Volcano Watch — Halema`uma`u Engine Shifts Gears, but Keeps on Chugging

Clattering sounds issue from the Halema`uma`u vent as gas pulses forth, forming a thick plume that rolls into white billows high above the caldera. On most days, the plume rises and turns above the summit, gently drifting southward and darkening the Ka`u landscape. In the distance, an endless train of fume wraps around the flanks of Mauna Loa, destined for Kona and other points on the island chain.

Since mid-October, this scene has been relatively steady. But on Monday, November 17th, something changed. The thick white plume that we have become accustomed to seeing suddenly vanished, leaving only transparent trails of fume issuing from the vent. It appeared as though the summit activity had stopped—the chugging engine had seized. But, just as suddenly as it paused, it resumed a few minutes later, and the thick plume returned, puffing high above the caldera. This cycle of robust and wispy plumes occurred several more times on Monday and into the next day.

On Tuesday morning, HVO geologists made their way to the crater's edge and were standing above the vent through four of these plume transitions. Instead of a continuous flow of fume and occasional clatter, they witnessed something far more curious. When the plume became weak and wispy, the faint roar of surf, usually a common sound, was barely audible. Then, suddenly, they heard the loud and intimidating sound of breaking glass, followed by the rushing return of a thick white plume.

During these changes, cameras captured images and instruments recorded temperatures as glow returned to the vent. The glow intensity and associated temperatures were the highest observed since mid-October.

The amount of material ejected onto the crater rim also increased to its highest level since mid-October. This material largely consists of Pele's hair and other glassy fragments suggestive of fresh lava.

So, what is happening at the vent and why has it changed? Our explanations of natural phenomena are always working hypotheses that are refined as more data become available. At this stage, our data suggest that lava may be pistoning (rising and falling) within the vent.

When pistoning occurs, lava rises within the conduit to a level perhaps within 100 meters (300 feet) of the vent rim. As the lava cools, a crust forms on its surface. This crust can impede the flow of gas, causing pressure to build within the lava. When enough pressure is built to break the crust, gas can rush forward. The reduction in gas pressure causes the lava piston to drop down within the vent, and the cycle starts again.

Scientists at HVO have witnessed lava pistoning within vents during other eruptions on Kīlauea. For example, seismic tremor recorded at the summit in recent days is reminiscent of tremor produced at Pu`u O`o several years ago, when lava filled and drained from vents within its crater.

The sound of breaking glass heard within the vent may be caused by shattering of the glassy crust on the lava-piston's surface. The increase in glassy lava fragments deposited on the rim of Halema`uma`u is probably the result of this sequence.

Periods of intense glow observed at night could also be explained by the pistoning. When the crust breaks up and the lava level drops, red-hot molten lava and incandescent vent walls are exposed, increasing the intensity of the glow.

An explanation for changes deep within the volcano causing this pistoning behavior is more ambiguous. Whatever the reasons are for the shift in summit activity, we are sure that the engine is still chugging.

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Volcano Activity Update

Kīlauea Volcano continues to be active. A vent in Halema`uma`u Crater is erupting elevated amounts of sulfur dioxide gas and very small amounts of ash. Resulting high concentrations of sulfur dioxide in downwind air have closed the south part of Kīlauea caldera and produced occasional air quality alerts in more distant areas, such as Pahala and communities adjacent to Hawai`i Volcanoes National Park, during kona wind periods. The have been several small ash-emission events from the vent, lasting only minutes, in the last week. For a few days mid-week, there were several brief periods of decreased plume vigor lasting minutes to hours. During these periods, the plume was the weakest it has been since the vent opened last March.

Pu`u `Ō`ō continues to produce sulfur dioxide at rates higher than the vent in Halema`uma`u Crater. Trade winds tend to pool these emissions along the West Hawai`i coast, while Kona winds blow these emissions into communities to the north, such as Mountain View, Volcano, and Hilo.

Lava continues to erupt from the Thanksgiving Eve Breakout (TEB) vent and flows toward the ocean through a well-established lava tube. Activity at the Waikupanaha ocean entry has fluctuated over the past week, with periods of small littoral explosions and a voluminous plume, followed by periods of no explosions and a wispy plume. Part of these fluctuations may have occurred in response to a deflation-inflation (DI) event at Kīlauea's summit on November 16-17. A western branch of the tube, which diverges from the main TEB tube at the top of the pali, continues to supply lava to breakouts on the coastal plain. These breakouts are scattered along the western margin of the TEB flow field and are slowly marching southward toward the National Park boundary.

Be aware that active lava deltas can collapse at any time, potentially generating large explosions. This may be especially true during times of rapidly changing lava supply conditions. Do not venture onto the lava deltas. Even the intervening beaches are susceptible to large waves generated during delta collapse; avoid these beaches. In addition, steam plumes rising from ocean entries are highly acidic and laced with glass particles. Check the Civil Defense Web site or call 961-8093 for viewing hours.

Mauna Loa is not erupting. Two earthquakes were located beneath the summit this past week. Continuing extension between locations spanning the summit indicates slow inflation of the volcano.

There were no felt earthquakes reported on Hawai`i Island for the past week.

Visit our Web site for daily Kīlauea eruption updates, a summary of volcanic events over the past year, and nearly real-time Hawai`i earthquake information. Kīlauea daily update summaries are also available by phone at (808) 967-8862. Questions can be emailed to askHVO@usgs.gov.