August 6, 1998

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

It's Dirty Work, but Someone's Got to Do It

In 1790 a group of Hawaiian warriors in the Ka`u Desert was killed by an eruption of hot gas and flying rocks that originated from Kilauea caldera. Scientists have studied the layers of tephra visible in gullies along the southwest rift zone in Hawai`i Volcanoes National Park and have concluded that this event was the only major explosion of the last 2000 years.

Recent studies by geologists of the U.S. Geological Survey's Hawaiian Volcano Observatory suggest that Kilauea's explosive eruptions may happen more often than we think. The explosions form deadly clouds of ash, rocks, and pumice (collectively called tephra) that fall and blanket the ground in layers. These layers provide clues to the eruptions - where they came from, how big they were, and the order in which events happened. Each layer must have been deposited before the layer on top of it.

There are aspects of these tephra layers, called the Keanakako`i deposits, that do not make sense if all the tephra were deposited during the explosive event in 1790. Two circles of rocks, which look like Hawaiian structures, were found built on one layer of tephra and partially buried by later deposits. People generally would not build in a place where ash and rocks were raining down on their heads. The structures imply a quiet time between eruptions, long enough for people to feel this location was safe. A discovery like this is an exciting moment for a scientist, like the light bulb flashing in a comic character's thought bubble.

Excitement is tempered by hours and days of slow and persistent work, seeking answers to small questions that add up to the larger solution. One question is how to distinguish separate layers of tephra. Individual layers of the Keanakako`i deposits can be identified by their color and the type of tephra - ash, rocks, or pumice - within them. The layers are often accessible only by the digging of pits. The ditch-diggers get coated in dust and sand from the Ka`u winds but return triumphantly with small, labeled bags of ash and rock samples.

The samples are dried in the lunch room oven (50 minutes at 200 degrees) and poured through a series of sieves stacked like a multi-layered double boiler. The sieves separate the samples of ash and rock by grain size. Analysis of the grain sizes helps identify what type of explosion deposited the tephra layer. This increases the understanding of Kilauea's explosive eruptions.

Another part of the puzzle came from scientists, armed with the willingness to see Keanakako`i deposits in a different light, who found new evidence for multiple eruptions. They observed that some layers of tephra had been cut by gullies and later filled in by more tephra from explosive eruptions. Some scientists argue that explosive blasts from the volcano could gouge gullies in existing tephra layers. Newly discovered gullies, however, could not have been carved by explosive blasts because they are perpendicular to the direction of the blasts. This suggests the gullies were carved by water, a process which could take many years. Because the gullies were filled by newer ash, their formation must have occurred in quiet periods --between eruptions.

When the current study of the Keanakako`i deposits goes public, some will disagree with the results and seek new evidence to disprove them. This is good. Close scrutiny motivates scientists to be accurate and prepared to defend their findings. Scientific understanding increases when past assumptions are questioned and examined. New findings about the Keanakako`i deposits may reveal that explosive eruptions occur more frequently than previously thought and help Big Island residents prepare for this destructive threat.

Eruption and Earthquake Update

No earthquakes were reported felt since the last update.