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.

Want to see classic volcano features in California? Check out Lava Beds National Monument...

At least when the Republicans come to their senses and reopen the government. The park is closed right now.

Lava Beds National Monument was one of the central locales for our exploration of the northern California Cascades on our recent field studies class. The park was established on the northern flank of Medicine Lake Highland, a shield-like edifice that is the most voluminous volcano in California (about 130 cubic miles of lava as compared to 108 at Mt. Shasta). The park is known primarily for the richness and variety of its lava tubes, and for being the ancestral homeland of the Modoc People, who fought against impossible odds for the right to live on the land in 1872 and 1873. They lost everything.

Lava Beds is also one of the best places I know of outside of Hawaii to see the classic features of basaltic lava flows. The park preserves a number of extensive flows, some as young as 1,150 years. In the dry high desert environment, they look as if they erupted yesterday.

We drove up the park road from Captain Jack's Stronghold and found a marvelous example of an a'a lava at the Devil's Homestead flow, which erupted about 12,000 years ago (above). I was trying to imagine crossing the lava flow barefoot, saying "ah, ah" the whole way. Basalt is often quite liquid (non-viscous), but with distance and dropping of temperature, the lava can crust over and break up into a blocky rough surface.

A short distance later we encountered one of the youngest flows in the park, the Black Crater flow, dated to about 1,250 years ago. The short trail to the spatter cone crosses some excellent examples of pahoehoe flows, formed by basalts that had low viscosity .

Black Crater is actually quite colorful. It is not really a crater at all, but is instead a group of spatter cones, which formed as lava droplets popped out of the vents, forming small cones of popcorn shaped basalt.

A bit further up the road we found the Fleener Chimneys, a group of spatter cones that were the source of the large Devil's Homestead flow, seen in the first picture. The Dragon's Mouth is a small lava tube opening that provides a clue of how basalt lavas can flow for many miles. They crust over into well-insulated channels.

There are nicely developed spatter cones at the Chimneys, that like the other features we'd seen, looked as if they erupted practically yesterday instead of 12,000 years ago.

The pahoehoe flows seem to still retain a glassy veneer, a feature that is quickly lost in more humid environments.

Three vents from the Devil's Homestead flow can still be seen (the "chimneys"). One was originally 50 feet deep. Morons over the years kept throwing rocks in the hole, filling it. Volunteers recently pulled 35 TONS of rocks from this particular chimney. Looking down the vent made me think of the Sarlaac in Star Wars...

Our next stop was our first exploration of the lava tube system. The 3/4 mile stroll to Big Painted Cave offered an excellent view of Schonchin Butte, a classic cinder cone, one of many scattered across the flanks of Medicine Lake Highland.

Up next...going underground in Lava Beds!

The Other California: Waiter! There's A'a in my Pahoehoe!

Medicine Lake Highland is one of the finest places in California to compare and contrast volcanic landforms, lava flows, and pyroclastic debris. The volcano has erupted a complete range of different lavas in the last few thousand years, from silica-poor basalt to silica-rich rhyolite, resulting in the presence on the mountain of practically every kind of volcanic landform: a shield, cinder cones, plug domes, a caldera and tuff cones. And every kind of lava flow....

Today we are looking at basalt flows. Basalt can be highly fluid (non-viscous), forming smooth-surfaced flows that ancient Hawaiians compared to shark skin. When such lavas flow over edges, they form structures that look like coiled rope, as seen in the photo below. These are called pahoehoe lavas ("pa-hoy-hoy"). I have to admit I took the photo in Hawaii on the 1974 Maunu Ulu flow, not at Medicine Lake. As many times as I have visited Medicine Lake Highland, I have failed all those times to simply look down and take a picture of some pahoehoe lava...

As the lava cools, or if it has a different gas or silica content, the basalt lava can become more viscous. As it flows, it crusts over and the unstable surface breaks up repeatedly into an uneven rubble, even as the interior remains hot and molten. The Hawaiians called these a'a lavas ("ah-ah"). Some a'a is visible on the right side of the photo above, and fills the picture below. This shot is the Devils Homestead Flow in Lava Beds National Monument on the north flank of Medicine Lake Highland. It erupted around 12,000 years ago from an area in Lava Beds called the Fleener Chimneys.

The youngest basalt lava in Lava Beds National Monument, the Callahan flow, originated at Cinder Butte just south of the monument only about 1,100 years ago. A small quarry (below)reveals a cross section of an a'a flow, with the rubbly base, a solid interior, and the rubbly surface (a sketch map showing the location of Cinder Butte and the Callahan flow can be found in yesterday's post; it's a narrow dirt road, but accessible to cars with reasonable clearance). The barren surface of Cinder Butte is a wonderful example of a young cinder cone, formed as small fragments of lava (cinders) were explosively ejected from the eruption site.

A'a and pahoehoe flows can show transitional forms. In the photo below, from Black Crater in Lava Beds, the pahoehoe surface was beginning to break up as it congealed, forming slabby pahoehoe. This particular flow is probably only a few decades older than the Callahan Flow.

Medicine Lake Highland also has silica-rich rhyolite lavas. We will explore these in the next post.

In the interest of full disclosure, I stole today's title quip from an absolutely fantastic DVD called Lava Flows and Lava Tubes which is one of the best movies I have ever seen illustrating the dynamics of lava flows. It intersperses active Hawaiian flows with volcanic locales across the western United States, including Lava Beds. If you are curious about lava flows or are a teacher, it is a great resource.