Journal Article

Geophysical and Geochemical Constraints on Magma Storage Depths Along the Cascade Arc: Knowns and Unknowns

Wieser, Penny E., Till, Christy B., Abers, Geoff A., Kent, Adam J. R.

2023

Overview

The iconic volcanoes of the Cascade arc stretch from Lassen Volcanic Center in northern California, through Oregon and Washington, to the Garibaldi Volcanic Belt in British Columbia. Recent studies have reviewed differences in the distribution and eruptive volumes of vents, as well as variations in geochemical compositions and heat flux along strike (amongst other characteristics). We investigate whether these along‐arc trends manifest as variations in magma storage conditions. We compile available constraints on magma storage depths from InSAR, geodetics, seismic inversions, and magnetotellurics for each major edifice and compare these to melt inclusion saturation pressures, pressures calculated using mineral‐only barometers, and constraints from experimental petrology. The availability of magma storage depth estimates varies greatly along the arc, with abundant geochemical and geophysical data available for some systems (e.g., Lassen Volcanic Center, Mount St. Helens) and very limited data available for other volcanoes, including many which are classified as “very high threat” by the USGS (e.g., Glacier Peak, Mount Baker, Mount Hood, Three Sisters). Acknowledging the limitations of data availability and the large uncertainties associated with certain methods, available data are indicative of magma storage within the upper 15 km of the crust (∼2 ± 2 kbar) beneath the main edifices. These findings are consistent with previous work recognizing barometric estimates cluster within the upper crust in many arcs worldwide. There are no clear offsets in magma storage between arc segments that are in extension, transtension or compression, although substantially more petrological work is needed for fine scale evaluation of storage pressures. Plain Language Summary The Cascade arc contains a number of large volcanoes, which present a significant hazard to human populations and infrastructure (e.g., Mount St. Helens, Mount Rainier). Until now, there has been no wide‐scale review of where magma (molten rock) is stored in the crust beneath these volcanoes, even though understanding where magma is stored is very important to help monitor unrest at these volcanoes and to predict future activity. We compile all available data on magma storage for each volcano, and find that many volcanoes have had very few studies investigating them, despite the risk they pose to society. The available data (albeit sparse) suggest that most magma is stored at 0–15 km depth before eruption. Key Points The availability of magma storage depth constraints along the Cascade arc is highly variable and not well correlated to volcano threat level Available geophysical, mineral‐melt and melt inclusion (MI) constraints cluster at 0–15 km depth (∼2 ± 2 kbar), consistent with global compilations Investigating the potential for deeper storage of the most mafic magmas will require studies accounting for MI vapor bubble CO2

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Barometers / Cascade arc / Compression / Depth / Geochemistry / Geophysical data / geophysics / Glaciers / Heat flux / Heat transfer / Human populations / Inversions / Lava / Magma / magma storage / melt inclusions / mineral barometry / Petrology / Saturation / Storage conditions / subduction zones / Synthetic aperture radar interferometry / Volcanic belts / Volcanoes