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.

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Yellowstone hosts a large magma reservoir that was the source of some of the largest volcanic eruptions in Earth’s recent past. The current magma reservoir is located beneath Yellowstone Caldera, which is a large basin-shaped depression in the Earth’s surface that formed during eruption of the ~1000 km³ Lava Creek Tuff about 631,0000 years ago. Over the past 50 years geologists have studied the volcanic rocks in Yellowstone to better understand when past eruptions occurred and the conditions in the magma reservoir at the times of those eruptions. Thanks to this work, we have a pretty good understanding of what the magma reservoir probably looks like today—a mostly solid mass—and we have also learned that it has not always existed in this present-day form.

Geophysical studies of Yellowstone’s current magma reservoir indicate it contains a small proportion of melt and is best described as a crystal mush—a large, semi-rigid region that is composed of mostly crystals and with melt distributed within this crystalline framework, rather than a large tank of molten rock. Geochronology studies that determine the age of volcanic rocks and the minerals they contain suggest that the current magma reservoir at Yellowstone is likely only about 300,000 years old. However, given that Yellowstone has been volcanically active for much more than 300,000 years, this begs the question: how did Yellowstone’s magma reservoir look prior to its current form, and how has it changed through time?

Thankfully, detailed studies of the age and composition of Yellowstone’s volcanic rocks have revealed how the magma reservoir has evolved since the formation of Yellowstone Caldera about 631,000 years ago. Yellowstone’s magma reservoir is a dynamic entity that changes its physical characteristics, size, and composition through time as material is erupted form the magma reservoir and new, deeper sourced magmas are added. Yellowstone’s magmatic system likely alternates between less active periods where the magmatic system is cooler and more crystalline, and more active periods where it contains large volumes of eruptible magma.

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The sequence below represents one possibility for how the Yellowstone magma system operated through time:

• Prior to eruption of the Lava Creek Tuff and the formation of Yellowstone Caldera 631,000 years ago, a large magma reservoir existed in the crust beneath Yellowstone. This magma reservoir was probably a crystal mush that contained regions rich in liquid magma.
• 631,000 years ago: Eruption of the Lava Creek Tuff evacuated much of the magma reservoir and formed Yellowstone Caldera as the overlying ground surface collapsed.
• 631,000 to 580,000 years ago: No known eruptions occurred within Yellowstone Caldera, suggesting the magmatic system cooled off and solidified as input of deeper magmas into the crust waned.
• 580,00 to 480,000 years ago: At least 7 eruptions, referred to as the Upper Basin Member rhyolites, occurred within Yellowstone Caldera. These rhyolites are thought have formed by melting of older, solidified Yellowstone magma in the shallow curst. Melting was caused by injection of deeper, hotter magmas into the mid or shallow crust.
• 480,000 to 350,000 years ago: No known eruptions occurred within Yellowstone Caldera, suggesting the magmatic system again cooled off as input of deeper magmas into the crust slowed.
• 350,000 years ago: Geochemical studies of young volcanic rocks suggest that there may have been an episode where deeper magmas were injected into the crust and caused melting to form one or more magma bodies; however, these magmas were not erupted.
• 350,000 to 255,000 years ago: No known eruptions occurred within Yellowstone Caldera and there is no evidence from younger volcanic rocks of any melting during this time, suggesting once more that input of deeper, hotter magmas into the crust was limited.
• 255,000 years ago: Two rhyolites erupted within Yellowstone Caldera, near the current location of Old Faithful. Recent research has shown that these eruptions were triggered by intrusion of deeper, hotter magmas into the shallow magma reservoir. These rhyolites contained crystals that were formed during the melting event 350,000 years ago, suggesting the same regions of the magma reservoir were melted in both events, even though the events occurred nearly 100,000 years apart.
• 255,000 to 160,000 years ago: No known eruptions occurred within Yellowstone Caldera, but unlike previous time periods of no eruptions, injection of deeper magmas into the shallow magma reservoir probably continued and caused the region of melting to expand, forming a large, integrated magma reservoir.
• 160,000 to 70,000 years ago: Over 20 rhyolites erupted in five brief episodes separated by 7,000 to 40,000 years (collectively referred to as the Central Plateau Member rhyolites). The exact trigger for these eruptions is debated, but probably involved injections of deeper, hotter magmas.
• From 70,000 years ago to the present day, no eruptions have occurred within Yellowstone Caldera. This suggests that the current magma reservoir beneath Yellowstone Caldera is the mostly solidified remanent of the magma reservoir that erupted the Central Plateau Member rhyolites from 160,000 to 70,000 years ago.

Although there are still many questions to be answered about how Yellowstone’s magma reservoir has changed over time, it is clear that Yellowstone and other magmatic systems around the world are ever changing, dynamic natural features. Fluctuations in the amount of heat and mass provided to the shallow magmatic system from deeper, hotter magmas plays a key role in the size, composition, and physical state (liquid vs. solid) of the magmatic system. So the next time you think of Yellowstone’s magma reservoir, remember that it is more than a unchanging tank of liquid magma!