Volcano Watch — The June 27th lava flow: It's more than just a flow front

The flow front position is the simplest indicator of the activity and its potential threat. But the June 27th flow, like other pāhoehoe lava flow fields, is expansive and dynamic, and the flow front location is only part of the story.

For the broader picture, let's look at the June 27th lava flow as a whole. The flow covers an area of about 1,200 hectares (about 3,100 acres), but only a tiny fraction of this area consists of active, flowing lava on the surface. Based on measurements taken on January 29, for example, the area of active lava was about 4 hectares (10 acres). In other words, only about 0.3 percent of the June 27th flow field surface consists of active, flowing lava. The rest of the surface is warm, but solidified.

Much of the active lava on the surface, what we call "breakouts," is focused around the farthest portion of the flow—near, but not always at, the flow front. The lava gets from the vent to this distal part of the flow via a subsurface lava tube. For much of the length of the flow field, this lava tube is a single "master" tube, perhaps a few meters (yards) wide. The June 27th lava flow is about 23 km (14 mi) long, but this master tube appears to reach no farther than about 20 km (12 mi). Over the final 3 km (2 mi) of the flow length, the lava moves beneath the surface via a complicated network of minor lava tubes. The master tube can be thought of as an artery that branches out into small capillaries (minor tubes). This network of minor tubes is what supplies lava to the surface breakouts near the flow front.

When the rate of lava supply from the vent is high, lava moves efficiently through the network of minor tubes, feeding vigorous surface breakouts that reach the flow front. In this case, the flow front advances downslope. But when the lava supply rate is low, the lava on the surface and within the system of minor tubes may cool to the point of solidifying and stalling, and active lava may not be able to reach the flow front. In this scenario, the flow front stalls, but weak breakouts can nevertheless persist a short distance upslope, closer to the end of the master tube.

It is this second scenario—a lower lava supply and stalling flow front—that appears to have been in play over much of January. If people only hear that the flow front has stalled, they might get the false impression that the flow as a whole is inactive. But scattered breakouts have persisted upslope of the stalled flow front for weeks now.

HVO and Hawai‘i County Civil Defense keep a close watch on these upslope breakouts because they can help determine the future path of activity. The network of minor lava tubes supplying the breakouts is unstable and can change through time, shifting the location of breakouts on the flow field. Once the breakouts have shifted, an increase in the lava supply rate can re-energize the breakouts and feed a new lobe of lava traveling in a new direction.

HVO's daily eruption updates (http://hvo.wr.usgs.gov/activity/kilaueastatus.php) describe these upslope breakouts, but to better show their locations and extent, we have also recently begun posting thermal maps of the flow field (http://hvo.wr.usgs.gov/maps/). These thermal maps are made with images collected by a handheld thermal camera during our helicopter overflights. HVO also posts satellite images, when available, that show "hotspots" (red pixels) indicating the areas of active breakouts.

Keeping an eye on the flow front position is an important part of staying abreast of current activity and the ongoing lava flow hazard. But for sure, it is important to be mindful of what's going on upslope as well.

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Volcano Activity Update

Kīlauea's East Rift Zone lava flow remained active with breakouts scattered across the leading 3 km (2 mi) of the flow, in an area roughly 6 km (4 mi) upslope of the flow front, and also farther upslope near Pu‘u ‘Ō‘ō. The leading tip of the flow remains stalled roughly 500 meters (0.3 mi) upslope of Highway 130 as of Thursday, February 5.

The summit lava lake level was roughly 40–45 m (130–150 ft) below the rim of the Overlook crater over the past week.

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