The Art and Science of Geologic Mapping — USGS Volcano Watch
Geologic mapping has been one of the most fundamental mandates of the U.S. Geological Survey (USGS) since its establishment in 1879. Congress created the USGS to "classify the public lands and examine the geological structure, mineral resources, and products within and outside the national domain."
Examples of Hawaii geologic maps. The lefthand map is a USGS Hawaiian Volcano Observatory map created on September 17, 2024, within hours of remote sensing (helicopter overflight) of the eruption that occurred from September 15–20, 2024. Several age ranges for lava flows of interest are noted by color changes, with those that erupted from 1790–2018 in purple (older lava flows are gray), those erupted on September 15 in pink, and those erupted September 16–17 in red (with the active fissure as a yellow line). This map also shows roads and the boundary of Hawaiʻi Volcanoes National Park, as these would be of interest to those using this map. The righthand geologic map is that of the Island of Hawaiʻi from the Geologic Map of the State of Hawaii by Sherrod and others (2021). This map was compiled through decades of work and displays the various lava flows, tephra deposits, and other rock and sediment types mapped through fieldwork and remote sensing. The primary purpose of this geologic map is to show all mappable geologic units with their colors denoting their source volcano and age. This map is free to download at https://pubs.usgs.gov/publication/sim3143.
The famous western US explorer John Wesley Powell (also the second director of the USGS) convinced Congress into authorizing the "preparation of the geological map of the United States." The first geologic maps were uniform in size and contained all available information on topography and geology, with accompanying text describing mapped geology. But sometimes too much information on a map can make it difficult to read and interpret. Modern maps tend to be more versatile; displaying geologic deposits (also commonly referred to as map units) and features of special interest for a project or investigation.
Modern geologic mapping efforts have centered more around investigations that are deemed of critical importance, such as those vital to energy (oil and gas) and economic (minerals and ore deposits) resources, surface and groundwater resources, urban development and land use. The last one is of particular importance as the Earth is a dynamic place and understanding the locations and ages of past volcanic (lava flows and tephras) and tectonic (faults and fractures) features gives us a blueprint for where and when future activity may occur.
Geologic maps are not an exact depiction of the Earth’s surface. Instead, geologic maps display a generalization of interested units and features in correspondence with the scale of a map. If mapping is done at a small scale, which would be zoomed out to show a large area, then smaller geologic units and features might not be able to be accurately displayed. In the end, everything depends on the scale being used and what the geologic mapper is trying to portray.
In the case of USGS Hawaiian Volcano Observatory geologic mapping on the Island of Hawaiʻi, the primary map units of interest are volcanic vents (such as fissures and scoria cones) and their associated lava flows and tephra deposits divided by age. It’s easy to make these determinations for young eruptions that have been witnessed, with eruptions over the last few years being mapped within hours or days of activity starting using Geographic Information Systems (GIS) software. Remote sensing techniques using aerial and satellite imagery have also made this much quicker.
While some older lava flows can be mapped using remote sensing, others exposed to the elements for hundreds or thousands of years can be harder to tell apart. Therefore, diagnostic criteria from the field or lab is usually required to distinguish their geographic extents when compiling a geologic map. Geologists make field excursions to document minerals present in the lava flows and their abundances, and collect samples to analyze chemistry, radiometric ages, and paleomagnetism. Usually, a combination of these is needed to put together a full picture on a map sheet.
For geologic maps published by the USGS, there is a standardized set of symbols, patterns, and colors that are used: the Geologic Map Schema, or GeMS for short. Whereas symbols and lines tend to be objective on a geologic map (for example, set line thicknesses, styles for things line contacts, faults, and ground cracks), colors used for geologic map units can be more subjective. It is common for geologic maps that portray volcanic terrains with young lava flows and tephras to have the ‘hottest’ colors, such as reds and oranges, and those colors gradually get ‘cooler’ going to greens, blues to purples as the map units get older. This is makes it easy to glance at a geologic map and get a quick sense of the relative age of volcanic activity.
These USGS geologic maps are generally printed but all are now also published as GIS digital databases that are freely available to be downloaded. A recent geologic map addition to get a sense for scale, color, and symbology used is the Geologic Map for the State of Hawaii and its digital database published in 2021. The next time you are looking at a geologic map, you can appreciate both the art and science that went into its presentation.
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Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates. This week's article is by HVO geologist Drew Downs.
