March 27, 2008
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
A new ash plume rises from Kilauea's summit
Last week's "Volcano Watch" detailed the development of the new gas vent - and small explosive eruption - on the south wall of Halema`uma`u Crater. During its first week, this vent emitted a continuous stream of hot gas, forming a conspicuous white plume drifting to the southwest. On Sunday, March 23, the plume turned from white to dusty brown and now billows from the vent in an ever-changing display. This ash plume is impressive and fascinating?but seems a bit out of place at a volcano which normally oozes lava. So how did this ash plume come to be?
Before examining the source of the brown coloration, we should understand why it was white to begin with. When the new vent opened around March 12, it started emitting a large amount of volcanic gas into the air. One of the primary components in volcanic gas is water. At the vent, where the plume is still very hot, this water is in vapor form and the plume is transparent - you can see right through it. A short distance above the vent, however, the plume cools and the water vapor condenses into tiny droplets. This makes the plume an opaque white, like any cloud in the sky.
But last Sunday, something changed. The plume turned brown because it contained a large quantity of ash particles. Ash, in a volcanological sense, refers to particles smaller than two millimeters (0.08 inches). A clue to the source of the ash came last week, when the plume was still white. On several occasions, our Webcam and seismometers captured collapses of rock around the vent, followed by brief periods of ash in the plume - and the same dusty brown color we saw recently. So we inferred that the broken-up blocks falling in and filling the conduit were the source of ash. With the plume continuously brown, we think that a large amount of such collapse material remains in the conduit. Gas streams through this conduit rubble at high speed, picking up the fine bits to create the ash plume. The plume still contains a lot of water, but the ash provides most of the coloration.
The plume at Halema`uma`u, however, is different than many other ash plumes. This ash is mostly fine, broken bits of the conduit wall as described above, which is solidified lava from earlier eruptions. Old material like this is called "lithic." You are probably more familiar with ash plumes created during large explosive eruptions, such as at Mount St. Helens in 1980. In those cases, the ash in the plume originates from fluid magma in the conduit that has fragmented due to expanding gas bubbles in the magma. Ash originating from new magma is called "juvenile." So we have a "lithic-rich" ash plume here at the summit, whereas many other plumes are "juvenile-dominated." We did measure a small juvenile component in Halema`uma`u's plume - not surprising, because we have found spatter and Pele's tears thrown up on the crater rim.
One of the most mesmerizing features of the plume is its constant billowing motion. As the plume rises, bulbous portions of the exterior circulate and expand. Scientists call these features "turbulent vortices" or "eddies." These eddies actually play a major role in controlling the composition and density of the plume. When the plume exits the vent, it is entirely volcanic gas and ash. But once it enters the atmosphere, these rotating eddies very efficiently roll around and envelope air along the plume margin incorporating the air into the plume. Scientists call this process "air entrainment."
All of this air inside the plume gets heated up by the volcanic gas and ash and expands considerably. This lowers the bulk density of the plume by a large factor - a process which allows plumes to rise as high as they do. What's amazing is how much air is normally entrained into plumes. A small plume like the one at Halema`uma`u can actually incorporate many times its original weight in air over a short distance - so at a height of just a few hundred meters (yards), it can be composed mostly of entrained air. So if someone asks us what the plume is made of, a cute answer would be, simply, "air."
Halema`uma`u's new show is well worth a trip to the National Park, where you can view the plume from several areas along the east caldera rim.
For more information on volcanic ash, visit the USGS Ash website at volcanoes.usgs.gov/ash.
Kilauea summit and Pu`u `O`o are slowly deflating. Seismic tremor levels and sulfur dioxide emission rates at the summit have remained elevated at several times background levels since late 2007. 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 27, 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 Web site (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 flows, have burst from the lava tube on the steep slopes within the Royal Gardens subdivision. 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. 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 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 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:
One earthquake beneath Hawai`i Island was reported felt within the past week. A magnitude-2.4 earthquake occurred at 7:32 p.m., H.s.t., on Friday, March 21, 2008, and was located 5 km (3 miles) west of Kilauea summit at a depth of 6 km (4 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.
Updated: April 21, 2008 (pnf)