Anaheim, Calif., USA: For over a century, the concept of a shallow magma chamber—a large magma-filled cavity just ~5 km under the base of a volcano—has been the foundation for igneous petrology and volcanology. However, an upcoming presentation at the Geological Society of America's GSA Connects 2024 meeting might cause an eruption among the igneous petrology community.
Dr. Allen Glazner, an emeritus professor at University of North Carolina, will be presenting a culmination of evidence that argues against the presence of shallow magma chambers. "We all grew up with pictures of volcanoes with magma chambers under them, and the idea was the magma chamber feeds the volcano, and when the magma chamber cools down, it becomes a pluton," says Glazner. "[However,] a lot of what we see in the field doesn't match those predictions."
Glazner's story begins 20 years ago during an investigation of granite plutons in Yosemite National Park. His team conducted geochemical studies to understand when and how long plutons took to form. It had been assumed that the shallow depth of magma chambers would cause them to cool rapidly—over hundreds of thousands of years—but his team discovered that the Yosemite plutons formed over a 4-million-year period. Glazner explains, "We didn't believe it at first, so we collected more samples over three years and found a consistent, symmetrical pattern of getting younger and younger from the outside in with ages spanning millions of years."
The question of how a shallow magma chamber can stay molten for such a long period of time led him to conduct thermal modeling experiments to estimate how much magma needs to enter the chamber to keep it molten. The results indicate that the amount of magma needed to sustain a molten shallow magma chamber over a million years would cause substantial surface uplift and crustal thickening—far more than what's observed within the geologic record. Additionally, he reviewed geophysics literature and found that areas assumed to be shallow magma chambers are predominantly crystalline rock with less than 20% molten material. "You shouldn't call it magma unless it can move on geologic/volcanic timescales," says Glazner. "If it's more than 50% crystal, it doesn't move, it's a solid."
Glazner proposes that magma chambers are deeper in the crust, and that transportation of magma through the crust by dikes led to the formation of plutons and supply magma for eruptions. This interpretation addresses the identified issues with shallow magma chambers and provides a stronger explanation for well known phenomena like magma mixing. According to Glazner, "If you let go of the shallow magma chamber idea, then a lot of problems we [igneous petrologists] had disappear, and this [interpretation] solves a lot of them." Glazner expects pushback at his presentation but welcomes discussions and hopes that other researchers feel compelled to question the status quo.
Are Shallow Magma Chambers a Thing?
