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Volcano Watch — At least some footprints in the Ka`u Desert are older than 1790

September 5, 2002

Luckily, Hawai`i experiences volcanic ash much less often than it does lava flows. When it comes, though, it can be anything from a nuisance to a disaster for those beneath the falling ash. The most recent ash fall, from Halemaumau in 1924, was minor but affected residents from Maku`u to Pahala. The latest major ash fall, in 1790, resulted from explosions at the summit of Kilauea.

HVO and Smithsonian volcanologists are investigating past ash eruptions to understand better their consequences and cause. This many-sided study ranges from the scientifically esoteric to the socially serious. Today's column describes two of the most recent findings, one of interest to the local community and the other a reminder to volcanologists that it's not always as simple as we'd like.<> The human footprints in the Ka`u Desert are internationally renowned. They were made in two different layers of wet ash, each containing numerous small spherical pellets of ash termed accretionary lapilli. Each layer has mud cracks and so must have dried in the sun before it was covered with more ash. This observation shows that the two layers are from two different eruptions, not surprising, since glassy sand-a different kind of ash-occurs between the two layers and suggests yet another explosion.

The footprints were discovered (or perhaps rediscovered) in 1920 by HVO seismologist Ruy Finch. Thomas Jaggar thought that all of the prints were made in 1790 by Keoua's warriors, many of whom perished that year during an explosion. The thinking of Jaggar's time was that few people ever visited the summit area of Kilauea, so the odds seemed high that the warriors made the footprints. Jaggar recognized but didn't worry about the fact that the footprints occurred in two distinct layers.

Recent discoveries by national park archaeologists and HVO volcanologists raise serious doubts about this interpretation. Archaeologists found ample evidence for people being in the area often, and volcanologists showed that the deposits containing the ash layers resulted from a number of eruptions between 1500 and 1790. The question now becomes this: Were the footprint-bearing ash layers deposited in 1790 or at an earlier time?

By tracing individual beds of ash, HVO scientists can now show that the earlier footprint-bearing ash was formed by eruptions before 1790, perhaps decades or more. This was done by identifying the deposits of the 1790 eruption (some of which are perhaps even older than 1790) and finding that the first footprints occur below those deposits. The age of the younger footprints is still uncertain, and we have more work to do before we can say whether or not they formed in 1790.

This work was comparatively easy, because the ash fell on bare lava flows and formed continuous layers of consistent thickness, much as snow falls on an empty parking lot. But imagine snow falling on a parking lot filled with cars. Some cars leave, some stay, some are bigger than others, some are parked at odd angles, the snow slides off some-uneven, discontinuous snow cover results. The same is true for ash falling on thick forest; the resulting deposits can be very challenging to understand.

An example comes from a trench recently excavated between KMC and Keauhou Ranch. Near the caldera, the sides of the trench showed obvious ash layers in deposits ranging in age from several hundred to several thousand years. Beyond the Volcano Golf Course Subdivision, however, the deposits are very complicated. Individual layers are discontinuous or even absent in many places. Lots of organic debris is intermixed in the deposits. Most likely, ash fell on thick forest with tangles of logs on the ground, such as in the nearby Ola`a Tract of the national park today. Ash became hung up on leaves and branches, sifting down at various times in the wind and rain. Some ash never reached the ground below logs. When the logs rotted away, ash slumped and sloughed to the ground in a chaotic way, mixing with organic debris. The end result is a messy deposit that makes the volcanologist long for the desert.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. From the end of the Chain of Craters road, lava can be seen flowing down Pulama pali and cascading over Paliuli. Many surface breakouts can be found in the coastal flats, and the National Park Service is allowing visitors to hike out and get up close to these active flows. At 6:17:42 a.m. on September 3, lava entered the ocean near the east end of the Wilipe`a bench. The ocean entry activity is now taking place along the leading edge of the Wilipe`a bench.

One earthquake was reported felt during the week ending on September 5. A resident of Glenwood felt an earthquake at 4:00 p.m. on Thursday, August 29. The magnitude-2.2 earthquake was located 27 km (16.2 mi) south of Ka`ena Point at a depth of 35 km (21 mi).