Volcanoes and volcanism are one of the defining features of subduction zones, and contributes greatly to the range of hazards associated with convergent margins. Subduction zone volcanoes – such as those in the Cascades, Aleutians, and Andes – erupt magma ultimately derived from the mantle, however there is also abundant evidence that most subduction volcanoes are underlain by shallow crustal magma storage regions (≤ 10km ) (Huber et al. 2019, Wieser et al., 2023; Pang et al. 2024) and that magma can reside in these for extended periods - 100,000 years or longer - prior to eruption (e.g. Claiborne et al., 2010; Cooper and Kent, 2014). The conditions of this storage greatly influence the size, style, and recurrence rate of eruptions, and so also strongly impact volcanic hazards.

One particular area of interest are the temperatures of magma residing in the shallow crust. Temperature is linked to magma viscosity and mobility because cooler magmas are more crystalline and more viscous (Figure 1), and thus harder to mobilize and erupt (e.g. Shamloo et al., 2024). Recent application of a range of geochemical tools (e.g. Cooper and Kent, 2014; Barboni et al., 2016; Szymanowski et al., 2017; Anderson et al., 2017) have constrained the temperature of magma storage in a range of subduction and other volcanic systems, and two contrasting and paradigms of magma storage have emerged. In cold storage magmas are crystal-rich and spend the majority of storage times at temperatures and viscosities where they are unable to erupt, and are then rapidly mobilized through intrusion of hotter magma or other means (Kent and Cooper, 2014; Kent et al., 2023). For warm storage magmas spend significant storage in a liquid-rich eruptible state (Barboni et al., 2016). The difference suggests variations in the thermal balance between heat input and loss in shallow magma reservoirs. In addition, for cold storage geophysical detection of eruptible liquid-rich magma bodies in the shallow crust would suggest that eruption might be imminent as these are only a transient state of a magmatic system.

A recent review (Cooper and Kent, 2026) provides some further insight. The cold vs. warm storage paradigm is widely used, but variance in the individual definitions of the terms has also produced considerable “semantic drift” in the literature. Application of a more consistent nomenclature tied directly to the rheological transitions experienced by a cooling magma (Figure 1) suggests that most subduction magmas in the crust spend most of the time pre-eruption in relatively crystal-rich and immobile/uneruptible state. This is also consistent with a broad range of geophysical, geochemical and field evidence.  However, forcing a simple binary choice between cold and warm storage paradigms, also means we miss a much richer reality, where magma storage in individual magmatic systems may vary in both space and time (e.g. Anderson et al., 1919). Future developments can look to take advantage of this by looking at volcanic systems with greater temporal and spatial resolution, and also increase the accuracy of our constraints. 

Cooper KM, Kent AJR. 2026 Petrological Constraints on the Thermal History of Magma Storage in the Crust. Annu. Rev. Earth Planet. Sci. 54:79–104

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References

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Andersen NL, Singer BS, Coble MA. 2019. Repeated rhyolite eruption from heterogeneous hot zones embedded within a cool, shallow magma reservoir. J. Geophys. Res. Solid Earth 124:2582–600

Barboni M, Boehnke P, Schmitt AK, Harrison TM, Shane P, et al. 2016.Warm storage for arc magmas. PNAS 113: 13959–64

Claiborne LL, Miller CF, Flanagan DM, Clynne MA, Wooden JL. 2010. Zircon reveals protracted magma storage and recycling beneath Mount St. Helens. Geology 38:1011–14

Cooper KM, Kent AJR. 2014. Rapid remobilization of magmatic crystals kept in cold storage. Nature 506:480–83

Cooper KM, Kent AJR. 2026 Petrological Constraints on the Thermal History of Magma Storage in the Crust. Annu. Rev. Earth Planet. Sci. 54:79–104

Huber, Christian, Townsend, Meredith, Degruyter, Wim and Bachmann, Olivier 2019. Optimal depth of subvolcanic magma chamber growth controlled by volatiles and crust rheology. Nat. Geosci. 12: 762-768 

Kent AJR, Till CB, Cooper KM. 2023. Start me up: the relationship between volcanic eruption characteristics and eruption initiation mechanisms. Volcanica 6:161–72

Pang G, Abers GA, Moran SC, Thelen WA. 2025. Long-lived partial melt beneath Cascade Range volcanoes. Nat. Geosci. 18:184–90

Shamloo H, Kent AJR 2024. An appraisal of the observed crystallinities of volcanic materials. Volcanica 7:105–15

Szymanowski D, Ellis B, Wotzlaw JF, Bachmann O. 2019. Maturation and rejuvenation of a silicic magma reservoir: high-resolution chronology of the Kneeling Nun Tuff. Earth Planet. Sci. Lett. 510:103–15

Wieser, P.  E., Kent, AJR, Till, CB, Abers, GA. 2023. Geophysical and Geochemical Constraints on Magma Storage Depths Along the Cascade Arc: Knowns and Unknown. Geochemistry, Geophysics, Geosystems, 24, e2023GC011025.