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Volcano Watch — Metis Shoal, Tonga

June 23, 1995

The formation of new islands invariably sparks the imagination. On June 6th a submarine eruption began at Metis Shoal in the Tonga Islands. A passing ship confirmed that an eruption was continuing on June 9th.

The formation of new islands invariably sparks the imagination. On June 6th a submarine eruption began at Metis Shoal in the Tonga Islands. A passing ship confirmed that an eruption was continuing on June 9th.

Also on June 9th, the New Zealand Hydrographic Office issued a navigation warning to all shipping in the area. A more recent report indicates that the eruption has built a small island and that the eruption is now subaerial. The island is apparently between 200 and 500 meters across; a cone has formed that reaches 50 to 80 meters above sea level. Ash columns are small, reaching only 500 meters high. The activity is centered at the southeastern corner of the island.

Metis Shoal has erupted nine times since 1851. The most recent prior eruption was in 1979, and the eruption before that was in 1967-1968. On at least three previous occasions, in 1858, 1967-68 and 1979, eruptions produced small islands. The island produced in 1967-68 attained a size of 700 by 100 meters and a height of about 15 meters; it disappeared within a month after the eruption was over.

The island formed in 1979 attained a size of 300 by 120 meters and a height of about 15 meters; it, too, disappeared within a few months of the end of the eruption. The present eruption has already produced a much higher island that should last longer than the previous attempts at island-building.

In 1979, the first evidence that an eruption was underway was a large pumice raft located northwest of Metis Shoal. Shallow submarine eruptions commonly produce large volumes of pumice that float on the ocean surface, sometimes so closely packed that the raft looks like an island. This apparently occurred at Metis Shoal during the 1979 eruption, when pilots reported that the newly formed "island" appeared to be 3 kilometers in diameter on June 21, 6 kilometers in diameter on June 22, and 16 kilometers in diameter on June 24.

In mid-July, scientists investigating the eruption site found the island to be less than a kilometer across. The pilot had observed an expanding pumice raft. These pumice rafts can travel great distances before the pumice fragments finally sink or wash ashore.

Occasionally, such pumice rafts are seen in Hawaiian waters where they may be mistaken for local submarine eruptions. Several historical submarine eruptions around the Hawaiian Islands, such as one reported betweeen O'ahu and Kaua'i in 1956, may actually have been such rafts from distant eruptions. Part of the 1956 raft was reported to have washed up on the Waianae coastline, but we have been unable to locate a fragment in any of the archives in Hawaii.

A chemical analysis of such material would easily distinguish the pumice of an undersea Hawaiian eruption from pumice produced elsewhere. If you have a piece of the 1956 pumice that washed ashore on O'ahu, we would be interested in hearing from you.

Here in Hawai'i, Lo'ihi Seamount grows closer to sea level with each eruption. Eventually, it, too, will grow above sea level, although this passage probably will not occur for several hundred thousand years. There have been some estimates of its emergence that are much sooner. Our estimate is based on the combination of slow lava accumulation from small, infrequent eruptions, rapid sinking of the entire Island of Hawai'i as Lo'ihi grows on its flank, and landslides, which reduce Lo'ihi's height.

What we do know is that, as Lō‘ihi approaches sea level from its present summit depth of about 970 meters, the eruptions will become more explosive and that small islands may form and be destroyed by wave erosion several times before the island becomes established.

Our County Civil Defence responded to the Volcano Surveillance group at the Institute of Geological and Nuclear Sciences in New Zealand about the current eruption in Tonga by providing information about LAZE (LAva haZE), produced by the interaction of hot lava and sea water. LAZE is a mix of strong hydrochloric acid and steam. It is of concern at Metis Shoals because boat tours are being run to the area. A more severe hazard from shallow water eruptions is that sea water with abundant gas or steam bubbles may become so low in density that it will not support the weight of a boat. Many years ago, a Japanese research vessel sank when it steamed over a shallow submarine eruption in the western Pacific. Since Metis Shoals has already constructed a volcanic cone above sea level, this hazard is no longer a concern.