The 20-km-wide volcano that dominates Heard Island is about a million years old. Its present summit, Mawson Peak, appears to have formed only within the last 10,000 years after collapse of the western side of the mountain. Because Heard Island is seldom visited and usually shrouded in cloud, observations of volcanic activity are rare. There were no recorded observations before 1881 and 1910 — long after the seal hunters had left — when clouds of smoke were reported. About 30 observations of activity were recorded during the Atlas Cove station years (1948–54).
There is a 31-year gap in records before a lava eruption with bluish-coloured plumes was observed from aboard Marion Dufresne on 14 January 1985. Red glows around the summit were seen from Nella Dan on 1 and 2 October 1985 and from Atlas Cove on 4 November 1985. Plumes were seen from South West Bay on 17 November 1985 and from the Space Shuttle between 7 October and 3 November. Australian investigations at the summit in 1986–87 found a lava lake gently emitting smoke and steam, and a small explosive emission occurred on 4 January 1987.
During the year-long research presence at Spit Bay in 1992, Big Ben was particularly active, with many sightings of steam issuing from the summit. Eruptions of plumes from the summit, hidden from Spit Bay by a ridge 4km to the east, were vigorous enough to be seen many times, sometimes rising hundreds of metres above the ridge. Two earthquakes were experienced. The largest of these, on 18 December 1992, may have been associated with an eruption producing a 4.5km lava flow subsequently found on Big Ben’s southern flank.
Kevin Kiernan