyell, 1858; Cucuzza Silvestri,

0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 2 4 - 4

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S. CalÕari, H. Pinkertonr Journal of Volcanology and Geothermal Research 90 (1999) 263–280

1977.. Greeley Ž1971. concluded that 18% of the flows on Etna have been emplaced at least partially through lava tubes. However, more recent studies have shown that this may be an underestimate. Brunelli and Scammacca Ž1975. compiled a comprehensive catalogue of all known lava caves in Sicily, including many in ‘a‘a flows. More recently, Pinkerton and Sparks Ž1976.; Guest et al. Ž1980.; Frazzetta and Romano Ž1984. and Calvari and Pinkerton Ž1998. have discussed the importance of lava tube development in ‘a‘a lava flows on Etna. Finally, many tubes have been discovered and surveyed during the past three decades by the Gruppo Speleologico Etneo. Thus, lava tubes play an important role in the formation of many lava flows on Mount Etna, and many of these lava flows are ‘a‘a. While single ‘a‘a flow units on Etna are typically 1 to 10 m thick, the resulting flow fields are commonly 10 to 50 m thick ŽRomano and Sturiale, 1982., and at least one Ž1991–1993. attained a maximum thickness of 100 m ŽCalvari et al., 1994a; Stevens et al., 1997.. Occasionally, and for a short time before a new vent opens from its margins, the flow front of a single inflated flow can attain a thickness of 20–30 m ŽCalvari and Pinkerton, 1998.. Tubes that form inside arterial lava flows during the first few weeks of an eruption can easily be related to their parent channels. During long-lasting eruptions, however, later flows cover features related to previously formed tubes, and they become very difficult to detect. Only when the tube is obstructed, or pressure increases cause breakouts along the roof or sides of the tube, does its path become visible ŽMattox et al., 1993; Calvari and Pinkerton, 1998.. Such breakouts can remain localised at changes in slope, producing tumuli. Post-eruption or post-drainage collapse of part of the roof can then form useful entrances to the tube system. Because of the compound nature of many flow fields on Etna, the flows in which many tubes form are covered by subsequent flows, and there may be little relationship between the slope and direction of surface flows and the underlying tubes. This is in agreement with observations on pahoehoe flow fields in Hawaii ŽPeterson et al., 1994.. While our observations confirm the importance of tube formation in both pahoehoe and ‘a‘a lava flows on Etna, our observations suggest that at least some

of the tubes that are considered to have formed on pahoehoe formed in large ‘a‘a lava flows. This complication arises because the type of lava that finally forms on a flow field or along its margins may differ from the type of lava within which the tube system developed. We know, for example, that pahoehoe and toothpaste morphologies on Etna are common on flows that develop at the margins of ‘a‘a lava flows through third-order ephemeral vents ŽCalvari and Pinkerton, 1998. with very low discharge rates ŽPinkerton and Sparks, 1976; Calvari et al., 1994a.. These small flows form during drainage of primary ‘a‘a lava, and are typical of the later stages of long duration eruptions on Etna. Thus flow fields that have been active for years, and whose actual shape was formerly controlled by the emplacement of large ‘a‘a lava flows ŽWadge, 1978; Kilburn, 1989; Kilburn and Lopes, 1991., may eventually have a surface of mostly pahoehoe and toothpaste lava. For example the pahoehoe and toothpaste morphologies common in the proximal part of the 1991–1993 flow field contrast with the predominantly ‘a‘a flows that produced extensive systems of lava tubes during most of this eruption ŽCalvari and Pinkerton, 1998; Calvari et al., 1994a.. We conclude that some of the lava tubes previously considered to form in pahoehoe lava flows on Etna and elsewhere may have formed initially in ‘a‘a lavas which have subsequently been covered by late-stage pahoehoe lava. This will be investigated further during future fieldwork on Etna. In view of the importance of lava tubes during the 1991–1993 and earlier eruptions on Mount Etna, we have undertaken a systematic study of a number of lava tubes. These studies confirm the complexity of underground structures and lead to an improved understanding of how tubes control the development of many long-lived flow fields on this volcano.

2. Lava tube morphology During August and September 1997, September 1998, and October 1998, we examined 16 lava tubes on Etna ŽFig. 1. with the assistance of members of Gruppo Speleologico Etneo. Our observations confirm that underground observations are an essential component of research into emplacement mecha-


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Fig. 1. Map of Mount Etna and location of the lava tubes described in the paper. 1 s Micio Conti tube, prehistoric lava flows; 2 s Tre Livelli-KTM tube system, 1792–1793 eruption; 3 s Grotta Cassone, 1792–1793 eruption; 4 s La Fenice tube, 1792–1793 eruption; 5 s and 6 s Cutrona and Salto della Giumenta tubes, 1991–1993 eruption; 7 s Gallobianco tube, 1050–1100 eruption; 8 s Intraleo tube, 1225 eruption; 9 s 1985 proximal tube system.

