Abstract
As our ability to detect volcanic unrest improves, we are increasingly confronted with the question of whether the unrest has a magmatic origin (magma on the move) or a non-magmatic origin from a change in the hydrothermal system (fluids that are not magma on the move) or tectonic processes. The cause of unrest has critical implications for the potential eruptive hazard (e.g. used in constructing Bayesian Event Trees), but is frequently the subject of debate, even at well-studied systems. Here, we propose a set of multi-disciplinary observations and numerical models that could be used to evaluate conceptual models about the cause of unrest. These include measurements of gas fluxes and compositions and the isotopic signature of some components (e.g. H2, He, C, SO2, H2 O, CH4 and CO2), the spatial and temporal characteristics of ground deformation, thermal output, seismicity, changes in gravity, and whether there is topographic uplift or subsidence spanning hundreds to thousands of years. In several volcanic systems, both magmatic and non-magmatic unrest is occurring at the same time. While none of these observations or models is diagnostic on its own, we illustrate several examples where they have been used together to make a plausible conceptual model of one or more episodes of unrest and whether eruptions did or did not follow the unrest. This article is part of the Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.
Highlights
This special issue explores an emerging conceptual model of trans-crustal magmatic systems (TCMS) where magma storage and differentiation occurs at several locations throughout the entire crustal column [1,2]
Most eruptions are presaged by at least a few days to weeks of volcanic unrest [5,6]—how does this unrest relate to the TCMS paradigm? some volcanic eruptions have little to no measured unrest before eruption [7,8]—is this consistent with the TCMS conceptual model? We define volcanic unrest as ‘the deviation from the background or baseline behaviour of a volcano towards a behaviour which is a cause for concern in the shortterm because it might prelude an eruption’ from Phillipson et al [6]
If unrest is caused by magma layer destabilization and movement, Sparks & Cashman [1] consider it more ominous than volatile accumulation and release from magma that moved during a previous destabilization, or changes in the shallow hydrothermal system [1]
Summary
This special issue explores an emerging conceptual model of trans-crustal magmatic systems (TCMS) where magma storage and differentiation occurs at several locations throughout the entire crustal column [1,2]. To quantify the possible eruptive threat from unrest episodes, several groups have developed Bayesian Event Tree (BET) (figure 2) that include an assessment of the likelihood that the unrest is caused by ‘magma on the move’, geothermal or tectonic activity, or some other process [9,11,12,13]. Non-magmatic unrest includes ‘fluids on the move’ [11], where the fluids include brines, gas, supercritical fluids or a combination of these as long as partially molten rock (magma) is not migrating [11] These fluids could have their origin from magma, for example, volatiles derived from stagnant [9] cooling/crystallizing magma batches (e.g. figure 1a,b) and are called ‘magmatic fluids’ [14], from meteoric/groundwater water mixing in the hydrothermal system, or a combination of the two that are referred to as ‘hydrothermal fluids’. Our focus is on silicic systems because these are the sites of the most explosive and dangerous eruptions on Earth, but we include some useful examples from other systems as well
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