Formation of lava tubes and extensive flow field during the 1991–1993 eruption of Mount Etna

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. Bll, PAGES 27,291-27,301, NOVEMBER 10, 1998

Formation of lava tubes and extensive flow field

during the 1991-1993 eruption of Mount Etna

Sonia Calvari 1 Istituto Internazionale di Vulcanologia, Consiglio Nazionale delle Ricerche, Catania, Italy

Harry Pinkerton Environmental Science Department, Institute of Environmental and Natural Sciences Lancaster University, Lancaster, England, United Kingdom

Abstract. Detailed mapping during the 1991-1993 eruption of Mount Etna has shown that there is a relationship between tumuli, ephemeral vents, lava tubes, and their parent lava flows. During this eruption, many tubes formed in stationary, inflated 'a'a lava flows. Ephemeral vents at the fronts of these stationary flows and above lava tubes fed secondary lava flows, many of which subsequently developed new tubes. The resulting complex network of tubes, ephemeral vents, and secondary flows was responsible for most of the widening, thickening, and lengthening of the 1991-1993 Etna lava flow field. The supply of relatively uncooled lava via tubes to distal parts of this flow field allowed lava to flow 3 km farther from the vent than the longest channel-fed lava flow. Our observations suggest that lava tubes play a more important role in the formation of extensive 'a'a flow fields on Etna than has previously been recognized.

1. Introduction

During eruptions on Mount Etna, the maximum lengths of 'a'a lava flows are related to effusion rate [Walker, 1973; Wadge, 1978; Kilburn, 1993]. It could therefore be argued that published length-effusion rate relationships may be useful for hazard as- sessment on Etna. However, the flow field that formed during the 1991-1993 Etna eruption (Figures 1 and 2) reached a maximum distance of 8.5 km from the source fissure, more than 3 km

farther than the distance predicted by length-effusion rate relationships. One of the flows threatened the town of Zafferana, and considerable effort was required to prevent the flow from entering the town. Detailed observations of this flow field have shown that it contained a complex network of lava tubes which permitted the formation of a flow field that was considerably more extensive than would have been possible if all the flows had been channel-fed.

The 1991-1993 eruption is not unique. Lava tubes have also been recognized during previous Etna eruptions [Ponte, 1949; Cumin, 1954; Pinkerton and Sparks, 1976; Guest et al., 1980; Greeley, 1987]. While the mechanisms responsible for the formation of tubes in pahoehoe flows have been studied extensively on Hawaii [Peterson and Swanson, 1974; Greeley, 1987; Peterson et al., 1994; Kauahikaua et al., 1996; Kauahikaua et al., this volume], tube formation in 'a'a lava flows is more difficult to observe because of the frequent eruption of thick flows above parts of the embryonic lava tube system.

•Also at Environmental Science Department, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, England, United Kingdom.

Copyright 1998 by the American Geophysical Union.

Paper number 97JB03388. 0148-0227/98/97JB-03388509.00

Consequently, there has, until now, been no detailed study of the formation of lava tubes in 'a'a lava flows on Etna, and previous evidence has been gathered from studies on inactive flow fields [Guest et al., 1980, 1984; Cavallaro et al., 1985; Corsaro and Cristofolini, 1989].

In this study, we (1) show how a major tube system developed during the 1991-1993 eruption on Mount Etna, the largest eruption on Etna for 300 years (Figures 1 and 2), (2) describe flow inflation and the role it plays in the development of tubes in distal sections of individual flows, (3) discuss the factors that influence the dimensions of lava flows and tubes, and (4) assess the role of tubes and ephemeral vents in the growth of the complex 1991-1993 'a'a flow field on Etna.

2. The 1991-1993 Etna Eruption

In view of the potential hazard from flows during the 1991- 1993 eruption, extensive studies were carried out by scientists from the Instituto Intemazionale di Vulcanologia (IIV) of Catania and the Gruppo Nazionale per la Vulcanologia (GNV) of Italy. Our work is based on more than 2000 slides and photographs, 20 hours of videotapes, field observations made by one of the authors (S. Calvari), and daily reports by IIV and GNV scientists during the 1991-1993 eruption. This unique data set has allowed us to model the development of the tube system.

Detailed accounts of the 1991-1993 eruption [Calvari et al., 1994] and measures taken by the Civil Defense to protect the town of Zafferana [Barberi et al., 1993] confirm that the eruption can be divided into five phases. The first and second are very short and correspond to the rapid emplacement of large arterial 'a'a flows from a fissure at the headwall of the Valle del Bove

(Figures 1 and 3a-3d). During the third phase, a major basin, the Val Calanna, was filled by innumerable small pahoehoe flows (Figure 3e). The fourth phase was marked by flows that overlapped the man-made dam at the exit of Val Calanna (Figures 2 and 3f-3g) and rapidly expanded towards the town of Zafferana.

27,291

27,292 CALVARI AND PINKERTON: TUBES IN AA LAVAS

Summit Craters SI•Etna

La Montagnola

• ....... 1991-93 lava flows

• •ield of VALLE DEL BOVE

Mt. Calanna N,,,• ,Eruptive •I1ssure

Val

Ztx•olaro

ano • de11'Acq•

A 2 km E•.ea Figure 1. Upper eastern flank of Mount Etna showing the area covered by the flow field that formed during the 1991-1993 eruption.

We observed a progressive and continuous expansion of the tube system from the first to the fourth phase (Figures 3c-3g). This expansion ended when the main tube system was successfully blocked at the base of the eruptive fissure [Barberi et at., 1993]. During the final (fifth) phase, flows were restricted to the upper part of the Valle del Bove (Figures 3h-31). This change was the result of the artificial diversion and a decrease in effusion rate. A

short tube network formed during tl•e fifth phase (Figures 3h-31). In section 3 we discuss the methods used to detect tube

systems. We also describe the different mechanisms of formation of tubes and tube networks during the 1991-1993 eruption on Etna, and we show how the tube system was instrumental in helping to construct an extensive compound lava flow field.

