The Glencoe caldera ring fault(s) as a detailed record of a magmatic sub-surface transport system and its collapse mechanics.

Abstract

Some of the largest and most hazardous natural events on Earth are silicic caldera-forming volcanic eruptions, where the eruption of vast volumes of magma results in the collapse of crustal material that forms the magma reservoir’s roof (Cole et al., 2005; Geyer and Martí, 2014; Sigurdsson et al., 2015; Kennedy et al., 2018; Geshi et al., 2021). Volcanic eruptions include 3 key regions of activity: magma storage, sub-surface magma transport, and surface magma eruption. The behaviour of the sub-surface transport system throughout a caldera-forming eruption is poorly constrained due to limitations in methods of study available, and because it suffers from being complicated by its interaction with the structural caldera collapse system (Wiebe et al., 2021). Along caldera ring fault surfaces the two separate movements occur (the upwards transport of magma, and the downwards collapse of the caldera block), so, it is difficult to unpick the relationship between these two processes and whether this interaction causes any changes to the mechanics of caldera-forming eruptions.
In order to address the poor understanding of caldera transport systems, this thesis addresses two questions. (1) How do the processes of caldera slip and magma transport initiate and interact with one another when both occurring along the common surface of the caldera ring fault? (2) In what state is magma transported through the crust? This study applies these questions to the ancient Glencoe caldera (Scotland) where erosion has allowed for exposure of the caldera on many levels including the ring fault/conduit fill (Kokelaar and Moore, 2006). Therefore, this study uses field, textural, petrological, geochemical, and theoretical evidence to discuss these key questions in order to build a model for the evolution of the sub-surface transport system throughout an eruption which is applicable to calderas worldwide where this exposure cannot be seen.
Here we show that friction along caldera ring faults is not high enough to produce frictional melts, as proposed by previous ‘superfault’ theories (Spray, 1997; Kokelaar, 2007; Han et al., 2019; Kim et al., 2019). Instead, we suggest that collapse is incremental, piecemeal, and dilational, and that ring faults are dominated by magma transport processes throughout an eruption. Hence, this provokes the idea that the naming of such ring faults may have a misleading emphasis. This work also shows that the subsurface transport of pyroclastic magma is deep, widespread and long-lived during caldera eruptions which progressively tap deeper magma chamber components. We anticipate this thesis will encourage future research to not focus on caldera models which rely on high friction along caldera ring faults, but instead understand that these systems are long lived and dominated by eruptions of pyroclastic material.