Volcano Watch: The art and science of geologic mapping

“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 Hawaiian Volcano Observatory geologist Drew Downs.

Geologic mapping has been one of the most fundamental mandates of the U.S. Geological Survey since its establishment in 1879.

U.S. Congress created the agency to “classify the public lands and examine the geological structure, mineral resources and products within and outside the national domain.”

The famous Western U.S. explorer John Wesley Powell (also the second director of the U.S. Geological Survey) 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 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 might occur.

Geologic maps, however, are not an exact depiction of the Earth’s surface.

Instead, they display a generalization of interested units and features in correspondence with the scale of a map.

Everything depends on the scale being used and what the geologic mapper is trying to portray. 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 case of 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 during the past few years being mapped within hours or days of activity starting using geographic information systems 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.

There is a standardized set of symbols, patterns and colors that are used for geologic maps published by the U.S. Geological Survey: 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 like 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,” shifting to greens, blues and purples, as the map units get older.

This makes it easy to glance at a geologic map and get a quick sense of the relative age of volcanic activity.

These U.S. Geological Survey geologic maps are generally printed, but all are now also published as geographic information systems digital databases and 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 Hawai‘i and its digital database published in 2021.

Now, the next time you are looking at a geologic map, you can appreciate the art and science that went into its presentation.

Volcano Activity Updates

Kīlauea on the Big Island is not erupting. Its U.S. Geological Survey Volcano Alert Level is at Advisory.

During the past week, earthquake rates beneath Kīlauea’s summit and upper-to-middle East Rift Zone area were slightly lower than the previous week, with about 25 earthquakes located beneath the summit and about 70 in the upper-to-middle East Rift Zone area.

Ground deformation rates continue to show slow inflation at the summit and near the Sept. 15-20 middle East Rift Zone eruption site.

Future intrusive episodes and eruptions could occur with continued magma supply.

Mauna Loa on the Big Island is not erupting. Its Volcano Alert Level is at Normal.

A brief increase in earthquake activity at Kamaʻehuakanaloa volcano (formerly Lōʻihi Seamount), submerged in waters south of the Big Island, took place Nov. 1-2. More than 100 events were detected, the largest of which was a magnitude-4.3 earthquake.

Activity could have been related to magma movement, but past eruptions at Kamaʻehuakanaloa have been accompanied by thousands of earthquakes throughout a period of days to weeks.

One earthquake was reported felt in the Hawaiian Islands during the past week:

• A magnitude-4.8 earthquake 3 miles southwest of Pāhala at a depth of 23 miles Nov. 5 at 1:42 a.m. This event was part of the ongoing swarm of deep seismicity that has been occurring beneath the Pāhala area since 2019.

The Hawaiian Volcano Observatory continues to closely monitor Kīlauea and Mauna Loa.

Visit the volcano observatory’s website for past “Volcano Watch” articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Email questions to askHVO@usgs.gov.