Saturday, July 29, 2017

Walking on Broken Glass, Literally: And what is slabby pahoehoe?

Some places on our planet are just not like anywhere else.

Kilauea, on the Big Island of Hawai'i is one of the most volcanically active places on Earth, and very few parts of its surface are older than a thousand years, and most are much younger. It's a place where erosion is essentially a non-existent process. There is only the addition of rock during eruptions, or the collapse of rock (into caldera or pit craters).

Walking on a landscape that has existed for only a few years is truly an otherworldly experience. A raw surface, unsoftened by soil formation or plant cover, is not a normal experience for most human beings. I was at Kilauea in May for a conference and field trip, and we had a chance to walk across one of Kilauea's more recent extrusions, a lava flow near Halemaumau Crater in the caldera that erupted in 1982. The eruption destroyed several hundred feet of the highway that travels around the caldera. It was quickly repaired, but has been closed more recently because of the ongoing (since 2009) eruption in the depths of Halemaumau that is putting out deadly fumes and gases, and the occasional explosion of debris. We were allowed to enter the area on this particular day.

Even if you have walked on basalt flows on the U.S. mainland, the experience is not like this one was. I've always thought of lava flows as being firm and solid, but when they are new, that is not always the case. The entire flow seemed to be covered by loose flakes about the size of potato chips.

One could never sneak up on an adversary on a surface like this. Every step produced loud crunching sounds. A close inspection of the loose chips showed why. The chips were composed not of basalt, but of glassy obsidian!

The more familiar kinds of obsidian most often seen around volcanoes is richer in silica, and is usually associated with lava domes or plug domes, a type of volcano not found in Hawai'i. But all lavas can form glass when they cool rapidly, and that is what happened at the surface of this flow. As the lava was exposed to the cooler air, it quickly solidified, forming a glassy crust. One would think that would be the end of it, and that the whole flow would solidify almost as quickly. But it doesn't. Six weeks after the eruption ended, bulldozers were realigning the highway, and overturned slabs still glowed red.

Such flows continue to be mobile after the crust forms, and lava beneath the crust is still quite molten (it is insulated by the crust). The flow can actually inflate with molten lava, becoming a great deal thicker before a breakout occurs, and the lava drains away. The surface can rise and fall, breaking here and there into large slabs. At the same time, the cooling rock at the surface contracts, and the surface breaks up into the chips we were walking on. In videos of such flows, one can often make out the tinkling sound of the rocks snapping off.

The amount of deflation in this particular flow was considerable, as can be seen in the photograph above, exceeding seven meters or so (about 20 feet). The lava reminded me of a sheet on an unmade bed, partly pulled up over a pillow.

The cliffs were unusual for a Hawaiian-type volcano. They were made up of distinct layers that looked almost sedimentary. They didn't result from any sedimentary process, however. They record a series of violent ash eruptions that culminated in a very explosive event in 1790 that killed dozens, maybe hundreds of Hawaiian warriors, and changed the course of history on the island (the army was facing off against King Kamehameha, and the tragedy was seen as a sign from the gods). Kamehameha soon thereafter united all the islands under his rule.

We climbed the cliff for a perspective of the main part of the flow and steaming cauldron of Halemaumau in the near distance. Halemaumau is the abode of the volcano goddess Pele, and many people take her existence seriously, providing offerings and prayers.

I was fascinated by the swirls that became visible from our vantage point a hundred feet or so above the lava flow. It is much smoother than the lavas I experienced on previous trips along the southeast rift zone of Kilauea. Compare it, for instance, with the pahoehoe flows near the former village of Kalapana that I visited in 2004, only days after they were extruded. They have a more "ropy" lobe-like shape, but note the silvery color. That silver color is also a thin layer of glass. After a few decades or centuries, that silvery luster will weather away, and the flow will have the forbidding black color that most people associate with basalt lava.

Post a Comment