Weekly Volcano Watch

Something had to change after 25 years

Reprinted from the USGS website.

The Halema`uma`u gas plume at sunrise. March 15, 2008.

2008 will be known as the year that Kilauea Volcano had a mid-life crisis. After 25 years of almost continuous, effusive eruption from its east rift zone, an incandescent gas vent mysteriously appeared in Halema`uma`u Crater sometime on March 11 or 12, and exploded early on the morning of March 19, 2008.

Now that it happened, we are looking back to see whether we missed any precursors to the explosion. We're also trying to piece together exactly what happened. Here's what we know so far:

The event started at 2:55 a.m. with a few seismic events that were either small earthquakes, very small explosions, rockfalls, or a combination of all three. At 2:56 a.m., the steady incandescence of rocky rubble covering the top of the gas vent started to founder. At 2:58 a.m., a second series of even larger explosive earthquakes began.

At 2:59 a.m., the newly opened vent started to vibrate strongly at a frequency too low to hear but recorded by instruments operated by our colleagues at the Infrasound Laboratory of the University of Hawai`i at Manoa. Their sensors detect infrasound, or acoustic energy, that travels through the air. Seismic energy traveling through the ground was also recorded by our seismometer network as increased tremor.

At 3:02 a.m., HVO's automatic earthquake-location software concluded that the combination of energy from the earthquakes and explosions was a magnitude-3.7 earthquake in the upper east rift zone and posted that to our Web site. Several hours later, when an analyst had a chance to look at the signals, he realized that the vibrations were explosions, not earthquakes, and our Web site was updated.

At first light on March 19, scientists rushed to the rim of Halema`uma`u and found the nearby area scattered with angular rock debris that was coarser and thicker closer to the vent. Not surprisingly, the rock debris lay directly beneath the gas plume from the new vent. And that's not all! Some folks in Pahala and Wood Valley found their cars covered with a light coating of fine white grit 30 km (19 miles) downwind probably courtesy of this early-morning Halema`uma`u explosion.

Debris blanketing the area was composed of rock fragments blasted from Halema`uma`u Crater, probably from the rubble pile covering the gas vent. Many of the rock fragments were too hot to touch more than 5 hours after the explosion. But there was no lava. This was an explosive eruption that blasted out old rocks.

The largest rock observed on the rim of Halema`uma`u above the new gas vent was about 90 cm (2.6 ft) in average dimension. Assuming typical rock densities, that rock weighed about 2,000 kg (2.7 tons), and the explosion was powerful enough to propel it more than 72 m (230 feet) into the air!

We are still gathering details and trying to deduce the exact nature of the explosion. One explanation that HVO scientists are discussing is a "throat clearing" hypothesis. In this scenario, hot gas coming up along the east wall of Halema`uma`u Crater progressively undermined the overlying rubble and created an open conduit underneath. Early Wednesday morning, the surface rubble layer collapsed into the underlying open conduit and temporarily blocked gas from escaping. Pressure built up and, two minutes later, reached a level that blasted the rubble from the gas conduit.

Science is a process of asking questions, and the above hypothesis has raised many. The biggest question is whether pressure can build to explosive levels in only two minutes. Another is whether an explosion could happen at all without involving water that explosively flashes into steam as it did during the last series of explosive Kilauea eruptions in 1924.

Once we feel confident that we understand what happened on March 19, we will be able to assess whether future explosive eruptions are likely. Right now, we cannot rule them out.

Activity update

Kilauea summit and Pu`u `O`o are slowly deflating. Seismic tremor levels at the summit are elevated to nearly moderate levels. Summit sulfur dioxide emission rates have remained elevated at nearly 10 times background levels since early January 2008. Earthquakes were located primarily beneath the general summit area, the southwest rift zone, and the south flank faults.

Lava from the 2007 Thanksgiving Eve Breakout (TEB) flow, erupting from fissure D of the July 21 eruption, continues to flow through what remains of the Royal Gardens subdivision and across the coastal plain. On Wednesday, March 5, the flow entered the ocean in the vicinity of Kapa`ahu. The Waikupanaha delta has since grown to a width of about 1,000 m (3,280 ft) and has multiple entry points. On March 15, another branch of the flow reached the ocean farther to the east, within a few hundred meters of the lava viewing area. As of Thursday, March 20, both the Waikupanaha and Ki entries remained active, though the Waikupanaha entry is far more vigorous.

