Eruption rate for July-August lava flows was high but may have diminished toward the previous long-term rate

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The ongoing eruption of Kīlauea has spewed forth nearly 58 million cubic meters (76 million cubic yards) of lava in the days between July 21 and October 16, 2007, the early days of volcanic episode 58.

This is a photo of lava flowing in channel under one of two bridges left after lava level dropped.

Lava flowing in channel under one of two bridges left after lava level dropped.

(Public domain.)

This volume corresponds to about 500,000 cubic meters per day (about 92,000 gallons per minute, gpm), averaged over the 87-day duration. In contrast, the eruption prior to this, in recent years, had been producing an average of 300,000 to 400,000 cubic meters per day (55,000-73,000 gpm). A large dump truck holds a piddling 7 to 8 cubic meters (10 cubic yards).

How did we derive the new flow-field volume? By modeling the preexisting topography and subsequently changed landscape. The method is similar to measuring the swelling of a wasp sting by knowing the shape of your arm before and after the insect makes her charge. Topographic maps show the lay of the land, how steeply it slopes, and the altitude above sea level for points on the surface. Map accuracy, however, may be poor in highly forested areas, like Kīlauea's east rift zone. Also, the maps become outdated in a rapidly changing volcanic terrain.

Fortunately, satellite remote sensing provided us with topographic data current as of 2005, and no lava flows had entered the area of interest in the two intervening years. The method used, known as Synthetic Aperture Radar (SAR), works well for mapping barren ground but cannot penetrate the dense forest canopy. Thus, like other methods for creating traditional topographical maps, it leads to a map that may misrepresent ground elevations in forested areas.

In our case, the Pu`u `O`o and Kupaianaha vents had shed lava across much of the terrain in the late 1980s and early 1990s. The resulting nearly tree-free surface could be depicted with one- to two-meter accuracy.

To measure the ground surface built up by new lava flows, we used a precise but labor-intensive surveying method based on the Global Positioning System (GPS). Two geologists, with special antennas mounted on their backpacks, spent six days traversing the episode-58 flow field. In their packs were receivers that recorded their precise location every second. When processed back in the office, the GPS readings produced locations and altitudes accurate to within one meter.

The rest of the task was simplified by desktop computers. The traverse data were converted into a contour map of the modern lava-flow surface using Geographic Information System (GIS) software. The software also grids the surfaces of the newly mapped and preexisting terrain and then calculates the volume bound between them (the swelling, in the wasp-sting analogy). The result is the bulk volume, 57.7 million cubic meters (75.5 million cubic yards). An average rate over time is obtained by dividing the volume by the number of days of eruption.

Other methods for determining rates are based on dense-rock volumes-the volume that the lava would occupy if all of the gas bubbles were removed. The gas-discharge method, for example, estimates the volume of bubble-free magma needed to produce all the gas measured downwind from the vent. Lava on the flow field, however, is anything but bubble free. For consistency in comparing our new GPS volume estimates with other methods, the bulk volume was reduced to dense-rock equivalent, using a factor of 0.75, determined by experiments elsewhere on Kīlauea.

To finish the story, the 57.7 million cubic meters (75.5 million cubic yards) of bulk volume corresponds to a dense-rock equivalent of 43.3 million cubic meters (56.6 million cubic yards). The dense-rock eruption rate, averaged over 87 days, is roughly 500,000 cubic meters per day (92,000 gpm). This estimate compares favorably with other rates, including that determined by the gas-discharge method.

Some preliminary estimates, obtained immediately after the July 21 fissure opened, suggested a higher initial eruption rate-and the rate may have fluctuated during the months that ensued. Recent short-term estimates suggest that it has since diminished to more closely matching the long-term rate of 300,000-400,000 cubic meters per day (55,000 to 73,000 gpm) estimated before the 2007 activity. Volcanic conditions are subject to change, which makes meaningful forecasting so difficult, but the eruption-rate data suggest the current activity may have settled in for the long term.

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Volcano Activity Update

Kīlauea summit and Pu`u `O`o deflation changed to slow inflation on November 19. 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 from eruptive fissure D, 2.3 km (1.4 mi) northeast of Pu`u `O`o. For the last several months this lava was directed entirely into a perched channel, consisting of separate pools. On the morning of November 21 (6am), lava began issuing from a new location on the south flank of the perched channel, over fissure D. Lava supply to the perched channel appears to have been mostly redirected through this new outlet, dropping lava levels in the perched channel and cutting off supply to the eastward tube which had been feeding flows in the vicinity of Pu`u Kia`i. These flows near Pu`u Kia`i were still active on the afternoon of Nov 21, reaching the 1500 ft elevation, but will probably stagnate over the next several days.

The new eruptive outlet over fissure D is currently feeding a channelized pahoehoe flow directed southeast, along the south flank of Kupaianaha. This flow had advanced 1 km (0.7 mi) as of the afternoon of November 21. While there is not immediate threat directly from this lava flow, residents of the Royal Gardens subdivision are advised to stay informed.

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).

Two earthquakes beneath Hawai`i Island were reported felt within the past week. A magnitude-3.0 earthquake occurred at 4:07 p.m. H.s.t on Friday, November 16, 2007 and was located 4 km (2 miles) southeast of Kukuihaele at a depth of 36 km (22 miles). A magnitude-2.9 earthquake occurred at 3:11 a.m. on Saturday, November 17 and was located 23 km (14 miles) southwest of Waikoloa Village at a depth of 40 km (25 miles).

Mauna Loa is not erupting. One earthquake was located beneath the summit. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates.