November 8, 2007
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Perched lava channel elevates the flows
Despite the potential hazards, the ongoing eruption from Kilauea's east rift zone is a remarkable and fascinating sight. It is startling to see a river of lava coursing between broad levee walls that tower above the surrounding land surface. What the casual observer may not know, however, is that this is a one-of-a-kind phenomenon. A perched lava channel has never been seen before. This makes it particularly difficult to forecast what might happen next.
The dynamic system we have today developed from a channelized `a`a flow that took its first baby steps this summer on July 26. Lava, erupting from fissure D of the current eruption, poured downslope to feed an advancing `a`a flow. The open lava channel that formed along the axis of the `a`a flow fed a series of short-lived `a`a lobes that traveled up to 3 to 4 miles to the northeast before stagnating. By mid-August, all this `a`a piling up in front of the lava channel caused the channel to back up and overflow its banks, building them up in the process. The perched lava channel we see today had begun to form.
Scientists often look for analogies in other scientific disciplines to understand or explain phenomena they observe in their own discipline. In the case of lava flows, volcanologists sometimes have to pull out their old hydrology textbooks. Did you know that big, slow-moving rivers, like the Mississippi, can build up their banks and become perched above the surrounding landscape? When a river like this is in flood stage, water spills over the river banks and slows down. When the water slows down, it dumps the sediment it was carrying. All that sediment adds up. If the river does this enough times, it builds its own levees and can become elevated. Standing well beyond the banks of the river, you would have to look up to see ships sailing by.
The currently active lava channel has developed by a similar process. When the channel fills up, lava spills over the sides. These overflows are thin and, as such, cool very quickly. They typically travel no more than a few hundred feet beyond the channel. Thus, each overflow serves to build the channel levee up a little higher. If this happens enough times, the lava channel, like a river, becomes elevated above the adjacent land surface. When compared to a river, the process of forming a perched lava channel is different, but the result is the same. Since the lava channel began overflowing two-and-a-half months ago, it has developed into a mile-long channel perched some 37 m (120) feet above the pre-existing surface.
Occasionally, the channel wall is not strong enough to hold back the huge weight of the lava flowing through the channel. The result is the failure of the channel wall, usually near the end of the lava channel. We have seen several of these breaches since August, each resulting in a significant drop in the level of lava in the channel as lava drains out to feed `a`a flows. To use another hydrology analogy, this is similar to the levee breaches that devastated the Ninth Ward and Lakeview neighborhoods in New Orleans during Hurricane Katrina in 2005.
Unlike levee breaks on rivers, though, the breaches we have seen on the perched lava channel heal themselves. When the lava level is low, slices of the channel wall break off and fall into the lava stream. These chunks, along with other large pieces of debris already submerged in the lava stream, get carried along the bottom of the channel. When they reach a breach in the channel wall, these partly molten, car- to garage-sized boulders pile up and choke off the escaping lava. Throw in countless plates of cooled lava crust from the surface of the lava channel, like blood platelets healing a bleeding wound, and before you know it, the breach has been plugged. In most cases, this healing process has taken no more than a couple of days.
We continue to monitor and document this unique perched lava channel that currently defines the eruption. Though we can not be sure, it is likely that, through the coming weeks to months, the lava channel will continue to do what it is doing now. It will produce frequent, brief overflows that will mantle the channel levees and add to the channel's already elevated position. Only time will tell.
Kilauea summit and Pu`u `O`o continued to deflate slowly, although the changes have been quite small over the past week. Seismic tremor levels continued to be low. Earthquakes were located mostly beneath Halema`uma`u Crater and the south flank faults.
The July 21 eruption continues to supply lava into a perched channel from eruptive fissure D, 2.3 km (1.4 mi) northeast of Pu`u `O`o. The channel forms a discontinuous, north-northeast-trending molten stream about 1.4 km (0.9 mi) long. The channel has been segmented into separate pools due to narrow sections of the channel roofing over to form bridges. Overflows from both sides of the channel continue to build up the channel walls. Lava backing up behind the bridges has elevated each upstream section of the channel above its downstream neighbor, giving the channel a stair-stepped appearance dropping downstream to the northeast.
Near the end of the channel, lava drains into two lava tubes--one carrying lava to the north, and the other carrying lava to the east. Over the last week, the southeastern tube was the more dominant and was supplying lava to a slowly advancing pahoehoe flow. This thin, dispersed flow was moving east along the crest of Kilauea's east rift zone over lava erupted in 1977, but had stagnated at its farthest extent (4.6 km, or 2.9 miles, from the north end of the channel) by Sunday, November 4. Further lava supplied by this easterly tube has been active about 1 km (0.7 mi) upstream from the terminus, near Pu`u Kia`i. Besides these flows, small `a`a flows continue to be fed from intermittent seeps breaking out from the north and south sides of the channel.
At Pu`u `O`o, no incandescence has been seen on the Webcam at night since August. The heavy fume coming from Pu`u `O`o completely obscures any view into the crater. As in years past, Pu`u `O`o likely is serving as a large chimney, beneath which lava is stored briefly and degassed substantially enroute to the erupting fissure. Sloughing of Pu`u `O`o into its own crater since late August has left numerous fresh cracks on the north rim and south flank of the cone.
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).
Four earthquakes beneath Hawai`i Island were reported felt within the past week. A pair of magnitude-3.4 earthquakes occurred at 2:01 and 2:27 a.m., H.s.t., on Sunday, November 4, 2007, and were located 15 km (9 miles) south of Volcano village at a depth of 11 km (7 miles). A magnitude-3.1 earthquake occurred at 7:35 p.m. on Monday, November 5, and was located 13 km (8 miles) northeast of Mauna Kea summit at a depth of 29 km (18 miles). A magnitude-3.0 earthquake occurred at 5:00 a.m. on Thursday, November 8, and was located 3 km (2 miles) northeast of Captain Cook at a depth of 13 km (8 miles).
Mauna Loa is not erupting. Three earthquakes were located beneath the summit. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates.
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: November 13, 2007 (pnf)