Wednesday, July 26, 2017

What the Heck is a Superelevated Lava Flow Anyway? 1974 Basalt Flow near Keanakāko‘i Crater

Talk about catnip for a geologist... "Do Not Enter", "Stop Here", "Go no closer to the eruption", and "Roads and Trails Closed Beyond This Point". How could any self-respecting geologist ignore such signage? And yes, that is a volcanic plume emanating from a crater on the far right side of the picture. We headed right on by the signs and headed down the road...

I suppose I should mention that we were there legally as guests of the park service, on an officially sanctioned field trip. I know that takes a bit of fun out of the story, but I suppose it's better than documenting an adventure in trespassing. We were exploring the evidence for explosive eruptive activity at the Kilauea caldera in Hawai'i Volcanoes National Park, and were walking along the edge of Keanakāko‘i Crater.

Keanakāko‘i Crater ("cave of the adzes"), probably formed in the 1400s and for centuries it was an important source of a particularly hard form of basalt that could be used to make adzes, the tools used to carve canoes and logs for walls. The adze quarry was covered by a lava flow in 1877, and a later flow in 1974 buried the quarry even deeper. It was the 1974 lava flow that we were having a look at.

The flow emanated from a rift opening just up the slope and traveled down a gully towards the main part of the Kilauea caldera. It coated the gully walls with a thin layer of pahoehoe (smooth-surfaced) basalt as it drained into the deeper crater. In other words, it was less of a lava flow than a covering layer on the gully walls. This odd occurrence revealed something interesting.
Downflow, the lava appeared to be thicker, but as we walked lower, it was apparent that it was only thick on one side. What did that mean? It meant that we were looking at a superelevated flow. And no, I'd never heard of the term either.
This was a fast-moving flow, and as it moved down the gully it made a tight turn. With an estimated velocity of 30 kilometers per hour (19 mph), the flow climbed high up the slope as it banked to the right. That's how one bank ended up around 10 meters higher than the other. It was superelevated...

The flow must have been quite a sight as it careened down the canyon but I strongly doubt there were any witnesses, as it would have been incredibly dangerous and the radiant heat would have been deadly. Then again, someone would have put up warning signs, and any geologists would have found the signs irresistible. Maybe someone saw it after all!

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