[VOLCANO WATCH]

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Mapping lava tubes
can help avert
future disasters

Tubes under buildings can
collapse or may contain burial
sites of native Hawaiians

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U.S. Geological Survey
Hawaiian Volcano Observatory

HAWAIIAN VOLCANO OBSERVATORY scientists have developed techniques to study conditions in the interior of active lava tubes and flows by making measurements on and above their surfaces. These techniques take advantage of the high temperature and electrical conductivity of molten lava.

For example, an infrared camera (sensitive to heat rather than visible light) can be used to map the active tube systems by imaging the slightly enhanced temperature over their surfaces. Estimates of the amount of lava flowing through a tube can be made by measuring the amount of distortion the electrically conductive lava produces in a radio wave passing over its surface coupled with a measure of the speed of the lava.

Hawaiian lava contains much iron and can form magnetic minerals. These crystals can make lava quite magnetic when it cools below 1,020 degrees Fahrenheit. Therefore, we can also study active lava tubes by measuring the variation in magnetic fields over them.

Long after tubes are abandoned by molten lava, they take on a different importance.

For ancestral Hawaiians, lava tubes were a source of fresh water. They also made excellent shelters and pathways for the living, and secure repositories for the dead.

Several species of insects, arthropods and spiders have adapted to life in cracks and voids beneath the ground. Modern peoples look to lava tubes for recreation -- spelunking, or cave exploration, is growing in popularity as a hobby.

Lava tubes also have become a nuisance to some, who worry about whether buildings or vehicles will collapse into an unknown lava tube below. More recently, lava tubes have become things to preserve during development.

Advocates demand that the government take extra measure not to collapse lava tubes while constructing roadways in order to preserve their possible recreational and scientific value. This has recently been an issue with the Puna emergency access road and the Puainako Street extension.

Large hotel and golf course developments have been slowed recently when Hawaiian burials were discovered within lava tubes. The courts have ordered 100-foot stand-off buffers around all known burial caves within the Hokuli'a development in Kona. Development must proceed around these buffered areas.

Researchers are trying to find a way to find and map lava tubes before developments are planned. Spelunkers map tubes with measuring tapes and a compass while walking or crawling inside, but the process is slow. A better solution would be to use measurements on the ground to reveal tubes beneath.

[UPDATE] Lava flows reach ocean Eruptive activity of Kilauea Volcano continued unabated at the Pu'u 'O'o vent during the past week. Lava flows down Pulama pali in two distinct areas. The cascade near the Royal Gardens subdivision has subsided in volume and crusted over since last week, but it continues to supply the ocean entry east of Kupapa'u. A lava channel descends the pali about 0.9 mile to the west of the boundary of Hawaii Volcanoes National Park. Surface flows from this source on Thursday reached the ocean west of Kamokuna. This second ocean entry is 2.1 miles to the west of the Kupapa'u entry. One earthquake was reported felt during the week ending Thursday. A resident of Volcano village felt an earthquake at 8:02 a.m. Thursday. The magnitude-3.2 earthquake was located 5.4 miles south of the summit of Kilauea Volcano at a depth of 14.3 miles.

Of our arsenal of study techniques for active tubes, measurements of the magnetic field might work best for these older tubes, as well. The absence of magnetic rock in the tube is detectable by mapping detailed magnetic field variations over the ground surface.

Field tests indicate that this can be an effective and economical way to map tubes. The variations produced by even medium-size lava tubes are approximately 5 percent of the total intensity of the earth's magnetic field in Hawaii.

The biggest technical difficulty is that many other lava flow structures (as well as anything that has even small amounts of iron) can also produce large magnetic field variations. Tubes are long, continuous features, but most other structures are not, so we can differentiate tubes from most other disturbances by preparing a map of magnetic field variations. Such a survey determined that the Kona airport runway extension was not being built over any large lava tubes.

It would be naïve to think that the difficulties between developments and lava tubes are only technical. Political and cultural issues exist whether or not we know where all the lava tubes are. If the issue is finding burial sites, mapping all lava tubes narrows the search but does not eliminate it, because not all lava tubes contain burials. Judicious use of technology to our advantage is an art that few have mastered.

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This article was written by scientists at the U.S. Geological Survey's Hawaiian Volcano Observatory. Contact the observatory at P.O. Box 51, Hawaii National Park, HI 96718; or call (808) 967-7328. "Volcano Watch" runs every Saturday.