Report on Monowai (New Zealand) — January 1998

Bulletin of the Global Volcanism Network, vol. 23, no. 1 (January 1998)
Managing Editor: Richard Wunderman

Monowai (New Zealand) Acoustic signals from submarine explosions detected

Please cite this report as:

Global Volcanism Program, 1998. Report on Monowai (New Zealand). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 23:1. Smithsonian Institution. http://dx.doi.org/10.5479/si.GVP.BGVN199801-242050.

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Monowai

New Zealand

25.887°S, 177.188°W; summit elev. -132 m

All times are local (unless otherwise noted)


The French Polynesian Seismic Network detected short episodes of acoustic signals from the Kermadec area during both 16-17 January and 31 January-3 February. According to the signal arrival times at various stations, the source was Monowai Seamount. The two events were interpreted as later stages of the eruptive activity of 15-17 December 1997 (BGVN 22:11).

The first episode, which lasted from 1739 on 16 January until 1034 the next day (figure 5), included weak, long, high-frequency tremors. At 0200 on 17 January, the tremors increased in intensity and short explosive waves appeared. The total number of acoustic waves recorded during the event was 69; nine of the waves during the first two hours were interpreted as explosive. The strongest wave, recorded at 0927 on 17 January, was not explosive.

Figure (see Caption) Figure 5. Acoustic waves from Monowai Seamount during 16-17 January (times are GMT). Courtesy of O. Hyvernaud.

The second episode of acoustic activity consisted of 86 waves during 31 January-4 February (figure 6). Explosive waves accounted for 18 of the total. The strongest, at 0250 on 4 February, was not explosive. The main activity was concentrated on 3 February but stopped suddenly.

Figure (see Caption) Figure 6. Acoustic signals from Monowai Seamount during 31 January-4 February (times are GMT). Courtesy of O. Hyvernaud.

Monowai Seamount, also known as Orion seamount, lies midway between the Kermadec and Tonga Islands, ~1,400 km NE of New Zealand. Small parasitic cones occur on its N and W flanks. Numerous eruptions have been detected from submarine acoustic signals since 1977. Surface observations have included water discoloration, vigorous gas bubbling, and areas of upwelling water accompanied by rumbling noises.

Geologic Background. Monowai, also known as Orion seamount, rises to within 100 m of the sea surface about halfway between the Kermadec and Tonga island groups. The volcano lies at the southern end of the Tonga Ridge and is slightly offset from the Kermadec volcanoes. Small parasitic cones occur on the N and W flanks of the basaltic submarine volcano, which rises from a depth of about 1500 m and was named for one of the New Zealand Navy bathymetric survey ships that documented its morphology. A large 8.5 x 11 km wide submarine caldera with a depth of more than 1500 m lies to the NNE. Numerous eruptions from Monowai have been detected from submarine acoustic signals since it was first recognized as a volcano in 1977. A shoal that had been reported in 1944 may have been a pumice raft or water disturbance due to degassing. Surface observations have included water discoloration, vigorous gas bubbling, and areas of upwelling water, sometimes accompanied by rumbling noises.

Information Contacts: Olivier Hyvernaud, BP 640, Laboratoire de Geophysique, Papeete, Tahiti, French Polynesia.