Harmon Craig provided additional observations of the 11-12 October eruption. When the RV Melville first arrived . . . on 11 October, bright green discolored water had spread in a crescent-shaped zone ~2.5 km wide that extended 1.5 km from a point source. The water remained discolored during the 2 days that the ship was in the area. Episodes of erupting gas bubbles were most intense during the afternoon, with bubbles rising sporadically at 10-minute to 1-hour intervals. Following eruptions of bubbles, an area of chocolate-brown water, perhaps 100 m across, appeared in the center of the green discolored water. Craig saw a total of ~30-40 basaltic rocks brought to the surface by gas bubbles. The depth to the summit was 30-100 m; the summit had a flat area at 100 m depth and pinnacles rose as much as 70 m above this surface.

The RSP reported that a seismic crisis began again on 14 October at [1200]. Continuous intense noise was recorded until the onset of explosive phenomena at about [1730], persisting until [0320 the next morning] when it was followed by weak but continuous activity that was ongoing as of 16 October.

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: H. Craig, Scripps Institute of Oceanography; J. Talandier, LDG Tahiti.