Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 10, October 1995
Ruby Seamount (Northern Marianas)  Submarine eruption

Ruby Seamount
Mariana Islands, W Pacific
15.62N, 145.57E; summit elev. -230 m (submarine)

Ruby is a prominent, active submarine volcano in the Mariana
Arc (2,300 km S of Tokyo) located NW of the Island of Saipan
(figure 9). Although signs of an eruption were first noted
by fishermen about 11 October, initial attempts to confirm
their early observations failed. On 23 October fishermen
reported that they could hear submarine explosions in that
vicinity. A vessel from the Wildlife and Emergency
Management Office of the Commonwealth of the Northern
Marianas Islands confirmed these reports. An Associated
Press news report stated that early on 25 October observers
had seen dead fish and bubbles, and had smelled a sulfurous
odor. On 27 October the Pacific Daily News reported the
eruption site as 15 deg 36 min 22"N, 145 deg 34 min 33"E
(15.6061N, 145.5758E). This spot clearly lies on the edifice
identified by Bloomer and others (1985, p. 215) as Ruby
(only ~1.7 km from the point specified in this report's
heading).

Prior to the eruption, published estimates of the summit
elevation suggested a 230-m depth, a refinement an earlier
estimate of 549 m (Bloomer and others, 1985, p. 215). On 6
October 1995, the Pacific Daily News report stated the
summit was measured at 185-m depth. This newly reported
depth remains unconfirmed. According to Mike Blackford, on
23 October a marine depth finder reportedly measured a depth
of ~60 m. Although this could be a reflection off the
eruptive plume, in the absence of any discussion of
instrument type and calibration, this depth remains
equivocal.

According to Koyanagi and others (1993), the two seismic
stations nearest the eruption were on Saipan (~50 km SE of
Ruby) and Pagan islands (~130 km N of Saipan), both  too
distant to detect subtle seismic effects. Despite the lack
of a nearby seismic station, tremor appeared on seismic
records at the time of the eruption and the next day. Given
the temporal coincidence between the eruption and the
tremor, the two were probably associated.

A fish recovered at the eruption site was found to have
small particles of ash in its gills and HVO researchers
planned to analyze this ash. News of the eruption caused
concern about a possible local tsunami and on 25 October,
the Commonwealth of the Northern Marianas Islands issued an
alert.

       Evidence for Ruby~s active status came from 1966
hydrophone data, followed later by dredging of extremely
fresh volcanic rocks bearing plagioclase, clinopyroxene, and
olivine (Bloomer and others, 1985). The larger and much
shallower Esmeralda Bank (figure 9) has been the site of 6
possible eruptions in a record starting in 1944.

       References: Koyanagi, R., Kojima, G., Chong, F., and
Chong, R., 1993, Seismic monitoring of earthquakes and
volcanoes in the Northern Mariana Islands: 1993 summary
report: Prepared for the Office of the Governor,
Commonwealth of the Northern Mariana Islands, Capitol Hill,
Saipan MP 96950 (revised 21 February 1993), 34 p.

Reference: Bloomer, S.H., Stern, R.J., and Smoot, N.C.,
1989, Physical volcanology of the submarine Mariana and
Volcano arcs: Bull. Volcanol., no. 51, p. 210-234.

Information Contacts: Robert J. Stern, Center for
Lithospheric Studies, University of Texas at Dallas, Box
830688, Dallas, TX 75083-0688 USA (Email:
rjstern@utdallas.edu); Robert Koyanagi, USGS Hawaiian
Volcano Observatory, Hawaii Volcanoes National Park, HI
96718 USA; Ramon C. Chong, Commonwealth of the Northern
Mariana Islands (CNMI), Disaster Control Office, Capitol
Hill, Saipan, MP 96950 USA; Mike Blackford, Pacific Tsunami
Warning Center, 91-270 Fort Weaver Road, Ewa Beach HI 96706,
USA; Associated Press; Pacific Daily News.