3. Field Evidence of Tube Systems

A number of methods have been used to study tube development on Hawaii including Landsat thematic mapper (TM) images [Flynn and Mouginis-Mark, 1994], thermal infrared remote sensing [Realmuto et at., 1992], VLF electromagnetic induction [Kauahikaua et at., 1996], and detailed mapping of breakouts [Mattox et at., 1993]. During the 1991-1993 Etna eruption, VLF and magnetic measurements along limited sections of the second

tube system defined the cross-sectional area of the tube [Bozzo et al., 1994;Budetta and Del Negro, 1995]. The usefulness of these measurements, however, was limited by the large thickness of lava (up to about 100 m; see also Figure 2) that accumulated in the upper Valle del Bove [Calvari et al., 1994; Stevens et al., 1997] and by the complex morphology of the flow field. Similar though less acute problems developed along other parts of the tube system; thus an alternative method of locating tubes was devised. This involved extensive mapping of new flows some of which became pathways for new tubes, together with detailed mapping of skylights, tumuli, and ephemeral vents. By combining all of these data we have been able to locate tube systems that could not be detected by any other means. Underground surveys of one tube system 12 months after the end of the eruption [Giudice and Leotta, 1995] are in agreement with the location based on our mapping.

3.1. Skylights

Skylights were very common on the proximal 1991-1993 flow field. They formed by roof collapse along arterial tubes or by in- complete bridging of lev6es. Skylights helped define the pathways of proximal tubes. The dimensions of the tubes were measured after drainage.

2500

;:ii•,••-•eruptive fissure Mt. Zoccolaro earth dam of 2000 41!iiiiiiiii•'"'"•5•:•• P. C•edd/ \ Sabo Giumenta Portella Calanna I :ii :.• •11!i I Mr. F. di Cosimo / Zaffe• Etnea 1500

2ooo

0 '"l I I I I I ! I I ........ 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10,000

Distance (m)

Figure 2. Longitudinal section of the 1991-1993 Etna lava flow field (thickness data after Stevens et al. [ 1997]) along the path of the first tube system. The vertical scale is twice the horizontal scale.

CALVARI AND PINKERTON: TUBES IN AA LAVAS 27,293

December

o 1 2

December 20 1 991

ß

0 1 2 3 4 5 6 7

December 24 1 991

o 2 3 4 5 6 7

January 31-1 0 1 I I

I 0 1 2 3 4 5 6 7

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0 1 2 3 4 5 6 7•(• .May 2 3 1 992 . I I_ 6 June 1-25 1 992 • i 6 7

0 1 2 3 4 5 1 2 3 4 5 '"•

0 1 2 3 4 5 F•st tube system: Dec.'91 to May'92 Seomd tube system: from May 27 '92

..:

3 0 1 2 3 4 5 • '• Figure 3. Sketches showing the formation of the lava tube systems during the emplacement of the main lava flows, 1991-1993. Active flows are dark grey, the flow field is light grey, and heavy solid lines indicate the path of the main lava tubes. (a) to (g) Development of flows and tubes prior to the diversion on May 27, 1992. (h) to (1) Flows and tubes that formed after the diversion. (m) and (n) The entire first and second tube systems. The grid is 1 km x 1 km square.

3.2. Tumuli

Tumuli formed in the medial part of the 1991-1993 Etna flow field. Their size decreased both with distance from the source

vent and with local discharge rate. Both size and shape of Etna tumuli are comparable with tumuli on pahoehoe flow fields on Hawaii [Walker, 1991]. Tumuli on Etna formed along the path of main tubes, mostly where there was a marked reduction in slope. This association between tumuli and tubes has been found

previously on Etna [Guest, 1982; Guest et al., 1984] and Hawaii [Mattox et al., 1993; Honet al., 1994].

3.3. Ephemeral Vents

Another feature that indicates the presence of a tube system is the distribution of ephemeral vents [Guest et al., 1980] or breakouts [Mattox et al., 1993]. Persistent vents that changed only slightly in position were normally located above active tubes.

However, it is necessary to distinguish between different kinds of "ephemeral vents." The main feature of these vents is that they were not connected directly with the main fissure system or vent [Cumin, 1954]. While many ephemeral vents were active for only a small fraction of the lifetime of the flow field, others lasted sev-

eral weeks and they fed flows with lengths similar to those produced by the main vent. Our mapping has revealed that there were three types of ephemeral vents during the 1991-1993 eruption on Etna. These are classified on the basis of age and position with respect to the arterial flows and the tube system.

First-order ephemeral vents opened at the front of 'a'a lava flows whose fronts slowed and eventually stopped because of cooling but which continued to be supplied with lava from the source vent. These vents produced secondary flows that lasted for many days or months, depending on the magma supply rate from the source and on episodes of blockage and obstruction of the main flow. Some of these flows had lengths that were more than


half that of the parent flow. Normally, these secondary flows were 'a'a, with pahoehoe surface textures in the proximal part of the flow. First-order vents played a major role in the formation and propagation of the main arterial tube system.

Ephemeral vents that opened above the main arterial tube are classified as second-order ephemeral vents. These vents typically produced 'a'a flows lasting from days to months. Many second- order ephemeral vents were located at the base of a steep slope, where they acted as "pressure release valves" during periods of tube blockage or temporary increases in effusion rate. These vents commonly supplied short flows which spread and accumulated forming fans of lava. They were accompanied by tumuli, many of which oozed lava from cracks along their crests. Vents of this kind were the surface expressions of perturbations in discharge rate or blockages within the main arterial tube.

