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Field Geologists are Always Students and it Pays Off

August 9, 2013
The following is this week’s edition of “Volcano Watch” from the USGS Hawaiian Volcano Observatory:

Explosive interaction between lava and seawater blasts a tephra jet consisting of steam, hot water, black tephra, and molten fragments into the air. (USGS Photo by J.D. Griggs February 3, 1988)

No matter how old, a field geologist is a student. The teacher is the earth, the classroom the field area, and the textbook the deposit under study. Some books are harder than others, frustrating, confusing, and even downright intimidating. Then you come to the paragraph that makes everything clear, and you wonder why the teacher didn’t just say so in the first place!

Recently, an HVO geologist took a field class that was way over his head. It should have been simple. Beginning geologists learn that when an explosive eruption occurs, fragments called “tephra” (volcanic ash and pumice, for example) fall to the ground. To find evidence of the eruption, all you have to do is poke around on the surface to find tephra, which will form a continuous deposit wherever it fell. Erosion by wind and water will remove parts of the deposit, but you search for, and find, remnants that allow you to piece together the deposit. You don’t look in gullies, where running water destroys it.

Explosive eruptions, like this one from Halema`uma`u on May 23, 1924, produce downwind ash that can become rich soil if abundant enough. (USGS)

South of Kīlauea Caldera, the ground is barren of pumice bombs except where they fell during the 1959 eruption of Kīlauea Iki. Now the class starts getting hard. Beyond the limit of the 1959 deposit, our student found pumice in very strange places—not on the general ground surface but, instead, low on the sides of gullies eroded into the ground surface. The pumice is plastered on the gully sides, starting a few centimeters (about an inch) above the bottom of the gully and reaching 20-40 cm (8-16 in) up the sides. The pumice is absent higher on the gully sides and on the ground surface between gullies.

The first thought was that water had washed the pumice along the gully from the 1959 deposit. But that didn’t square with the absence of 1959 pumice in the headwaters of the gullies, and it also didn’t match the details of the pumice deposit itself. The deposit is crudely layered, its lower third containing a lot of Pele’s hair and tears and its upper two-thirds containing little. How could water that washed pumice into scores of different gullies always leave a similar deposit? The student was stumped.

Then he realized that 1-2 cm (up to an inch) of rocky tephra (we’ll call it unit H) overlies, and therefore is younger than, the pumice! That meant that the pumice could not be from 1959, since no eruptions since 1959 could have formed unit H. The pumice and unit H both had to be older than 1959! This was the “Aha!” realization, the clarifying paragraph.

Current webcam image of Halema’uma’u Crater. Click to enlarge. (USGS)

The student had already discovered that the pumice overlies tephra from an eruption in 1790. He couldn’t be sure exactly how old the pumice is, but let’s say 1800. It turns out that this was the first pumice erupted from Kīlauea Caldera since about 1500. For 300 years, many explosive eruptions had occurred without pumice, because the caldera was so deep that groundwater interacted with magma and inhibited the formation of pumice. The newly discovered pumice indicates that, finally, the caldera was filling.

There are loose ends to tie before the student’s hard-won lesson is over. Why doesn’t the pumice appear on the ground surface between gullies? Unit H is there, but no pumice underlies it. The reason, the student thinks, is that the pumice is so light that it was eroded away by strong wind and rain before unit H was erupted—except in protected settings. Low in a gully is a protected setting. The student often rests in gullies to escape the fierce trade wind; so, in a sense, did the pumice.

This class took months to complete. The story above is true but simplified, leaving out dead ends. The final lesson is that pumice, because it is so light and easily eroded, can be swept away and leave only a bare trace of its previous existence. Low on the side of a gully is where to find it—the very place beginning students are told to avoid. Realizing that, an important eruption was discovered that told of a big change in Kīlauea’s eruptive style 200 years ago. The class was tough and the teacher unreceptive to dogma, but the learning experience was exhilarating!

Hawaiian Volcano Observatory

One Comment leave one →
  1. August 9, 2013 5:51 pm

    Great post! This geology buff would love to see more like it.

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