Volcano Watch —Lava flow hazard map revisited

Maps showing volcanic hazard zones for the Big Island were first prepared in 1974 by the U.S. Geological Survey. The mapping project was instigated by the Department of Housing and Urban Development (HUD), which was required to consider natural hazards before extending its programs to new areas. At HUD's request, the USGS prepared maps delineating hazard zones for lava flows, ashfall from explosive eruptions, ground rupture, and ground subsidence.

The original map showing lava flow hazards was updated in 1987 because a wealth of new information was available from geologic mapping on Kīlauea, Mauna Loa, and Hualālai Volcanoes. By mapping and dating prehistoric flows, geologists now had a better idea of the eruption history of each volcano and of how often any given area had been overrun by lava.

The 1987 lava-flow hazard map was incorporated into a booklet entitled, "Volcanic and Seismic Hazards on the Island of Hawaii", published in 1990. This booklet is still available at no cost from the Hawaiian Volcano Observatory.

Partly in response to demand for a larger version of the lava-flow hazard zone map, it was revised slightly in 1992 on the basis of more information from geologic mapping and published in cooperation with the Hawaii Office of State Planning. The new map has a scale of 1:250,000 and can be overlain on the USGS topographic map of the Big Island, which is at the same scale. The hazard map is currently for sale at "Basically Books" bookstore in Hilo.

The lava-flow hazard zones are based solely on geological criteria, including the location of eruptive vents, past lava flow coverage, and the topography of the volcanoes. Hazard zone boundaries are approximate, and the change in degree of hazard from one zone to the next is generally gradual rather than abrupt.

The current map divides the island into nine zones based on the likelihood of coverage by lava flows. Zones 1-3 are limited to the active volcanoes of Kīlauea and Mauna Loa. Zone 1, the most hazardous, includes the summits and rift zones of Kīlauea and Mauna Loa where vents have been repeatedly active in historic time and lava flows will originate in the future. Areas adjacent to and downslope of active rift zones make up zone 2. All of the 181 houses destroyed in the 1983-present eruption of Kīlauea were located in zone 2, about 8 miles from the vent.

Zone 3 includes areas gradationally less hazardous than Zone 2 because of greater distance from recently active vents or because the topography makes it less likely that flows will cover these areas.

Zone 4 comprises all of Hualālai Volcano, where the frequency of eruption is lower than on Kīlauea and Mauna Loa. Hualālai is in a single zone because its slopes are steep and flows could rapidly cover the distance between potential vent sites and the coast. Therefore, the hazard is considered to be essentially equal anywhere on the volcano.

Zones 5 and 6 are areas on Kīlauea and Mauna Loa currently protected from lava flows by the topography of the volcano. Zones 7-9 include the dormant volcanoes of Mauna Kea and Kohala. The younger part of Mauna Kea, which last erupted about 3000 years ago, makes up zone 7; the rest of the volcano is included in zone 8. Zone 9 consists of Kohala Volcano, which last erupted over 60,000 years ago.

We can do little to reduce or eliminate the volcanic hazards that have always existed on the Big Island, but we can greatly reduce the risks to life and property posed by these hazards. Our best tools for reducing risk are proper land-use planning, which limits development in high hazard areas, and education, which gives people a rational basis for deciding where to build a home, develop commercial property, or locate a public facility.