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Volcanowatch

May 14, 2009

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Pele plays the plume in a recent multisensory performance

Plume from Halema`uma`u on May 8, 2009.
Plume from Halema`uma`u on May 8, 2009.

This weekend marks the return to the Big Island of ukulele virtuoso Jake Shimabukuro. His shows encompass a musical variety from Hawaiian to bluegrass, classical to rock, flamenco to jazz, and just about everything in between. Jake's fans and critics alike declare that these wide-ranging performances demonstrate the enormous potential of the tiny instrument.

Kīlauea Volcano, a more permanent but similarly wide-ranging entertainer on the Island, has also shown a remarkable capacity to dazzle, teach, amaze, and mystify her audience. Just a week ago this past Friday, park visitors and staff at the Jaggar Museum and HVO had front row seats at a mesmerizing ash-plume "happening."

The brief ash-producing episode began at 12:27 pm H.s.t., with a Very Long Period (VLP) seismic event. VLP events at Kīlauea are literally too slow to feel but produce a characteristic instrumental signature easily discerned in the earthquake record. About a half minute after the seismicity began, brown ash became apparent in the plume. The plume cloud rose energetically several hundred meters above the floor of Halema`uma`u Crater.

Over the next several minutes, prevailing tradewinds carried the ash to the southwest, further dusting the already closed and rock-strewn Crater Rim Drive. During the course of the event, the plume color changed from white to brown to grey, then to brown, and, finally, back to white again.

To visitors and staff occupying the Jaggar Museum "front row seats," Pele's brief Technicolor performance was spectacular, but for an HVO staff member who happened to be "on stage" at the rim of Halema`uma`u the experience might be better characterized as sublime. During the awe-inspiring event, the sounds associated with the towering plume were only those of continuous rock-sliding and gas-rushing.

The ash/VLP event was a hot topic for discussion at the HVO staff meeting this week. One scenario explaining the ash/VLP sequence involves a process that can be thought of as working from the top downward. Beneath Halema`uma`u, we might envision a column of magma extending upward from Kīlauea's shallow summit reservoir to the Overlook vent.

Gases trapped within the melt at depth in this column will tend to bubble out as pressure is reduced during magma convection (think lava lamp). Large gas bubbles can be formed by combining lots of smaller ones. When these bubbles rise and collect near the top of the magma column, the system is primed for an event.

Several possibilities exist for what might trigger an event. The Overlook Vent itself is a remarkably unstable environment. Rockfalls and rim crumblings are frequent. A rim collapse or a substantial rockfall could conceivably dump rubble onto the top surface of the magma column, triggering the vigorous release of the collected gas bubbles and ash. This sudden gas release could, in turn, generate a pressure shockwave downward that is reflected in the seismic record as a VLP event.

Another potential trigger for gas release in the magma column could be a depressurization of the magmatic system itself. In a recent Volcano Watch (April 2) column, we suggested that the rapid deflation of Kīlauea's pressurized summit reservoir could have caused the rapid release of sulfur dioxide we saw in June 2007.

Coincidently, the warm-up act for the ash/VLP show seen from the Jaggar Museum overlook and elsewhere last Friday was a rapid deflation/inflation (DI) cycle that began at the volcano's summit only a few hours earlier. An HVO field crew making gas measurements at the start of deflation recorded a doubling of sulfur dioxide emission rates during the day--consistent with the notion of deflation-triggered gas release.

DI events, as we call them, while not a regular instrumental gig for Kīlauea, have become at least a recurring one during the past several years. The rapid sulfur dioxide gas increase we observed last week is supporting evidence, though not proof, that the DI event caused the decompression, ash and VLP display. We eagerly await the next show.

One thing we can say with certainty is that the spectacular range of performances offered to Kīlauea's fans—from ropy lava flows to vigorous ash eruptions, from quiescent gas release to thousand-foot-high lava fountains—have demonstrated, over many years, the enormous capability and range of this comparatively young volcano virtuoso.

Activity update

The Waikupanaha and Kupapa`u ocean entries remain active, and no breakouts have been reported on the coastal plain since late April. There have, however, been a couple of minor, short-lived breakouts on the pali in the past few weeks. At Kīlauea's summit, the vent within Halema`uma`u Crater continues to emit elevated amounts of sulfur dioxide gas, resulting in high concentrations of sulfur dioxide downwind. Bright glow and loud vent noises during the past week indicate that a small lava lake is still present at shallow levels below the floor of Halema`uma`u Crater.

Visit our Web site (http://hvo.wr.usgs.gov) for detailed Kīlauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call (808) 967-8862 for a Kīlauea activity summary; email questions to askHVO@usgs.gov. Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey's Hawaiian Volcano Observatory.

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Updated: May 26, 2009 (pnf)