# Volcano Watch — Volcano mapping among most detailed on Earth

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Geologic mapping has come a long way since the first map accurately showing the locations of lava flows on the Island of Hawaii was published by Stearns and Clark in 1930. A more detailed geologic map of the entire island was produced by Stearns and McDonald in 1946.

Volcano mapping among most detailed on Earth

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Geologic mapping has come a long way since the first map accurately showing the locations of lava flows on the Island of Hawaii was published by Stearns and Clark in 1930. A more detailed geologic map of the entire island was produced by Stearns and McDonald in 1946. These earlier maps shows the "historic" (post-1790) lava flows of Mauna Loa, Kīlauea, and Hualālai, but lumped most of the older flows into geologic units that included flows separated in age by thousands of years.

Today, through the combined use of earlier geologic maps, aerial photos, satellite images, and extensive fieldwork, the geology of Kīlauea, Hualālai, and Mauna Loa (in progress) has been mapped at the 1:24,000-scale to produce geologic maps that are among the most detailed of any volcano on earth.
In 1991, the U.S. Geological Survey's Hawaiian Volcano Observatory entered a cooperative venture with the State Department of Land and Natural Resources to convert the detailed geologic maps of Kīlauea and Mauna Loa into digital (computer) files for use in a geographic information system. To date, all of Kīlauea and nearly half of Mauna Loa has been converted to digital files for the GIS (See Map 1). Storing the maps in a GIS gives map users capabilities that were never before possible or were extremely time-consuming.

A GIS is a computer system designed to efficiently enter, store, update, manipulate, analyze, and display all forms of geographically referenced information. A GIS should not be confused with other types of computer graphics systems, such as computer-aided design systems that are often used to store engineering or architectural drawings, as well as map data. The major difference between the two types of systems is that GIS can integrate map and descriptive data for subsequent map analysis; CAD systems do not have this capability.

At the Hawaiian Volcano Observatory, GIS has revolutionized the progress of large-scale geologic mapping. The advent of GIS has revolutionized the process of geologic mapping by allowing us to compile maps on the computer with greater accuracy and efficiency. Until very recently all additions and corrections had to be hand-drafted in ink.

Information about each lava flows' age, mineral content and texture (for example, whether it is aa or pahoehoe), is stored in the GIS. With this information, derivative maps - such as those showing the distribution of all aa flows that were erupted less than 1,000 years ago - can be readily displayed and plotted. A series of such maps can show patterns in the eruption of different lava flows and provide us with a better understanding of a volcano's history. In the not-so-distant future, the GIS will also be used to map active lava flows, enabling us to disseminate information quickly and easily to the various state and county agencies. The lava flow maps can be combined with census maps on the computer to obtain an estimate of the number of residents who need to be warned about the eruption. This is particularly important for residents on the southwest flank of Mauna Loa, where lava flows tend to advance more rapidly, leaving little warning time for those who need to evacuate.

Digital geologic maps are currently being used to assess lava flow hazards. The "Map Showing Lava-Flow Hazard Zones, Island of Hawaii" (USGS MF-2193) by Wright and others, was published from a digital map in 1992. Precise estimates of the percent of area covered by lava since 1790 within each hazard zone can now be calculated by overlaying the hazard boundaries with a geologic map stored in the computer.

The GIS is also being used to estimate the probability of lava flow inundation in specific regions. This is done by first calculating the number of times the region has been affected by lava. Once calculated, the lava flow recurrence interval can then be used to estimate the probability of lava flow inundation occurring in, say, the next 50 years. On Kīlauea's East Rift Zone, for example, the probability of inundation by a lava flow within a 50-year time span is 92% for hazard zone 1, 71% for hazard zone 2, and 22% for hazard zone 3.

The application of GIS to geologic and hazard assessment problem is still in its infancy. We expect to find many other ways to use this powerful tool in the years to come.