New perspectives on ice forcing in continental arc magma plumbing systems

Abstract

Determining how and why eruptive outputs are modulated by the loading and unloading of ice is key to understanding whether ongoing and accelerating deglaciation across mid- to high-latitudes will impact future activity at many volcanoes. Here, we address two central questions. First, does decompression of the upper crust during rapid thinning of ice sheets propel increases in eruption rates? Second, does surface loading during ice sheet growth, followed by rapid unloading during deglaciation, promote changes in magma storage conditions and compositions within the underlying magma plumbing systems? To provide new perspectives on these questions, we address the mechanics and dynamics of ice sheet-arc magma plumbing system interactions at a regional-to-local scale within the Andean Southern Volcanic Zone. Here, piedmont glacier lobes, forming the northernmost extension of the Patagonian ice sheet, have enveloped dozens of large, active, composite volcanoes as these glaciers reached local thicknesses of nearly 2 km during the local Last Glacial Maximum (LGM) between ∼35 and 18 ka, before retreating rapidly between 18 and 15 ka. Our multi-faceted review features a synthesis of existing and new field observations, laboratory measurements, and numerical simulations. Advances in ⁴⁰Ar/³⁹Ar radioisotopic and ³He surface exposure geochronology, in conjunction with geologic mapping, facilitate reconstructions of volcanic eruptive histories spanning the last glacial-deglacial cycle and in places provide constraints on the thickness of ice at specific time slices. The magnitude and geometry of the glacial loading and unloading is captured in a climate model-driven numerical simulation that reveals spatial and temporal heterogeneities in the configuration of the northernmost Patagonian ice sheet retreat. Geological observations including dated moraine complexes, dated lava-ice contact features, and glacial erratic boulders at high altitude on volcano slopes, are consistent with this model. Deep valleys imply intense localized erosion on volcano flanks, and deposited sediment in nearby floodplains implies narrow regions of rapid sediment deposition. These observations, in conjunction with dated lava flows, provide constraints on rates and patterns of crustal loading and unloading by sediment redistribution.

The ice loading model, cone growth, and a sediment redistribution history inform numerical simulations of intra-crustal stress changes below the volcanic arc in response to the ice-driven and sediment-driven changes. In turn, the modeled surface loading is central to designing numerical simulations of magma reservoir responses to intra-crustal stress changes beneath the volcanoes. Following periods of subdued volcanic output, upticks in eruptive rates are found at three volcanoes during, or shortly after, the LGM. A numerical magma chamber model suggests that this behavior could be the result of a delicate balance between the timescales of magma cooling, the rate of magma recharge from depth, and viscous relaxation of surrounding crustal rocks. Depressurization of the crust increases eruptive mass flux to the surface only if: (1) the rate of recharge just outcompetes the rate of cooling, (2) the rate of recharge is barely large enough before loading to overcome viscous relaxation of overpressure by creep around the chamber, and (3) magmas are volatile undersaturated, and exsolve volatiles via second boiling during the long repose associated with the high ice loads that precede rapid deglaciation. Existing and newly developed thermobarometers that constrain magma crystallization and storage depths can be applied to eruptive products spanning a glacial-deglacial transition, such that not only secular changes in rates of volcanic eruption, but also changes in the depths of pre-eruptive magma storage and in magma composition can each be interpreted in the light of intra-crustal stress changes associated with glacial loading and unloading.