Third-order ephemeral vents were located at flow margins, and the resulting flows were predominantly pahoehoe or toothpaste lava [Rowland and Walker, 1987]. They formed around flows that were no longer fed. These parasitic flows, which were supplied by lava that drained from the fluid interior of the parent flows, were very short lived and typically had very low discharge rates.

While third-order ephemeral vents give little information about lava tubes, second-order ephemeral vents are very useful indicators of the position of large arterial lava tubes, and they played an important role in locating tube pathways. First-order ephemeral vents generally form at the exits of newly formed lava tubes, and they control the development of arterial tube networks.

4. Formation of Lava Tubes During the 1991-1993 Eruption on Mount Etna

By combining all field, photographic, and video data onto detailed maps, we recognized two main tube systems. The first formed a few days after the beginning of the eruption and it mainly followed the paths of flow channels on arterial flows. The second tube system formed 6 months later, in new flows that were fed directly from the eruptive fissure following the artificial diversion that blocked the first tube system.

In the following discussion we use "effusion rate" for the vol- umetric flow rate from the main fissure based on measured ve-

locities and depths, in contrast to the local "discharge rate" which is the discharge from a local breakout from a flow margin or tube. We calculated "discharge rate" for single flows by dividing the regular (daily where possible) measurements of the new lava volume by its emplacement time. "Eruption rate" is the mean value of output during an eruption, calculated by dividing the total volume of the flow field by total emplacement time.

4.1. First Tube System: December 1991 to June 1992

The eruption began on December 14, 1991, and the first flow from the base of the fissure was erupted on December 15. This flow travelled rapidly down a steep (30 ø) slope along the western wall of the Valle del Bove, then along a 10 ø slope at its base (Figures 2 and 3a). The main flow slowed down as the gradient decreased to 4 ø at the base of Monte Zoccolaro. By December 20, the flow was 4.1 km long (Figure 3b), and during the following 4 days the flow front inflated and advanced slowly. (The mechanism of inflation in 'a'a lava flows is described in section 5.1.) A second arterial flow propagated from the stationary flow front through two first-order ephemeral vents, and the first segment of the tube system formed during this period (Figure 3c). The first

part of the tube system was about 1.5 km long and located between Poggio Canfareddi (1600 m above sea level (asl)) and the base of Monte Zoccolaro (1530 m asl; see Figures 1 and 2).

The next part of the tube system formed in the proximal region of the December 15 flow (Figure 3d). Along the eruptive fissure, where the ground slope was 30 ø, a lava channel was established only a few hours after the start of the eruption. The eruptive fissure and the proximal channel began to roof over at the end of December 1991, 15 days after the start of the eruption (Figure 3d). This mechanism was also observed a few months later along the channels that formed on the 30 ø slope of Salto della Giumenta (Figure 2).

The three main flows that covered the Salto della Giumenta

from February to April 1992 formed additional tube branches (Figures 3e-3f). One tube disappeared in a few days because it was no longer fed and its roof and walls collapsed. The other two remained active until the artificial diversion at 2000 m asl suc-

cessfully drained lava from the main tube [Barberi et al., 1993]. In Val Calanna, two parallel tube sectors merged on the low

ground slope in the middle part of the valley (Figure 3g). Inside this valley, the path of the active tube system was largely ob- scured by numerous overlapping flows. However, on May 7, 1992, a large second-order vent opened at the base of Monte Fior di Cosimo (Figure 2), confirming that a long tube system had formed in Val Calanna. From the new vent a 10-m-wide flow

spread out towards the NNE and attained a final length of about 500 m. Along the path of the new flow, many second-order ephemeral vents opened during the next days. Although they were not always in the same position, their distribution during May marked the path of the main tube system at the end of the Val Calanna (Figure 3g).

On April 10, 1992, the first of many lava flows overlapped the man-made dam at Portella Calanna (Figure 2). The flow stopped, thickened, and widened when it reached the flat zone of Piano

dell'Acqua, but many others flows followed the first one during the ensuing days, and a complex network of small tube branches formed during May along these flows (Figure 3g). Downslope of Portella Calanna, a roof formed rapidly over the narrow lava channels. The eruption of many small flows in this region and consequent variations in discharge rates along individual channels eventually formed a braided tube system (Figure 3g).

4.2. Second Tube System: After the Artificial Diversion of May 27, 1992

After the successful diversion at the base of the eruptive fissure in May 1992 [Barberi et al., 1993], supply to the first tube system was cut off, though drainage from the tube allowed small distal flows inside Val Calanna to continue advancing slowly for 8 days after the diversion. On May 27, a new flow emerged from the skylight at 2000 m asl and expanded rapidly along the southern margin of the Valle del Bove. The flow propagated eastward, stopping at Poggio Canfareddi on June 1, about 2.5 km from its source (Figure 3h). A well-defined channel with an estimated width of 5 m rapidly formed in the middle of the flow.

Effusion rates measured before and after the diversion testified

to a decrease from 30 to 15 m 3 s -1 between the end of May and the first days of June [Barberi et al., 1993]. As a consequence, the May 27 flow slowed down, the channel cooled along its margins, and an overflow formed from the main channel on June 4. The fluid inner part of the first flow drained and emerged through third-order ephemeral vents 2.5 km from.the source around the 20-m-high obstacle of Poggio Canfareddi. On June 7, surface


cooling of the May 27 flow propagated upslope along the main channel, and from the base of the eruptive fissure, a new flow overflowed from the main channel. By June 15, the southern branch of the new tube system was completely roofed over, and another flow propagated eastward from the main vent at the base of the fissure (Figure 3h).

By the end of June, the northern branch of the new tube system was established along the June 15 flow (Figure 3h), and tumuli began to form along the two tubes at the base of the steep slope. During July and August, the two branches of the new tube system propagated downslope (Figure 3i), reaching a maximum distance of 2.5 km from the diversion point. During September 1992, large flows were erupted from second-order ephemeral vents at the ends of the tubes, confirming that both tube branches were active (Figure 31). The average effusion rate during September was 5 m 3 s -1 [Calvari et al., 1994]. During October a large overflow from the upper part of the eruptive fissure was observed, indicating either an obstruction of the tube sector downslope or increased cooling caused by a reduction in effusion rate. Similar reductions in effusion rates have been found during the final phase of previous eruptions on Etna and Hawaii [Wadge, 1981; Kauahikaua et al., 1996]. During December, two tumuli were active along each branch of the tube system. After January 1993, large overflows along the upper part of the eruptive fissure became more common, confirming the stagnation of lava in the tube system. The eruption ended on March 31, 1993 [Calvari et al., 1994].

One year after the end of the eruption, a team from the "Centro Speleologico Etneo" surveyed the lava tube formed after the diversion [Giudice and Leotta, 1995]. The accessible part of the tube system was composed of two branches, each about 400 m long. These formed along the two main flows that were erupted on the surface of the pre-diversion flow field: the flow of May 27 to June 1 to the south, and the flow of June 15 to 25 to the north. The mean tube width was about 5 m, the same as the channel

width of the parent flow, and the mean tube height was about 3 m. By contrast, the height of the first tube system (Figure 3m) measured after drainage at a distance of 2-3 km from the source, was 32-33 m from the top of the skylight to the bottom of the tube (L. Villari, personal communication, 1996). Although this value may be an overestimate because of internal roof collapse beneath this skylight (L. Villari, personal communication, 1996), it is clear that tubes that formed early during the eruption were considerably larger than tubes formed after the artificial diversion (Figure 3n). This difference in size cannot be attributed to different age or maturity of the two tubes, because the largest was active for 6 months, whereas the second, smaller tube produced output of lava up to the end of the eruption, remaining active for 9 months.

Our observations suggest that tube dimensions are a function of effusion rate and local ground slope, although secondary effects, including roof collapse, infilling of the tube, and erosion of the flow at the braiding points also cause large localized variations.

5. Mechanisms of Tube Formation

Unlike previous descriptions of tube formation, our observations show that the first part of the tube system to form on Etna 'a'a lava flows was in the distal portion of stationary flows which fed first-order ephemeral vents. This was followed by roof formation in proximal channels, and the final part of the tube formed along the wide channels that characterized the medial

zone. The mechanisms of formation are different in each of these

sections of an arterial flow and required different times to develop. They are shown in Figures 4a and 4b, where we consider three sections of a typical 1991-1993 Etna 'a'a lava flow: a proximal zone (AA'), which commonly formed on a steep slope (-30ø); a medial zone (BB'), where the ground slope was typically about 10ø; and a distal zone (CC'), where the ground slope was less than 4 ø. In sections 1 to 8 (Figure 4) the inside of the flow is divided into three parts: an outer crust with an irregular surface of 'a'a blocks (light grey), an intermediate viscoplastic layer (dark grey), and a fluid inner core (black). In sections 5.1 to 5.3, we describe the mechanisms of tube formation

in different parts of the flow field.

5.1. Low Gradient Zones (<5 ø )

All tube systems we observed formed initially on the distal part of arterial 'a'a flows, where they encountered a reduced ground slope, allowing the flow front to cool and expand laterally. When the first flow reached the lower gradient uphill of Monte Zoccolaro, it thickened and spread laterally before advancing in a different direction. This cycle of reduced advance rates followed by inflation was observed twice during the emplacement of the flow. This cycle was interrupted by the emergence of a new flow from first-order ephemeral vents (Figure 4b). The new vents opened at the flow front along the boundary between the fluid core and the lower thermal boundary layer. Drainage of the parent flow caused the collapse of the upper crust (Figure 4b, section 7), as described previously for smaller flows on Etna [Pinkerton and Sparks, 1976]. When the supply rate to the parent flow was significantly lower than the effusion rate from a new vent, the new vent drained the channel

of the parent flow, and the upper crust collapsed, closing the pathway and preventing further drainage of lava within the flow. This happened during the second phase of the eruption, when the January 3 flow expanded rapidly surrounding Monte Calanna in a few days (Figure 3d). Third-order ephemeral vents opened around its margins at the base of Monte Calanna, completely draining its channel. Conversely, when the rate of supply remained constant, for example, during the eruption of the December 15 flow (Figures 3a-3g), the pressure of lava within the flow prevented total roof collapse. Under these conditions the upper crust, which was supported initially by the pressure of the fluid lava beneath it, thickened, cooled, and eventually became self-supporting. Once drainage from a first-order vent attained a steady state and a stable sector of the tube formed (Figure 4b, section 8), the level of the fluid lava inside the tube could decrease without causing roof collapse. Although at a different scale, the mechanism of tube formation outlined here is very similar to that described for pahoehoe flow lobes [Peterson and Swanson, 1974; Greeley, 1987; Mattox et al., 1993; Hon et al., 1994; Peterson et al., 1994] where tubes took between 2 and 4 weeks to form [Hon et al., 1994].

