Volcano Watch — Eleven years of activity at Kīlauea volcano - part II

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Editor's note: The following is part two of an 11-year retrospective look at Kīlauea Volcano - October 1992 through the end of 1993.

Changes at Pu‘u ‘O‘o

Since the eruption shifted to Kupaianaha in 1986, the Pu‘u ‘O‘o conduit has enlarged from 20 meters to a gaping crater 300 meters across. At the same time, the Pu‘u ‘O‘o cone has been reduced in height by about 20 meters, due to collapse of the unstable structure. A lava pond has been active at the bottom of the crater throughout much of the last several years, but no lava flows have originated from Pu‘u ‘O‘o since 1986. The active pond occupies about a third of the crater floor and varies in height from about 30 to more than 75 meters below the lowest point on the crater rim.

Episode 52, More of 51

A new eruptive fissure opened on the night of October 2, 1992, in response to a magnitude 4.3 earthquakelocated 7 kilometers beneath Kīlauea's south flank between the eruption site and the ocean. The two main vents on the fissure fed a vigorous pahoehoe flow that turned to channeled ‘a‘a as it advanced to the south. The episode 51 vents were in repose at the time of the earthquake, but on the following day, they resumed erupting. Both the 51 and 52 vents were active for the next three days, and then the 52 vent began to wane. By mid-October, the 52 vents were dead, while 51 continued to erupt.

The earthquake that initiated episode 52 apparently altered the plumbing to the episode 51 vents. Following the earthquake, the activity became more constant. As a result, the lava tube leading to the south edge of the 51 shield extended in October to the top of the steep slope above the coast, and surface flows advanced rapidly downslope. By November 8, lava had crossed the Chain of Craters Road and entered the ocean at Kamoamoa. In December, virtually all of the lava erupted at the 51 vents was fed directly to the ocean via lava tubes. At the coast, flows buried the Kamoamoa campground, picnic area, and the new black sand beach formed earlier in this eruption by Kupaianaha flows entering the ocean farther to the east. A new lava delta extended 300 meters out to sea and formed 60 acres of new land. On the evening of November 24, spectacular steam explosions formed a 7-8-meter tall littoral cone where lava entered the sea. This cone has since been partially buried by subsequent flows. The lava delta has begun to break apart with the formation of cracks parallel to the coastline. On December 26, a piece of the lava delta slid into the ocean.

On January 3, 1993, Kīlauea again paused, perhaps to celebrate the tenth anniversary of the eruption, but activity resumed the following day and by the 5th, the volume was back to normal (50-100,000 cubic meters per day). At about the time of this brief pause in activity, a number of new collapse pits formed on the west flank of Pu‘u ‘O‘o. Late on the night of February 14, magma began to intrude into the upper East Rift Zone near Makaopuhi Crater. Soon afterward, eruption tremor recorded near Pu‘u ‘O‘o decreased in amplitude and by 4:00 a.m. had reached background levels. A series of steam explosions shook the coastal lava delta as seawater entered the drained tube. The next night, the floor of Pu‘u ‘O‘o collapsed. The eruption would remain shut down for nine days.

Episode 53 - Yet More of 51

The volume slowly increased after activity resumed on February 16, but did not reach high levels until the 20th, when a new vent opened adjacent to the main episode 52 vent. Fountain heights increased to 50 feet by the following day, and the lava ponded over a broad area at the base of the episode 51 shield. On the 23rd, the ponded lava had overtopped a skylight in the old episode 51 tube, and lava from both the episode 53 and 51 vents commingled in this tube and was transported toward the coast. Another small collapse pit formed on the west side of Pu‘u ‘O‘o on Februry 23.

Throughout the remainder of 1993, lava has erupted from both the episode 51 and 53 vents, and it has flowed to the ocean through the old episode 51 tube. There have been occasional breakouts along the pali, and the tubes have undergone collapse leading to new surface flows and new ocean entry points. The volume has been high since episode 53 began, but has also been variable, with high volumes of perhaps 300,000 cubic meters per day and lows of about 150,000. No additional structures have been destroyed, although more archaeological resources in Hawai‘i Volcanoes National Park have been buried, particularly at Lae‘apuki.

On April 19, a visitor to Hawai‘i Volcanoes National Park was killed when the lava bench he was standing on collapsed into the ocean. The same event injured 22 other visitors when steam explosions scattered incandescent lava blocks on the lava delta by the ocean entry. All the injured visitors and the one person killed were beyond "area closure" signs posted by the National Park Service to keep people safely away from the unstable lava benches along the ocean. During the remainder of the year, additional lava benches collapsed into the ocean, but fortunately, no one else was injured.

Summary

Since the Pu‘u ‘O‘o-Kupaianaha eruption began, about 1,100 million cubic meters (1.1 cubic kilometers) of lava has erupted. (In comparison, Mauna Ulu, the previous longest-lived historic eruption on the East Rift Zone, erupted about 350 million cubic meters of lava between 1969 and 1974.)

The eruption has destroyed 181 homes and a number of other buildings, with total losses of about $61 million. As long as the eruption is confined to the episode 51 and 53 vents, it poses no immediate threat to residential areas, but it continues to impact natural resources in Hawai‘i Volcanoes National Park.

One year ago, we suggested that the relatively low extrusion rates might indicate that the eruption was winding down. The trends from mid-1991 to early-1993 were toward progressively smaller eruptive volumes and migration of the active vents toward the summit. The activity that began in February clearly reversed this trend, and the volumes are now nearly as high as they were at any time during the 11 years of the eruption.

The east with which magma returns to Pu‘u ‘O‘o following pauses suggests that this eruption will not stop easily. As soon as a pause begins, magma begins to accumulate beneath the summit, and pressure slowly builds up until the magma finds the weakest exit, which has always been the same conduit to Pu‘u ‘O‘o. To end the eruption, the magma pressure at the summit must remain low for long enough that the conduit feeding magma to Pu‘u ‘O‘o can cool.

The last long-lived rift eruption, at Mauna Ulu from 1969-1974, stopped following an eruption on the southwest rift zone in 1974 and after the magnitude 7.2 Kalapana earthquake in 1975. Both events depressurized the summit reservoir. This is not to say that this eruption will follow the same script, but similar large events may have to occur before this long-lived eruption will end. Predicting when, and even where, eruptions will start is easier than predicting when and how they will end.