May 8, 2008
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Ashes, ashes, all fall down
Volcanic ash can provide important evidence about processes that drive volcanic unrest. These processes include the intrusion of new magma into a volcano and phreatic activity-the potentially explosive interaction of steam and hot water. The hazard implications of these two types of unrest depend on the type of volcano. In composite volcanoes, like Mount St. Helens, movement of magma could potentially lead to a large, explosive magmatic eruption, while phreatic eruptions are typically much smaller and, therefore, less hazardous.
Volcano petrologists, scientists who specialize in the study of volcanic rocks, are keen to examine ash erupted from a volcano that reawakens after a period of dormancy. The main question they want an answer to is this: "Is juvenile (new) lava present in the ash sample?" The presence of juvenile lava in an ash sample is solid evidence that the unrest is being driven by the injection of new magma. If the ash consisted only of old, pulverized rock, the unrest could be driven by phreatic activity.
An early sign of unrest at Mount St. Helens in 1980 was the occurrence of small eruptions with the characteristic appearance of phreatic eruptions: stark white eruption plumes with contrasting zones of dark gray. The white parts were from condensed steam, while the dark parts of the plumes were filled with wet ash.
Petrologists carefully examined the Mount St. Helens ash samples, looking for juvenile material. They didn't find any-results consistent with the outward appearance of the small eruption plumes. Other evidence, chiefly large-scale bulging of the side of the volcano, suggested that magma was indeed forcing its way into the volcano. The ambiguity was resolved on May 18, 1980, when a catastrophic magmatic eruption occurred.
Years later, reanalysis of the early ash samples revealed that juvenile material was actually present, though most of the ash was composed of old rock fragments. If the juvenile material had been quickly identified in 1980, the volcano would probably been assigned a greater explosive potential.
In contrast to composite volcanoes, movement of magma in a Hawaiian volcano is much less likely to lead to an explosive eruption and phreatic activity is more likely to lead to explosive behavior. When ash from the March 19 Halema`uma`u explosion was examined, no juvenile material was found. Accordingly, HVO scientists speculated that the unusual activity might be driven by phreatic activity. But that possibility was put to rest when ash erupted just a few days later was found to contain juvenile material. This means that the activity is magmatically driven and that the explosive potential is relatively low.
The eventual course of this magmatic activity is uncertain. So far, we have witnessed the formation of a volcanic vent-an irregular pipe that allows gases from shallow magma to escape to the surface. Pele might be content with this arrangement for a long time to come. Once volcanic gases open a passageway to the surface, the escaping gases tend to keep the passage open. Another possible scenario is that this might be an early stage in the formation of a new lava lake in Halema`uma`u Crater.
Kilauea summit and Pu`u `O`o resumed deflation after completing four DI tilt events this past week. 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 within a well-established lava tube. Disruptions in the lava supply to the TEB flow, caused by deflation/inflation (DI) events at the summit of Kilauea, have resulted in breakouts from the TEB rootless shields and tube over the past week. These disruptions choked off the lava tube and caused the ocean entries at Waikupanaha to diminish in vigor for periods of several hours.
The public should be aware that lava deltas could collapse at any time, potentially generating large explosions in the process. This may be especially true during times of rapidly changing lava supply conditions as have been seen lately. Do not venture onto the lava deltas. Even the intervening beaches are susceptible to large waves that are suddenly generated during delta collapse; these beaches should be avoided. In addition, the steam plumes rising from the ocean entries are highly acidic and laced with glass particles. 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.
No incandescence was observed at night in Pu`u `O`o in the past week, though minor incandescence has been sporadically present throughout the past few months. As in years past, Pu`u `O`o is likely serving as a large chimney, beneath which lava is briefly stored and substantially degassed on its way to the eruption site.
On March 12, 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., H.s.t. 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 fence was damaged by rocks 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. On April 9, another small explosion occurred, depositing dense blocks and particles of fresh lava on the overlook area. A third small explosion from the vent occurred at 3:57 a.m. on April 16, depositing a thin dusting of pale red ash west of the crater. The vent continues to vigorously vent gas and miniscule amounts of ash. Fresh lava spatter, Pele's tears, and Pele's hair have been collected at the rim (though none recently), 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:
Two earthquakes beneath Hawai`i Island were reported felt within the past week. A magnitude-3.2 earthquake occurred at 1:20 a.m. H.s.t, on May 4, 2008, and was located 7 km (5 miles) northwest of Mauna Loa summit at a depth of 12 km (7 miles). A magnitude-3.2 earthquake occurred at 9:15 a.m. on May 6, and was located 15 km (9 miles) southeast of Na`alehu at a depth of 39 km (24 miles).
Mauna Loa is not erupting. Eleven earthquakes were located beneath the summit. Extension between locations spanning the summit, indicating inflation, has resumed following a period of stagnation.
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.
Updated: May 12, 2008 (pnf)