nisms of lava flows on Etna. They have led to a greater appreciation of the complex processes that take place during the emplacement of long-lived ‘a‘a lava flows, and they help to confirm the importance of many of the processes that we described during our earlier work on the 1991–1993 flow field on Etna ŽCalvari and Pinkerton, 1998.. In the following account, we describe structures within selected lava tubes on Etna, and we show how studies of their internal dimensions, morphology and field relation-

ships can provide important information on the mechanisms of formation and propagation of tubes and their parent lava flows. 2.1. Longitudinal sections of laÕa tubes The most important feature of many longitudinal profiles of lava tubes is the alternation of very wide chambers and narrow, low passages. We have observed this situation in many lava tubes on Etna and

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note that this is a common feature in lava tubes elsewhere ŽOllier and Brown, 1965; Atkinson et al., 1975.. Often constrictions are so pronounced that they produce an insurmountable blockage within the tube, although the tube may continue downslope and perhaps be accessible through a different entrance. Our underground observations lead to the following conclusions. Large hemispherical chambers within a tube system commonly represent different stages of inflation of a flow front. The small, narrow passages that are encountered at the distal end of some of these chambers are first-order ephemeral vents ŽCalvari and Pinkerton, 1998. through which the tube system propagated downslope. When the inflationary stage lasted long enough to develop a stable crust, the resulting chamber was hemispherical. If inflation continued and propagated upslope before an ephemeral vent formed, the chamber assumed a more elongated shape. If drainage through a first-order ephemeral vent took place before the crust of the inflated flow became self-supporting, the roof collapsed and blocked further expansion of the tube. Occasionally, partial collapse formed a downward bulge on the tube roof. Longitudinal profiles of lava tubes can also be compound, where two or more tubes coalesce. In such cases, tube size typically decreases vertically towards the upper part of the compound tube. This can arise either from a decrease in effusion rate during the eruption ŽWadge, 1981., or because successive flows in any region tend to be smaller Žbecause there are more of them., and hence have reduced discharge rates. While many lava tubes have similar mean diameters for several hundred metres, some gradually increase in size with increasing distance whereas others decrease. There is no evidence to suggest that gradual increases in mean diameter are a consequence of inflation. Instead they are inferred to arise from reduced flow velocities because of the combined effects of increased apparent viscosities and reduced topographic gradients. In order to maintain the same discharge rate under those conditions, the mean diameter of a tube will need to increase downflow. However, this increase can be offset by the tendency for flows Žand hence tubes. to split with increasing distance from source. Each splitting event

will reduce the mean diameter. It is therefore important to recognise that not all changes in mean diameter are induced by inflation and tube coalescence. As an example of increasing mean diameter with increasing distance from the source, the 1792–1793 Tre Livelli tube close to the eruptive fissure has a mean diameter of 1.5 m and a mean slope of 208 ŽCorsaro et al., 1990.. By contrast, the Grotta Cassone, a tube that formed in the same flow field at a distance of 2.5 km from the main vent, has a mean diameter of 3 m and a mean gradient of 58 ŽBalsamo et al., 1994.. If we assume Ž1. a constant discharge rate between these tubes; and Ž2. that Jeffrey’s equation can be used to relate flow velocities to tube dimensions and viscosity, we can readily show that h 4 sin arh is constant, where h is the thickness of a flow, a is the gradient and h is the apparent viscosity. For the 1792–1793 tube system, under conditions of full tube flow, discharge rates in the proximal region will be 11 m3rs for an apparent viscosity of 10 3 Pa s and a density of 2500 kgrm3. To maintain a similar discharge rate in the lower part of the tube system, the apparent viscosity will rise to 4000 Pa s. Assuming similarities in rheological properties with the 1983 Etna lava, this change can arise through a decrease in temperature from 10978 to 10858C ŽPinkerton and Norton, 1995.. While this decrease is considerably higher than suggested by previous workers ŽSwanson, 1972; Calvari et al., 1994a; Peterson et al., 1994; Keszthelyi, 1995., it does not take into account crystallisation during degassing ŽSparks and Pinkerton, 1978.. 2.2. Internal structures and other features in laÕa tubes During our surveys we noted the presence of several features that help to unravel the flow and thermal history of lava tubes. These include single or multiples layers of lava on the outer walls and roof of many lava tubes, different types of stalactites, lateral lava benches, peel-off and rolling-over structures, longitudinal and transversal cracks, and different surface morphologies on the floor of tubes. 2.2.1. Lining on the walls and roofs of laÕa tubes All lava tubes that we have studied are lined with one or more layers of lava. These layers preserve a


