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Volcanowatch

May 31, 2001

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


Lava tube magnetism

volunteers hard at work with magnetometer
USGS Volunteers for Science demonstrate use of the cesium magnetometer over a tube skylight. | large |

HVO 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 550 degrees C (1,020 degrees F). 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. We can see these creatures in lava tubes because tubes are the only voids in Hawai`i that we can both fit into. Modern peoples look to lava tubes for recreation - spelunking, or cave exploration, is growing in popularity as a hobby.

Lava tubes have also become a nuisance to some. One worries about whether buildings or vehicles will collapse into an unknown lava tube below. More recently, lava tubes have become a thing 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 Pu'ainako 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.

What is desirable is 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.

Of our arsenal of lava tube study techniques for active tubes, measurements of the magnetic field might work best for these older tubes. 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-sized lava tubes are approximately five percent of the total intensity of the earth's magnetic field in Hawai'i. 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.

We would be na?ve 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 burials, 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.

Eruption Update

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 1.5 km (0.9 mi) to the west of the boundary of Hawai`i Volcanoes National Park. Surface flows from this source reached the ocean on May 31 in an area west of Kamokuna. This second ocean entry is 3.5 km (2.1 mi) to the west of the Kupapa`u entry.

One earthquake was reported felt during the week ending on May 31. A resident of Volcano village felt an earthquake at 8:02 a.m. on May 31. The magnitude-3.2 earthquake was located 9 km (5.4 mi) south of the summit of Kilauea Volcano at a depth of 23.8 km (14.3 mi).


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Updated: June 4, 2001 (pnf)