Pāhoa Near Miss: Serendipity? Or Science?

Abstract:

The “June 27th” flow, named for the date on which it started, threatened communities on the lower East Rift Zone (ERZ) of Kīlauea Volcano, Hawaii, for several months during 2014–2015. The flow encroached on Pāhoa, threatening to cross the main street and sever Highway 130, which serves the rural communities on Kīlauea's south and east flanks.

The rate of flow advance was influenced greatly by magma supply from the summit. Increases in ERZ discharge, presaged by inflationary summit tilt, resulted in flow advance of up to several hundred meters per day. When summit tilt switched to deflation, discharge waned and the flow slowed and often stalled, repeatedly stopping short of destroying critical infrastructure. When activity resumed, it typically did so as many small breakouts as far as a few km behind the flow front, which then had to resurface previously covered ground to extend the overall length of the flow.

At the same time, lava near the flow front had a very spiny texture and was noticeably less fluid than lava closer to the vent, leading to speculation that physiochemical changes along the flow were limiting its length. In December 2014, we collected a suite of lava samples along the flow from near Pu`u `Ō`ō to its distal end (3 km to 18 km) to better understand the physiochemical changes along its length. The magma flux on that day was calculated at 90,000– 100,000 m³/day (~1 m³/s).

Preliminary analyses of the samples reveal a down-flow decrease in lava vesicularity and temperature and a corresponding increase in density and microcrystallinity. We propose that the lava flow could only attain a length of ~21 km because of the changes in its apparent viscosity, for the calculated magma flux. The flows could not overcome the increasing downstream yield strength, and breakouts from the flow margins farther upslope were easier to achieve than forcing the front forward. These upslope breakouts were themselves limited in length by the same factors.