Hydrophones recorded a submarine eruption from Macdonald Seamount 29 May 1967 (Johnson, 1970). After more than 10 years of apparent inactivity, acoustic waves (T-phase) from six eruptions were detected by French Polynesian seismic stations (at Tubuai, 23.3°S, 149.5°W; Tahiti, 17.6°S, 149.5°W; Moorea, 17.5°S, 149.8°W; Rangiroa 15.0°S, 147.7°W; Hao, 18.1°S, 141.0°W; and Rikitea, 23.1°S, 135.0°W) between December 1977 and February 1981. The acoustic record of each eruption began with intense explosive signals, followed by a few hours to a few days of amplitude-modulated noise that was frequently punctuated by brief periods of additional explosive activity, less commonly by strong (but apparently non-explosive) increases in noise amplitude.

The first eruption, which was the longest and most vigorous, was first detected at 1630 on 10 December 1977 and returned continuous noise for about 92 hours, during which nearly 50 periods of explosive activity could be recognized. Occasional bursts of noise continued for another 30 hours before activity ended at about 1830 on 15 December. A strong explosion recorded at 0246 on 30 September 1979 was followed by fewer than 5 hours of diminishing activity. About 20 minutes of frequent explosions first recorded at 1330 on 12 February 1980 were followed by roughly 12 hours of eruption noise. Explosions that began to appear on the records at 0109 on 10 November 1980 remained strong and frequent for about 7 hours and eruption noise continued for about 15 additional hours. On 24 December 1980, seismic instruments detected intermittent explosions from 0610 until roughly 0900, continuous eruption noise for about 4 more hours, then intermittent noise that lasted until early the next morning. Explosions at 0611 and 0620 on 15 February 1981 were followed by about 12 hours of eruption noise. No additional activity had been recorded as of late October.

Macdonald was discovered after the 1967 eruption and bathymetric work in December 1973 defined a submarine edifice reaching to within 49 m of the ocean surface (Johnson, 1980). More recent bathymetry by the French National Marine vessel La Paimpolaise indicates that further growth of Macdonald has occurred, bringing its summit to 23 m below sea level.

Reference. Johnson, R.H., 1970, Active submarine volcanism in the Austral Islands: Science, v. 167, p. 977-979.

Further Reference. Talandier, J. and Okal, E.A., 1982, Crises sismiques au Volcan Macdonald (Ocean Pacifique Sud): C.R. Acad. Sci. Paris, ser. II, v. 295, p. 195-200.

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.