Volcano Watch — What will the next eruption of Hualālai be like?

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Hualālai, looming majestically above Kailua-Kona, is Hawai‘i'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 Hualālai will next erupt, but we can offer clues about the nature of its next eruption.

We get these clues from Hualālai'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 Hualālai is lava flows.

It appears that Hualālai'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 Ka‘ūPULEHU (to the northwest) and the Hu‘ehu‘e (Kona airport) flows—have been examined in detail, and the results are surprising.

The Ka‘ūPULEHU flow erupted from the second highest vent, and the Hu‘ehu‘e flow came from the lowest two vents. Both eruptions produced complex lava flows in phases that started with channelized ‘a‘ā flows and ended with pāhoehoe flows.

The Ka‘ūPULEHU 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 Ka‘ūPULEHU flow to be about 300 cubic meters per second (4 million gallons per minute). Using this eruption rate and the volume of the Ka‘ūPULEHU 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 "Hu‘ehu‘e flow" is actually two flows. The earlier one progressed from ‘a‘ā to pāhoehoe, while the later flow was entirely pāhoehoe. This later pāhoehoe flow is very similar to today's active flows on Kīlauea's coastal plain, suggesting, by analogy, that the Hu‘ehu‘e 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 Hualālai.

The initial lava flow that erupted from Mauna Loa in 1984 had advanced 15 km (9 mi) in little more than 20 hours. So, if Hualālai's next eruption produces lava flows similar to the Ka‘ūPULEHU or the Hu‘ehu‘e flows, we speculate that they could reach the ocean in less than a day.

Because developed coastal areas are 15 km (9 mi) or less from the vents on Hualālai'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 September 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 Pu‘uwa‘awa‘a cone, and two had magnitudes estimated at more than 6. The earthquakes alone produced "hundreds of thousands of [1929] dollars in damage." But no eruption occurred.

The Ka‘ūPULEHU 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 Hualālai, even when there are no apparent changes on the volcano. Should any occur, we will immediately let you know.


Volcano Activity Update

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 (August 22), the lake level was dropping again with the onset of another DI event.

On Kīlauea'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 Hawai‘i Volcanoes National Park boundary. The Kahauale‘a 2 flow, fed from a spatter cone on the northeast edge of the Pu‘u ‘Ō‘ō crater, continues to be active and is burning forest north of Pu‘u ‘Ō‘ō.

There were no felt earthquakes in the past week across the Island of Hawai‘i.