Answer to a Hawaiian Mystery, and a Cautionary Tale

If you've had a geology or earth science course, do you remember what you learned about basalt? Basalt, the low-silica volcanic rock, the one that flows instead of exploding. The one that isn't all that dangerous. Even if you haven't had such a class, you've heard that visiting volcanoes on the Hawaiian Islands is one of the things tourists can do. Helicopters fly over the lava flows, and people watch lava pouring into the sea from a few hundred yards (or feet) away. Basalt is the black volcanic rock with holes in it.

Pele's Hair collected near the edge of Kilauea Caldera

In my last post, I provided a bit of a mystery, a series of circles found in the barren plains near the summit of the Kilauea caldera on the Big Island. I'm going to provide the answer (and yes, someone accurately solved it), but first I'd like to show you some unusual things you'll see if you get access to some parts of the Kilauea volcano complex.

First off, the stuff in the first two pictures. These fibers are found around the summit area of Kilauea, and in protected hollows they can accumulate in large masses. It's called Pele's Hair, and it's made of natural volcanic glass, otherwise known as obsidian. Glass is not usually associated with basalt in the minds of most people, but glass can form around any lava that cools so quickly that crystals can't readily form. This odd feature develops around spattering edges of lava lakes like that which currently resides in the crater of Halemaumau. As globs of liquid are thrown into the air, some of the liquid trails behind as a thin fiber, which then breaks off and floats away in the turbulent hot air currents. If you visit the Big Island, you can usually find some near the Jagger Museum on the crater rim.

Reticulite from Kilauea Caldera

Then there is this weird material that can also be found around the summit region of Kilauea. It made me think of old weathered sponge rubber, but it is no such thing. It is a rock. It's composed of volcanic glass, and could be described as a sort of golden pumice, but it is distinctly different from any pumice I've ever seen. It's lighter, for one thing, and that is hard to believe, even while holding it in your hand. Most pumice is between 64-94% air bubbles, but this material exceeds 95% air. The walls of the bubbles are so thin that many are open, and this rock will not float the way that pumice can because it fills with water too quickly. It is called reticulite. It's so light that it can be blown a long ways from a crater by high winds. It is so delicate it can be crushed between one's fingers, and it can't be expected to last long in most geological environments.

Close up of reticulite from Kilauea

And finally there is this rock outcrop on the rim of Kilauea Caldera. It looks, well, almost like sedimentary layers! That is most decidedly not the kind of thing one expects to find on the edge of a basaltic shield volcano, the edifice that is supposedly constructed by multitudes of basaltic lava flows. What the heck is going on here, and what does it have to do with the strange circles of our little mystery?

A closer look reveals that these are layers of volcanic ash and scoria, the smaller particles that are associated with explosive eruptions, the kind we expect to find on the slopes of a Mt. St. Helens or a Mt. Shasta, the stratovolcanoes found on continental landmasses near subduction zones. What was going on here? The layers are more than 30 feet thick, and have been named the Keanakāko‘i Tephra.

Keanakāko‘i Tephra partially covered by a 1983 basalt flow.

It's clear that what we get taught about basaltic lava is not the entire story. Sometimes basalt erupts violently, and as such it can be exceedingly dangerous. An eruption in 1790 killed several hundred Hawaiian warriors on the eve of a major battle, and the event changed Hawaiian history, as the tragedy was seen as the judgment of the gods. These deposits were once thought to be the results of the 1790 eruption, but it turns out that they include dozens of explosive eruptions that took place between about 1500 and the early 1800s.

Exposures of the Keanakāko‘i Tephra on the margin of the Kilauea Caldera. A 1983 basalt flow can be seen below on the right.

What caused this explosive activity? In a word: water. When rising magma encounters groundwater, the water can flash to steam, causing intense explosions. Apparently the caldera collapsed to a depth great enough to reach the regional water table, and huge explosions ensued. Something like this happened at Kilauea in 1924 (see the picture below), but the massive explosions totaled only about 1% the volume of the 1790 and earlier eruptions. There have been some seriously dangerous eruptions throughout time on this volcano.

1924 eruption of Kilauea Caldera, courtesy of the USGS and Bishop Museum

And that brings us to the strange circles of the mystery. In 1924 some huge blocks were thrown out of Halemaumau crater and were thrown a thousand or more meters. When they landed, they produced bowl-shaped craters. One of the biggest from 1924 weighed 8 tons, and can be seen in the picture below.

Subsequent eruptions produced Pele's Hair, reticulite, and small cinders or coarse ash. All of these particles blew across the landscape, and accumulated in the shallow craters. The blocks remain visible in the centers of some of the craters, while others are buried. The filled craters in some cases trap water more efficiently than other surfaces, so plants are able to gain a foothold (roothold?) in the craters.

The evidence of 300+ years of explosive eruptive activity around Kilauea is sobering. Such eruptions have the potential to do serious damage to surrounding communities around the caldera and in the Puna District to the east. Current research is seeking to better understand the cycle of activity surrounding these periods of violence.

It was a real privilege to explore the flanks of Kilauea Caldera with Don Swanson, Tina Neal, and Frank Trusdell of the Hawai'i Volcano Observatory during my visit to the islands last week. It was a fascinating learning adventure. More stories to come!