record of periods when lava partially or completely filled a lava tube. Sections through these layers are visible only when part of it collapses from the roof or walls of a lava tube. Multiple concentric layers have been observed in several cases and they commonly show an increase in thickness both towards the base of the tube and downward along the flow field. These layers have thicknesses that vary from a few millimetres ŽFig. 2. to 1 m, and we conclude that their thickness is a function of flow rheology, flow duration, discharge rate, and thermal history of the tube. Thinner layers form when a single pulse of fluid lava flows through a tube and is rapidly drained. Thicker, generally multiple, layers of lava form when several surges of relatively cool lava pour down a tube. The lining of lava tubes is generally black or dark grey in colour, and occasionally red, indicating different states of oxidation. Each layer corresponds to a cycle of infilling and emptying of the tube. This cyclic process can be caused by Ž1. variations in vent discharge rate; Ž2. partial or complete blockages upflow or downflow; Ž3. tube coalescence; or Ž4. cycles of flow inflation at the margins or flow fronts. A revealing example of

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the latter process can be seen in La Fenice tube, where four layers of comparable thickness in the upper tube are clearly mirrored by four connected flows ŽFig. 3..

2.2.2. Lateral benches Lateral benches are a very common feature of lava tubes Že.g., inside the Cutrona tube, the Tre Livelli tube and the tube along the 1991 eruptive fissure.. They indicate that a stable lava level was maintained for long enough to allow lateral solidification. Sometimes multiple benches are observed ŽWood, 1974., and these record different stages of stable magma supply rate. The inward accretion of lateral benches can produce a complex system of stacked tubes that can resemble several generations of tubes formed in successively higher flows. This occurrence is common along tubes that formed in eruptive fissures, where tube width is generally less than 2 m and flow level decreases with time. Merging of lateral benches to form ‘tubes within tubes’ is more common when peeling-off and rolling-over structures develop. Leotta Ž1994. considers these

Fig. 2. Multiple linings with individual mm-thick layers at the entrance of the Cassone tube, 1792–1793 eruption. Photo by Alfio Amantia, IIV-Catania.

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Fig. 3. Multiple layers, each 10 to 15-cm thick, along La Fenice tube, 1792–1793 flow field. Note the section of the tube indicating sealing of narrow channel and the tube roof section formed by X X jamming of a a lava blocks. The tube width is ;1 m.

structures to be very common in tubes formed inside eruptive fissures Že.g., the 1792 fissure. where, because of their width, peeling-off and rolling-over structures can readily coalesce and effectively fuse to form an inner tube roof. 2.2.3. Longitudinal and transÕersal cracks Longitudinal cracks along the roof of lava tubes are a common feature, and they form as a result of roof deformation, either as a consequence of deformation under the weight of the roof, or due to the combined weight of the roof and an overlying flow. Cracks can form in the middle of the roof, producing a downward directed bulge, or they can open at the sides of the collapsing central belt. Extreme deforma-

tion of longitudinal cracks can produce pillars, sometimes elongated in the flow direction with ellipsoidal section. They are made of ‘a‘a rubble or ropy blocks from the flow surface, coated by lava flowing into the tube. Such pillars are common in the Micio Conti prehistoric lava tube at the outskirts of Catania ŽFig. 4.. Transverse cracks are less common and where present they are not accompanied by inward deformation of a tube’s roof, and appear to be related to thermal contraction. Transverse cracks are more surficial structures than longitudinal ones: they develop only in the innermost lining of tubes, and are restricted to places where the ground slope increases. If a tube suddenly drains and cools, thermally-induced contraction cracks will develop preferentially on the steepest slopes. Steep gradients assist in the generation of locally high gravitational stresses and this may explain their higher concentrations in these regions. The number of transverse cracks per unit length appear to be proportional to lining thickness and local ground slope. Two examples are the northern branch of the Cutrona tube, which has transverse cracks every 1–2 m on a slope of 258 ŽGiudice and Leotta, 1995., and the upper part of the Tre Livelli tube, which has cracks every 0.5 m on a slope of 308.

2.2.4. GrooÕes Horizontal and inclined grooves and striae are common features on the walls of lava tubes ŽFig. 5.. They form when cool ‘a‘a lava blocks on the surface of the flow move past the plastic walls of the tube, softened by the high temperature of the lava flowing into it, and they record the flow direction. In some places, grooves and striae record sudden changes in flow direction. An excellent example can be seen in the Intraleo tube where, on both sides of a lava tube, grooves change from horizontal to almost vertical over a distance of 10 m. Because this sudden change in direction commonly takes place at the point where the floor of an upper tube has collapsed into a lower tube, such sudden changes in groove direction are considered to be indirect evidence of tube coalescence. Different groove directions are preserved on multiple layers in the lining of lava tubes and channels. Along the 1983 flow field close to Mt. Vetore,


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Fig. 4. Close to the entrance of the Micio Conti lava tube, formed in prehistoric pahoehoe lava flows. The wall partially separating two parallel chambers may have formed either by Ža. lateral coalescence of two parallel tubes; or Žb. by downward collapse along axial cracks of the still plastic roof of the tube; or Žc. by multiple vent opening along a fingering pahoehoe flow front. Photo by Alfio Amantia, I.I.V.-C.N.R.

there is a lava fall along a channel where grooves have different inclination in each layer. In this case,

they appear to record a progressive increase in slope of the step below the lava fall due to mechanical

Fig. 5. Horizontal grooves above the roadside entrance to the Tre Livelli tube, 1792–1793 flow field.

