Studying Yellowstone’s volcanic system at the microscopic scale

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Some of the most valuable data used to understand the evolution of the Yellowstone volcano come from the microscopic world. What are some of the tools that researchers use to study the microscopic products of the volcano’s multiple eruptions?

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Brandi Lawler, Ph.D. student, and Kenneth Sims, Professor, both with the Department of Geology and Geophysics at the University of Wyoming.

Thin section made by slicing a small layer off the surface of a hand sample of Yelowstone lava.

Thin section made by slicing a small layer off the surface of a hand sample of Yeloowstone lava. Note the marker for scale.

(Credit: Brandi Lawler, University of Wyoming. Public domain.)

The footprint of the Yellowstone volcanic system during the last 2.1 million years covers an area that is nearly 17,000 km2 (or roughly 10,560 square miles) – greater than that of Maryland and 8 other states! Indeed, Yellowstone is quite large compared to many other well-known volcanoes around the world, such as Mount St. Helens in Washington, or Mount Fuji in Japan. The extremely large size of the volcano prevented the early explorers of the region from recognizing it as a single volcano. Those who have been fortunate enough to spend time in Yellowstone National Park would likely agree that when inside the Park, it is easy to forget you are in the midst of a volcano unless you happen to be near one of the many spectacular hydrothermal features or large lava flows.

Given the volcano’s immense size, some might find it surprising that one of the most useful ways to learn about its eruptive history involves looking at objects so small that they often cannot be seen without the aid of a microscope. These objects are the tiny minerals which make up the lava flows that cover the Yellowstone Plateau. While in some instances it is possible to identify the minerals just by looking at a piece of rock in your hand, it is often necessary to use special equipment to slice and polish a piece of rock until it is only about 0.03 mm (0.001 inches) thick. That tiny slice of rock is then mounted on a microscope slide that is typically 27 mm by 46 mm (1 inch by 1 and 7/8 inches) to make what is referred to as a “thin section” (someone clearly put a lot of creative thought into naming it!).

Thin section image of a lava sample from Yellowstone using a polarizing microscope.

Thin section image of a lava sample from Yellowstone using a polarizing microscope. The mineral assemblage is representative of many of Yellowstone’s basaltic rocks. The three large and colored crystals (known as phenocrysts) in the center of the image are the mineral olivine. The olivines are surrounded by a groundmass of small plagioclase feldspar crystals (white-grey blades) and micro-olivines (colorful specs). The black blebs within the olivine phenocrysts are inclusions of a mineral known as spinel.

(Credit: Brandi Lawler, University of Wyoming. Public domain.)

Researchers are then able to place thin sections on a transmitted light microscope. By changing the orientation of polarizers on the microscope, it is possible to reveal visible characteristics that are unique to each specific mineral, thereby allowing the researchers to determine the type and abundance of minerals within each lava flow. This information can then be used to understand such things as the potential depth of magma formation or the relative speed at which the magma cooled during its ascent to the surface.

For example, the presence of the larger olivine crystals with spinel inclusions in thin sections of a Yellowstone basaltic lava flow suggests that the magma was stored at significant depth within the crust for a period of time. This storage allowed the magma to cool slightly and grow both the spinel and surrounding olivine crystals. When a magma erupts without having cooled significantly as it rose through the crust, the minerals in the resulting solidified lava will remain microscopic and will have a somewhat glassy appearance in a hand-sized sample.

Thin sections can also be placed within another special type of microscope, known as an electron microscope, where individual minerals are bombarded with electrons to reveal the chemical composition of the minerals at the atomic level.

The amount of information that can be revealed by examining a single 0.03 mm slice of rock is vast. When one considers the massive size of the Yellowstone volcanic system, it is plain to see that there is a whole lot more left to explore.