The New York Times reported that the seamount erupted on 11 October as the RV Melville was over the vent. When the ship first reached the area, a large crescent-shaped zone of sea water was discolored green. While researchers dredged rock specimens from the summit and took water samples, large bubbles suddenly broke the sea surface and turned the water dark brown. Gas bubbles shook the ship, causing loud "clangs and clamors" and a sulfur smell. Harmon Craig (Univ of California, San Diego) described one large bubble that rose 2 m above the ocean surface and burst, forcefully ejecting gas and exposing 20-30 volcanic clasts in its core. The rocks floated briefly, and one piece of dark volcanic glass ~20 cm long was recovered while still too hot to touch. When sampled after the eruption, the seamount was covered with fresh volcanic glass. Coral was also found on the volcano, suggesting significant repose periods between some eruptive episodes. Eruptive activity was declining as the ship departed 12 October [see 12:10 for more details].

RSP stations have been detecting acoustic waves (T-phase) since June. Intense seismic crises occurred 4-7 June and 6-15 July; both began explosively. Activity began gradually 7 August and was continuous but weak until 10 August. Sporadic moderate-level activity was recorded 10-12 August and 19 August-4 October. The 10-13 October activity was continuous and of high intensity. On 3 July 1987, divers from the Cousteau Society's RV Calypso photographed and filmed intense hydrothermal activity.

The August-October seismicity did not include crises that began explosively, indicating to French seismologists that semi-continuous activity could have been occurring during the entire period. This episode brings to 20 the number of distinct crises detected since 1977.

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, LDG Tahiti; The New York Times.