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The Etna guides continue to produce ash trays of fresh lava as effusive activity on the 4 February 1999 fissure is going on. This photo, taken on 4 June 1999, shows an Etna guide ripping a chunk of incandescent lava from an effusive vent.

Four months after its beginning, the effusive activity at the southeastern base of the SE Cone is continuing at a relatively low but apparently stable rate. In the past week or so, lava flowing down the western slope of Valle del Bove has no longer been visible from Catania because it is now flowing behind a large ridge of lava accumulated during the previous weeks. Nonetheless the effusion rate does not appear to have diminished since the last visit by Behncke and others on 19 May (see 20 May update).
The following information regarding the activity from 30 May to 2 June was kindly provided by John Guest (Department of Geological Sciences, University College London, UK) and Angus Duncan (Centre for Volcanic Studies, University of Luton, UK) who did fieldwork on Etna in late May-early June.
"Several explosions heard from the summit craters, probably the Bocca Nuova, on 30th May at 6.30 pm. On 1st June a bright red glow, lasting several seconds, was seen over the Bocca Nuova at 9.45 pm. Visited the lava vent area at the foot of the SE Crater on 2nd June. The active lava pile had increased in thickness since our previous visit on 30th May. Fresh lava now partly buried the 'old' tumulus of altered lava blocks. A new tumulus had formed a few metres downflow (to the south) of the old tumulus. During the afternoon of 2nd June there was an apparent increase in effusion rate from the vent being observed - due either to an increase in supply or blockage of other channels. This led to an increase in the rate of advance of a proximal flow to the west from virtually 0 to 0.15 m/min. There was also inflation of lava in the proximal area and numerous overflows."
There have been some signs of slightly increased at NE Crater as well in the past week. On 2 June, Boris Behncke, while doing fieldwork on the western flank of Etna, observed occasional forceful emissions of vapor from the crater, and on the afternoon of 3 June, shortly before 1700 h, Sandro Privitera (IGGUC) observed three emissions of reddish gray ash to a height of more than 500 m above the crater. This information was confirmed the day after by local mountain guides.
On 4 June, Boris Behncke and Sebastiano Giuffrida (IGGUC), Ken Hitchen (Geological Survey of Great Britain, Edinburgh, Scotland) and Oonagh O'Loughlin (Petroleum Affairs Division, Department of the Marine and Natural Resources, Dublin, Ireland) visited the eruption area. The two main sites of effusive activity are generally the same as observed on 19 May: an effusive vent about 25 m below the hornitos at the upper end of the eruptive fissure which became active on 4 February, and a cluster of ephemeral vents located at about 2600 m elevation on the western slope of the Valle del Bove. While the former is located on the 4 February fissure, the latter is not a true eruption vent, but it is rather the point where lava issues from a lava tube about 1 km long. Both, however, are of very similar appearance and are spots where lava wells to the surface.

4 June 1999 photos (more to follow)
Left: Tourists crowding at the effusive vent just below the hornitos formed early during the current effusive activity. The active effusive vent is at left. Right: A close look at the active effusive vent after one group of tourists has left. Flow is from left to right.
Left: Secondary effusive vent on the side of a recent, but stagnant lava flow (vent is to the left of the large rock in center of image), feeding a very slow flow to the right. Right: View northwestwards from the 1991 scoria rampart across the southwestern part of the lava flow-field emplaced since 4 February. The lava outlined in red has been added to the margin of the flow field since late-May. The summit cone complex, dominated by the SE Cone from this perspective, is visible in the background.
Left: Looking from "Belvedere" to the active flows on the western Valle del Bove slope. Near continuous rock falls from a flow front advancing on a very steep part of the slope creates the light brown dust clouds visible in the center of the image. Right: This is the area of the tumulus that collapsed on 12 May, seen from above (and looking eastwards). The two sites of lava effusion on 4 June are highlighted by numbers 1 and 2.
Photo mosaic of the area of effusive activity on the western slope of the Valle del Bove whose floor can be seen in the background. Larger vent (1 in previous figure) ) is at left, vent 2 forms small tumulus in central left of image, and feeds two small, narrow lava tongues.
Left: Close-up of the larger of the effusive vents on the western Valle del Bove wall. Note the concentric structures around the vent, formed as it shifted downflow. Right: Two smaller vents in the southern part of the tumulus collapsed on 12 May, feeding small, sluggish lava flows with ropy surfaces.

