Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Richard Tollo, emeritus Professor of Geology at George Washington University.

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Yellowstone National Park ranks among the finest classrooms in North America for learning geology through outdoor field trips.  This distinction results from a unique combination of geological events and characteristics developed especially throughout the past 2.1 million years.  Most of the major geological units can be visited in a day by taking a drive along the iconic Grand Loop Road and offshoots, bolstered by short hikes along well-maintained trails that are accessible from these roadways.

The primary geologic feature of Yellowstone National Park is perhaps that which is most difficult to observe in its entirety: that is, Yellowstone Caldera, a volcanic collapse feature that formed as a result of a major explosive eruption 631,000 years ago.  Two similar calderas formed as a result of comparable eruptions that took place about 1.3 and 2.1 million years ago.  Because each caldera is centered over an associated magma reservoir, pyroclastic deposits and lava flows are in close proximity to one another within or close to the caldera.  This geological concentration results in a dazzling array of closely spaced features that support productive field trips and produce many opportunities for geological education. 

Moreover, as visitors arriving from mostly lower elevations in the eastern, central, and southern United States rapidly discover as they find themselves short of breath while hiking, the Yellowstone region is an elevated plateau, with an average elevation of 8,000 ft (2,400 m).  The high elevation is caused by uplift due to its location atop a zone of mantle upwelling (hotspot) that transports mantle heat to the overlying crust and causes upward expansion.  As a result, nearly all streams in the headwaters of the Yellowstone River actively erode their channels, forming deeply incised valleys with steep canyon walls where rocks are exposed, providing unmatched three-dimensional views of the geology.  In this way, river erosion and tectonic uplift make a powerful combination acting to produce numerous and invariably instructive geological exposures throughout the park.

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The presence of roadways—especially the Grand Loop Road—provide benefits for field trips beyond facilitating transportation because construction involved the creation of roadcuts.  These cuts often expose bedrock, furnishing insightful views of primary geological features that might otherwise be altered or destroyed by erosion or chemical alteration.  Roadcuts have the added advantage of providing access to fresh, relatively less weathered rock, which is useful for collecting samples for laboratory studies, such as geochemical, paleomagnetic, and geochronologic investigations.

Construction projects support field trips in the Yellowstone area in other ways. A case in point is Grassy Lake Dam, which was built in the 1930s to impound local stream flow to create Grassy Lake Reservoir located just south of Yellowstone National Park.  A quarry that was excavated close to the eventual dam site provided rocks for use in the project.  The quarry, located less than 200 m (650 ft) south of the southern boundary of Yellowstone National Park, was developed in the Lava Creek Tuff (the welded ash unit that formed during the eruption that created Yellowstone Caldera) and provides considerable information regarding the genesis of that important eruptive formation that is not available elsewhere.  For example, the columnar-jointed tuff at the quarry site is hard and glassy, unlike parts of the same unit exposed elsewhere in the Yellowstone region. Such textures and field characteristics suggest that ash accumulated at relatively high temperatures, in agreement with insights from another nearby but different unit of the Lava Creek Tuff.  This interpretation, which in turn implies thar some silicic magmas at Yellowstone were unusually hot, might not be reached without the information provided by this locality. 

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Field trips allow geologists to share their findings with a broad audience, and also to educate the next generation of geoscientists in both Earth’s history and how to conduct geological investigations.  Field trips at Yellowstone are especially productive because of the many types of exposures—each with a story to tell.  Sharing knowledge among scientists and between geologists and the public is a bountiful way to augment our collective understanding of how the Earth works, and how Yellowstone came to be a geologic Wonderland.