The public should be aware that the ocean entry areas could collapse at any time, potentially generating large explosions in the process. The steam clouds rising from the entry areas are highly acidic and laced with glass particles. Do not venture onto the lava deltas. Even the intervening beaches are susceptible to large waves suddenly generated during delta collapse; these beaches should be avoided. Check the County of Hawai`i Civil Defense website (http://www.lavainfo.us) for information on public access to the coastal plain and ocean entry.

In the past week, sporadic breakouts, some large enough to form channelized `a`a flows, have burst from the lava tube on the steep slopes within the Royal Gardens subdivision. A few of these have reached the base of the pali before stalling. Other breakouts have been spotted at the top of the pali near the upper boundary of the Royal Gardens subdivision. Closer to the TEB vent, an area of persistent breakouts on the northeast side of the shield complex also continues to produce small flows. These northeast-directed flows are restricted to a broad, flat area on the south side of Kupaianaha.

Weak incandescence has been intermittently observed at night in Pu`u `O`o in the past week. As in years past, Pu`u `O`o likely is serving as a large chimney, beneath which lava is briefly stored and substantially degassed on its way to the eruption site.

On March 11, a new fumarole appeared low on the southeast wall of Halema`uma`u Crater, within Kilauea's summit caldera. The new vent is located directly beneath the Halema`uma`u Overlook about 70 m (230 ft) down. Incandescence could be seen at this vent, starting on March 13, and, by March 18 incandescence had grown to cover an area about 30 m (98 ft) across. At 2:58 a.m. on March 19, a small explosion occurred from this fumarole. The explosion scattered rock debris over an area of about 75 acres, covering a narrow section of Crater Rim Drive, the entire Halema`uma`u parking area, and the trail leading to the overlook. The overlook was damaged by rocks that reached up to 90 cm (3 ft) across. No lava was erupted as part of the explosion, suggesting that the activity was driven by hydrothermal or gas sources. The new explosion pit continues to glow at night, with incandescence reflecting on the fume emitted from the vent.

Sulfur dioxide (SO₂) emission rates from the summit area have been substantially elevated at 2-10 times background values since early January. During these conditions, SO₂ concentrations frequently exceed 1 ppm for much of Crater Rim Drive between Halema`uma`u parking lot and the southwest rift zone. SO₂ concentrations exceed 20 ppm for approximately 200 m (650 ft) of the road between the Halema`uma`u parking lot and the south caldera pullout.

The increase in sulfur dioxide emission rates at the summit means that SO₂ concentrations are much more likely to be at hazardous levels for visitor areas downwind of Halema`uma`u, especially during weak wind conditions or when winds blow from the south. Most people are sensitive to sulfur dioxide at these levels, especially children, individuals with asthma, chronic obstructive pulmonary disease (COPD), or other breathing problems. Stay informed about SO₂ concentrations in continuously monitored areas (Jaggar Museum and Kilauea Visitor Center) by visiting the Kilauea Visitor Center and the web at:
http://www2.nature.nps.gov/air/webcams/parks/havoso2alert/havoalert.cfm. To minimize these potentially harmful effects, the Hawai`i Volcanoes National Park has closed all access to the southern half of Kilauea caldera.

Two earthquakes beneath Hawai`i Island were reported felt within the past week. A magnitude-3.4 earthquake occurred at 2:22 a.m., H.s.t., on Friday, March 14, 2008, and was located 6 km (4 miles south of Mauna Loa summit at a depth of 3 km (2 miles). A magnitude-2.4 earthquake occurred at 2:28 p.m. on Saturday, March 15, and was located 2 km (1 mile) northwest of Pahala at a depth of 13 km (8 miles).

Mauna Loa is not erupting. One earthquake was located beneath the summit. The rate of extension between locations spanning the summit, indicating inflation, has decreased to values below current detection limits.

For complete information on visiting and touring the volcano, visit www.tourguidehawaii.com.