On 14 March, four RSP stations recorded acoustic waves (T-phase) from activity interpreted as a shallow submarine eruption at Macdonald Seamount. The signals were received at Moorea, Vaihoa on Rangiroa, Tubuai, and Rikitea in the Gambier Islands. Strong explosive signals began at 0914, and were followed by continuous noise of varying intensity. The activity began in the same manner as the seven previous eruptions, and was probably comparable in strength and development to that of February 1981. However, high background noise levels caused by the passage of Cyclone Reva over Polynesia made interpretation difficult.

Geological Summary. Discovered by the detection of teleseismic waves in 1967, Macdonald seamount (also known as Tamarii seamount) rises from a depth of about 1,800 m to within 27 m of the ocean surface at the eastern end of the Austral Islands. The basaltic submarine volcano marks the site of a hotspot that was the source of the Austral-Cook island chain. The summit, named after volcanologist Gordon Macdonald, consists of a flat plateau about 100 x 150 m wide with an average depth of about 40 m. The summit plateau is capped with spatter cones that form steep-sided pinnacles. Most eruptions have been seismically detected, but in 1987 and 1989 pumice emission was observed from research vessels. Pumice rafts observed in the South Pacific in 1928 and 1936 may also have originated here.

Information Contacts: J. Talandier, Lab. de Géophysique, Tahiti.