June 11, 1998

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

Bench collapse sparks lightning, roiling clouds

Four of us HVO lava junkies had the rare opportunity to witness a partial bench collapse on Monday evening, June 8. The collapse began at 7:40 p.m. when a slab of incandescent lava fell outward from the bench edge into the ocean. The hot rock was fragmented by steam explosions as it hit the sea water, and the steam cloud became abruptly darker as the rock fragments were blasted upward.

The initial explosion disrupted the adjacent bench, enlarging the explosion and darkening the cloud with more debris. Each event led rapidly to another, so that within seconds the collapse progressed from inconsequential to catastrophic. It was during this period that the 15 or 20 visitors near the site recognized a life-or-death situation and, choosing life, moved back to safer ground. Fortunately, no one had ventured onto the active bench.

The collapses continued sporadically for the next two hours as slabs of incandescent lava calved away into the surf. Bench width was diminished by 40 m, or nearly one-half of its initial extent. Small tremors, accompanied by dull, audible thuds, were felt early in the two-hour event, chiefly by people sitting on the ground. The largest tremor was felt by people standing and sitting.

Littoral cone-forming explosions began in the first five minutes and continued through the evening as sea water invaded the core of the disrupted lava flow. The cone grew by small explosions that hurled incandescent bombs and fragmented bedrock upward and outward for tens of meters. The growing cone was positioned at the edge of the bench, so its growth was halted every few minutes as part of it slid away. When the bench collapses ceased, the cone began building in earnest. By 10 p.m. it was 5-10 m high.

Most thrilling to us was the lightning in the eruption cloud, forming jagged spikes that reached down nearly to the ocean from heights of 30 m or more. Accompanying thunder brought no roar but, instead, the sound of popping electric arcs, reminiscent of electronic bug killers that zap when insects fly too close.

Only those clouds darkened by disrupted solid lava sparked with lightning. We suspect that the older solid lava carried the charge imbalance that led to lightning, whereas the hot, spattery lava was perhaps incapable of maintaining electrical charge. This phenomenon probably differs from the lightning characteristic of ash clouds from large explosive volcanoes. That lightning results because the larger ash particles carry a different charge than smaller particles. Perhaps this interpretation explains why lightning was never observed during the cinder-rich high-fountaining episodes from early eruptions at Pu`u `O`o.

The bench remains a dangerous but enchanting site on the Big Island. In the past weeks, surface flows on the coastal plain have draped the low cliffs that once formed an insurmountable backdrop to the active bench. Foolishly, visitors have been climbing down this lava drapery to traverse the bench. Those of us in the volcano business can only ascribe this idiocy to a death wish.

Anyone on the bench during the recent partial collapse would have been engulfed in acidic steam clouds that billowed back from the lava entry to the sea cliff. The steaming white-out prohibits retreat even as explosion debris rains down. And if the collapse is wholesale, opportunity to escape changes to tragedy in a heartbeat.

Eruption and Earthquake Update

The east rift zone eruption of Kilauea Volcano from the Pu`u `O`o vent continued unabated during the past week. The lava flows through a network of tubes to the seacoast and enters the ocean at two locations-Waha`ula and Kamokuna. The public is again reminded that these two areas are extremely dangerous. The National Park Service has restricted access to them because of frequent explosions that accompany collapses of the growing lava bench as described above.

There were no earthquakes reported felt in the past week.