Volcano Watch — Big Island geologic map

This week's column was written by Edward W. Wolfe, co-compiler of the recently released geologic map of the Island of Hawaii. Ed was a staff geologist at the U.S. Geological Survey's Hawaiian Volcano Observatory from March, 1982 to June, 1984. After leaving the staff of HVO, Ed served as principle scientist of the Big Island Mapping Project which produced this detailed map. Ed is currently a geologist at the U.S. Geological Survey's Cascades Volcano Observatory where his project is the geologic map of Mount St. Helens.

Volcanoes, glacial deposits, and faults related to huge undersea landslides are three of the remarkable geologic features depicted on a colorful new U.S. Geological Survey geologic map of the Island of Hawaii.

The map is the first to show in detail the age and distribution of the lava flows that form the surface of the Big Island. The new map is a compilation of geologic mapping undertaken from 1975 to 1995 by approximately 20 geologists from the Geological Survey and various universities. This mapping has greatly refined our understanding of the natural history and growth of the island. The map's emphasis on the chronology of the lava flows reflects the application of age-determination techniques that were unavailable when the map's classic predecessor was published by Harold Stearns and Gordon Macdonald in 1946.

The chronologic information is highlighted by map-unit colors that are increasingly bright with decreasing lava age. The three volcanoes that have been active in the last 200 years, Kīlauea, Mauna Loa, and Hualālai, are largely covered by lavas younger than 10,000 years old. Lavas younger in age than 750 years pave most of Kīlauea, cover less than half of Mauna Loa, and are represented by only a few flows and vent deposits on Hualālai. The map shows a handful of postglacial (see below) lava flows and cinder cones on Mauna Kea, all older than 4,000 years. Kohala, the northernmost volcano, has been inactive for more than 100,000 years. These differences from one volcano to the next are related to changes in eruptive style and decrease in eruptive vigor as the volcanoes age.

Most people don't think about glaciers in Hawaii, but the new map shows glacial deposits that formed high on Mauna Kea during the ice age. The map shows glacial moraines that formed about 70,000 years ago and younger ones deposited approximately 40,000 to 13,000 years ago by a more recent ice cap. Mauna Kea is the only volcano in the Hawaiian chain known to have been glaciated. If glacial deposits ever formed on the similarly high summit of Mauna Loa, they have long since been buried by younger lavas.

The map shows landforms that are related to the enormous submarine landslides that have been the subject of much attention recently. Westward-facing palis at Kealakekua Bay and on Mauna Loa's southern cape are fault scarps that are interpreted as remnants of the breakaway zone for a massive submarine landslide complex that was active more than 100,000 years ago. On Kīlauea's south flank, the prominent pali is part of the Hilina fault system, which is apparently the breakaway zone for a massive, active submarine landslide that extends about 70 km offshore. Abrupt downward and seaward motion of this landslide mass during the 1975 magnitude-7.2 earthquake resulted in 11 feet of vertical and 26 feet of horizontal displacement of Kīlauea's south coast.

The USGS publication consists of two maps, the geologic map and a sample-data map. They may be ordered separately or as a set, by telephone at 1-800-USAMAPS or by mail from the U.S. Geological Survey Information Services, Box 25286 Federal Center, Denver, CO 80225 Orders must contain the following information: Geologic map of the Island of Hawaii, U.S.G.S. Miscellaneous Investigations Series Map 1-2524-A, 1:100,000-scale, 3 sheets with descriptive pamphlet, Wolfe, E.W., and Morris, Jean, compilers, 1996; or Sample data for the geologic map of the Island of Hawaii, Miscellaneous Investigations Series Map 1-2524-B, 3 sheets and pamphlet, Wolfe, E.W., and Morris, Jean, compilers, 1996.

The new geologic map may also be purchased in Hawaii Volcanoes National Park, at the main Visitors Center or the Jaggar Museum, from the Hawaii Natural History Association.

Volcano Activity Update

The eruption from flank vents on the western side of Pu`u `O`o continues unabated, with lava flowing through the six-mile long tube system and entering the ocean at Lae`apuki. The HVO seismic network recorded a series of large rockfalls from 8:00 p.m. on Sunday, December 8 to 4:30 a.m. on Monday, December 9. The signals originated from Pu`u `O`o, and upon inspection of the cone, geologists found that the circular collapse pit on the west flank was greatly enlarged. The pit is now a trough that extends from the episode 51 vent collapsed area up to within 35 feet of the summit of Pu`u `O`o. The continued collapse of Pu`u `O`o is expected, and hikers are warned to stay away from the unstable cone.

Two earthquakes were reported felt during the past week. Residents of Leilani Estates were shaken at 8:20 p.m. on Monday night, December 16, by a shallow temblor located five miles east-southeast of Pahoa. A magnitude of 2.4 was registered for the earthquake. The shallow depth of focus and proximity to a populated area accounted for such a small earthquake being felt. Earlier that day at 52 minutes past midnight, residents of the Volcano Golf Course subdivision felt an earthquake which was located in Kīlauea caldera at a depth of one mile and had a magnitude of 2.7.