Is this foretelling the start of an eruption?

(Editor's Note: This article was written by scientists at the U.S. Geological Survey's Hawaiian Volcano Observatory. UniSci appreciates being able to bring it to its readers, and almost wishes we lived near that volcano just to read their newsletter regularly.)

Indeed, steam often does rise from the ground before lava comes out. We have excitedly observed that happening several times as an erupting fissure along Kilauea's east rift zone gradually lengthened by splitting open the ground.

The steam is generated by ground water that is heated just ahead of rising magma. Other gases may also be emitted in advance of a growing fissure, methane being the most common. Methane is also released from the ground, sometimes explosively, near the edges of lava flows moving through vegetation.

In most cases, however, the new steam reported on residential or farm land is not related to anything volcanic. Instead, the steam, sometimes accompanied by methane, records rotting vegetation just below the ground surface.

Many of you are familiar with compost piles and with the precautions that must be taken to prevent them from smoldering and even bursting into flame. The same thing can occur in areas of landfill, land clearing, or even completely naturally.

Dead vegetation rots, and some of the chemical reactions that take place release heat. With proper ventilation, the heat is dissipated to the air without any problem. A dense mat of rotting vegetation may, however, trap the heat. That is why it is always a good idea to keep your compost pile rather loose and airy.

Many of the reports of new steaming holes come from locations 200-300 m (a few hundred feet) or more about sea level, where the mornings are cool. Warm air escaping from the ground may condense to steam on such mornings and become visible to onlookers.

Many of the reports also come from areas that were cleared by bulldozers or other heavy equipment that shoved vegetation into holes in the ground surface. The buried vegetable debris forms a kind of compost pile, which rots and heats. Most of the heat is lost passively, but if the pile is dense, or if it is trapped within a small lava tube, tree mold, or natural ground crack, it may build up and become quite warm.

Only rarely does a fire result. Noticeable amounts of steam can be generated, however, if the ground surface is broken by collapse of weak soil, cave-in of a lava tube, or small explosions resulting from increased pressure. More often, the pressure simply drives the warm, light gas upward through pores in the soil. In any case, there may be enough steam to be quite evident on cool or humid mornings and days.

There are several ways that we can check a caller's observations to see whether an eruption might be coming. The seismic monitoring done by HVO is the first thing we fall back on. Lacking seismicity in the general area of the report, it is very unlikely that magma is nearing the surface, either rapidly or slowly.

Check our website for up-to-date seismic information for your part of the island.

Secondly, we consider the location. Is the steam coming from one of the rift zones or is it on the flank of one of the island's volcanoes far from most volcanic vents?

Thirdly, we ask the caller to describe the nature of the steam source; is it a hole, a long crack freshly cutting the ground surface, a skylight in the roof of a small tube, etc. Of these possibilities, only the new long crack would be a possible cause for concern.

Finally, we can visit the site and take samples to analyze for volcanic gas if necessary.

Backyard steam is mostly benign and unlikely to be a harbinger of something volcanic without other evidence of unrest.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lava moves away from the vent toward the ocean in a network of tubes and descends Pulama pali in two separate areas.

Breakouts from the tube system above Pulama pali fed ephemeral flows on the pali. Small surface flows, primarily ooze-outs from inflated areas in the coastal flats, are occasionally observed. Lava continued to enter the ocean in the area east of Kupapa`u throughout the week.

The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the new land. The steam cloud is extremely hot, highly acidic, and laced with glass particles.

Swimming in the area can be a blistering or even deadly experience.

One earthquake was reported felt during the week ending on August 2, 2001. A resident of Papa`aloa felt an earthquake at 11 minutes after midnight on July 29. The magnitude-2.7 temblor was located 9 km (5.4 mi) west of Honoka`a at a depth of 35.84 km (21.5 mi). There were no earthquakes from Lo`ihi Volcano during the past week.

[Contact: Carolyn C. Bell]

13-Aug-2001