April 10, 2008

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

Eventful week at Kilauea's summit culminates in small explosion at Halema`uma`u

This was the view from the Mauna Loa Strip Road in Hawai`i Volcanoes National Park at 7 a.m. this morning. Stagnant surface winds, combined with higher elevation winds from the southwest, caused the plume to rise straight up and then shear off to the northeast. April 10, 2008

It was a busy week atop our favorite volcano as Halema`uma`u continued to emit a towering white plume of gas and ash that was visible for miles around.

As the week began, the activity showed little change. The amount of ash in the plume continued to decline, and ash collectors placed near the vent were yielding only a meager sampling of Pele's hair and tears each morning. Sulfur dioxide emissions also declined, with measurements in the range of 500-800 tonnes per day, compared to the highs of 2000 tonnes per day several weeks ago. The current levels are still 3-4 times higher, however, than normal (prior to December 2007) emission rates.

Just when the Halema`uma`u plume was beginning to seem a little ho-hum, a shift in wind direction caused everyone's adrenaline to spike. The prospect of the national park and nearby communities being invaded by high concentrations of sulfur dioxide, along with the potential for ash fallout, sent emergency managers into full swing. The park was evacuated and closed for two days as shifting winds alternately lofted the plume well overhead or forced it to hug the ground. Nearby residents experienced intervals of high sulfur dioxide concentrations, but fortunately, none of these periods lasted for more than 1-2 hours. The ash content of the plume remained low, and the fallout was mostly undetectable.

For the last two weeks, the evidence suggests that molten lava is probably within 100-200 m (325-650 ft) of the surface within the new vent. Numerous discussions among scientists at the Hawaiian Volcano Observatory have focused on the question of what it will take to make the lava rise to the surface. One suggestion was that a deflation-inflation (DI) event might be the trigger.

DI events occur 1-2 times per month at Kilauea's summit, where tiltmeters record an abrupt deflation, usually lasting about 12 hours, followed by an abrupt inflation. The tilt events are usually accompanied by an increase in summit tremor during the deflation phase. Careful analysis of these events suggests that they are related to changes in the supply of magma supply to a storage reservoir at less than 1 km (0.6 mi) depth, just east of Halema`uma`u Crater. Many of the DI events at Kilauea's summit are also recorded at Pu`u `O`o tiltmeters, after a delay of 1-2 hours, indicating that the magma-supply changes also affect the ongoing eruption.

At midday on April 8, just after the park was evacuated, a deflation-inflation event began at Kilauea's summit, accompanied by an increase in seismicity. Night-duty scientists at HVO watched Halema`uma`u with bated breath as the deflation ended around 10 p.m. H.s.t on the 8th, and the summit began to inflate steeply. As usual, the vent glowed brightly through the night, but nothing else happened. The inflation ended by noon on April 9.

About eleven hours later, at 11:08 p.m. on April 9, a small explosion took place at Halema`uma`u that was similar to that of March 19, though considerably smaller. A few blocks were thrown onto the rim of the crater, along with a substantial deposit of finer fragments. Most of the debris consisted of fragments of older rock, but pieces of solidified lava were also blown from the vent. Many of the ejected fragments were hot enough to melt through our plastic ash collectors.

The explosion enlarged the vent, but lava has yet to take advantage of this change. Meanwhile, to the entire island's relief, the trade winds are back. Stay tuned for the next installment in this unusual chapter in Kilauea's eruptive history.

Activity update

Kilauea summit and Pu`u `O`o continued to deflate. Sulfur dioxide emission rates and seismic tremor levels have remained elevated at several times background levels. Earthquakes were located primarily beneath Halema`uma`u Crater and the adjacent areas, 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 (Waikupanaha entry) 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 (Ki entry) farther to the east, within a few hundred meters of the lava viewing area. As of Thursday, April 10, both the Waikupanaha and Ki entries remained active.

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 few weeks, sporadic breakouts have burst from the lava tube on the steep slopes within the Royal Gardens subdivision.

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. 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. At 11:08 p.m. on April 9, another small explosion occurred, depositing dense blocks and particles of fresh lava on the overlook area. The new explosion pit continues to vigorously vent gas and ash, with the plume alternating between brown (ash-rich) and white (ash-poor). Fresh lava spatter, Pele's tears and Pele's hair have been collected at the rim, indicating that magma resides at shallow depths in the new conduit.

Sulfur dioxide (SO₂) emission rates from the summit area have been substantially elevated up to 10 times background values since early January; the emission rates for the past week have been decreasing but are still elevated. 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, Hawai`i Volcanoes National Park has closed all access to the southern half of Kilauea caldera.

Three earthquakes beneath Hawai`i Island were reported felt within the past week. A magnitude-1.9 earthquake occurred at 11:06 p.m., H.s.t., on Thursday, April 3, 2008, and was located 6 km (4 miles) east of Honoka`a at a depth of 1 km (1 mile). A magnitude-2.4 earthquake occurred at 4:30 p.m. on Friday, April 4, and was located 3 km (2 miles) northwest of Pa`ia, Maui at a depth of 10 km (6 miles). A magnitude-2.5 earthquake occurred at 2:23 p.m. on Saturday, April 5, and was located 5 km (3 miles) northwest of Mauna Kea summit at a depth of 32 km (13 miles).

Mauna Loa is not erupting. No earthquakes were located beneath the summit. The rate of extension between locations spanning the summit, indicating inflation, has decreased to values below current detection limits.

Visit our Web site (http://hvo.wr.usgs.gov) for daily Kilauea eruption updates and nearly real-time Hawai`i earthquake information. Kilauea daily update summaries are also available by phone at (808) 967-8862.