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Pushing the boundaries: Montana State University updates Yellowstone’s geologic map

September 6, 2020

What better way to celebrate the upcoming 150th anniversary of Yellowstone National Park than with an updated geologic map? Scientists and students from Montana State University are spending this summer doing just that!

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Raymond Salazar and Jackson Grimes, undergraduate students in the Department of Earth Sciences at Montana State University.

Back in the 1960s and 70s, a group of United States Geological Survey mappers set out to determine what rocks made up the Yellowstone Volcanic Field. Identifying the rocks that make up a region is not only useful from a resource management standpoint, but also for reconstructing the geologic history of the park. Yellowstone National Park is very large (about the same size as Puerto Rico!), making it impossible to cover the entire area completely. To make mapping more efficient, the park was split into 34 separate smaller sections which were then stitched together to make a complete map, which was published in all its glory back in 1972 (an updated version is available as part of a major USGS Professional Paper about Yellowstone published in 2001).

Example of a boundary problem in the Yellowstone geologic map
An example of a boundary iproblem n the Yellowstone National Park geologic map, which was stitched together from many smaller mapped sections. The red line highlights the contacts that contain different units across the boundary.

The challenge, however, was that having multiple mappers working in different areas of the park often resulted in various interpretations, names, and levels of detail being positioned next to each other, leading to areas where rock units don’t line up across smaller map boundaries. While not necessarily a problem at large scale, where mappers were able to discuss their differences and agree on the major characteristics of the geology, it did highlight challenge areas in the geologic mapping at smaller scales.

This summer, a team of graduate and undergraduate researchers from Montana State University set out to address some of these challenges and increase the accuracy of the geologic map at small scales.  The team was led by the courageous M.S student Natali Kragh. The new map will increase the resolution of our current map and address many of the boundary problems mentioned above.

Correcting these problems is no small feat, however, as it requires geologists to visit hundreds of sites to search out contacts between rock units. To put this into perspective, there are currently over 700 known boundary problems! Some of these are easily fixable, such as areas where rock touches water. In many cases, however, the problems are not as simple. For instance, the team often visited boundary areas where rocks had been mapped incorrectly, or where mapped contacts were offset by hundreds of meters across a boundary (perhaps not surprising, since there was no GPS in the 1970s to aid with the precise positioning of contacts on a map!). Combining field work with modern technology, the Montana State University team aims to correct problems which were hard (or impossible) to fix previously.

Geologists from Montana State University study an outcrop of diorite.
Geologists from Montana State University study an outcrop of diorite (a silicic igneous rock that cooled slowly beneath the ground). An outcrop of this size is important because it is easier to see any structural relations along with rock composition.

Most of the problems are located in hard-to-reach areas deep in the backcountry, requiring the team to hike long distances through dense forests littered with deadfall, spiderwebs, wild animals, and who knows what else. Geologists had to cross raging rivers, battle giardia, and protect themselves from grizzlies and black bears. It wasn’t all bad, however. Sometimes the scientists found themselves skirting around seldom-visited hydrothermal areas or witnessing porcupines climbing trees! 

Upon arriving within several hundred meters of a boundary problem, the geologists began looking for suitable outcrops for data collection. Outcrops ranged from 30-meter cliff faces to scatterings of cobbles. When an outcrop was found (or was found to be missing!), a data point was collected. On an average day in the field, anywhere between 5 and 20 points were collected depending on availability of outcrop and the complexity of the problem. If the outcrop was too complex to be interpreted in the field, a small representative sample was taken under an official research permit (Milestones Permit #2020-50) for further analysis at Montana State University.

After six weeks in the field, enough data were collected to correct roughly 10% of problems found on the map (may we remind you, Yellowstone is the size of Puerto Rico!). But fear not, several problems can be addressed using remote datasets (like Google Earth and aerial photography) during the winter, which will set the team up for additional mapping in the future. With more trips planned for this fall and next summer, there is strong hope that the map will be done before the 150th Anniversary in 2022!

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