August 30, 2007
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
New Report Details July 21 Fissure Eruption Hazards
For the past six weeks, lava erupting from a fissure east of Pu`u `O`o has been flowing on the surface through an open channel and feeding a series of `a`a flows. The incandescence reflected on clouds from these flows and the fires they have ignited when advancing through nearby forest have caught the eyes of many Puna residents and visitors after dark. Understandably, the flows have also caused concern.
Throughout the entire six weeks, as the Hawaiian Volcano Observatory has been doing since 1912, we have been closely monitoring this eruption and assessing what some future outcomes of the current activity might be. We record a short daily update summary available at (808) 967-8862 and post a detailed daily updates on our website (hvo.wr.usgs.gov), along with almost-live webcam images. We update our maps and photos 2-3 times per week. Hawai`i County Fire personnel are flying over the flows each day to assess the fire danger. On this basis, our assessment is that there is currently no direct threat from lava flows. Here?s why:
The current eruptive activity has been somewhat repetitious over the last few weeks. Lava is fed into a channel. The channel feeds an `a`a lava flow that advances 5-6 km (3-4 miles) from the vent, and then stalls. The channel backs up, overflows, and starts a new `a`a lava flow that advances along the north side of the previous one. As of August 30, we have seen this happen at least six times. The flows have not progressed farther downslope in the last few weeks, but keep spreading out in the same area.
The map of the lava flows looks something like your forearm with fingers together laid palm down flat on a table. If the vent is halfway between your elbow and wrist (ignore your arm above that point), the channel is the rest of the forearm to the wrist, the end of the channel that overflows is the back of your hand, and the `a`a lava flows are your fingers.
An `a`a flow is usually supplied by an open channel. The surface of moving lava in the channel is not completely crusted over and so loses a great amount of heat into the air. Data so far indicate that the temperature of lava in a channel can drop at least 6-7 degrees C. per km (17-20 degrees F. per mile). That?s more than 10 times the temperature loss per km (mile) for flow through an open channel than through a lava tube. The greater temperature loss in channels promotes the formation of microscopic crystals in the lava, making it stickier. This is something like thickening gravy with flour.
Ultimately, an `a`a flow loses so much fluidity that it can?t move forward any more. The core of the flow is molten and is just too sticky to advance, so the flow front stops or stalls. When this happens, the channel is blocked, and it starts to overflow its levees or banks.
What would it take to get `a`a lava flows to advance farther before stalling? Based on studies of such conditions, volcanologists have found that the length of `a`a flows is directly related to supply rate. It would take an increase in the eruption rate to get the currently forming `a`a flows to advance more than the 5-6 km (3-4 miles) limit currently observed.
Through most of the 1983-2007 eruption of Pu`u `O`o and Kupaianaha, the long-term average eruption rate has been about 300,000 cubic meters per day (55,000 gal/min). Our estimate of the current eruption rate is somewhere between that and one million cubic meters per day (180,000 gal/min).
Longer `a`a flows can be produced if the eruption rate increases substantially. For example, the Mauna Loa 1984 eruption produced lava at an average rate of about 25 million cubic meters per day (4.8 million gal/min) and fed an `a`a flow that was nearly 25 km (16 miles) long. Eruption rates like this are very unlikely for Kilauea, based on previous eruptions.
There is another way that lava flows can advance farther. If the open channel crusts over and forms a lava tube, the lava moving through it will be much better insulated and able to retain its ability to flow farther, even at the current eruption rates. This has been the eruption mode throughout most of the ongoing Pu`u `O`o eruption that successfully delivered lava to the coastline for many years.
HVO has just published these ideas and supporting information in a report available on-line (http://pubs.usgs.gov/of/2007/1264/). The report reiterates that there is currently no direct threat from lava flows, and also provides some details about future possibilities. Even with no current threat, it is important for members of the public to stay informed about this eruption and the hazards it poses.
Kilauea summit and the July 21 fissures continue to deflate. Seismic tremor levels continue to be low. Earthquakes were located beneath Halema`uma`u crater and the south flank area. The summit monitoring network recorded the third pressure variation, known as a DI or deflation-inflation tilt event in the last two weeks. The pressure variation shows up at Pu`u `O`o between 1-2 hours later, demonstrating the excellent hydraulic connection between the two locations.
Fissure D of the July 21 fissure eruption remains active. The lava enters an open lava channel that transitions into an `a`a flow moving toward the northeast. Because `a`a lava flows cool relatively quickly, each flow has been able to reach only 3 - 4 miles from the fissure before stagnating. Lava then piles up behind the stalled flow and is forced to jump out of the channel to make a new `a`a flow. This process has repeated several times in the past month. For about the last week and a half, when the flow has stalled, the lava has ponded up to form an elongate, perched channel with pahoehoe overflows. This ponding is taking place along the channel from about a half-mile to a mile northeast of the fissure. After a day or two of ponding, the lava has been breaching the lower wall of the perched channel and gushing out to feed new `a`a flows. The channel then plugs back up again and a new perched channel begins to form.
There has now been a total of six `a`a flows that have advanced toward the northeast, each robbing the supply from the previous flow. There have also been a few smaller flows that quickly stagnated and did not capture the flow. The currently active `a`a flow, as of Thursday, August 30, is advancing east across the top of `a`a flows a few weeks old. This flow still has about a mile to go to reach the stagnant tips of the `a`a flows from early August. Because of the `a`a flow-length limitation imposed by cooling, it is likely that the current flow will stagnate in the next day or two and that a new flow will break out from the perched channel closer to the fissure.
Despite the heavy steam and fume, occasional glimpses of incandescence are still seen at night in the Pu`u `O`o crater on the Pu`u `O`o Webcam (http://hvo.wr.usgs.gov/cam/index.htm). As has been seen in years past, Pu`u `O`o could be acting as temporary storage for lava that passes beneath the cone on its way to the erupting fissure. There have also been a number of collapses in the Pu`u `O`o crater in the past few weeks. If you look at the Pu`u `O`o Webcam, you will see mud plastered onto the Webcam window from one of these collapses.
Vent areas are hazardous. Access to the eruption site, in the Pu`u Kahauale`a Natural Area Reserve, is closed (http://www.state.hi.us/dlnr/chair/pio/HtmlNR/07-N076.htm).
Two earthquakes beneath Hawai`i Island were reported felt within the past week. A magnitude-2.1 earthquake occurred at 4:53 a.m. H.s.t. on Thursday, August 23, and was located 3 km (2 miles) southeast of Ke`anae, Maui, at a depth of 16 km (10 miles). A magnitude-2.5 earthquake occurred at 6:35 p.m. on the same day and was located 13 km (8 miles) northeast of Pahala at a depth of 34 km (21 miles).
Mauna Loa is not erupting. Two earthquakes were located beneath the summit; both were short-period quakes. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates, which have slowed further since May 2007.
Visit our Web site (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: September 5, 2007 (pnf)