Is It Time For A Paradigm Shift In How We Think About Igneous
Petrology?
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?
Contact: Allen Glazner, University of North Carolina at Chapel Hill,
afg@unc.edu
T131. Petrology and Volcanology of Earth and Other Planets
Wed., 25 Sept., 3:45–4 a.m.
About the Geological Society of America
The Geological Society of America (GSA) is a global professional society
with a membership of more than 17,000 individuals in over 100 countries. GSA
serves as a leading voice for the geosciences, promoting the understanding of Earth's dynamic processes and fostering collaboration among scientists, educators, and policymakers.
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