Stromboli Volcano

"Strombolian" activity is today a generic term used to describe mild explosive activity producing pyroclasts (molten rocks) and ash (see also videos of Strombolian activity at Fogo Volcano). The explosions are caused when bubbles of gas burst explosively at the top of the conduit. These gas pulses together with the continuous lower level degassing result in estimated emission of 6-12 thousand tonnes of gas per day. The name strombolian results from persistent observation of such activity at Stromboli volcano.

Stromboli volcano is also known as the Lighthouse of the Mediterranean. Historical accounts dating as far back as the report of Timaeus in the 4th century BC describe activity at the volcano. The accounts are however not accurate enough to unequivocally ascribe a particular eruption style thereto. By studying layers of volcanic deposits and carbon dating organic materials trapped therein, Rosi et al., 2000 (Bulletin Volcanology 62:294-300) determined that the onset of strombolian activity probably occurred in the 3rd to 7th century AD. Before then, it appears that the volcano was highly active between the 4th century BC and 1st century BC, with strombolian periods intermingled with episodes of intense lava fountaining which emplaced discrete decimeter thick layers of lapilli on the flanks of the volcano.

Stromboli Island with San Vincenzo village at base of volcano. The Island rises 2000m off the sea floor, and reaches a height of 924m a.s.l.	Stromboli volcano. The crater terrace is located at the lower flat area right of the summit. Erupted material often slides down the scree slope (Sciara del fuoco) on the right.

Thanks to numerous geological studies of stromboli, it is possible to follow the 200,000 year history of the volcano much further into the past. Two notable features that are clearly visible to visitors to stromboli are the steep-sided scree slope (Sciara del fuoco) and the steep cliff under the Pizzo Sopra la Fossa which overlooks the present crater terrace. These features mark the perimeter of a huge landslide which is thought to have occurred in the last few thousand years. The volume of the landslide is estimated at over 0.7 cubic kilometers and involved detachment of a section of the NE slope of stromboli from a height of about 800m down to a similar depth under the sea. It is now known that during the evolution of stromboli volcano at least 8 sector collapses occurred between which eruptive activity rebuilt much of the structure (Tibaldi 2001 (Bulletin Volcanology 63:112-125)). Most of these, in particular the four most recent ones (Upper Vancori, Neostromboli, Pizzo Sopra la Fossa and Sciara del fuoco sector collapses), occurred in the same NW direction. The reasons for this are complex.

Stromboli volcano is predominantly fractured in a SW-NE direction. It is thought that structural failure causing the Upper Vancori collapse 13000 years ago was the result of (i) gravity after the volcano had reached a critical mass, (ii) magma intrusion into the SW-NE fractures (or dykes) which pressed the flanks in a NW to SE direction, and (iii) the fact that the sea floor at the SE side of the volcano is higher and thus more able to support that flank, preventing collapse in that direction. The subsequent 3 collapses are thought to have accurred in the past 5000 years and have been progressively smaller in volume. This is thought to be due to the fact that the Upper Vancori collapse introduced a plane of weakness into the volcano but also due to the gradual shift in activity from lava emission toward explosive eruption of pyroclasts as seen in the present day. Pyroclastic deposits are less stable than lava deposits and thus may fail when a lower volume of material has been emplaced.

There are currently 2 conflicting models used to explain strombolian activity. Both agree that large slugs (bubbles) of gas resulting from solute exsolution cause the strombolian explosions when they reach the top of the conduit and burst explosively to the surface. Solute exsolution means that gas dissolved in the magma (such as water vapour or carbon dioxide) exsolves (i.e. comes out of solution) as a result of the falling pressure as the magma rises towrds the surface. This gas forms bubbles with gradually grow in size as they merge with other bubbles. The conflicting models disagree on where these bubbles merge to form the large gas slugs responsible for the eruptions. In the Rise Speed Dependent (RSD) model, bubbles merge into an increasingly large slug as they rise up along the conduit. The model assumes that the magma in the conduit is rising slower than the bubbles and that the slug rises and progressively grows in size until it reaches the surface. In the Collapsing Foam (CF) model, gas exsolves in a specific chamber (e.g. a subterranean cavity) connecting to the conduit. The gas is thought to accumulate as a foam layer at the top of this chamber, with partial collapses of the foam layer leading to release of large gas slugs which then rise up through the conduit to the surface. The two models are discussed in e.g. Parfitt, 2004 (J. Volc. Geotherm. Res. 134, p.77-107). Interestingly, tremors at a depth of several hundred meters can be directly correlated with explosions at Stromboli (Chouet et al., 1999. Geophys. Res. Lett. 26, p.1937-1940). These could be the result of foam layer collapses as proposed in the CF model.

