The volcano entered a new eruptive phase 19-20 January after 2 weeks of quiet. T-waves from a series of explosions were registered by the RSP. After 3 days without activity, eruptions began just as the support ship for a joint French-German oceanographic study of hot spots in the Society and Austral Islands (with the submersible Cyana) arrived . . . . On 24 January at [0345], flashes of light, probably resulting from gas combustion within surface bubbles, accompanied a strong H₂S odor. Bubbling at the sea surface emitted water vapor and H₂S gas [at 0400-0430 and 0530-0600]. A green slick formed on the ocean surface, rapidly growing to nearly 2 km, while the seismic station at Papeete detected explosion signals from Macdonald. The green slick (apparently 5-25 m thick) drifted away . . . at roughly 0.5 km/hour and disappeared during the following night.

No surface activity was observed early 25 January, but the seismic network registered small signals. At [1215], the appearance of a green surface slick coincided with an explosive event recognized on the seismographs. The slick was composed of floating patches of foam, volcanic ash, and orange-yellow bits of sulfur with dark grey interiors. Numerous dead fish floated on the ocean surface in and around the slick, their gills choked with volcanic ash.

No further activity was observed at Macdonald until 27 January. At [1400], sprays of water and vapor were observed on the sea surface as Cyana was submerging to study the volcano. These were accompanied by large black bubbles of foam, composed of an emulsion of volcanic ash and gas, iron sulfide, and steam. The bubbles exploded at the surface, emitting a large volume of gas. Degassing from fissures at the base of one of the craters was observed from Cyana at the sea bottom. Similar surface activity occurred again [at 1730-1900], accompanied by short, red flashes.

At [0435] the next day, three green surface slicks formed, accompanied by large explosions that were felt on the support ship. Many explosive events that were registered on board the ship by a 3.5 kHz echosounder were also recorded at the RSP in Tahiti. By the time the eruption ended at [1501], a number of events had been detected by the echosounder during surface activity. Two additional dives by Cyana allowed reconnaissance studies of Macdonald's volcanic structures between 1,500 m depth and the summit, 50 m below sea level. During the eruption, 16 sets of water samples (each consisting of 12 samples from different depths) were taken. While the support vessel was in the area, a hydrophone was deployed, recording all underwater acoustic events. (see figure 1)

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-L. Cheminée, IPGP; P. Stoffers, Christian-Albrechts Univ zu Kiel, Germany; J. Talandier, LDG Tahiti; G. MacMurtry, Univ of Hawaii; H. Richnow, Univ Hamburg, Germany; N. Binard, Lab de Géologie Marin, France; R. Huber, Univ Regensburg, Germany; E. Okal, Northwestern Univ, IL.