October 4, 2007
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
How much lava is being erupted at the July 21 fissure?
This has been a popular question posed by scientist and citizen alike but is currently a difficult one to answer accurately. It is also one of the most important quantities to monitor during an eruption. For example, the advance rate and ultimate length of `a`a flows are controlled by eruption rate. Because of its importance and its challenges, we are using a number of techniques to get a better estimate. (WARNING: math ahead).
Our first guesses of the eruption rate used the simplest technique: estimate the area covered by new lava flows, multiply it by an average thickness, and divide the product by the number of days since the eruption began. We realized in the early days of fissure D that the effusion rate was at least 1,000,000 cubic meters per day (cmpd) or 180,000 gallons per minute (gpm), which is twice the output of Pu`u `O`o prior to shutdown in mid-June and three times Kilauea?s long-term average lava production rate. Though quick, these methods provide only crude estimates.
A much more accurate estimate can be made using digital elevation models (DEMs) of the flow field made at different times. Light Detection and Ranging, or LIDAR, measurements made from a plane or helicopter can quickly produce these DEMs. Differencing them can yield an accurate estimate of the volume change since the earlier DEM. We hope to experiment with this approach soon.
Another technique takes advantage of the spectacular lava channel that has been a persistent component of the July 21 eruption. The discharge rate of the channel can be estimated in the same way that water discharge in a river is estimated: estimate the cross-sectional area of the channel and multiply by the average velocity. The lava channel width and the average velocity are easy to measure; the depth is harder. If we don?t need the eruption rate estimates immediately, we can wait until the channel drains out and measure its depth when empty. However, if we are interested in the eruption rates right away, we need some way of measuring channel depth with lava flowing in it. We have not yet worked this out.
A variation of this technique was used to accurately estimate lava flow rates through lava tubes over during the Pu`u `O`o-Kupaianaha eruption. The cross-sectional area of the lava in the tube was estimated by measuring the extent that radio (Very Low Frequency or VLF) waves were distorted by the electrically conductive lava inside the tube. These measurements were made over the top of the tube near a skylight through which a speed of the lava was also measured. If all lava was flowing through this one tube, the cross-sectional area multiplied by the average speed equaled the eruption rate.
Monitoring like this allowed HVO to forecast the demise of Kupaianaha in 1992 and track other important eruption developments. The technique cannot be used for open channels because measurements cannot be made across the top.
Another method to estimate eruption rate is based on the emission rate of sulfur dioxide gas (SO2) from the eruption site. SO2 is dissolved in magma but starts to bubble out just before the magma erupts and continues to do so as the lava flows at the surface. Because we know the amount of gas originally dissolved in the magma and assume that we measure all the gas bubbling out of the lava, we can estimate the eruption rate: more gas means more lava is erupting.
Of course, it?s never that simple. For reasons that we don?t yet understand, SO2 emissions were unusually low at the start of the July 21 eruption but more than doubled about three weeks later. We know that the increase did not reflect an eruption rate change. Since then, the gas-derived eruption rate has fluctuated between 600,000 and 1,400,000 cmpd (110,000 to 250,000 gpm). In comparison, The Wailuku River in Hilo discharges between 100,000 and 170,000 gpm of water.
There is no clear answer yet to the eruption rate question although HVO scientists are working hard to improve estimates in the coming weeks. Nevertheless, all estimates for the July 21 eruption rate thus far are higher than average eruption rates for Kilauea and the last several years of the Pu`u `O`o eruption.
Kilauea summit and Pu`u `O`o continued to slowly deflate. Seismic tremor levels continued to be low. Earthquakes were mostly located beneath Halema`uma`u Crater and the south flank area.
The July 21 eruption remains active. The erupting lava flows toward the northeast through an open, perched lava channel about 1.4 km (0.9 miles) long. The channel can be naturally divided into four distinct, wide sections separated by constrictions, two of which are roofed over. At the end of the perched channel, the lava turns sharply east and cascades into a lower channel. The lower channel is about 600 m (~2000 ft) long and disappears as it transitions into an `a`a flow.
The `a`a flow fed by the lower channel has been reburying older `a`a flows erupted in August. The terminus of the `a`a flow extends less than a kilometer (0.6 miles) beyond the end of the lower channel. Lava seeping from the south side of the perched channel wall, active now for several weeks, also continued to creep forward during the past week. As of Thursday, October 4, lava from this flow was burning into a kipuka along the southeastern edge of the current flow field. Small lava flows fed by seeps on the north side of the channel are now inactive.
At Pu`u `O`o, no incandescence has been seen on the Webcam at night for the last several weeks. The heavy fume coming from Pu`u `O`o completely obscures any view into the crater. 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. It is, basically, a big chimney. There has also been a number of collapses in Pu`u `O`o?s crater since late August, and numerous fresh cracks cut 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).
No earthquakes beneath Hawai`i Island were reported felt within the past week.
Mauna Loa is not erupting. Two 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: October 16, 2007 (pnf)