Since the 19 May visit, the upper site of effusive activity has shifted about 30-40 m upslope and lies close to the lowermost of the large hornitos formed in February. During the 16 days between the two visits, the site of lava emission has shifted constantly from one place to another, sending surface lava flows in various directions. During the past week, new lava flows have advanced downslope along the southwestern margin of the lava field emplaced since 4 February, enlarging it by 5-10 m. the farthest flow front has arrived near a spatter rampart formed during the first day of the 1991-1993 eruption. During the 4 June visit, lava oozed from several ephemeral vents on the margins of fresh but stagnant flows and on top of them, forming small, slow-moving lobes only 5-20 m long.
One or two blocky flows fed by the main effusive vent near the hornitos were slowly advancing across previous lavas in the middle of the lava flow-field, but they did not extend farther than 100-150 m from that vent, and none of the recent flows has reached the Valle del Bove rim.
The area of the hornitos and of the active effusive vent is visited by hundreds of tourists daily, and measures have been taken to prevent tourists from venturing away from the marked paths, and in the direction of the SE Cone. Signs set up by Etna Natural Park personnel warn tourists to leave the area delimited by fences. The summer tourism machinery is running smoothly, and the ongoing effusive activity together with an increased tourist flux in the Catania area promise a better summer season than in previous years. This is especially important for the tourist facilities at Etna which have suffered from a bad skiing season with abnormally little snow and frequent tephra falls (from SE Cone) onto ski tracks.
The other site of effusive activity, on the western Valle del Bove slope, lies far away from the present tourist paths, in a hardly accessible area amidst lavas that have flowed down the slope since 4 February. Until 12 May, a large tumulus had grown in the area of the ephemeral vents; on that day Behncke and Scarpinati observed how the tumulus spectacularly collapsed (see 13 May update). One week later, the collapse depression, though partly filled with new lavas, was still perfectly discernible. This was not so when visited again on 4 June. While there appears to have been further subsidence at the site of the former tumulus, lava has issued from numerous closely spaced ephemeral vents and covered most traces of the collapse depression. At the time of the visit, lava flowed vigorously from two ephemeral vents on the northern side of the former tumulus and two very small lava rivers were fed from two ephemeral vents on its southeastern side. Most lava flowed down on the northern side of the large ridge of lavas accumulated during the previous weeks. One flow front was advancing on a particularly steep slope resulting in phases of near-continuous rockfalls generating conspicuous dust clouds.
No active lava extended to the Valle del Bove floor; and no lava has again reached the length of the farthest flows which came close to the Monti Centenari (two cinder cones formed in 1852-1853) in late February.
The overall impression is that the effusion rate has remained nearly constant since about 2 months (at about 1 cubic meter per second or little more), and the output is probably in balance with the amount of magma rising within the central conduit system in this period. As long as this flux remains more or less stable, the eruption is likely to continue. The record of Etna's activity during the past 25 years shows that slow effusive activity in the summit area may continue for years without significant variations in the effusion rate. However, in the long term the rate of magma rising to the surface is not constant at all, and this effusive activity will surely not continue forever.
About 25-30 million cubic meters of lava have accumulated since 4 February on the western Valle del Bove rim and the slope below, forming a large "ridge", or morphologic "shoulder" which has not only altered the outline of the mountain when seen from south, but it has possibly also increased the structural instability of this area which has been considered prone to major collapse for some time before the current activity. The western Valle del Bove rim is subject to appreciable horizontal (and, in part, vertical) displacements caused by intrusions of magma that precede flank eruptions in that sector. It is estimated that the western Valle del Bove rim was displaced eastwards by 5-6 m during four major dike intrusions, or rifting events, since the early 1980's. The area has thus grown more unstable during that period, and some researchers have voiced concern that large-scale collapse may occur at the Valle del Bove rim in the foreseeable future. While since 1991 there has been no further rifting, the northernmost part of the area in question has now received a heavy load of newly emplaced lava which itself has created a new, and much steeper slope. Nothing can be suspected about additional load provided by the rapid growth of the SE Cone in the past year. In any case, even if the potential increase of structural instability caused by the accumulation of new lava is neglected, the lava field on the western Valle del Bove slope is in itself unstable. Since the 19 May visit, the area of the former tumulus has subsided by about 2 m, indicating that the lava may be compacting under its own weight, or part of it is slowly sliding downslope.

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(more than 10,000 during the first 3 weeks!)
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Page set up on 27 May 1997, last modified on 14 June 1999