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erosion triggered by turbulence.

2.2.5. LaÕa stalactites While some stalactites form from mineral precipitates at the end of eruptions ŽGiudice and Leotta, 1995., the majority form in response to thermal and mechanical processes during flow and subsequent drainage. Stalactites on Etna have shapes and sizes that differ significantly from some of their Hawaiian counterparts. For example, the delicate, worm-like structures found on the roofs of lava tubes on Hawaii ŽJaggar, 1931. have not been observed on Etna. On Etna four kinds of stalactites can be distinguished, and there are excellent examples on the walls and roof of the Tre Livelli and Cassone tubes. Stalactites with very smooth surfaces form on ridges that are elongated in the flow direction ŽFig. 6.. These stalactites are typically red in colour, and they are considered to form by remelting by gases accumulating below the roof ŽJaggar, 1931; Kauahikaua et al., 1998.. On Etna they are typically up to a few centimetres long and at most 2 cm wide at the base,

and they are conical in shape. Another type is rough, grey in colour and spiky, and it is usually found in constrictions of the tube. This second group of stalactites was also recognised by Jaggar Ž1931.. They are generally less than 0.5 cm wide, a few centimetres long, and they are considered to have formed where lava completely filled a tube and then drained, either partially or completely. The resulting stalactites record the dribbling of lava from the roof. The spiny nature of these stalactites is due to the presence of crystals Žmainly plagioclase. and small amount of interstitial glass. The third group of stalactites are morphologically similar, and they form when part of the roof or wall lining drops or rolls off, leaving rough ‘pull-apart’ stalactites ŽFig. 7.. The final type of stalactite, which is not very common, is characterised by bulbous shapes. Thin sections reveal that they are composed of multiple layers with an external boundary marked by a very thin film of oxides. We interpreted these as stalactites that have been repeatedly coated by lava. Sometimes stalactites have an unusual shape due to welding, on the lower tip, of small and delicate ‘a‘a scoria. This is

Fig. 6. Smooth lava stalactites Žright. formed by melting of ridges Žparallel to the flow direction. on the roof of the 1792–1793 Cassone tube, close to the entrance.


clear evidence that the tube was, at one time, filled with ‘a‘a lava. Beautiful examples of these structures are exposed along the upper part of the Tre Livelli tube. 2.3. Master tubes and laÕa tube networks Our observations confirm that many lava tubes on Etna form complex three-dimensional braided networks. ‘Master tubes’ tend to form in zones of high ground slope, in narrow valleys, and along eruptive fissures. The term ‘master tube’ refers to an arterial tube that delivers lava to the network of tube distributaries throughout most of the flow field. The relationship between the location of master tubes and ground slope or surface morphology has been observed in other volcanoes, for example in Iceland ŽWood, 1971., Mexico ŽKeszthelyi and Pieri, 1993. and Australia ŽAtkinson and Atkinson, 1995; Atkinson et al., 1975.. Where ground slope decreases, tubes generally split into smaller branches ŽCavallaro et al., 1985.. In this situation they are difficult to detect, unless a higher gradient improves drainage ŽWood, 1975..

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Master tubes form in cooling-limited lava flows ŽGuest et al., 1987. when a steady supply allows the flow front to inflate, and where topography permits a number of first-order ephemeral vents to open from successive front regions of connected flows ŽCalvari and Pinkerton, 1998.. The effect of these vents is to drain the upslope portion of the early-formed tube sector. If the inflated frontal zone has a self-supporting roof, a lava tube forms within this region. Two other mechanisms discussed by Calvari and Pinkerton Ž1998. occur Ža. along narrow channels, where a slightly fluctuating effusion rate allows levee ´ growth, eventually leading to complete roofing-over of the channel, and Žb. along wide channels, where cooling rate and crustal growth play the main role ŽCalvari and Pinkerton, 1998.. The latter two mechanisms have been found to be less efficient than the first. During the initial phase of the 1991–1993 eruption, at similar effusion rates, it took 15 days to form a tube along a narrow channel, and 1 month along a wide channel, whereas tubes developed in inflated frontal zones in 1 week ŽCalvari and Pinkerton, 1998..

Fig. 7. Pull-apart stalactites Žarrows. formed in the 1792–1793 Cassone tube by detachment of an inner lining. The ruler is 25-cm long. Photo by Alfio Amantia, IIV-Catania.

