The latest eruptive episode from Stromboli began on 28 December 2002 (BGVN 28:01) and included a significant explosion on 5 April (BGVN 28:04). This report includes field observations provided by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) through mid-June 2003. Thermal alerts based on infrared satellite imagery over the course of this eruption have been compiled and summarized by scientists at The Open University.

Activity during 17 April-16 June 2003. Effusion of lava from vents located at ~600 m elevation, on the upper eastern corner of the Sciara del Fuoco, continued until 16 June with a generally decreasing effusion rate. This caused a significant increase in the thickness of the lava field formed since 15 February to over 50 m. Since the 5 April eruption, the summit craters of the volcano have been blocked by fallout material obstructing the conduit. Small, occasional, short-lived explosions of hot juvenile material were observed on 17 April during a helicopter survey with a hand-held thermal camera, and on 3 May from the SAR fixed camera located at 400 m elevation on the E rim of the Sciara del Fuoco.

The effusion rate from the 600-m-elevation vents on the Sciara del Fuoco showed a significant decline between 1 and 4 May, when inflation of the upper lava flow field was detected through daily helicopter-borne thermal surveys. Inflation stopped on 6 May, when two new vents opened on the inflated crust of the flow field, causing drainage and spreading new lava flows along the Sciara del Fuoco. Between the end of May and early June, gas-rich magma was extruded from the 600 m vents on the upper Sciara del Fuoco. Spattering built up two hornitos, which in a few days reached an estimated height of over 6 m. This activity was accompanied by lava flow effusion along the upper Sciara del Fuoco, with lava descending to 150 m elevation.

On 1 June, Strombolian activity resumed at Crater 1 (NE crater). It was revealed first through helicopter-borne thermal surveys, and then by direct observations from the eastern Sciara del Fuoco rim. Most of the ejecta fell within the crater, and from the lower slopes of the volcano only pulsating dark ash emissions were observed. Strombolian activity stopped around 6 June, and occasional lava flows occurred at the hornitos at 600 m elevation on 11 June. The summit craters showed discontinuous ash emission until mid-June, and the SAR fixed camera at 400 m elevation showed a Strombolian explosion with abundant ash emission on the night of 15 June.

MODVOLC Thermal Alerts. MODIS thermal anomalies for Stromboli covering the period from the start of MODIS data acquisition over Europe in May 2000 until the present were compiled using data available at http://modis.higp.hawaii.edu/.

With the exception of single-pixel alerts on 8 July and 19 September 2000 (with alert ratios of -0.798 and -0.794, both barely above the -0.800 automatic detection threshold of the thermal alerts algorithm), activity at Stromboli remained below the automatic detection threshold until November 2002 (figure 74). In that month there were two single-pixel alerts, barely above detection threshold (-0.790 on 12 November and -0.792 on 28 November). Thermal infrared radiance was higher than ever before at the time of the MODIS overpass on 20 December 2002, when there was a two-pixel alert, with alert ratios of -0.667 and -0.749.

Figure 74. Alert-ratio, number of alert pixels, and summed 4 µm (MODIS band 21) spectral radiance for MODIS thermal alerts on Stromboli between 1 November 2002 and 13 May 2003. MODIS data courtesy of the HIGP MODIS Thermal Alert Team.

These five dates were the only MODIS thermal alerts prior to the start of effusive activity on 28 December 2002 (BGVN 27:12 and 28:01). The source of the radiance to trigger these alerts was evidently incandescence at one or more of the active vents. In the case of a volcano such as Stromboli, prior to December 2002, isolated thermal alerts are more likely to represent the chance coincidence of a short-lived peak of incandescence with the time of MODIS overpass, rather than a sustained emission of infrared radiation. However the November-December 2002 thermal alerts can with hindsight be seen to have been indicators of enhanced activity in the lead-up to the 28 December effusive eruption.

On 28 December 2002 MODIS recorded its highest ever alert ratio at Stromboli (+0.419) and highest summed radiance at 4.0 µm (MODIS band 21) in a seven-pixel alert, corresponding to the daily MODIS overpass at 2115 UTC. This is a record of radiance from 300-m-wide lava flows from the NE crater (BGVN 27:12). Subsequent to that date, thermal alerts have occurred persistently at Stromboli, and evidently reflect ongoing lava effusion. The general trend of the highest alert ratio on each date, the number of alert pixels, and the summed 4.0 µm radiance for all alert pixels on each date shows an exponential decline.

There are no thermal alerts for 3-7 April 2003 inclusive, which could be because of cloud cover. There is thus no direct record of the explosion on the morning of 5 April that completely covered the upper 200 m of the volcano with bombs. However, the mild intensification of subsequent thermal-alerts indicates slight re-invigoration of the on-going lava effusion.

Geological Summary. Spectacular incandescent nighttime explosions at Stromboli have long attracted visitors to the "Lighthouse of the Mediterranean" in the NE Aeolian Islands. This volcano has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period took place between about 13,000 and 5,000 years ago. The active summit vents are located at the head of the Sciara del Fuoco, a prominent scarp that formed about 5,000 years ago due to a series of slope failures which extends to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild Strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: Sonia Calvari, Istituto Nazionale di Geofisica e Vulcanologia, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/); David A Rothery and Diego Coppola, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom. MODIS data courtesy of the HIGP MODIS Thermal Alert Team.