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July 23, 1998

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


Lava Flows Make Good Time Markers

Although the study of volcanoes is, in itself, fascinating and is more than a full-time job, volcanologists also work closely with researchers in other sciences. One of the things we contribute to the work of other scientists is the ages of the lava flows around the island.

As described in a recent "Volcano Watch," the age of a lava flow can be obtained by dating the charcoal found underneath it. The charcoal is all that remains of plants burned by the advancing lava, so that the charcoal and the lava flow have the same age. The ages of lava flows erupted since the early 1800s are known from historical accounts.

Other scientists use those ages as starting points to figure out the rate of growth of things that are found on those same lava flows. We think in a similar way at home when we find unrecognizable items in the back of our refrigerators: "Wow, this looks like a big oreo cookie, but it must be that spamburger I bought last week! That green, furry mold with the pink flecks must grow pretty fast."

The growth rates of plants, corals, and even beaches can be studied in the same way. For example, most of the trees on the rim of Kilauea caldera must be less than 200 years old because the soils they are growing in date from about 1790. On or about that year, Kilauea erupted explosively and apparently devastated most plant life in the vicinity. The trees near the eastern portion of Saddle road are even shorter than both those on Kilauea's rim and those farther away from the road because that roadway was built on the 1855 and 1881 lava flows from Mauna Loa.

Dated lava flows can also be used as markers in Hawaiian archaeology. Structures built on a lava flow must be younger than the lava flow. Structures partially covered or surrounded by a lava flow must be older than the lava flow. Using these rather obvious notions, petroglyphs found on a recent Hawaiian flow can be used to question published ideas about their interpretation.

Several books on Hawaiian petroglyphs point out that human figures are generally found as either stick figures, figures with a triangular body, or filled-in figures with muscular arms and legs. They then hypothesize that, because the styles range from simple to complex, the simpler ones must be older. The most recent petroglyphs should, according to this notion, be the filled-in figures with musculature. However, both stick and triangular human figures are found carved into the Hu`ehu`e lava flow erupted from Hualalai in 1801! The geologic perspective would seem to support the idea that the style of petroglyphs may be just that - a style.

Using these lava flow age markers, we have also raised questions about the building of Hawaiian trails. For example, a beautiful, curbed trail extends north from Kailua town along the coastline to the edge of the Hu`ehu`e flow. The trail appears to continue as a slightly crooked path worn into pahoehoe marked only by occasional bits of white coral. The curbed part of the trail is thought to have been completed in the 1830s but ends at the edge of the 1801 flow. Could the curbed trail be older than 1801? Unfortunately, the place where the curbed trail meets the edge of the Hu`ehu`e flow may have been destroyed during the construction of the Kona airport. We cannot now tell whether 1801 lava flowed over the curbed trail.

Using flow features, we can sometimes guess about the environment into which a particular lava flowed. Tree molds and wood remnants in those molds can tell us what trees were growing there. Beach rocks frozen into the surface of a lava flow can outline a former coastline. Using the Hu`ehu`e flow example again, we know that both `ohi`a and wiliwili were common (only wiliwili is present now) and that the pre-1801 coastline was as much as 3/4 mile inland of the present coastline.

We may only be scratching the surface of all the ways in which volcanological information can be used in other scientific studies. Rest assured that we will continue to seek them out.

Eruption and Earthquake Update

Kilauea's ongoing eruption paused briefly at the end of last week. The pause started early on the morning of July 17 and ended about 48 hours later on the 19th. During the pause, the lava tube leading from the vent to the ocean drained, and blockages developed as the unsupported roof and walls of the tube collapsed. When the eruption resumed, lava overflowed from skylights in the blocked tube, forming several surface flows on Pulama pali and the coastal plain. By evening on the 19th, lava had reoccupied the tube all the way to the ocean, where it entered the water at the Kamokuna site near the eastern boundary of Hawaii Volcanoes National Park. One surface flow on the pali persisted through July 22.

The Big Island experienced one felt earthquake in the last week. The magnitude 3.8 quake was located 28 km (17.5 mi) northeast of Kailua at a depth of 28.6 km (17.7 mi).

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Updated: 31 July 1998