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2.4. Tube coalescence Mature tube systems typically form complex three-dimensional networks, and upper tubes can remain separated from lower, previously formed, tube systems. Our observations suggest that the stability of the intervening crust depends on its tensile strength, thickness, and on the pressure exerted by the flow above it. Occasionally, an upper tube can break through the roof of a lower tube. This can result in the reactivation of the lower tube system because the increased pressure at the snout allows breakouts at the front, roof or margins of the previously inactive lower system ŽPeterson and Swanson, 1974; Mattox et al., 1993.. Tube coalescence generally occurs in a vertical direction. Two classic examples are seen in the 1985 and 1792–1793 proximal lava tubes ŽFig. 1., where respectively four and three levels of tubes can be observed. The Zafferana-Rifugio Sapienza road cuts through the 1792 Tre Livelli tube and reveals a section through the upper two tubes of the complex three-level master tube. The uppermost tube did not merge with the middle one because it was small, it had not inflated, and it appears to have been em-

placed at a late stage of the eruption, giving sufficient time for the upper crust of the middle tube to become self-supporting. The upper tube has a stable base 0.5 m thick, is about 1 m wide, and the final flow that occupied the tube had a ropy surface. Our observations indicate that vertical coalescence generally takes place when the direct stress applied by lava within a new upper tube causes tensile failure of the roof of a lower tube. In Hawaii, tube coalescence was found to be more common in near-vent areas ŽPeterson and Swanson, 1974., where the crust had insufficient time to cool, thicken, and become self-supporting before being covered by a new flow. We observed the same on Etna, where tubes that formed at less than 2 km distance from the eruptive fissure produce complex three-dimensional networks. Horizontal coalescence also takes place, as for example in the Micio Conti and Gallobianco I lava tubes on Etna ŽFigs. 1, 4 and 8. ŽPuglisi, 1981; Fanciulli et al., 1989.. Along the large 1991–1993 flow field, tube coalescence was common on the Cutrona tube ŽGiudice and Leotta, 1995. at about 2 km from the main vent, and also on the Salto della Giumenta tube that formed in Val Calanna, 5 km from the source

Fig. 8. Plan view of Gallobianco I lava tube after Fanciulli et al. Ž1989.. The kink close to Section 2 is due to lateral coalescence.


ŽFig. 1.. The septum between superimposed tubes is typically in excess of 0.5 m on Etna. The minimum thickness required to prevent roof collapse is clearly a function of the relative size of superimposed tubes and the intervening crustal thickness. It is also a function of distance from the source because of the increase in thickness and strength of the upper crust during flow. 2.5. A classification system for Etna laÕa tubes Our underground observations suggest that, on Etna, lava tubes can be split into two main morphological types: simple and compound ŽFig. 9.. A simple lava tube forms by roofing over of a single lava channel ŽFig. 9a. or by cooling of a stable crust around an inflated lava flow ŽFig. 9b.. A simple lava

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tube can be either symmetric or asymmetric. The degree of asymmetry is commonly related to the flow direction of the tube. If a tube is straight, it is usually symmetric ŽFig. 9a–b.. Asymmetry develops when a tube changes direction ŽFig. 9c.. This asymmetry is a consequence of greater deposition of lava on the more slowly moving inner side of the bend compared with the outer side. Asymmetry in simple tubes can also arise from differential loading of the tube roof ŽFig. 9d., accumulation of lava flows on one side of the roof, opening of ephemeral vents on the walls of the tube, or secondary collapse events. Compound lava tubes form by coalescence of adjacent tubes. In this way we apply to tubes the same distinction suggested by Walker Ž1971. for lava flows. Compound tubes have symmetric transverse sections when tube coalescence is vertical

Fig. 9. Simple Ža to d. and compound Že–f. transverse sections of lava tubes. Ža–b. Simple and symmetric sections; Žc. simple, asymmetric section due to bending of the tube; Žd. simple asymmetric section due to roof loading; Že. symmetric, compound tube section with typical key-shape, indicating coaxial capture; Žf. asymmetric compound tube section.

