Scientists Wonder Why Lava Won’t Advance Beyond “Invisible Line”

PUNA – The June 27th lava flow has not advanced since last Monday, but it remains active and continues to widen.

Below the flow, in the town of Pahoa, life seems to be returning to normal despite the constant volcanic threat. Stores that closed in anticipation of destruction are now re-opening. Facilities that were impacted – like the transfer station – have been restored. Emergency meetings and conference calls have been scaled back or cancelled altogether. Its a stark contrast to the fear and apprehension that prevailed when the lava first approached the town. All seemed lost when the lava crossed Apa’a Street early in the morning on October 25th. But then the flow slowed down, eventually stopping short of Pahoa Village Road. And a few months later, when another lobe of lava looked like it would cut off Highway 130 to the north, it also stalled out, a mere 550 yards away from highway.

Scientists are wondering why. Why can’t the lava find the strength to go beyond this invisible line, less than a mile from Puna’s main arterial road? University of Hawai’i at Hilo geologists Ken Hon speculated on this during a recent talk story, filmed by the National Park Service.

“This lava flow is a little bit strange compared to other lava flows we’ve seen,” said Hon, who has spent decades studying and filming the behavior of pāhoehoe lava. He told the crowd that came out to hear his Volcano Month presentation on January 27 that this lava flow is different from the flows that destroyed Kalapana in the 1990s.

Pahoa, the lava flows have been coming down and then stopping. So two possible ideas on this: One that is related to the amount of lava that is going to the lava front, which the Volcano Observatory guys have been measuring. But it’s hard to get a real accurate measurement on that. And the other one is that it’s getting too sticky… The viscosity or the properties of the lava have changed and that maybe its reaching the furthest distant point it can get. If that was the case then you could just kinda sit there and not worry about it.Ken Hon, Jan. 27 talk story

Hon has observed the June 27 lava flow up close. He and his wife Cheryl filmed and distributed many of the clips that were seen around the world as the current flow entered Pahoa in late 2014. He has some theories as to why the June 27th lava flow only seems to be able to go so far.

We need to have supply rate, the amount of lava, the viscosity, and a thing called yield strength – how the fluid behaves, and then how steep the slope is is gonna control how it advances. And then the mode of propagation. To us there’s actually several different kinds of pahoehoe flows. We have channelized flows; you find those those on steeper slopes. There’s sheet flows; they spread out on flat ground and they crust over but they have a lot of lava going underneath them. And then we have low lava supply and a flat or steep slope we get a kind that’s called hummocky pahoehoe. The hummocky pahoehoe moves very slowly and has a hard time advancing. When the lava flows in Pahoa are moving fast they’re moving as sheets. When they slow down at the front they start making hummocky pahoehoe.Ken Hon, Jan. 27 talk story

Hon noted the Aila’au flows, which stretched from Kilauea all the way to the ocean from the years 1410 to 1470, were about 1145 degrees centigrade in the vicinity of Pahoa. The current June 27th lava flow is running at about 1135 C.

“The lava’s been cooling,” Hon said. “This lava is a lot stickier than any lava we’ve seen during the course of this (30+ year eruption).” Jon said the lava supply rate is about half of what it was during Kalapana, so its not as “thermally efficient” and is therefore cooling quicker.

However, Hon added that scientists have also observed that the flows seem to be supply starved. “We wish we could say ‘oh yeah, they’ve hit their viscosity limits and they’re not going to go any further,’ but we know lava will flow from this.

It’s getting down towards the end where – if it’s gonna flow, if it gets much cooler than this – it’ll have to change over to a’a, it won’t be able to flow as pahoehoe very well anymore. There’s a discussion going on in the scientific community… we’re wondering which one of these things is right. And there’s people talking on both sides of it. I think there’s a number people who think it’s supply rate limited, and I’m kind of in that camp. But there’s also people who reason and are thinking that maybe it’s viscosity or behaviorally limited. It will be interesting to see how it plays out.”Ken Hon, Jan. 27 talk story

Only time will tell what the June 27 lava flow has in store for Puna. In the meantime, the Hawaii County Civil Defense Agency and Hawaiian Volcano Observatory are maintaining close observations of the flow, and – as they say everyday in a daily radio message to the community – they will keep us all informed of any changes in flow activity and advancement.

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