203 Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder

We encourage papers that challenge and give insights into the interplay between timescales, methodologies, and processes to build a cohesive understanding of the lifespan, evolution, storage, and eruption of magma, from plutonic to volcanic systems.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 216AB

What’s Under the Hood: Petrologic Mapping of the Magmatic Plumbing System Prior to the Most Recent Mount Hood Eruptions

Paper number: 203-3
Program: Technical Sessions
Time: 2:15 PM – 2:30 PM

Authors

Abstract

Understanding how volcanic systems change in response to magma recharge is critical, and changes in crystal composition can be used to track this response.  Mount Hood, OR is generally thought to erupt through cycles of mafic recharge, as has been noted for the Old Maid (~240 ya; OM) and Timberline (~1.5 ka; TL) eruptions. However, it is currently unclear whether the subsurface magmatic recharge system filters the injected mafic magma in the same way every time during recharge events. Plagioclase is a long-lived mineral in magmatic systems; thus, tracking plagioclase composition variations allows us to investigate how the magmatic system of Mount Hood filters recharge events. By combining anorthite composition and systems analysis, we track magma migration prior to OM and TL eruptions.  

Our preliminary results indicate that all zoning types, normal, reverse, and oscillatory, are present in both eruptions. However, when comparing core and rim compositions, we note that each eruption has variable modal core and rim compositions for the various zoning patterns, which suggests that different magmatic bodies were volumetrically dominant when contributing to the crystal cargo. In normal zoning patterns, OM rims consist of An40-45, whereas TL rims are An40-55. The proportion of mafic magma mixed could have varied, which could have then impacted the composition of the liquid that precipitated the rims, or even changed the temperature or water content of the magma, all of which could impact An content. Recent data from zircons show Mount Hood eruptions are becoming more silicic over time [1]. Although directly comparing Polallie (~12-30 ka) to OM and TL, this trend suggests that our results are also the result of less mafic magma being mixed, leading to a more silicic eruption.

The possibility of different magmatic reservoirs being involved or differing proportions of a poorly mixed reservoir resulting in different compositions in eruptions at Mount Hood has implications for future eruptions, as, instead of effusive eruptions, they could become more explosive with higher silica content. Diffusion modeling of Mg and Sr gradients in select plagioclase grains will elucidate typical mixing timescales before eruption. This could be useful for hazard monitoring to advise evacuation timescales in the event of future unrest.

[1] Klemetti et al. (2025). Geosphere.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414412

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