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ŽFig. 9e., and asymmetric sections when the upper tube was not directly above the lower one when the intervening roof collapsed ŽFig. 9f.. In addition to these end-members, we have observed many examples where multiple processes have operated, resulting in symmetric simple or symmetric compound lava tubes, and asymmetric simple or asymmetric compound lava tubes. An examination of transverse sections of primary lava tubes and of their modification with distance from the source is essential in understanding the mechanism of flow development. For example, longitudinal downward bulges ŽFig. 6., often accompanied by medial cracks, are a characteristic feature of many lava tubes. 3. The role of mechanical and thermal erosion on Etna Thermal and mechanical erosion are considered to play an important role in the development of komatiite lava flows ŽGreeley and Hyde, 1972; Huppert and Sparks, 1985; Huppert et al., 1984; Williams and Lesher, 1996. and in basaltic lavas and tubes in Hawaii ŽPeterson and Swanson, 1974; Kauahikaua et al., 1998. and Etna ŽCumin, 1954; Greeley et al., 1998.. On Hawaii, combined rates of thermal and mechanical erosion between 5 and 10 cmrday have been measured by Kauahikaua et al. Ž1998.. On Etna, apparent erosion has been reported for only two tubes. The first was in a channel on ‘a‘a lava that flowed over the December 15, 1991 flow on the southern margin of Valle del Bove ŽCalvari et al., 1994a.. This channel was located at the base of Serra Pirciata, 1 km from the main vent. On January 3, 1992 the lava stream was flowing inside a welldefined, 10-m wide channel with vertical walls and a slope of 118. Because the levees ´ were 5 m high, and the level of the flow inside the channel was 2.5 m below the channel rim, the flow thickness was estimated to be 2.5 m for a length of about 100 m, and the measured mean flow velocity of 0.4 m sy1 resulted in a discharge rate of 8 to 10 m3 sy1 ŽTable 2 in Calvari et al., 1994a.. Measurements were repeated at the same place on January 5 and 7, giving flow rates of 18.8 and 18.8 to 25 m3 sy1 , respectively ŽCalvari et al., 1994a.. On January 7, 1992 the upper level of lava in the lower half of the channel was over 3 m below the channel rim. The upper

surface which, on January 3, was gently sloping, on January 7 had a step with nearly vertical lava cascades that produced an increase in the surface flow velocity upstream and may have induced local turbulence at the base of the step ŽKauahikaua et al., 1998.. Bonforte Ž1994. concluded that this increase in flow depth resulted from combined thermal and mechanical erosion of the lava flowing on a substratum made of debris and reworked pyroclastics and lavas from previous eruptive centres ŽCalvari et al., 1994b.. However, our detailed underground observations support an alternative explanation. As noted above, our surveys in old lava tubes on Etna have revealed that tube coalescence is very common. The abrupt changes in the direction of grooves on the walls of some lava tubes suggests rapid changes in the flow direction of lava within these tubes. Good examples are the Cutrona tube Žformed inside the June 2, 1992 lava flow., the Intraleo tube Ž1225 eruption, Tanguy, 1981., the Gallobianco tube Ž1050–1100 eruption, Tanguy, 1981., the Tre Livelli tube Ž1792–1793 eruption, Corsaro et al., 1990., and the Salto della Giumenta tube Žformed inside the flow of December 24, 1991. ŽFig. 1.. The Intraleo tube is particularly revealing because of the clear relationship between surface morphology of the lava flow field and internal structures within the lava tube. Here, we observed a 10 m wide lava flow with a tube inside, whose roof and floor had collapsed in the frontal region where it merged with a previous, deeper lava tube. Grooves were well developed on the walls of the lower half of the upper tube. About 5 m from the tube junction, striae were almost horizontal, but their inclination gradually increased downslope and became vertical where the upper tube broke through the roof of the lower tube. This tube coalescence occurred at the front of the upper flow, which, at this point, had a thickness of about 6 m. Because the roof of the upper flow also collapsed, it is probable that this flow was still inflating when it merged with the lower tube. In this case, the upper crust had no time to solidify and become self-supporting. We believe that the mechanism we have just described is similar to what took place on the January 3, 1991. The January 3 lava flowed along the channel of the December 15, 1991 flow. Stable crust along the December 15 flow was first observed after


a few days of emplacement of the flow. The crust was only a few days old and probably less than 1 m thick when it collapsed below the weight of the new overlapping flow. Greeley et al. Ž1998., in their very comprehensive review of erosion within lava tubes, described one locality which, they argue, supports mechanical erosion. Their locality lies within the proximal part of the Tre Livelli tube at a height of about 1700 m a.s.l. ŽFig. 1.. This tube formed along the 1792–1793 flow field between 1850 and 1450 m a.s.l. ŽCorsaro et al., 1990.. The eruptive fissure, which is still visible on the ground, extends between 1850 and 1600 m a.s.l., and the flow field reaches the lowest altitude of 750 m a.s.l., close to the town of Zafferana Etnea ŽRomano et al., 1979; Corsaro et al., 1990.. We re-examined the tube system in September 1997 and October 1998. The lining on the wall of the lowest of the three tubes comprising this tube system has collapsed, exposing a layer of pyroclastics that lie beneath the 1792–1793 flow field. Greeley et al. Ž1998. use this as evidence of mechanical erosion at the base of the flow. However, because the collapse is very close to the eruptive fissure, it is possible that the pyroclastic layer has been exposed by the open-

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ing of the fissure rather than by erosion of the substratum. We confirm the conclusion of Greeley et al. Ž1998. that there is no evidence of thermal erosion inside lava tubes or channels on Etna. If it occurs, it is only close to the vent area, and it is much less extensive than in Hawaiian lava tubes ŽKauahikaua et al., 1998.. However, we have unequivocal evidence of tube coalescence. This was observed in virtually all tubes that we surveyed, and we argue that this mechanism plays a pivotal role in prolonging and lengthening lava tubes on Etna.

