At 10:02 p.m. Saturday, Aug. 6, a section of altered, thermally stressed rock enclosing the Halema‘uma‘u Crater lava lake detached from the vent wall and plummeted into the molten lava.
Tons of rocky debris impacting the lake surface triggered a violent explosion of volcanic gas, incandescent spatter (blobs of molten lava), and pieces of solid rock that sent a jet of glowing debris skyward.
Within seconds, tephra (airborne volcanic rock fragments) began falling to the ground, blanketing the rim of Halema‘uma‘u Crater — about 120 m (400 ft) above the lava lake surface — with a continuous layer of spatter and dense rock fragments that covered an area of about 50 m by 80 m (165 ft by 260 ft). In places, this tephra layer was up to 20 cm (8 in) thick.
Almost certainly, anyone who had been near the crater rim in this area would have been killed or severely injured.
Trade winds influenced the trajectory of the fallout. So, while most of the tephra fell just east of the vent, pebble-sized debris also pelted the Halema‘uma‘u Crater parking lot, about 500 m (0.3 mi) to the southwest.
As testimony to the heat and violence of the event, images captured two days later (posted at http://hvo.wr.usgs.gov/multimedia/index.php) show the blanket of spatter and solid rock fragments, with individual pieces up to 70 cm (28 in) across, on the crater rim.
Spatter that landed on a plastic case housing batteries and electrical components for a gravity monitoring instrument about 35 m (115 ft) from the rim of Halema‘uma‘u melted the case and ignited a fire that incinerated its contents. The gravimeter itself survived, but is being systematically assessed for possible unseen damage. Other nearby HVO monitoring instruments remain operational.
The event was recorded by HVO web cameras, and HVO seismometers around Halema‘uma‘u Crater detected a distinctive long-period signal related to sloshing of the lava lake set in motion by the rockfall. Following the 10:02 p.m. event, the lava lake surface remained agitated for a few hours, something also observed following past rockfall-triggered lava lake explosions.
Late-night visitors at the Jaggar Museum Overlook in Hawaii Volcanoes National Park likely witnessed a dramatic and rapidly ascending bright orange glow as the explosion cloud rose above the summit vent rim. They also might have heard a low rumble followed by a loud boom as the vent wall gave way and impacted the lava lake.
Diners at Volcano House Hotel, 3.5 km (2.1 mi) across the caldera, reported seeing an especially bright glow above the east margin of the lava lake. The distant glow also was noted by residents of nearby subdivisions.
In the aftermath of the event, HVO scientists and University of Hawaii colleagues carefully mapped and sampled the debris field before rain and wind could take a toll on the deposit. Analyses of the tephra might provide insight into how these lava lake explosions happen and what conditions favor their occurrence — information that could enable HVO to quantify the probability of future events and the likely range of dangerous impacts.
Rockfalls and the resulting lava lake interactions that produce a severe hazard are challenging, if not impossible, events to forecast. To date, HVO scientists have seen no evidence of precursory signals before an explosion, and the magnitude of the event likely depends on the location and size of the rockfall, the lava lake level at the time of the rockfall, wind velocity and other dynamic factors.
Most rockfalls from the vent wall occurred during rising lava lake levels, when large areas of the wall rock are heated and develop internal cracks because of expansion. But some rockfalls, such as the Aug. 6 event, occur after the lake level drops, possibly when the buttressing effect of the lake is lost, facilitating wall failure.
These ideas are part of ongoing research examining the evolution of the summit vent since it opened in March 2008.
What is certain is that the recent explosive event reinforces our awareness of inherent dangers in the vicinity of an active lava lake in a deep crater. Residents and visitors alike are reminded to heed National Park Service and USGS advisories regarding volcanic activity and ongoing hazards.
Volcano
activity updates
Kilauea continues to erupt at its summit and East Rift Zone.
During the past week, the summit lava lake level varied between about 35 m and 46 m (115-151 ft) below the vent rim within Halema‘uma‘u Crater.
On the East Rift Zone, the “61g” flow continued to advance across the coastal plain and enter the ocean at multiple points. The lava flow does not pose an immediate threat to nearby communities.
Mauna Loa is not erupting. In the past week, earthquakes have occurred mostly in the south caldera and upper Southwest Rift Zone at depths less than 5 km (3 mi), with the overall seismicity rate about the same as last week, but still elevated relative to the long-term background rate.
GPS measurements show deformation related to inflation of a magma reservoir beneath the summit and upper Southwest Rift Zone, with inflation occurring mainly in the southwestern part of the magma storage complex.
No earthquakes were reported felt on the Big Island this past week.
Visit the HVO website (http://hvo.wr.usgs.gov) for past Volcano Watch articles, Kilauea daily eruption updates, Mauna Loa weekly updates, volcano photos, recent earthquakes info, and more; call for summary updates at 808-967-8862 (Kilauea) or 808-967-8866 (Mauna Loa); email questions to askHVO@usgs.gov.
Volcano Watch (http://hvo.wr.usgs.gov/volcanowatch/) is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.
