Volcano Watch — Kīlauea eruption status, September 1, 1995

The long-lived eruption on Kīlauea's East Rift Zone took a short break starting Tuesday, August 22. Before the pause in activity, we observed that the lava pond inside Pu'u 'O'o Crater was unusually deep (more than 310 feet below the rim).

In addition, starting about noon, our instruments began to record changes at Kīlauea's summit that included an increase in seismic tremor (ground vibration caused by magma movement) and a deflation, or downward tilt, of the caldera.

We have observed the same pattern before previous pauses in the eruption, and therefore anticipated the pause in the eruption.

At about 10 p.m., the eruption shut down, as determined from additional changes in seismic activity. The following morning, no lava was entering the ocean at Highcastle, and no active flows could be found on the surface.

By midday on August 23, the summit had begun to reinflate, or tilt upward, and the tremor beneath the summit subsided to background levels.

On Friday, August 25, several small swarms of earthquakes occurred beneath the upper East Rift Zone in the vicinity of Pauahi Crater; all the earthquakes were too small to be felt. By 10 a.m., lava was again flowing from a breakout point in the old lava tube at about the 2,200-foot level. This flow was a small, channelized pahoehoe flow that advanced down the eastern side of the Kamoamoa flow field.

By Saturday, several additional breakouts had occurred at elevations of about 2,150 feet, 2,010 feet, and 1,700 feet. The lowest breakout was the largest and fed an a'a flow that had advanced to about 700 feet elevation by the end of the day. This flow lobe was igniting fires as it advanced into vegetated areas between the Waha'ula and Kamoamoa flow fields.

The eruption restarted slowly and the volumes did not reach pre-pause levels until at least Monday. The pond in Pu'u 'O'o apparently rose dramatically during the pause and during the next few days when the eruption was sluggish. Spatter was deposited on the rim of the cone, perhaps when a part of the wall of the crater collapsed into the pond.

By Thursday, August 31, this eastern flow had advanced down to 250 feet and was characterized by open channels from near the breakout point at the top of Pulama pali. On Friday, the pond in Pu'u 'O'o was below a newly-formed crust created when the pond rose during the pause. The active pond was very small and lava streamed from west to east.

The flow near the eastern side of the Kamoamoa flow field had advanced to the 175-foot level, about 3/10 of a mile above Pali'uli. This flow should reach the ocean near Kamoamoa within the next week or so if it continues as the main flow lobe. In the meantime, the VOG (Volcanic smOG) should be less intense since there is no LAZE (LAva haZE) component produced where lava enters the ocean.

The pauses in the eruption appear to be caused by a rapid depressurization of the summit magma chamberthat occurs when magma intrudes into the upper East Rift Zone. As magma rapidly flows out of the summit reservoir, tremor is produced, and the summit deflates (the ground surface drops).

These small intrusions do not have enough force to push to the surface and produce an eruption in the upper East Rift Zone. After the intruded magma stagnates within the rift zone, the tremor subsides, and the summit begins to inflate as additional magma from deeper in the system repressurizes the summit magma chamber.

Each of the intrusions that triggers eruptive pauses at Pu'u 'O'o is a potential eruption elsewhere on the volcano. At some future time, a similar, but more forceful, intrusion could lead to the breakout of a new eruptive fissure. The most likely places are where we see such intrusive events again and again. These sites are located near Pauahi Crater and just west of Makaopuhi Crater. Such an event could well signal the end of the Pu'u 'O'o eruption.

Near the end of the long-lived (1969-1974) Mauna Ulu eruption, several new fissures formed in the upper East Rift Zone, within Kīlauea's summit, and along the Southwest Rift Zone. Activity returned to Mauna Ulu after the outbreaks in the upper East Rift Zone and in the summit.

The Southwest Rift Zone eruption was quite voluminous and depressurized the summit magma chamber to such a degree that the pressure had not built up enough to cause a new eruption before the 1975 Kalapana earthquake disrupted the underground magma storage system. Pu'u 'O'o may go through a similar sequence of stops and starts, interrupted by eruptions elsewhere on Kīlauea, before activity finally ceases.