Cooling-induced inflation was observed at the front of 'a'a flows formed between December 15, and 24, 1991; December 24, 1991, and January 2, 1992; April 10, and May 27; and from May 27, to June 1, 1992. In the first and fourth cases, the flows encountered obstacles (Monte Zoccolaro and Poggio Canfareddi, respectively) against which the flow front thickened and widened (Figures 3b and 3h). The December 24, 1991, flow did not produce new ephemeral vents from its front region, because the supply along its path was cut off by the formation of the January 3 flow. The April 10 to May 27 flows behaved in a manner that


l

b

Figure 4. (a) Plan view of an arterial 'a'a lava flow (left) and of six idealized sections along its proximal (A- A', sections 1 and 2), medial (B-B', sections 3 and 4), and distal (C-C', sections 5 and 6) portions. In all sections, the isothermal interior of the flow is black, the viscoplastic layer is dark grey, and the brittle crust is light grey. Sections to the right (sections 2, 4, and 6) show a more evolved situation compared to sections to the left (sections 1, 3, and 5). (b) Plan view of a lava flow showing the opening of a first-order ephemeral vent at flow front (left), and cross sections (7 and 8) along its frontal region (C-C') showing the evolution (from 7 to 8) of the inflated portion of the lava flow when a new vent opens at its base.

was similar to the inflationary mechanism described for pahoehoe flows [Hon et al., 1994]. The flow thickness gradually increased due to expansion of the outer crust caused by continuous supply of lava to the fluid inner part of the core. Our observations of the mechanism allow us to exclude flow thickening by the accumulation of flow units or of 'a'a blocks on the flow surface.

Inflation in 'a'a flows on Etna is not a continuous, long-lasting process but happens for short periods of time, generally from hours to a few days. Flows advanced by periodic movement of the flow front as follows: (1) rapid advance of the flow front; (2) gradual reduction in advance rate of flow front accompanied by widening of the flow; and (3) the flow front stops advancing, and the flow thickens until it attains the critical height required to permit renewed movement of the flow front. This cycle was regulated by the ground slope and was repeated several times for many flows. The small flows expanding towards Zafferana on April 1992 were excellent examples of this mechanism. They encountered no obstacles apart from their cooling flow fronts, and they thickened by 2-3 times their mean value (from about 2-3 m to about 6-10 m) at the start of each inflationary cycle. The same cyclic movement of the flow front was described for the emplacement of wax flows emitted at low effusion rates [Hallworth et al., 1987].

While we have no direct measurements of inflation of Etna

flows, we can calculate inflation in two cases. For the two flows

erupted on December 15 and 24, 1991, daily measured increases in flow area coupled with a few measurements of mean flow thickness allowed us to calculate the daily mean discharge rates.

Our data suggest a gradual reduction in daily mean discharge rate from 22 to 6 m 3 s -1 during the period December 15, 1991, to January 2, 1992. Large cyclical variations in daily coverage (Figure 5a) reflect (1) the effects of thickness increases as the flow encountered abrupt gradient changes and (2) the effect of cooling of the flow front (Figure 5b).

Four peaks can be seen in Figure 5b: two major, where the flow fronts reached a maximum thickness of 30 m; and two mi- nor, where the flow fronts increased to 15 and 18 m. The two mi-

nor peaks can be explained by slower advance rates when the flow front encountered lower ground slopes, causing flow front thickening. This is confirmed by thickness estimation from video. The two major peaks were related to cooling of the flow front in conditions of continuous supply where no changes in slope were encountered. The first major peak corresponds to the large increase in thickness that preceded the cessation of the December 15, 1991, flow and the emergence of a new flow from the stationary inflated front on December 24. The second major peak corresponds to the final inflation of the December 24 flow inside Val Calanna. This flow finally died on January 2.

5.2. Steep Gradient Zones (15-30 ø )

Narrow channels (1 to 5 m wide) formed along and immedi- ately downslope of the eruptive fissure a few days after the start of the eruption, along Salto della Giumenta, and on the valley at the exit of Val Calanna because of their steep gradients (30", 25% and 15 ø, respectively). When lava drained through a narrow


0.6

0'41 0.2-.

0

0

I I I -

2000 4000 6000 8OOO

40

30-

20-

10-

b)

2000 4000 6000

o

o 8000

cumulative flow length (m)

Figure 5. (a) Relationship between area covered each day and cumulative flow length of the December 15-24, 1991, flow (0-4 km) and the December 24-January 2, 1992, flow (4-7 km). Solid circles indicate measured values. (b) Calculated thickness (open circles) of the distal flow, showing stages of inflation and deflation. The two small peaks (at 2100 m and 5900 m) show thickening resulting from reductions in the gradient of the underlying ground. The two large inflationary events are caused by significant cooling of the flow front. Calculation of thickness is explained in text.

channel, variations in local discharge rates commonly arose because of minor blockages in the supply system. The resulting changes in lava level inside the channel produced inward lev6e accretion [Sparks et al., 1976] (Figure 4a, sections 1 and 2), and the upper surface of the flow gradually roofed over in a similar manner to the zipper mechanism described for pahoehoe flows [Peterson et al., 1994]. On Etna, at least 2 weeks were required to produce a tube sector by this mechanism. Roof collapse and un- sealed portions of lev6es formed skylights that defined the path of tubes.

5.3. Intermediate Gradient Zones (5-10 ø)

The final stage of tube formation was confined to areas with intermediate slopes. These were typical of many medial sections of 1991-1993 Etna 'a'a flows. They are represented by sections 3 and 4 in Figure 4a. Here the gradual accumulation of 'a'a blocks on the flow surface of wide channels (typically 10 to 20 m) slowly increased crust thickness, and fluctuating movements of the fluid portion allowed the blocks to weld together forming a complex crust. This mechanism occurred on all major channels. Tubes took a minimum of 1 month to form by this mechanism.

The above observations of tube formation demonstrate the

close relationship between complex tube systems and their parent flows. The resulting tube systems in turn strongly influence the development of the flow field. In section 6, we assess the importance of factors controlling the lengths of individual, channel-fed lava flows, some of which form lava tubes.

