Volcano Watch — Sputtering lava flows build behind shield

The episode 51 vents on the west flank of Pu`u `O`o which were inactive for six days starting Tuesday, April 28, were in continuous eruption since Monday, May 4, starting at about noon, until Friday afternoon. During this 18-day interval, the eruption was nearly constant in level of activity, as seen in tremor of nearly constant amplitude.

The tremor we measure near the vent is a low-frequency ground vibration related to the flow of lava through the vent. The vents are sealed over with spatter and the lava flows from the vent through a tube to the pond. Between about 1:30 and 2:00 p.m. on Friday, the tremor decreased gradually as the eruption at the episode 51 vent apparently waned and then stopped.

The only time during this eruptive interval that the lava volume fluctuated greatly occurred last Monday, when the eruption rate declined to less than one-half of what it was for about eight to nine hours. During this period of lower eruption rate, the pond adjacent to the episode 51 vents drained, and the lava from the vent cascaded down about 15 to 25 feet into the empty pond, across the pond floor, and exited through a lava tube towards the south. The flows have not advanced beyond the limits of earlier flows from episodes 50 and 51. Instead, the flows have been building up a low shield surrounding the pond. This shield was measured on May 14 and found to be between 130 and 150 feet higher than the surface that existed before episode 50 began on February 17.

When the eruption rate is higher, as it has been during most of this eruptive interval, the pond has been full and overflowing or nearly full, with the lava exiting the pond through a tube headed towards the south. This tube transports the lava down the south flank of the shield, where it surfaces within a second perched pond surrounded by two- to five-foot tall levees. This second perched pond is currently about 300 to 350 feet in diameter.

For a long time, the lava lake inside Pu`u `O`o was at, or very near, the same elevation as the episode 51 eruptive vent. When the pond level rose, the eruption became more vigorous, and was characterized by low spattering and even occasional low-level fountains. When the pond level dropped, the eruption became less vigorous or stopped. This relation between eruptive activity and the level of the pond inside Pu`u `O`o no longer appears valid. The pond inside Pu`u `O`o is now located about 230 feet below the rim and well below the level of the episode 51 vent on the west flank of Pu`u `O`o. The continuous character of the current eruptive interval is probably related to the changes that have disconnected the lava lake inside Pu`u `O`o from the episode 51 vent.

During the last few days, the fume cloud from Pu`u `O`o has been particularly dense; the increase in fume is caused by the low level of the lava lake inside Pu`u `O`o. When the lava lake is higher, the lava in the lake blocks the escape of gas from underlying magma, with the result that the fume cloud is smaller. Some of the apparent difference is also that the light Kona winds have not blown the fume away from the vent as quickly as do the trade winds.