Volcano Watch — Pause in eruption gives way to flows again

Following the spectacular display of steam explosions and lava fountaining at the ocean entries last week, this week began with another pause in the eruption. By last Sunday morning, lava had stopped entering the ocean, and the lava tube upslope was drained and empty, although still brightly glowing.

The eruption recommenced very slowly starting Monday afternoon, when the first small surface flow was spotted at about the 2,000-foot level. The lava had reoccupied the tube system and flowed down the old tube until it broke through to form a surface flow. On Tuesday, Wednesday, and Thursday, a sequence of surface flows emanated from progressively lower elevations along the old tube system. However, throughout this period, the volume observed was small, and the flows were sluggish. On Thursday at 5:30 p.m., the lava finally reached the ocean once again as lava spilled from the tube system on the lava delta and created a channelized pahoehoe flow. This flow entered the ocean just west of the entries that formed the spatter cones last week. A second, smaller flow headed towards the ocean just east of the spatter cones and reached the water late Thursday evening.

Usually, when the eruption is going, strong harmonic tremor is recorded on our seismic stations near the vent. This harmonic tremor is a vibration of the ground caused by magma flowing underground. The effect is the same as blowing air through a wooden instrument, except that in the case of tremor, the fluid is magma instead of air, and the instrument is the vent. As the eruption started up again this time, the tremor was so weak that it failed to signal that the eruption had started again. Throughout the week, the tremor has been at low levels, indicating that little or no magma was flowing or that there was no eruption. This low-level activity probably reflects the small volume of the eruption.

When the eruption paused last Sunday, the summit region of Kīlauea immediately began to bulge upwards as new magma accumulated within the reservoir located a few kilometers beneath the surface. We measure this upward bulging using small changes in the slope, or tilt, of the ground surface. Instruments at the Observatory can measure changes as small as one millimeter over the length of one kilometer (the thickness of a dime over 0.6 miles). Usually, when the eruption is active, all new magma coming into the volcano from below is erupting, and the ground surface does not tilt upwards over the summit. However, when the eruption started on Monday, the summit continued to bulge upwards until Friday morning, indicating that much more magma was entering the volcano from below than was erupting. By Friday morning, the upward swelling of the summit region had stopped, and the volume of the eruption had increased to levels comparable to those seen during the last few months.

After more than a year of nearly continuous effusion of lava from the episode 51 and 53 vents, the two pauses in the eruption, large fluctuations in eruptive volume, increase in the depth of the lava pond inside Pu'u 'O'o, and highly explosive activity at the ocean entries all reflect changes in the magmatic system. The slow resumption of activity following the pause last Sunday also indicates that the system has changed. This activity may indicate that we are entering a period of rapid changes—with starts and stops, and possible the breakout of new vents—such as that which occurred between February 1992 and February 1993.