4. Relationship between surface features on lava flows and lava tubes There are a number of surface features on many lava flow fields that suggest the presence of lava tubes. The most common are skylights, tumuli, collapses, breakouts and secondary vents ŽGuest et al., 1980, 1984; Mattox et al., 1993; Calvari et al., 1994a; Calvari and Pinkerton, 1998; Kauahikaua et al., 1998.. However, in discussing the relationships between these features and tubes, it is important to

Fig. 10. Circular collapse zone along the southern margin of the 1991–1993 lava flow field, Mount Etna. The width of the collapse is approximately 80 m, and it formed after the end of the eruption by drainage of a lava pond.

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Fig. 11. Collapse depression formed by drainage of the flow front at the lower end of the 1792–1993 Cassone tube, a few metres downslope from the point where the tube stops. The wall on the right represents the remains of the inflated flow front. Flows on the flat ground and Žin section. behind Dr Calvari are secondary flows from third-order ephemeral vents ŽCalvari and Pinkerton, 1998..

Fig. 12. View from the NE of the southwestern wall of Valle del Bove, 2nd June 1992. The eruptive fissure of the 1991–1993 eruption is still degassing in the upper part. Note the alignment of skylights Žwhite patches on the black flow field.. These record the path of the first tube system which was active before the diversion of May 1992 ŽCalvari and Pinkerton, 1998..


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Fig. 13. Tumulus on the 1983 lava flow field. Note the small drained Žright and left. and undrained Žmiddle. tubes fed by the tumulus. The ruler on the undrained tube is 20 cm long.

clarify what constitutes a lava tube. If we define lava tubes as the cores of drained lava flows with a continuous and stable crust, we find that many tubelike structures never drained, or drained only a small proportion of their lava. This may be why Walker Ž1991. stated that, on Hawaii, tumuli are not related to lava tubes. Since these tumuli were observed on low gradient zones of the flow field in lava flows in the Cava Basalt, Mount St. Helens, Washington ŽGreeley and Hyde, 1972. and on Etna ŽCalvari and Pinkerton, 1998., it is highly probable that they did not drain. However they may have had an important role in delivering lava underground to the distal parts of the flow field Žsee Kauahikaua et al., 1998.. Depressions and collapse structures are often associated with tubes ŽFig. 10.. Sometimes collapsed zones appear to be much larger than the size of the tube ŽAtkinson et al., 1975., and this has been explained as a consequence of the drainage of a lava pond ŽStephenson, 1996; Kauahikaua et al., 1998.. Large collapse areas may also form during the complete drainage of inflated flow fronts at the point where an ephemeral vent opens. We have found two excellent examples of this mechanism. One is on the

1983 flow field of Etna close to Rifugio Sapienza Ž1900 m a.s.l..; the other is approximately 40 m downslope from the entrance of the Grotta Cassone, close to the margin of the 1792–1793 flow field ŽFig. 11.. Skylights ŽFig. 12. are extremely useful features because they allow direct observations and measurements of lava flowing inside tubes. Tumuli ŽFig. 13. usually form along main tubes where there is a sudden break in slope Žsee Greeley and Hyde, 1972.. Overpressure created by lava accumulating at the base of a steep scarp, or local collapses inside tubes ŽCalvari and Pinkerton, 1998. can give rise to continuous output of small flows that can accumulate to form large deltaic lava fans ŽGuest et al., 1984; Calvari and Pinkerton, 1998.. 5. Discussion In our study of the 1991–1993 lava flow field on Etna ŽCalvari and Pinkerton, 1998., we demonstrated the importance of ephemeral vent formation and successive budding of new flows from inflated flow fronts. Our recent detailed investigations of lava

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tubes, including cross-sections and longitudinal profiles, reveal that the processes of flow inflation recorded on the surface can be preserved underground in the form of inflated parts of a tube. We have also located numerous ephemeral vents emerging from the fronts of inflated tubes. Our underground surveys also reveal information on additional flow processes which are difficult to understand during eruptions. For example, we have documented several cases where the roof of a lava tube has collapsed, allowing lava from a surface flow to re-occupy a deeper level tube. Particularly striking are examples where tube systems at different levels intersect at right angles Žfor example the proximal 1985 lava tube system.. In some cases the evidence is subtle, and requires detailed investigations of changes from simple tube cross-sections to bellshaped sections. In other cases, grooves created by crustal material moving against still plastic lava walls can record sudden changes in flow direction from horizontal to inclined and often vertical flow. This change can be accompanied by an abrupt change in level of the floor of the lava tube. Additional evidence in support of this process includes abrupt changes in the number and thickness of layers that line the walls along different parts of a tube system. The sudden collapse of a tube roof and the subsequent rapid flow of lava into an older tube system has clear implications for hazard assessment. Firstly, it means that stationary flow fronts may suddenly be re-activated if a drained tube is re-occupied by new lava. Secondly, during future flow diversion measures, if a diverted lava flows on top of older lava streams, and if roof collapse permits lava to re-occupy an older tube system, the lava may be able to migrate downflow considerably more rapidly than if it continued to flow on the surface. In those circumstances flow diversion may exacerbate the hazards instead of reducing them. It is therefore important that old tubes are identified and their roof thickness determined before flow diversion measures are carried out. In addition, the probability of roof fracture must be established prior to any diversion measure. Considering a simplistic model for a flat, horizontal tube roof, we can calculate the minimum flow thickness required to fracture the roof of a tube from:

