Volcano Watch — A lull in Kīlauea's eruption

The nearly 12-year-long eruption along Kīlauea's east rift zone took another brief vacation this past week. This was the sixth pause in eruptive activity this year, and the third since the beginning of October.

At about noon on Tuesday, November 29, the flow of lava apparently stopped, or slowed to a trickle. The tube system was nearly empty the following morning, and the usual bright orange glow seen in the skylights had turned a dull orange. However, slightly before midnight on Wednesday, the tremor, or ground shaking related to magma movement, had increased near Pu'u 'O'o, signaling that the eruption had recommenced.

On Thursday afternoon, two pahoehoe lava flows were active above the top of Pulama Pali, and another small one was active along the western edge of the flow field at about the 550-foot elevation. The largest flow issued from the break-in-slope at the top of Pulama Pali at about the 1,900-foot elevation. This outbreak was feeding lava into a channelized 'a'a flow that was cascading down the face of the pali. On Thursday afternoon, lava had not reoccupied the tube below the 550-foot breakout, and no lava was entering the ocean. Later that evening, small flows broke out below Paliuli.

The total volume seen on Thursday was small compared to that observed before the pause in activity. Unfortunately, we were unable to measure the volume because the military very-low frequency radio signals we utilize were not in operation at the time. The rate of lava flow in the tubes, which we now measure with a radar-gun similar to that used to measure the velocity of pitches in baseball, was about 80-90 percent of that measured before the pause. Visual estimates of the level of the lava in the tube suggest that the volume was on the order of a few hundred thousand cubic yards per hour, compared with volumes perhaps three times greater before the pause. The eruption rate appears to be gradually increasing, in contrast to the abrupt start-ups we have seen after most previous pauses.

This pause was the first that we were able to predict before it took place. Following the last pause in late October, we recognized that the pause was preceded by a small sharp decrease in the tilt record at the summit. Tilt is a measure of the slope of the ground.The slopes of the ground surface around the summit change slightly as the summit inflates or deflates, like a balloon, as magma accumulates or is discharged from the summit magma reservoir. We can measure these changes in ground tilt accurately enough that we can see changes as small as would be caused by slipping a flat dime beneath one end of a half-mile-long board.

At about 4:00 pm Tuesday afternoon, the summit tilt began to sharply change, indicating that more magma was leaving the summit than was being resupplied from below. Hundreds of tiny earthquakes were recorded beneath the summit region of Kīlauea. These two changes were the same as those that preceded the last pause, and we began a friendly in-house bet as to when the pause would begin. As in the last pause, the movement of magma out of the summit reservoir indicated that magma was being intruded and stored elsewhere in the volcanic plumbing, probably beneath the upper east rift zone, where some small earthquakes were recorded.

This pause also had another feature we have not seen recently. The start-up coincided with a sharp increase in tilt at the summit. This change was so rapid that it was not caused by simply swelling, or inflating, the summit magma chamber. Instead, it is a ground surface change caused by emplacement of a shallow blade of magma, or dike, into the summit caldera. Such dikes, if they progress to the surface, become new eruptive fissures. In this case, the dike did not reach the surface, and no new eruption ensued. The change in summit magma pressure associated with this dike intrusion apparently caused eruptive activity to resume at the vents on the south and west sides of Pu'u 'O'o.

The latest pause may also be related to the magnitude 4.1 earthquake that took place on November 21 at 7:10 pm. This earthquake was located beneath the south flank of Kīlauea, about halfway between the coast and the east rift zone and slightly west of the active flow field. This earthquake was widely felt; we received reports from Pahala to Hakalau, including several from Hilo that reported minor damage to household objects shaken off shelves and walls. Several previous pauses in activity occurred following earthquakes in the flank adjacent to the active vents.

The three recent pauses in eruptive activity indicate that the magma supply system that feeds magma to the Pu'u 'O'o vents is becoming increasingly unstable. Such instability, characterized by frequent pauses in activity, preceded the end of activity at the Kupaianaha vent in early 1992.