Although previous eruptions have been recorded elsewhere in the South Sandwich Islands (Coombs and Landis, 1966), ongoing volcanic activity has only recently been detected and studied. These islands (figure 1) are all volcanic in origin, but sufficiently distant from population centers and shipping lanes that eruptions, if and when they do occur, currently go unnoticed. Visual observations of the islands probably do not occur on more than a few days each year (LeMasurier and Thomson, 1990). Satellite data have recently provided observations of volcanic activity in the group, and offer the only practical means to regularly characterize activity in these islands. These observations are especially significant because there has previously been no evidence of Holocene activity on Montagu Island (LeMasurier and Thomson, 1990).

Figure 1. The South Sandwich Island archipelago, located in the Scotia Sea. The South Sandwich Trench lies approximately 100 km E, paralleling the trend of the islands, where the South American Plate subducts westward beneath the Scotia Plate. Courtesy Hawaii Institute of Geophysics and Planetology and British Antarctic Survey.

Using Advanced Very High Resolution Radiometer (AVHRR) data, Lachlan-Cope and others (2001) observed apparent plumes and unreported single anomalous pixels intermittently on images of Montagu Island during March 1995 to February 1998. However, field investigations in January 1997 revealed that Montagu Island, as viewed from Saunders Island, was apparently inactive, with the summit region entirely covered in snow and ice. Hand-held photographs of the island obtained in September 1992 also showed the summit to be wholly inactive.

Significant volcanic activity may have begun on Montagu Island in late 2001 based upon analysis of thermal satellite imagery (1 km pixel size) from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. Using the automated MODIS Thermal Alert system (Wright and others, 2002), image pixels containing volcanic activity were detected and analyzed to characterize the eruption. From its location, the erupting center may be associated with a small hill on the NW edge of the ice-filled summit caldera, ~6 km from Mount Belinda (figure 2).

Figure 2. Map of Montagu Island with circles showing the location of all anomalous MODIS pixels detected since October 2001. Stippled areas show rock outcrop, the remainder is snow or ice covered. Relief is shown by form lines that should not be interpreted as fixed-interval contours. Map adapted from Holdgate and Baker (1979); courtesy Hawaii Institute of Geophysics and Planetology and British Antarctic Survey.

The first thermal alert on Montagu occurred on 20 October 2001 with a single anomalous pixel on the N side of the island. Subsequent anomalies generally involved 1-2 pixels, with the exception of several images in August and September 2002 that peaked at four pixels in size (figures 3 and 4). Visual inspection of the images revealed that the anomalies were all located between the summit of Mount Belinda and the N shore, changing in position either due to satellite viewing geometry or actual migration of hot material. We can generally discount other possible explanations for the anomalies, the most likely being solar reflectance influencing the short-wave bands, due to the presence of clear anomalies in nighttime imagery and the concomitance of apparent low-level ash plumes in several of the images. The persistence of the anomaly, and the lack of large ash plumes, suggests that activity here may involve a lava lake.

Figure 3. Selected MODIS images showing thermal anomalies on Montagu Island. Band 20 (3.7 µm) is shown here. The thermal anomalies appear to be located between the summit of Mount Belinda and the N shore. Images are not georeferenced for purposes of radiance integrity, therefore coastlines are approximate. Courtesy Hawaii Institute of Geophysics and Planetology and British Antarctic Survey.

Figure 4. Summed radiance of anomalous pixels in each image. Band 21 (3.9 µm) was used for these plots. Points show the result for each image, and the line is a three point running mean of values. Courtesy Hawaii Institute of Geophysics and Planetology and British Antarctic Survey.

References. Coombs, D.S., and Landis, C.A., 1966, Pumice from the South Sandwich eruption of March 1962 reaches New Zealand: Nature, v. 209, p. 289-290.

Holdgate, M.W., and Baker, P.E., 1979, The South Sandwich Islands, I, General description: British Antarctic Survey Science Report, v. 91, 76 p.

Lachlan-Cope, T., Smellie, J.L., and Ladkin, R., 2001, Discovery of a recurrent lava lake on Saunders Island (South Sandwich Islands) using AVHRR imagery: Journal of Volcanology and Geothermal Research, v. 112, p. 105-116.

LeMasurier, W.E., and Thomson, J.W. (eds), 1990, Volcanoes of the Antarctic Plate and Southern Oceans: American Geophysical Union, Washington, D.C., AGU Monograph, Antarctic Research Series, v. 48.

Wright, R., Flynn, L.P., Garbeil, H., Harris, A.J.L., and Pilger, E, 2002, Automated volcanic eruption detection using MODIS: Remote Sensing of Environment, v. 82, p. 135-155.

Geological Summary. The largest of the South Sandwich Islands, Montagu consists of a massive shield volcano cut by a 6-km-wide ice-filled summit caldera. The summit of the 11 x 15 km island rises about 3,000 m from the sea floor between Bristol and Saunders Islands. Around 90% of the island is ice-covered; glaciers extending to the sea typically form vertical ice cliffs. The name Mount Belinda has been applied both to the high point at the southern end of the summit caldera and to the young central cone. Mount Oceanite, an isolated peak at the SE tip of the island, was the source of lava flows exposed at Mathias Point and Allen Point. There was no record of Holocene activity until MODIS satellite data, beginning in late 2001, revealed thermal anomalies consistent with lava lake activity. Apparent plumes and single anomalous pixels were observed intermittently on AVHRR images from March 1995 to February 1998, possibly indicating earlier volcanic activity.

Information Contacts: Matt Patrick, Luke Flynn, Harold Garbeil, Andy Harris, Eric Pilger, Glyn Williams-Jones, and Rob Wright, HIGP Thermal Alerts Team, Hawai'i Institute of Geophysics and Planetology (HIGP) / School of Ocean and Earth Science and Technology (SOEST), University of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); John Smellie, British Antarctic Survey, Natural Environment Research Council, High Cross, Madingly Road, Cambridge CB3 0ET, United Kingdom (URL: https://www.bas.ac.uk/).