Caldera Chronicles: How to Cook a Yellowstone Rhyolite: Part Mantle, Part Crust | Open spaces
Mark Stelten U.S. Geological Survey
Over its 2.1 million year history, Yellowstone has erupted over 4,000 km3 of rhyolite magma. While it is well known that a vast magmatic system of rhyolite has existed beneath Yellowstone throughout its history, less is known how this rhyolite originated. In other words, how is a Yellowstone rhyolite made? Where does it come from? What ingredients does it require?
Before we can address this question, it is first necessary to have a general understanding of what rhyolite is and the components (or ingredients) available to make this composition. Rhyolite is a type of molten rock with a high silica content. The greater the amount of silica, the more viscous or sticky the molten rock becomes, and the more difficult it is for gases to escape from the molten rock. As a result, rhyolitic eruptions can be very explosive, such as the massive caldera-forming eruption that formed the Yellowstone Caldera around 631,000 years ago. If the gas is eventually removed from the magma, rhyolite eruptions can produce massive, thick lava flows, like those that have largely filled the caldera since its formation.
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In general, there are two types of components in volcanic systems like Yellowstone. First, there are basalt magmas – low in silica and therefore low in viscosity – that come from deep within the Earth – well below the shallow Yellowstone magma reservoir, which currently sits at a depth of 5 17 km (about 3 to 10 miles). Basalt magmas are usually generated by melting in the upper part of the Earth’s mantle (beneath the crust) at depths greater than 40 km (about 25 mi) and are injected into the Earth’s crust where they may burst to the surface or collapse. stall in the crust and crystallize. At Yellowstone, the basalts are believed to be related to a large thermal anomaly often referred to as a hotspot. These basalts provide the heat needed to begin the process of making a rhyolite. Second, there is the crust. The term crust, or peasant rock, refers to the rock that surrounds the basaltic and rhyolitic magma of Yellowstone. The crust surrounding the Yellowstone magma system is believed to consist primarily of approximately 2 billion year old rocks similar to those found in the Beartooth Mountains northeast of Yellowstone National Park.
There are several ways to produce rhyolite using basalt and/or crustal components. In one scenario, rhyolites can be derived by crystallization from basalt magma in a process called crystallization-differentiation. In this process, basalt 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 will have a rhyolite composition. In another scenario, heat from basalt injected into the crust can melt the crust and produce rhyolite. These scenarios are extreme and the actual process may be a combination of both. Now that we know the possible recipes, we can start exploring how rhyolites are “cooked” in Yellowstone!
Over the past several decades, geologists have examined this question by measuring the isotopic composition of elements such as oxygen, strontium, lead, neodymium, and hafnium in Yellowstone rhyolites, Yellowstone basalts, and rocks of the Beartooth Mountains northeast of Yellowstone. Isotopes occur naturally and refer to elements with a set number of protons but a different number of neutrons. For example, 176Hf has one less neutron than 177Hf, where Hf represents the element Hafnium and the superscript number is the atomic mass. Isotope ratios (e.g. 176Hf/177Hf) are particularly useful as they act as “fingerprints” for different rock types and are very different between basaltic magma and crustal rock. By knowing the isotopic composition of the Yellowstone basalts and surrounding crust and comparing them to the isotopic composition of the Yellowstone rhyolites, it is therefore possible to determine the relative percentage of basalt and crust that was required to make the rhyolites of Yellowstone.
Based on isotopic compositions, geologists estimate that to fire a Yellowstone rhyolite, approximately 50% to 70% basalt and 30% to 50% crust should be used. The mixing of these two components occurs in complex ways in the middle to lower crust, where hot (>900°C) basaltic magmas tend to stagnate, crystallize and differentiate. Hot basalt magmas melt the surrounding crust and mix these two components together, producing a magma with an isotopic composition between that of basalt and crust. This “hybrid” magma then continues to crystallize or is remelted as new basalts enter the crust, producing Yellowstone rhyolite.
It is not found in any cookbook, but thanks to the study of isotopic “fingerprints”, it is possible to reconstruct the recipe for rhyolite! And the story of Yellowstone is that it is a complex mix of magmas from deep below and the crust they interact with as they rise to the surface. The end result is a composition of magma that can fuel both the caldera-forming explosions and the thick lava flows found throughout the Yellowstone region.
Yellowstone Caldera Chronicles is a weekly chronicle written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Mark Stelten, a research geologist at the US Geological Survey.