Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Violet Turner and Faith Nolander, completing a summer undergraduate research experience with Dr. Madison Myers, Associate Professor of Earth Sciences at Montana State University.
Did you know that factors like pressure, temperature, water content, and chemistry of a magma all heavily control the way a volcano erupts? For instance, the hotter a magma, the less sticky (or viscous) it becomes. And as magma cools, gas bubbles can accumulate and cause an eruption to be more explosive. Unfortunately, there is no easy way to stick a thermometer into a magma chamber to take its temperature. Instead, volcanologists must rely on rock compositions to probe for this information.
Minerals that form in magma chambers record the conditions in which they grow. The best analogy for this idea is tree rings—in wet years, trees have wide spacing between their growth zones, whereas during hot years these spacings decrease. In a similar way, the growth of minerals is very sensitive to changes in the temperature and pressure of the melt over time. Those changes are recorded as slight shifts in chemical composition in mineral layers. To measure the chemical compositions of minerals, geochemists use a tool called electron microprobe analysis, where rock samples are placed in a sealed chamber and bombarded with electrons. This process is like taking an x-ray of a rock. Each crystal will emit different types of energy depending on what elements are present, and measuring this energy provides the composition of the area of the mineral that was bombarded!
By comparing the chemistry of the mineral and that of the melt, which can be preserved alongside a mineral as volcanic glass (melt that has rapidly quenched upon eruption), geochemists have found a way to determine the temperature history of the magma. This method is known as mineral-melt thermometry, and it involves equations that are created for different minerals based on high-temperature, high-pressure lab experiments. Most minerals have associated thermometers, meaning that this method can be applied to almost any volcano!
Determining how temperature varies between eruptions and deposit types is vital to understand factors like location of subsurface magma bodies. For instance, seismic waves that are produced during earthquakes travel at different speeds based on the temperature of the rock they are moving through, so having temperature estimates for a magma body based on mineral-melt thermometers can help geophysicists understand and image where magma is stored in the crust. The chemistry of a magma is also tied to the temperature at which it erupts. The viscous, silica-rich magmas that feed Yellowstone eruptions are several hundred degrees cooler than the fluid, less silicic ones that feed a volcano like Kīlauea in Hawai’i.
In Yellowstone there are several types of volcanic deposits. The most prominent are rhyolitic lava flow and welded ash flow tuffs. The tuffs and lavas can be nearly identical geochemically and have the same mineralogy—quartz and sanidine are the most abundant minerals, with lesser amounts of plagioclase feldspar, clinopyroxene, and zircon. While there are currently no mineral-melt thermometers or barometers available for quartz, there are several thermometers available for sanidine and plagioclase. This makes mineral-melt thermometry an accessible and vital tool for geochemists to deduce the temperatures of Yellowstone eruptions. The technique is currently being used by researchers at Montana State University to estimate the temperature conditions for the magma that fed the Lava Creek Tuff, which was associated with the formation of Yellowstone Caldera about 631,000 years ago. And guess what? The Lava Creek Tuff’s pre-eruptive magma was around 800 °C (1500 °F) before it erupted! That’s quite cool for a magma, which helps to explain why it was so explosive—at that temperature, the magma would have trapped a lot of gas bubbles.
While volcanologists do not expect Yellowstone to produce another super eruption anytime soon, understanding how magma properties influence eruption behaviors is key to characterizing the volcanic system. With an absence of other methods to determine magma temperature, mineral-melt thermometry is the most common way for volcanologists to take the temperature of a magma prior to an eruption.