Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Mark Stelten, research geologist with the U.S. Geological Survey.

Over its 2.1-million-year history, Yellowstone has erupted an impressive >4000 km³ (960 mi³) of rhyolite magma. While it is well known that a large rhyolite magmatic system has existed underneath Yellowstone throughout its history, it is less well known how this rhyolite came to be. In other words, how do you make a Yellowstone rhyolite? Where does it come from? What ingredients does it require?

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Before we can address this question, it is first necessary to have a general understanding of what rhyolite is and the components (or ingredients) that are available to make that composition. Rhyolite is a type of molten rock that is high in silica content. The greater the amount of silica, the more viscous, or sticky, molten rock becomes, and the harder it is for gases to escape from the molten rock. As a result, rhyolitic eruptions can be very explosive, like the massive caldera-forming eruption that formed Yellowstone Caldera about 631,000 years ago. If the gas is ultimately removed from the magma, rhyolite eruptions can produce massive and thick lava flows, like those that have largely filled the caldera since its formation.

In general, there are two types of components in volcanic systems like Yellowstone. First, there are basalt magmas—low in silica and therefore with low viscosity—that originate from deep within the Earth—far below Yellowstone’s shallow magma reservoir, which currently resides at 5 to 17 km depth (about 3 to 10 miles). Basalt magmas are commonly generated by melting in the upper part of the Earth's mantle (below the crust) at depths greater than 40 km (about 25 mi) and are injected into the Earth's crust where they may erupt at the surface, or stall within the crust and crystallize. At Yellowstone, basalts are thought to be related to a large thermal anomaly that is often referred to as a hotspot. These basalts provide the heat necessary to begin the process of making a rhyolite. Second, there is the crust. The term crust, or country rock, refers to the rock that surrounds Yellowstone’s basaltic and rhyolitic magma. The crust surrounding Yellowstone’s magmatic system is thought to be mostly made of >2-billion-year-old rocks similar to what is found in the Beartooth Mountains northeast of Yellowstone National Park.

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There are multiple ways to produce a rhyolite using basalt and/or crustal components. In one scenario, the rhyolites can be derived by crystallization of basaltic magma in a process called crystallization-differentiation. In this process, basaltic magma is injected into the crust and begins to solidify. When the basalt is almost entirely solid (~95% crystalline), the remaining liquid magma will be enriched in silica and have a rhyolite composition. In another scenario, the heat from the basalt injected into the crust can melt the crust and produce a rhyolite. These scenarios are extremes, and the actual process can be a combination of both. Now that we know the possible recipes, we can start to explore how are rhyolites “cooked” at Yellowstone!

Over the last several decades, geologists have examined this issue by measuring the isotopic composition of elements like oxygen, strontium, lead, neodymium, and hafnium in Yellowstone rhyolites, Yellowstone basalts, and rocks from the Beartooth Mountains northeast of Yellowstone. Isotopes occur naturally and refer to elements with a set number of protons but different numbers of neutrons. For example, ¹⁷⁶Hf has one less neutron than ¹⁷⁷Hf, where Hf stands for the element Hafnium and the number in superscript is the atomic mass. Isotope ratios (for example ¹⁷⁶Hf/¹⁷⁷Hf) are particularly useful because they act as “fingerprints” for different rock types and are vastly different between basaltic magma and crustal rock. By knowing the isotope composition of Yellowstone basalts and the surrounding crust and comparing them to the isotope composition of Yellowstone rhyolites, it is therefore possible to determine the relative percent of basalt and crust it took to make the Yellowstone rhyolites.

Based on isotopic compositions, geologists estimate that to cook a Yellowstone rhyolite you need to use approximately 50% to 70% basalt and 30% to 50% crust. Mixing of these two components happens in a complex fashion in the mid to lower crust, where hot (>900^oC) basaltic magmas tend to stall, crystallize, and differentiate. The hot basaltic magmas melt the surrounding crust and mix these two components together, producing a magma with an isotopic composition between that of basalt and the crust. This “hybrid” magma then continues to crystallize or is re-melted when new basalts intrude into the crust, producing a Yellowstone rhyolite.

It’s not something you’ll find in any cookbook, but thanks to the study of the isotopic “fingerprints”, it is possible to reconstruct the recipe for rhyolite! And the story from Yellowstone is that it’s a complex mix of magmas originating from great depth and the crust they interact with on their way to the surface. The end result is a magma composition that can feed both the caldera-forming blasts and thick lava flows that are found throughout the Yellowstone region.