Surface Sedimentary (Epiclastic) Processes and Deposits in Volcanic Environments

Many recent eruptions (Mt St Helens 1980, Mt Galunggung 1982, Mt Pinatubo 1991, Unzen 1991–1995, Soufrière Hills 1995–2013, Merapi 2010, and Fuego 2018) have been marked by major-impact sedimentary events both during and immediately after their respective eruptions. These secondary events have produced at least as much damage to infrastructure as the primary eruption processes, and have led to numerous deaths. In the case of the Mt Pinatubo eruption, most casualties were the result of post-eruptive hyper-concentrated flood flows that inundated the region around the volcano for at least 15 years after the eruption. Secondary sedimentary events initiated as a consequence of an eruption, or due to the instability of a high-relief volcanic edifice, therefore represent major hazards, and they must be understood as important processes in volcanic settings. In this chapter, we examine the types of surface sedimentary or epiclastic processes that operate in different volcanic terrains, including weathering and/or erosion, transportation, and deposition of volcaniclastic sediments at the Earth’s surface, in both subaerial and subaqueous environments. By reviewing surface epiclastic processes and the characteristics of the resultant deposits, we will highlight the features that make volcaniclastic sedimentary deposits in the rock record distinguishable from primary eruptive volcanic deposits. These can act in concert with other pre-, syn- to post-eruption processes. Surface sedimentary processes can lead to the syn-eruptive erosion and resedimentation of unlithified pyroclastic and autoclastic deposits (epiclastic sensu lato), or post-eruptive erosion and reworking of older volcanic source rocks (epiclastic sensu stricto).