s s War Ž bt 2 .

where s is the tensile strength of the roof, W is the weight of the overlying lava flow over a length b, a is the width of the tube roof, and t is its thickness. This can readily be re-expressed in terms of the thickness of the upper flow, h, as follows: h s Ž s t 2 . r Ž r ga2 . where r and g are lava density and gravitational acceleration, respectively. If we assume a value for tensile strength of 5 MPa, and a density of 2500 kgrm3 , then for a tube roof that is 3 m wide, the flow depth of the upper flow required to fracture the crust ranges from 0.9 m for a crustal thickness of 0.2 m, to 2.1 m for a crustal thickness of 0.3 m, 5.6 m for a crustal thickness of 0.5 m, to 22.7 m for a crustal thickness of 1 m. These figures are in general agreement with the dimensions of flows that caused roof collapse into underlying tube systems on Etna. 6. Conclusions On Etna, and on many other volcanoes, lava tubes are recognised only when the roof of a tube collapses, or when new roads cut through tubes. The thick, self-supporting crust of ‘a‘a lava flows makes most tubes in ‘a‘a lava flows difficult to detect. Moreover, lava flows emplaced through tubes that did not drain are virtually impossible to recognise. For these reasons, and because of the large increase, since 1971, of the number of known lava tubes on Etna, we conclude that the figure of 18% of tube fed flows on Etna ŽGreeley, 1971. is underestimated. We also conclude that at least some of the lava tubes on Etna previously considered to have formed on pahoehoe flows may have formed on ‘a‘a flows which were subsequently covered by late stage pahoehoe lava. Observations of active ‘a‘a lava flows emplaced during the 1991–1993 eruption suggested that inflation of the flow fronts of mature ‘a‘a lava flows was an important aspect of tube formation ŽCalvari and Pinkerton, 1998.. This statement is confirmed by features we observed underground. A striking feature in many etnean lava tubes is the association of large chambers with narrow passages. We interpret these features as the product of flow inflation in the frontal zone, followed by opening of first-order ephemeral vents on the snout region. This suggests that the


production of multiple flows connected by secondary vents is an essential mechanism of tube growth on Etna. This process can be observed on the surface where new, long-lived vents open at the margins of previously inactive ‘a‘a lava flows, and underground as multiple linings and lateral benches. Tube coalescence is an important process during the emplacement of long-lived flows. This occurs when an upper tube drains into a previous, lower tube because of roof collapse. The re-occupation of a deeper and possibly longer tube may cause a reactivation of distal parts of the flow field, and a consequent increase in flow field length. The possibility of new flows breaking through the roofs of previously inactive tubes has important consequences in hazard assessment. Our surveys indicate that a crust thickness of 0.5 m is the minimum required for ‘a‘a lava flows, and we propose a simple formula to calculate roof stability for a typical Etnean ‘a‘a lava flow. In conclusion, our underground surveys help to confirm Žsee Calvari and Pinkerton, 1998. that subterranean effusive processes play an important role in the development of ‘a‘a flow fields. They show how tube inflation and coalescence can result in considerable lengthening of lava flow fields beyond the distance that can be attained by channel-fed lava flows. This work has clear implications for hazard assessments during future effusive eruptions on Etna. Finally, in view of the ease with which lava can break through the roofs of lava tubes, we consider that the process of tube coalescence may be an integral part of tube development, not only on Etna, but also on other basaltic volcanoes. Acknowledgements We are very grateful to members of Gruppo Speleologico Etneo for their assistance in reaching some of the critical localities we describe above. Without their support and the photographic assistance of Alfio Amantia, IIV-Catania, this work would not have been possible. We also wish to thank Kathy Cashman, John Guest, Angus Duncan and members of Gruppo Speleologico Etneo for many stimulating discussions on the formation of lava tubes. We also thank Professors Lionel Wilson and Ron Greeley for their very useful comments on this paper. This study

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was undertaken with the assistance of a grant from the commission of the European Communities under the Fourth Framework Programme, Environment and Climate, Contract ENV4-CT97-0713.

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