Whilst Strombolian eruptions are considered as an explosive form of volcanism, each eruption does not actually involve a single abrupt explosion. For example, in 2004, the "explosions" at Stromboli were from 6-40 secs in duration (15 secs on average). Discharge rates of 10-500 kg/sec are commonly involved. Parameters of eruptions at Stromboli from a variety of sources are summarized in Houghton and Gonnermann, 2008 (Chemie der Erde 68, p.117-140).

Large ash eruption from Stromboli during phase of irregular activity, April 2007.	Crater terrace of Stromboli Volcano, June 2006.

	Nighttime eruptions of Stromboli, June 2006.

Close-up of unusually ash-rich eruption column, June 2006.	Close-up of unusually ash-rich eruption column, June 2006.

Looking at records of activity in recent times, one sees certain patterns of activity emerging. Stromboli has been in almost continuous eruption since proper documentation of activity began over 400 years ago. However, following phases of increased activity, eruptive pauses have been observed (a long pause being from May 1907 - May 1910; more recently from March - July 2007). Otherwise eruptive activity is strombolian with varying frequency and intensity, with frequent weaker ash emissions inbetween. At Stromboli, the lapilli-rich strombolian eruptions are referred to as type 1, and the ash-rich eruptions (which are not actually strombolian) as type 2 events. This general activity is occasionally punctuated by violent explosive eruptions, lava effusion in the crater terrace or down the sciara to the sea, or a combination of these. Unusually violent explosions occur about once or twice a year during periods of otherwise normal activity and may occur apparently without warning or precluded by general increases in activity. These explosions can throw large projectiles well out of the crater terrace and onto the surrounding flanks, rarely as far as inhabited areas.

The most violent historically recorded eruption of stromboli happened unexpectedly on 11. September 1930. The eruption was merely preceded by increased ash venting at 8:10. At 9:52, two extremely violent explosions blew much of the crater terrace over the NE side of the Island. Large blocks with volumes of up to ten cubic meters fell on Ginostra, destroying 14 houses. The explosions were followed by intense fountaining activity which lasted for over half an hour and lead to rapid accumulation of hot tephra on the upper slopes of the volcano. The accumulated material became unstable and two avalanches of glowing material slid down the N side of the volcano. One of these pyroclastic flow-like avalanches (comparable to those documented at Etna in 1999 (Behncke et al., 2003 (J. Volc & Geotherm. Res. 124: 115-128)) proceeded down Vallonazzo valley to the sea, killing 6 people. The eruption was concluded by a 15 hour effusive phase, with lava reaching the sea down the Sciara.

Strombolian activity from small vent in crater terrace, June 2006.	Strombolian activity from small vent in crater terrace, June 2006.

Another interesting period of activity was observed between 28 Dec. 2002 and 22 July 2003. An effusive eruption commenced on the 28th of Dec. Lava flows rapidly progressed down the Sciara del Fuoco reaching the sea. On 30th December, two landslides occurred carrying the lava flows and some underlying material into the sea. These landslides, with estimated volumes of around 5 Million cubic meters each, caused meter high Tsunamis which damaged property on the low-lying parts of the Island. Similar tsunamis occurred in 1930, 1944 and 1954 and are not uncommon at stromboli or other island volcanoes such as Augustine. Putting the 2002 tsunami into context, the Sciara del Fuoco collapse was 100-fold larger, so the impact of a similar collapse in the future would clearly be immense. Effusive activity continued after the landslides and persisted more or less continuously until 22 July. During this period, on 5. April, a violent explosive event occurred. This covered the top 200m of the volcano in a blanket of fresh pyroclastic deposits and propelled volcanic bombs as far as Ginostra village, where 2 houses were badly damaged.

Another effusive event occurred within the Sciara from 27 Feb. to 15 Mar. 2007. No strombolian activity was observed during this period. This is common during periods of effusion of lava from the flank of the volcano as it reduces the height of the magma in the conduit. Regular strombolian activity was reestablished again in late July.

Visitors should be aware that access to the summit of Stromboli has been restricted in recent years. When activity is considered sufficiently stable it is usually possible to climb, but only accompanied by a local guide. At other times, no access at all may be permitted. One should be aware that two tourists have been killed by volcanic bombs near the summit in the last 30 years and that it could have been many more if tourists had been at the summit during a major explosive event.

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