6. Factors Defining the Maximum Length of Individual Lava Flows

We investigated the effects of the main independent variables controlling the maximum length of channel-fed 'a'a lava flows on

Etna by performing a multiple regression analysis following the method adopted previously for Hawaiian flows [Pinkerton and Wilson, 1994]. For this analysis we used nine flows from the 1991-1993 eruption and eight sets of data from single flow units emplaced during recent Etna eruptions. The number of these data was necessarily constrained by the paucity of published information on single flow units on Etna. These data include Flow 7 and Flow 9 from the first phase of the 1974 eruption [Guest et al., 1974]; one flow emplaced on November 27, 1978, from the base of Monte Centenari [Romano, 1985]; the main flow from the 1981 eruption [Guest et al., 1987]; and four flows emplaced in 1990 from the South-East Crater [Calvari et al., 1991]. These flows are all individual lava flows that were erupted tbr 1 to 12 days. By considering individual flows instead of total flow field parameters, we can demonstrate that the maximum

flow length is related to the mean discharge rate, ground slope, and emplacement duration. In the following analysis, E is the mean discharge rate (in m 3 s -1) averaged for the whole time an individual flow was active, and L is the final length of the single channel-fed flow (in m):

L= 3.11 E 0'47 r2=0.86 (1)

The relationship between final flow lengths, and mean discharge rates is shown in Figure 6. The relationship improves if we add the total emplacement time of the flow (t, in seconds), and the mean ground slope (a, in radians):

L= 1.747 E 0'43 t 0'29 C• t0'26 r2=0.94 (2)

The relationship between measured, and calculated flow lengths using equation (2) is shown in Figure 7.

Since equation (2) is not dimensionless, it can be applied only to individual Etnean 'a'a lava flows with mean discharge rates, lengths, emplacement times, and ground slopes similar to those used for the model. Data used for the analysis involve lengths between 620 and 7500 m, mean discharge rates between 0.2 and 60 m 3 s -1, emplacement times between 1 and 12 days, and mean ground slopes between 2 and 20 ø. This model can be used during future Etna eruptions to estimate the maximum length that an individual channel-fed lava flow can travel from the source if a

maximum emplacement time of 12 days is assumed. Where lava tubes form, the same calculation can be repeated for every flow that emerges from first-order ephemeral vents. Figure 8 gives an example of the applicability of the model, showing lines calculated from equation (2) for common values of ground slope and emplacement times. By contrast, Figure 9 shows the effects,

10

0.32 • i 0.1 1 10 100

mean discharge rate (m3/s)

Figure 6. Relationship between final flow length and mean discharge rate for 17 selected individual 'a'a lava flows from Etna. The regression line is calculated from equation (1).


lO

3.2-

0.32

0.: !

3.2 10

calculated flow length (km)

Figure 7. Relationship between measured maximum lava flow lengths of the 17 selected individual 'a'a lava flows from Etna and calculated maximum lengths obtained from equation (2).

on length, of lava tubes by showing the trend followed by pri- mary and secondary flows emplaced between December 15, 1991, and April 10, 1992, on Etna. Figure 9 shows the reduction in measured discharge rates at different distances from the vent. It also shows how small flows coming out at low discharge rate can increase the final length of a tube-fed flow field.

7. Discussion

During the 1991-1993 Etna eruption, tubes formed in many 'a'a lava flows that were active for more than 4 days. A steady discharge rate was essential for tube formation, except when tubes formed on steep slopes. Here, narrow channels developed relatively quickly, and small fluctuations in lava level caused by transient blockages created overflows, inward levde accretion, and finally lava tubes.

The flow rate of lava through a tube defined the size of the tube system, wider and longer tubes forming at higher discharge rates. The main part of the first tube system, which was 6 km long and up to 32-33 m high (L. Villari, personal communication, 1996), formed when the discharge rate was 22 m 3 s 'l [Calvari et al., 1994]. The second tube system, which attained a length of 2.5

10 •o ' '

E I ..•.." .:;.,• !

m 3.2-[ ,"' ,".:"•-'• c: / ." ," ,." b•,- / • / ,';, •?" .:...'•' ],,"; ," o

= /

• •,.".,-" ,..•4 _u

u 0.32 0.1 1 10 100

mean discharge rate (m3/s)

Figure 8. Relationship between calculated flow length, mean discharge rate, slope, and time based on equation (2). Data from the 17 selected Etna flows used in our analysis have been sub- divided into flows lasting more than 2 days (black dots), and those lasting 2 days or less (grey dots). The maximum lengths of channel-fed Etna lava flows correspond to the top line.

lO

3.2-

0.32 ! I

0.1 1 10 100

mean dischar9e rate (m3/s)

Figure 9. Development of flows between December 15, 1991, and April 10, 1992. The single channel-fed flow that formed on December 15 (solid circles) was fed at a discharge rate that decreased slightly during emplacement. The final length of this flow was less than the maximum length calculated using equation (2) (shown as a dashed line) for a slope of 20 ø and an emplacement time of 12 days. The discharge rate from the breakout on December 24 (open circles) was smaller than that of the parent flow, and it decreased during emplacement because additional small breakouts formed in other parts of the flow field. Once the December 24 flow had attained its maximum length, many new breakouts piled up behind the dam at the exit of the Val Calanna (see Figure 3e). On April 10, a tube-fed flow emerged from the top of the dam (Figure 3f). This allowed the flow field to reach 8.5 km from the source, 3 km farther than the maximum length predicted using equation (2). The arrow shows the progressive growth of the flow field.

