The explosive event at Halemaumau Crater’s lava lake on Aug. 6 was the latest in a series that began in 2008. It is useful to view the event in the context of the entire series and to consider if another is likely.
The explosive event at Halemaumau Crater’s lava lake on Aug. 6 was the latest in a series that began in 2008. It is useful to view the event in the context of the entire series and to consider if another is likely.
The current summit eruption started explosively on March 19, 2008, when a rockfall temporarily impeded the release of volcanic gas from a new vent. Pressure built up, and an explosion ensued a few minutes later. The erupted material consists entirely of older rocks from past eruptions; the gas came from magma not yet in the vent.
Seven more explosive events took place between April 9 and Oct. 14, 2008. All involved both old rocks and new magma, which had risen into the new vent.
Since 2008, 19 explosive events deposited spatter on the rim of Halemaumau Crater. Many other smaller events weren’t large enough, or weren’t located close enough to the crater rim, to produce a recognizable deposit.
Every day, Pele’s hair, Pele’s tears, tiny hollow spherules, and other bizarre shapes are formed by thousands of gas bubbles bursting in the lake. Such bursts are technically explosive, but they constitute only background activity.
In this Volcano Watch, we consider only those explosive events that left a significant deposit on the rim of Halemaumau. How can we rank these events in terms of size?
Volcanologists use the Volcano Explosivity Index (VEI) to classify explosive eruptions by the volume of erupted material. The VEI, as currently defined, ranges from 8 to minus 6 (-6). A VEI of 8 has a volume of 1 trillion cubic meters or more—a cube 10,000 meters (more than 6 miles) on a side! A VEI of -6 has a volume of 0.1–0.01 cubic meters (a few ice cubes).
Three of the 2008 explosive events rate as VEI -2 (100–1,000 cubic meters; a house and garage), four as VEI -3 (10–100 cubic meters; a moderate-size living room), and one as VEI -4 (1–10 cubic meters; much smaller than an average room).
All explosive events since 2008 are either VEI -3 or VEI -4, mostly near the border between the two classes. None comes close to matching the three larger 2008 events. The August 6 event rates a VEI of -3.
Another important factor is the depth below the crater rim at which the explosive event takes place. For a given size, more material will reach the rim of Halemaumau if the depth is shallower; the ejected material has less distance to travel.
We don’t know the depths of the explosive events in 2008 or the two in 2009, except that they were certainly more than 150 m (492 ft). In contrast, all explosive events have been shallower than 150 m since 2011, when frequent measurements of the depth to the lava lake were started using a laser rangefinder. For example, the August 6 event occurred at a depth of 125 m (410 ft).
Three explosive events in April-May 2015 took place when the lake level was unusually high, 85–90 m (279–295 ft) below the Halema’uma’u Crater rim. Two of those events were so small that spatter might not have reached the rim had the explosion been deeper.
All the explosive events were triggered by rockfalls from the walls of Overlook crater, which contains the lava lake. Those walls are becoming more stable with time, as overhangs are destroyed. Evidence of increasing stability is the low amount of solid rock material erupted daily from the crater, now averaging less than 5 percent of the total daily ejecta.
Nonetheless, unstable portions of the wall remain. One is under the old visitor overlook on the Halemaumau Crater rim. Even small explosive events caused by failure of this part of the wall can throw spatter onto the overlook area. That happened on Jan. 8 and Aug. 6 of this year. Had these events taken place farther away, as did most earlier ones, the amount of spatter that fell at the overlook would have been much less.
Eventually this part of the wall will stabilize, but it probably hasn’t yet. We will not be surprised if another shower of spatter lands in the overlook area in the next few months.