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Volcano Watch — New Tongan island exemplifies potentially explosive shallow marine volcanism

November 22, 2006

A new Pacific island breached the ocean surface in August 2006, thanks to the eruption of the Home Reef volcano in the Tonga island chain. Even before land had built above sea level, the eruption was recognized by the presence of pumice rafts floating on the ocean.

Pumice rafts from an island-building eruption of the submarine Home Reef volcano. Photo by Fredrik Fransson.

Pumice is frothed volcanic glass that is buoyant because of the numerous encased voids or bubbles created by gases trapped in the magma. The pumice rafts form extensive sheets several tens of centimeters thick that float on the ocean's surface. Pictures of the floating pumice can be found by an Internet search with the words "pumice," "raft," and "Home." An easy portal to those recent pictures is the VolcanoWorld Web page.

Eruptions at the ocean-land interface are of great interest to volcanologists because they bring an added hazard--explosions. Liquid water expands greatly as it is heated to steam. Add to that some high eruptive rates for high-heat input, and one has the perfect catalyst for fueling an explosive eruption. Deep-sea eruptions, in contrast, tend to be passive. The confining pressure of the overlying water column limits the expansion potential.

Most Hawaiian residents probably look toward Lo`ihi when thinking about volcanoes or vents in the ocean setting. Lo`ihi is the submarine volcano growing 30 km (19 mi) south of the Island of Hawai`i. Its summit still lies about 980 m (3220 ft) below sea level. Although not currently erupting, it has been active several times in the past century. But Lo`ihi is the least of our concerns for the next nearshore eruption. It won't grow to sea level for another 120,000-160,000 years, an estimate derived by applying known growth rates for Kīlauea and Mauna Loa, which are in the range 6-8 m per 1,000 years. (School teachers: this growth-rate solution might be an interesting math problem for an 8th grade algebra course.) Besides, Lo`ihi's location places it at a great distance from shore and outside common shipping lanes.

More daunting might be eruptions along the shallow-water part of rift zones emanating from Kīlauea and Mauna Loa. The most likely place would be east of Hawai`i Island's easternmost point, Cape Kumukahi. Beyond 1-3 km (2 mi) from the coast, the water depth is great enough that an eruption would probably not result in explosive activity, unless the eruption lasted long enough to build a cone that brought lava close to sea level. But areas within 3 km (2 mi) are well within the range of scouring explosive blasts of ballistics and ash.

Fortunately, events like these bring advance warning. The abrupt onset of numerous small earthquakes should precede any volcanic activity and would be monitored by the Hawaiian Volcano Observatory.

The probability of nearshore eruptions diminishes elsewhere along the Hawaiian island chain, simply because the frequency of volcanic activity is substantially diminished. Looking beyond Kīlauea and Mauna Loa, the volcanoes Hualālai, on Hawai`i, and Haleakalā, on Maui, have the greatest potential for nearshore volcanic activity. Each has erupted several times in the past 1,000 years and each has rift zones that extend into the ocean. Haleakalā's coast has volcanic cones that contain evidence of past offshore explosive volcanism. For these two volcanoes, however, the abundance of vents decreases greatly near the shore, which reduces the probability of a hazardous offshore eruption.

The other islands are not free of hazard, although the threat becomes so small that its discussion serves more to remind us that eruptive activity could occur again than to raise a warranted concern. O`ahu, for example, has had sporadic volcanism throughout the past 1 million years, with the most recent activity producing the Koko Head spatter cones and, in consequence, Hanauma Bay, about 100,000 years ago.


Volcano Activity Update

This past week, activity levels at the summit of Kīlauea Volcano have remained at background levels. The number of earthquakes located in the summit area is low (usually less than 10 per day that are large enough to locate). Widening of the summit caldera, indicating inflation, stopped in early October.

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. Lava is fed through the PKK lava tube from its source on the southwest flank of Pu`u `O`o to the ocean. About 1 kilometer south of Pu`u `O`o, the Campout flow branches off from the PKK tube. The PKK and Campout tubes feed two widely separated ocean entries, at East Lae`apuki and East Ka`ili`ili, respectively. Both entries are located inside Hawaii Volcanoes National Park.

In the last week, intermittent breakouts from the Campout tube have occurred midway down Pulama pali and across the coastal plain to within a kilometer of the coast.

Access to the sea cliff near the ocean entries is closed, due to significant hazards. The surrounding area, however, is open. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

There were three earthquakes beneath Hawai`i Island reported felt within the past week. Only one of these quakes was an aftershock of the October 15 magnitude-6.7 earthquake. A magnitude-2.7 aftershock occurred at 3:15 p.m. H.s.t. on Thursday, November 16, and was located 14 km (9 miles) northwest of Hualālai summit at a depth of 12 km (8 miles). A magnitude-2.6 earthquake occurred at 2:19 p.m. on Saturday, November 18, and was located 12 km (7 miles) southeast of Waimea at a depth of 13 km (8 miles). A magnitude-3.0 aftershock occurred at 6:19 p.m. also on Saturday and was located 5 km (3 miles) southwest of Honoka`a at a depth of 14 km (9 miles).

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano's summit (one earthquake was located). Extension of distances between locations spanning the summit, indicating inflation, continues at slow rates.

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