km and a mean diameter of 5 m [Giudice and Leotta, 1995], formed when the discharge rate was 15 m 3 s -] [Barberi et al., 1993]. The portion of the first tube system downslope of the exit of Val Calanna was less than 1 km long and formed at a discharge rate of 1-4 m 3 s -1 [Calvari et al., 1994]. By comparison; the 1971 tube formed when the discharge rate was 10 m 3 s -• [Guest et al., 1980], and peak discharge rates of 0.2 m 3 s -• formed 300-m-long tubes during the 1975 Etna eruption [Pinkerton and Sparks, 1976]. Thus tubes on Etnean 'a'a flows form with discharge rates that vary by over 3 orders of magnitude. This is at variance with the restricted range of discharge rates (1 to 5 m3s -]) reported for tube formation on Hawaii [Peterson et al., 1994]. Our observations are also incompatible with the results from laboratory simulations using wax flows [Hallworth et al., 1987], where tubes were observed to form only at low effusion rate. However, laboratory experiments at different effusion rates were carried out for the same period of time (no more than 25 min) to prevent mixing of wax with water, and the size of the tank used remained the same, confining the front of the largest flow. It is conceivable that larger and longer tubes would have been obtained using larger tanks and longer-duration experiments with higher values of effusion rate.

Previous workers have argued that lava tubes form at the eruptive fissure and propagate downslope [Guest et al., 1980; Peterson et al., 1994]. While our observations reveal that this was valid at the scale of the entire 1991-1993 flow field (because tubes followed the emplacement of flows), it was not valid at the scale of a single flow. Formation of a tube network on Etnean 'a'a lava flows was not a continuous downflow process. Here, tubes roofed over at different rates on different sectors of 'a'a flows.


The complex process involved in the formation of a tube network was observed in its entirety only twice during the 1991- 1993 Etna eruption. On both occasions, the following events took place: (1) a large 'a'a flow propagated downslope until its continued advance was prevented, initially by topography and eventually by cooling of the flow front; (2) the flow stopped, the flow front thickened and widened (Figure 5), and the flow inflated from the front region backwards (Figure 4a, sections 5 and 6); (3) overflows emerged from second-order ephemeral vents a few tens of meters behind the flow front, where the crust was thinner than

in the distal part of the flow (beneath the overflow, lava continued to flow in the tube causing further inflation of the front); and (4) if the pressure of lava within the fluid core imme- diately behind the flow front exceeded the tensile strength of crust of the flow front, a new flow emerged through the breached flow front (Figure 4b).

Once the vent opened at the front of the first flow, drainage stabilized. The tube sector then propagated upslope by accumulation and welding of 'a'a blocks that stacked against the crust in the region behind the flow front. This was followed by roofing over of narrow channels on those parts of the flow with a steep gradient and finally by the sealing of 'a'a blocks along the wide lava channels in the intermediate gradient zones (Figures 4a-4b). Once the first arterial flow was roofed over, new tube segments could form by similar processes along secondary flows. This mechanism can be seen in Figures 3e and 3f.

The three mechanisms of tube formation recognized during the 1991-1993 Etna eruption reflect different rates of crust formation. Along the December 15, 1991, flow, which was fed at a rate of 22 m 3 s -1, inflation at the flow front required only 4 days to form a tube sector. On the same flow, tube formation along narrow channels and steep gradient zones required 15 days, while a stable crust over the wide channels of the middle portion of this flow required -1 month to form. These processes were shorter for the proximal flow of May 27, which was fed at 15 m 3 s '1. It was completely sealed over 14 days after the flow front stopped advancing.

The opening of first-order ephemeral vents from the fronts of inflated flows is the key to downslope propagation of the arterial tube network. If the discharge rate from the first-order ephemeral vent at the front of the lava flow is higher than the discharge rate along the channel, the channel and the frontal region will drain completely. The upper crust will then collapse, and the flow channel will be effectively blocked. If the discharge rate from the ephemeral vents is the same as along the channel behind it, the pressure inside the inflated front is sufficient to prevent the collapse of the upper crust. Provided that the lava supply rate is steady for the time required for the upper crust to become self- supporting, a sector of tube forms. If the discharge rate from the new ephemeral vent is low, the ephemeral vent remains active for a short time, feeding a small flow which cools and stops.

Our observations of first-order ephemeral vents confirm that they can form in two different situations. Major first-order ephemeral vents that are active across the entire flow front occur when there is a marked increase in gradient of the ground. However, most first-order ephemeral vents form after a period of flow inflation, during which time lava continues to be supplied through the active channel to the distal part of the flow. First- and third-order ephemeral vents form where localized breakouts of lava emerge from either single or multiple points on the flow front. These are morphologically analogous to the fingers that develop in viscous and granular flows [Huppert, 1982; Pouliquen et al., 1997]. It is conceivable that the fractal nature of 'a'a flow

boundaries [Bruno et al., 1994] reflects the shape of the front on the fluid core of distal flows. A new first-order ephemeral vent will open when the tip of a viscous finger penetrates the thinnest part of the crust. The two first-order ephemeral vents that opened at the front of the December 15 flow were located on the most

advanced portion of two such fingers (Figures 3b-3c). The importance of tubes during the development of the 1991-

1993 lava flow field on Etna can be seen in Figure 10, where we show the emplacement of selected lava flows during the 1991- 1993 eruption. The January 3, 1992, flow had a higher advance rate than the flows of December 15, 1991, when the main flow

split into two branches. The January 3 flow advanced initially very rapidly (about 1 krn/day, [Calvari et al., 1994]) and reached 5.5 km from the vent before its cooling flow margins prevented further advance. This is 3 km less than the maximum distance

eventually covered by the 1991-1993 lava flows. It is instructive to compare this eruption with the 1950-1951 Etna eruption, when a flow field of 171 x 106 m 3 covered the northern, central part of the Valle del Bove in 382 days. The mean eruption rate was 5.2 m 3 s '• and the total length of the tube-fed flow field was 9.7 km [Cumin, 1954]. By comparison, the 1991-1993 eruption produced 231 x 106 m 3 of lava in 473 days with an eruption rate of 5.7 m 3 s '1 [Stevens et al., 1997]. We argue that without the successful diversion in May 1992, the 1991-1993 flow field would have overrun Zafferana, 9 km from the source, and continued beyond.

