September 11, 2008
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
A lava lake is revealed within Halema`uma`u's new vent
"So, how far down is the lava?"
This is perhaps the most common question directed to HVO scientists since the new vent opened in Halemau`ma`u Crater this past March. With its persistently voluminous plume, nighttime glow and occasional explosions, the vent has been the subject of many recent "Volcano Watch" articles, each addressing a unique aspect of the ongoing activity.
But none of the articles, thus far, has focused on the Big Question, in large part because—until recently—we have only had a rough idea of how deep the lava surface was below the vent rim.
Before last week, views into the vent during our routine helicopter overflights did not provide any clear views of a lava surface, so we had to rely on geophysical and geochemical tools to infer the depth to the lava. Our best guess before last week was that the lava was indeed very shallow, on the order of several hundred meters.
As noted in the August 21 "Volcano Watch", precise locations of seismic tremor bursts indicated the tremor source (and thus perhaps the lava surface) to be 100-200 m (330-660 ft) deep.
But on Friday morning, September 5, the view that HVO scientists have been waiting months to see finally materialized, and uncertainty regarding the depth to the lava surface was finally put to rest. During our routine overflight, we were able to clearly see a 50-m (160 ft)-diameter lava lake about 100 m (330 ft) below the overhanging vent rim on the floor of Halema`uma`u.
Vigorous bubbling, sloshing and roiling disrupted the lava surface as globs of spatter were thrown onto the conduit walls, creating an awe-inspiring spectacle. Finally, we could see the lava lake we'd only been able to imagine during the preceding months.
Besides elation and awe, we also felt vindicated that two of our assessments regarding subsurface conditions were sound. First, the view confirmed that the lava surface was very shallow, as geophysical data suggested. Second, while hovering over the vent, we observed a cycle of activity that confirmed that the seismic tremor bursts we've been recording for several months were due to a process called "gas-pistoning," as we have speculated over the past few months.
The term "gas-pistoning" was introduced to the volcanological lexicon by HVO scientists observing activity during the Mauna Ulu eruption of Kilauea in the late 1960s. During sustained lava pond activity within the vent, the partially crusted lava surface would slowly rise toward the vent rim. When it reached its maximum height, spattering along the pond margins commenced and expanded until the whole pond surface was involved.
The lava level would then drop as lava drained back into the conduit. This activity, repeated in a cyclic manner, was interpreted as gas accumulating underneath the lava surface, leading to the surface rise, followed by gas release during the spattering, which caused the lava level to drop.
Gas-pistoning has been commonly observed within Pu`u `O`o's crater, as well, and for several months, it was present within the perched lava channel active in late 2007. Now, after seeing it within Halema`uma`u, it appears clear that gas-pistoning is a common process at Kilauea.
Our views into the vent on September 5 revealed gas-pistoning at Halema`uma`u. When we arrived at the vent, we heard by radio from the observatory that we were in an interval between tremor bursts. The lava surface at that point was moderately active, with a discontinuous crust and numerous scattered bubbling sources. Over the radio, we heard that a tremor burst was starting, and as we watched, the lava surface began to bubble and slosh more violently, until the whole lake surface was overturning and becoming a single roiling fountain as gas gushed more vigorously from the vent. A few minutes later, when the tremor burst and roiling had died down, we could see that the lava level had dropped several meters (yards), owing to all the escaped gas.
For now, Pele chooses to keep the lava surface out of sight from Jaggar Overlook, but, nonetheless, visitors during the past week have gotten some clear—though indirect—views of the gas-pistoning. Come to Jaggar at night and watch for the vent to darken as the lava crusts over, followed several minutes later by a dramatic increase in vent brightness as the pond surface is disrupted by intense spattering beneath the floor of Halema`uma`u Crater.
Frequently, gas-pistoning is replaced by steady tremor and vent glow that can last for days or weeks. Why the gas-pistoning comes and goes at the lava lake is not yet clear, demonstrating that—although the lava lake has been revealed—there are plenty of mysteries left to ponder.
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. On September 5, scientists observed a 50-m (160 ft)-diameter lava lake about 100 m (330 ft) below the vent rim on the floor of Halema`uma`u; the lava cannot be seen from the rim of Halema`uma`u Crater or Jaggar Museum Overlook. There have been several small ash-emission events from the vent, lasting only minutes, in the last week.
Pu`u `Ō`ō continues to produce sulfur dioxide at even higher rates 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 fissure D of the July 21, 2007, eruption and flows toward the ocean through a well-established lava tube. No significant incandescence was observed from the fissure to the top of the pali above the abandoned Royal Gardens subdivision or on the coastal plain, indicating little or no new surface flows in the past week. Lava continues to flow into the ocean at Waikupanaha.
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 Civil Defense Web site (http://www.lavainfo.us) 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.
One earthquake beneath Hawai`i Island was reported felt within the past week. A magnitude-3.6 earthquake occurred at 5:34 a.m., H.s.t., on Sunday, September 7, 2008, and was located 5 km (3 miles) west of Pahala, at a depth of 35 km (22 miles).
Visit our Web site (http://hvo.wr.usgs.gov) for daily Kīlauea eruption updates, a summary of volcanic events over the past year, and nearly real-time Hawai`i earthquake information. Kilauea daily update summaries are also available by phone at (808) 967-8862. Questions can be emailed to askHVO@usgs.gov.
Updated: September 24, 2008 (pnf)