Volcano Watch — Pāhoehoe lava makes for fitful advance of the June 27th lava flow

For the past few weeks, the flow has advanced in the way that slow-moving pāhoehoe flows are notoriously famous for—fitfully—as a series of budding lava "toes" and lobes (small flows) that break out and spread, then stop and inflate with fresh, molten lava, before breaking out again as new toes and lobes.

Advancing lava toes typically spread laterally, as well as downslope. As they do, they often merge with other nearby toes, forming a larger lobe made of coalesced toes topped by a thin crust of hardened lava.

Beneath the crust is a liquid lava core that is continually replenished with new lava from the main lava tube upslope. As more lava flows into the liquid core of coalesced toes, the crusted surface rises (inflates), making room for additional lava to travel beneath the crust.

The liquid core of coalesced toes becomes the conduit through which fresh lava is delivered to the front of an individual flow. This provides for the eventual downslope growth of a lava tube.

Such spreading of pāhoehoe lava has occurred not only at the tip of the June 27th flow (the part of the flow farthest downslope), but also at the leading edges of many separate lobes behind the flow front and along the margin of the flow. The lowermost 2 km (1.2 mi) of the June 27th flow has spawned many such breakouts of toes and lobes from the lava tube during the past month.

Lava lobes advance at varying rates over a period of time—from a few minutes to hours to days—depending mostly on the amount of lava that is supplied to them through the lava tube. Larger surface flows or lobes along flow margins upslope from the flow front may be active for days to weeks and sometimes overtake the flow front to form a new tip.

In this way, surface breakouts and coalescing toes of pāhoehoe lava form a complex patchwork of lobes and flows that contribute to the width, thickness, and length of the overall flow as it moves downslope. Scientists often refer to the resulting patchwork as a "flow field" instead of an individual flow.

With several surface flows moving simultaneously, but at different speeds along different parts of the June 27th flow, it is often difficult to determine the flow’s advance rate on a daily basis. This can lead to anxiety and confusion when the numbers keep changing.

During the past week, the leading edge of the June 27th lava flow slowed, stopped, and advanced again. Between Monday, Oct. 20, and Wednesday, Oct. 22, it moved about 40 m (45 yd) per day. During this same time, the flow front was passed by a narrow lobe, less than 50 m (55 yd) wide, that was moving nearly 200 m (220 yd) per day until Wednesday morning, when that rate nearly doubled. This lobe advanced quickly because it was confined, or channeled, by the local topography in a narrow gully.

The same pattern occurred about a month ago. Beginning about September 22, the leading edge of the June 27th flow stopped advancing for nearly a week, but subsequent upslope breakouts, moving along the north margin of the flow at a rate of about 120 m (130 yd) per day, overtook the flow tip.

Both of these slowdowns and temporary stalls in the forward advance of the June 27th lava flow correlate well with a measured decrease in the supply of lava from the Pu‘u ‘Ō‘ō vent into the lava tube that feeds the flow front. Repeated measurements of the cross-sectional area of the lava stream within the tube indicate that lava discharge from Pu‘u ‘Ō‘ō may have decreased by at least one-third in mid- to late September and by nearly two-thirds in mid-October.

Variations in flow rates are a normal part of pāhoehoe behavior but require close monitoring when flows approach residential areas and critical infrastructure. Therefore, the Hawaiian Volcano Observatory and Hawaii County Civil Defense continue to closely track the June 27th lava flow. Daily updates are posted at http://hvo.wr.usgs.gov/activity/kilaueastatus.php and http://www.hawaiicounty.gov/active-alerts/.

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Volcano Activity Update

The June 27th lava flow from Pu‘u ‘Ō‘ō remained active on Kīlauea Volcano's East Rift Zone. The previously sluggish flow front picked up speed this week, with the narrow flow front advancing towards AP‘A‘ā Street at a higher rate starting around October 20. By October 23, at 4:00 p.m., HST, the flow front was 340 m (0.2 miles) from the closest spot on AP‘A‘ā Street. Within the Pu‘u ‘Ō‘ō crater, glow was visible above several small outgassing openings in the crater floor.

The summit lava lake within Halema‘uma‘u Crater produced nighttime glow that was visible via HVO's webcam over the past week. The lava level was relatively steady over the past week, and was roughly 60 m (200 ft) below the rim of the Overlook crater as of this writing (Thurs., Oct. 23).

There were no earthquakes reported felt on the Island of Hawai‘i during the past week.