Given the complexity of 'a'a lava flow fields, especially those with lava tubes, there are currently no simple theoretical models that allow the maximum extension of tube-fed lava flows to be

calculated. However, using selected data from recent Etna eruptions, the lengths of individual Etnean channel-fed 'a'a lava flows can be calculated from mean discharge rates, mean ground slopes, and emplacement times. Our model is considerably better than the values that would have been obtained using existing length-effusion rate models based on eruption rates alone [Walker, 1973; Wadge, 1978; Kilburn, 1993]. The relationship we establish can be used during an eruption to predict the maximum length of individual flows. If conditions permit the formation of first-order ephemeral vents at the front of a flow, this method can then be used to determine the maximum length of each new flow.

8. Conclusions

Our observations suggest that major tube systems are important not only during the formation of pahoehoe flow fields [Oilier and Brown, 1965; Peterson and Swanson, 1974; Atkinson et al., 1975; Keszthelyi and Pieri, 1993; Mattox et al., 1993; Honet al., 1994; Peterson et al., 1994; Kauahikaua et al., this volume] but also during the development of long-lived 'a'a flow fields. The initial formation of tube networks during the 1991-1993 eruption on Mount Etna was a discontinuous process controlled by ground slope and discharge rate. Tube sectors formed on those lava flows that were active more than 4 days and that had first-order ephemeral vents along their margins, provided that lava continued to be supplied at a uniform rate. Discharge rate controlled tube size, whereas ground slope controlled the mechanism and rate of tube formation. Tubes formed more

rapidly in low-gradient zones, where inflation of the flow front and vent openings played important roles in forming and propagating the tube network.

The effect of discharge rate on tube size was highlighted by the two tube systems that formed during the 1991-1993 Etna eruption. At similar parts of the flow field, the first arterial tube system had a cross-sectional area that was an order of magnitude


lOOOO

8000-

6000 -

4000

2000 -

c E O.O. O. ..............

.0.0..0..0 -0" ..o .-ø .... ,•, 4,

,¾ ,

D

// F

I !

time (days)

$ Dec 15 '91

........ O ........ Dec 24 '91

.... .!, .... Jan 3 '92

---•[]--- April 10'92

115 120

Figure 10. Comparison between the increase in flow length with time of the three 'a'a lava flows shown in Figure 9 and a channel-fed flow that emerged from the base of the fissure on January 3, 1992. Letters refer to sketches in Figure 3, where the relationship between flow and tube formation is shown. The December 24 flow emerged from two first-order ephemeral vents that opened at the front of the December 15 main flow, 4 km from the eruptive fissure (see Figures 3c and 3d). Note that no new flow emerged from the front of the January 3 flow (the dark grey flow in Figure 3d). Also note that between January 10 (day 26) and April 9 (day 116), 1992, the Val Calanna was filled by small flows along which additional tube branches formed (compare with Figures 3e-3f). The April 10 flow (day 117) emerged from the exit of the first tube system when it reached the earth dam of Portella Calanna.

larger than that of the second tube system, and its mean discharge rate was about twice that of the second tube system.

Between October 1992 and the end of the eruption, there was a gradual decrease in effusion rate. During this period, breakouts along the upper part of the eruptive fissure became more common, indicating progressive cooling of the distal part of the second tube system. Similar upslope migration of breakouts along the Waha'ula tube system was observed during the final period of activity of the Kupaianaha vent in Hawaii [Kauahikaua et al., 1992, 1996].

The first tube system acted as an efficient pathway for lava from the main vent for 5 to 6 months, allowing a maximum length of 8.5 km to be reached by the flow field. Along the 8-km- long tube, the lava stream was close to isothermal [Calvari et al., 1994]. Comparison with the 1950-1951 flow field [ Cumin, 1954] suggests that the 1991-1993 flow field would have been considerably longer if the flow had not been diverted.

Using data for the 1991-1993 and earlier Etna eruptions, we show that the lengths of individual channel-fed Etna flows can be calculated given measured effusion rates and mean gradients. However, a more complex model is required when tubes form in lava flows. Figures 9 and 10 show that the gradual lengthening of a tube system by successive breakouts at stationary flow fronts allows a flow field to lengthen well beyond the length predicted using methods based on individual channel-fed flows.

While the processes described in this paper relate specifically to the 1991-1993 Etna eruption, we recognize many similarities with previous eruptions on Etna and other volcanoes. We see our mainly descriptive case study as an essential part of a more detailed and quantitative investigation of the role of lava tubes in the formation of extensive 'a'a lava flow fields.

Acknowledgments. This work has been based on the huge amount of data collected during the 1991-1993 Etna eruption by scientists from the Istituto Internazionale di Vulcanologia of Catania and the Gruppo

Nazionale per la Vulcanologia of Italy. We wish to thank Prof. L. Villari, R. Maugeri, and N.F. Stevens for allowing us to use their unpublished data and Prof. K.V. Cashman, Dr. J. Kauahikaua, and Dr. R.I. Tilling for their very useful comments on an earlier version of this paper.

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H. Pinkerton, Environmental Science Department, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster LA1 4YQ, England, U.K. (e-mail: [email protected])

(Received May 16, 1997' revised October 10, 1997; accepted November 11, 19973

Formation of lava tubes and extensive flow field during the 1991–1993 eruption of Mount Etna

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Transcript of Formation of lava tubes and extensive flow field during the 1991–1993 eruption of Mount Etna