Kīlauea in August 1998: Pauses, Lava Tubes and a Growing Pit at Puʻu ʻŌʻō

In August 1998 the long eruption of Kīlauea on Hawaiʻi's Big Island was in its 55th episode. Episode 55 had begun in February 1997, and its lava reached the sea that August. Since then the eruption had settled into a routine: lava rose quietly from vents on the southwest flank of the Puʻu ʻŌʻō cone and flowed 12 kilometers to the coast through lava tubes. But, as the USGS Hawaiian Volcano Observatory (HVO) pointed out in its update of August 19, 1998, even an apparently stable eruption changes from day to day.

Puʻu ʻŌʻō fuming, with the steam plume of the East Kamokuna ocean entry 12 kilometers away. Image from the USGS Hawaiian Volcano Observatory.

Pauses

The biggest changes came when lava stopped flowing altogether. These pauses struck at random, weeks or months apart, and lasted from hours to days. Their cause was poorly understood, but they ran a similar course:

• The steam plumes at the coast shrank over a few hours as the tubes drained into the ocean, the first sign the supply had stopped.
• Magma kept rising from the mantle into the reservoir a few kilometers under Kīlauea's summit, and with no outlet the summit swelled. HVO's tiltmeters measured the tiny change in slope. One microradian of tilt is about what a dime under one end of a kilometer-long beam would produce.
• Meanwhile, pieces of roof and wall fell into the empty tubes. When lava returned, it met these blockages and broke out onto the surface, often through skylights, and the breakouts crept downhill as the tube refilled. A breakout that ran for days could build a new tube.

The 19th pause of episode 55 began late on July 16 and lasted 53 hours, during which the summit inflated 6 microradians. When lava returned, it broke out in several places and covered 135 hectares with fresh flows. The 20th pause, starting August 12, lasted about 42 hours; again flows broke out on and below the Pulama pali, the largest from the same spot as in July.

Surface flows from the July 19 breakouts, about 3 kilometers from the coast to the base of Pulama pali. The August breakouts resurfaced much of this area. Image from the USGS Hawaiian Volcano Observatory.

July 23: a channel from a July 19 breakout, already partly roofed over, the first stage of a tube. Image from the USGS Hawaiian Volcano Observatory.

August 6: the same flow has become a tube, its skylights a chain of orange beads. Image from the USGS Hawaiian Volcano Observatory.

A pit eats the cone

On the southwest flank of Puʻu ʻŌʻō, several collapse pits were merging into one, 155 by 185 meters. A similar pit had begun on the west flank in 1993; its collapse ended in January 1997 with the failure of the cone's west wall, which took off the summit and left a deep notch.

The composite collapse pit is undermining the southwest side of Puʻu ʻŌʻō. Image from the USGS Hawaiian Volcano Observatory.

Earthquakes and the ocean entry

Dozens of earthquakes shake Kīlauea every month, a few large enough to feel, and HVO monitored them all with 61 seismometers across the island, along with the constant tremor of magma moving under the vent.

With the Wahaʻula entry declared dead on July 11, all the lava reaching the sea entered at East Kamokuna. There, new land formed as a lava bench, and it was dangerous: benches collapse without warning every few weeks, losing several hectares at a time, and when masses of red-hot rock hit the water, violent explosions can follow.

Sources

• USGS Hawaiian Volcano Observatory, "Photo and Video Chronology – Kīlauea – August 19, 1998," eruption update; rewritten in hubnx's own words. Photos and map from the USGS.