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September 23, 1994

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


The Rabaul Eruption

Rabaul caldera, an active volcano located on the northern tip of New Britain island in Papua New Guinea, erupted violently this past week. As of Friday morning, the eruption continues. The eruption has disrupted communications,and visibility and access are necessarily limited due to the ashfall, so many reports are unconfirmed. A few things are clear: this is a major eruption; and by Friday, the town of Rabaul, similar in population to Hilo, had been nearly totally destroyed.

Map of Rabaul caldera

The U.S. Geological Survey's Hawaiian Volcano Observatory and the Center for the Study of Active Volcanoes at the University of Hawaii at Hilo are indirectly involved with this eruption because many of the staff at the Rabaul Volcano Observatory were trained here in Hawaii. In addition, within the next week, the U.S. Geological Survey, as part of their Volcano Disaster Assistance Program, will be sending personnel to install seismic stations to replace those destroyed during the eruption.

The eruption began on September 18 after less than a day of intense seismicity and the Rabaul Volcano Observatory and the Papua New Guinea Emergency Services immediately declared a code red stateof emergency and evacuated the town of Rabaul. Because they had a volcanic disaster response plan, the evacuation took place prior to the start of the eruption. About 30,000 people were evacuated from Rabaul to the nearby towns of Kerevat and Kokopo before the eruption began. Two casualties have been reported, a boy hit by a truck involved with the evacuation, and a man hit by lightning from the ash cloud, but the casualty list is expected to grow.

The first vent to erupt was the Tavurvur cone on the east side of the caldera just before dawn (estimates range from 3:00 to 6:00 A.M.); it was followed by eruptions from Rabalanakaia, on the northeast side of the caldera. The eruptions were relatively mild at first, but were followed within a few hours by strong eruptions from Vulcan on the west side of the caldera. The initial Vulcan explosions sent an ash plume as high as 60,000 feet. The ash plume has deposited ash mainly to the west and southwest. The amount of sulfur dioxide emitted, as measured by satellite on the 20th, is 80,000 metric tons, again confirming that this is a large eruption.

At times on September 19, there may have been as many as five active vents along the caldera rim, including several that began below the sea. These secondary vents line up along the faults that bound the caldera. At Rabaul, the caldera encompasses the entire bay. However, most of the activity since September 19 has been centered at the Vulcan and Tavurvur vents. Within hours of the beginning of the eruption, heavily populated Matupit Island in the center of the caldera had sunk beneath the sea. Most of the bay is covered with floating pumice and at least part ofthe harbor has been filled in with volcanic material.

The ash clouds have triggered heavy rains and intense lightning. The rains have soaked the thick ash that has fallen on buildings, and the combined weight has collapsed an estimated 80% of the buildings in Rabaul. The ash is estimated to be at least 2.5 feet thick throughout the city with other areas having as much as 5-7 feet of accumulated ash. The heavy rains are causing flooding and mudflows in the surrounding areas. By the 21st, the eruption seemed tobe waning and the ash plume reached only to 20,000 feet elevation. However, ash has been falling on the Papua New Guinea mainland, more than 400 miles to the southwest. The people evacuated to Kerevat have now been evacuated farther away as ashfall had gotten quite heavy there. International air traffic has been diverted around the expanding ash column.

This eruption is similar to those at stratovolcanoes in the Pacific ring-of-fire in that the magma is rich in dissolved gases and is also pasty and thick. The expanding gases cannot easily escape and fragment the magma into minute particles. In contrast, Hawaiian lavas are poor in dissolved gases and the magma is fluid so the expanding bubbles can rise through the magma easily, thereby producing our more subdued eruptions. A caldera is a circular to elliptical collapse feature on a volcano. It overlies an underground magma chamber and downdrops when magma is removed from the chamber during a large eruption. For example, Kilauea's summit caldera overlies an underground magma chamber located a few kilometers below the surface; the present caldera may have formed during the last large explosive eruption of Kilauea in 1790.

On Thursday, the eruption intensified once again, although the ash plumes are not as tall as before. The maximum column height is 16,000 feet. Both Vulcan and Tavurvur are sending up plumes of ash, although the plume from Tavurvur is whitish in color whereas that from Vulcan is blackish-grey. The Vulcan cone is the source of hot pyroclastic surges that have now extended as far as 1.25-3 miles from the vent towards the northeast. In Kokopo, the main evacuation center, stores reportedly had run out of food by Thursday.

The town of Rabaul was located within the caldera of anactive volcano. The people who lived there were reminded of the inevitability of an eruption by intense earthquake activity and uplift of the ground within the caldera in the mid-1980's. Such activity in large calderas is called resurgence and describes the renewed uplift of the previously downdropped caldera floor. However, despite warnings and a declared stage-2 emergency in 1983 and 1984, Rabaul did not erupt and, in fact, activity waned and remained at low levels until hours before the latest eruption broke out this past week. The previous eruption, which lasted from 1937 to 1943, killed about 500 people. Vulcan and Tavurvur were the main active vents in that eruption as well. The Vulcan vent formed during an earlier eruption in 1878.

This eruption has several important lessons for volcanic hazards and riskmanagement. First, all large explosive eruptions are not preceded by weeks to months of increasingly intense seismic activity, as was observed at Mt.St. Helens and Pinatubo. The period of seismic activity at resurgent calderas may precede the eruption by many years and be followed by a period of quiescence and then the eruption. This may be particularly important in the United States where resurgent calderas occur at Long Valley, Yellowstone, and in several remote areas of Alaska. Such calderas, because of their low relief and infrequent activity are difficult to identify and more are being discovered each year. The second lesson is if we build cities and communities in areas with severe volcanic hazards, disasters not only can, but will, occur.


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Updated: 26 March 1998