Hualalai, looming majestically above Kailua-Kona, is Hawaii’s third most-active volcano (http://hvo.wr.usgs.gov/volcanowatch/archive/2009/09_10_01.html). The U.S. Geological Survey classifies it as a “high threat” volcano, based on its frequency of eruptions and the proximity of its vents to developed areas.
We can’t say when Hualalai will next erupt, but we can offer clues about the nature of its next eruption.
We get these clues from Hualalai’s past behavior. The geologic map of its volcanic deposits (http://pubs.er.usgs.gov/publication/i2213) shows that ash deposits are found around vents along the volcano’s rift zones and summit but rarely along coastal areas. The dominant features of the map are the several hundred lava flows covering the volcano’s surface, about 35 of which have been dated from the past 25,000 years. About 200 eruptions produced lava flows within the last 10,000 years. From this, we conclude that the predominant threat from Hualalai is lava flows.
It appears that Hualalai’s eruptions are clustered in time, separated by centuries of inactivity. The most recent cluster of activity included eruptions that ended in 1801 from six different vents within the volcano’s northeast rift zone. Lava flows from the largest of these eruptions — the Kaupulehu (to the northwest) and the Huehue (Kona airport) flows — have been examined in detail, and the results are surprising. The Kaupulehu flow erupted from the second highest vent, and the Huehue flow came from the lowest two vents. Both eruptions produced complex lava flows in phases that started with channelized ‘a‘a flows and ended with pahoehoe flows.
The Kaupulehu lava flow is similar in character to the flow produced by Mauna Loa’s 1984 eruption. If the comparison is correct, then, by analogy, the eruption rate of the two flows should be similar. Using this logic, we assume the eruption rate of the Kaupulehu flow to be about 300 cubic meters per second (4 million gpm). Using this eruption rate and the volume of the Kaupulehu flow — estimated to be about 160 million cubic meters (130,000 acre-feet) — the eruption must have lasted more than a week.
What we call the “Huehue flow” is actually two flows. The earlier one progressed from ‘a‘a to pahoehoe, while the later flow was entirely pahoehoe. This later pahoehoe flow is very similar to today’s active flows on Kilauea’s coastal plain, suggesting, by analogy, that the Huehue flows advanced similar distances to the ocean over several days to weeks.
We can further use the analogy with Mauna Loa to estimate possible advance rates of future lava flows from Hualalai.
The initial lava flow that erupted from Mauna Loa in 1984 had advanced 15 kilometers in little more than 20 hours. So, if Hualalai’s next eruption produces lava flows similar to the Kaupulehu or the Huehue flows, we speculate that they could reach the ocean in less than a day. Because developed coastal areas are 15 km or less from the vents on Hualalai’s summit and northeast rift zone, our next question is, will we be able to detect any eruption precursors?
We have only one possible example from which to draw conclusions: an earthquake swarm that was interpreted to be an intrusion or a failed eruption. Starting on Sept. 19, 1929, more than 6,200 earthquakes were recorded over the span of a month. Many of the earthquakes were strongly felt, especially near the Puu Waawaa cone, and two had magnitudes estimated at more than 6. The earthquakes alone produced “hundreds of thousands of  dollars in damage.” But no eruption occurred.
The Kaupulehu flow offers another possible clue about the duration of precursory signals: abundant, large xenoliths (rocks dragged up from depth with the ascending magma). From the size, shape, and mass of these xenoliths and the viscosity of the magma, we can estimate how fast the magma must have risen to carry these heavy rocks to the surface. Calculations suggest that the magma ascent was extremely rapid (hours to days), which, in turn, suggests a short period of precursory signals.
These are the reasons why the Hawaiian Volcano Observatory closely monitors Hualalai, even when there are no apparent changes on the volcano. Should any occur, we will immediately let you know.
A lava lake within the Halema‘uma‘u Overlook vent produced nighttime glow that was visible from the Jaggar Museum overlook and via HVO’s webcam during the past week. Two small deflation-inflation cycles (DI events) early in the week at the summit caused the lava lake level to fluctuate slightly. As of Thursday, the lake level was dropping again with the onset of another DI event.
On Kilauea’s East Rift Zone, scattered breakouts from the Peace Day tube remain active on the coastal plain. A small ocean entry is active just east of the Hawaii Volcanoes National Park boundary.
The Kahaualea 2 flow, fed from a spatter cone on the northeast edge of the Pu‘u ‘O‘o crater, continues to be active and is burning forest north of Pu‘u ‘O‘o.
There were no felt earthquakes in the past week across the Island of Hawaii. Visit the HVO website (http://hvo.wr.usgs.gov) for Volcano Awareness Month details and Kilauea, Mauna Loa, and Hualalai activity updates, recent volcano photos, recent earthquakes, and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.
Volcano Watch (http://hvo.wr.usgs.gov/volcanowatch/) is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.