Renewed submarine activity at Macdonald Seamount was detected 1 March by Polynesian seismic stations. Acoustic waves (T-phase) were recorded at Moorea, Tubuai, Vaihoa at Rangiroa, and Rikitea in the Gambier Islands. Explosive signals were first recorded at 1337 and were followed by continuous noise of varying intensity. The initial phase, a few hours long, was succeeded by sporadic activity that lasted until about 0400 on 2 March. The latest activity is comparable in length, intensity and development to that of February 1980.

Bathymetric work that ended in February 1982 and included dredging rocks from the summit peak and adjacent plateau precisely located the submarine volcanic edifice. Its top was 27 m below sea level.

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.