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Volcanowatch

July 15, 2004

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Anatomy of a murder-or at least a tragedy

In 1790, possibly November, explosions from Kilauea killed between about 80 and about 800 people belonging to Keoua's party traveling to battle Kamehameha in Ka`u. The uncertainties in the number of fatalities, and even the date, are large, and we will probably never know the correct details.

One thing that we can do is study the deposits left by the explosions in hopes of unraveling what actually caused the deaths. By figuring out the size of the explosions, in what direction the debris was strewn, and how many explosions there were, we might be able to say where the deaths likely occurred and what kind of explosion was responsible for the tragedy.

These speculations would have to result from intensive geologic study of the deposits, aimed at understanding the processes that formed them. Such studies take time and considerable effort, and it isn't glamorous to dig into dirt and rocks to try to figure out the esoteric details necessary to reach sound scientific conclusions. But it can be fun-as well as challenging--to those so inclined.

Currently just such a study is underway. Costanza Bonadonna (UH-Manoa and University of South Florida), Bruce Houghton (UH-Manoa), and Liana Carroll (Kona native and recent geology graduate of Manoa), aided by Simona Scollo (PhD student in Italy) and HVO scientists, are examining in great detail the latest deposits in the Keanakako`i Ash, the formal name for all explosive deposits formed between about A.D. 1490 and 1790.

Whether the latest deposits were actually formed in 1790 is an open question. Contemporary accounts suggest the possibility of some explosions occurring after 1790, but it is generally assumed that they were minor and would leave little trace. We are assuming that the deposits do, in fact, reflect the 1790 activity and are relevant to the tragedy. The actual date is not important to the scientific questions of how the deposits formed.

How is such a study conducted? First, we look at the field relations of the deposits. Careful observations lead to recognition of one or more beds of rocky and ashy material that have certain characteristics-grain size, color, thickness, and relation to deposits above and below them-that allow the beds to be recognized from outcrop to outcrop.

Once such beds are identified, each takes on the character of the page from a mystery novel. If we can understand what we are seeing, we can add clue to clue to find whodunit-in our case, to figure out the nature of the explosions that threw out the debris.

So, we look at shallow exposures of the deposits along gully walls and in small pits a few centimeters (inches) thick. We measure the thickness of each deposit and how the thickness changes with location. The thicker the deposit, the closer, probably, to the source of the explosion. We measure the size of the rocks in the deposits. The larger ones probably fell nearer the site of the explosion. We measure the relative proportions of rocks and ash. Well sorted deposits (mostly rock or mostly ash) suggest fallout from the air, and poorly sorted deposits (a mixture of coarse and fine material) suggest hot flows or surges traveling along the ground.

These are some of the clues we look for. Each is far more complex than just outlined. All this work takes place in the field, though some of the grain-size measurements are done in the laboratory on samples collected in the field.

All information is plotted on maps to see patterns of distribution of each bed, and how its thickness, grain size, and sorting change laterally. In this way we piece together where the explosion took place, where it was directed, which way the wind was blowing, and how large the explosion might have been.

Our work is in its middle stages. We have much information already, but more has to be obtained. Currently the distribution of what has been dubbed the "Mystery Unit" is puzzling, for the deposit is spread over a wider area than had been anticipated. We hope to have the mystery solved, or at least narrowed to a few possibilities, by summer's end-we'd better, for Costanza has to move to Florida then!

Activity update

Eruptive activity at Pu`u `O`o continues. The Banana flow, which breaks out of the Mother's Day lava tube a short distance above Pulama pali, is entering the ocean off the 2002 Wilipe`a lava delta. The national park has marked a trail to within a short distance of the active lava delta, and thousands have been enjoying the show. In addition, lava has been visible between Pulama pali and Paliuli for the past several weeks. Eruptive activity in Pu`u `O`o's crater is weak, with sporadic minor spattering.

One earthquake was reported felt on the island during the week ending July 14. Residents of Waikoloa felt a magnitude 2.3 earthquake at 6:45 a.m. July 9. The earthquake was centered 12 km (8 miles) north-northwest of Waimea at a depth of 15 km (9 miles).

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low but was higher than it has been in some time, with 10 earthquakes located in the summit area during the past week.

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Updated: July 15, 2004 (pnf)