Volcano-Hydrologic Hazards from Volcanic Lakes

Abstract

Volcanic regions typically host multiple lakes developed in explosion craters, volcano-tectonic collapse structures, and valley systems blocked as a result of eruptive activity, their boundaries and dimensions shifting in response to renewed activity and modification by background processes of erosion, sedimentation and tectonism. Such water bodies are a potent source of a wide range of complex and inter-related hydrologic hazards owing to their proximity to active volcanic vents, the consequent potential for violent mixing of magma with water, and the frequent fragility of their impoundments. These hazards arise as a result of water displacements within or from the lake basin and can be broadly sub-divided into 3 main types: (I) phenomena sourced within the lake basin as a direct or indirect consequence of subaqueous or subaerial volcanic activity; (II) floods from volcanic lakes triggered by volcanic activity, including induced breaching; and (III) floods from volcanic lakes with a non-volcanic cause. Type I hazards include subaqueous explosive volcanism and associated Surtseyan jets, base surges and tsunamis, which can impact lake shorelines and displace water over basin rims and through outlets. This results in Type II lahar and flooding hazards. Both types have been historically responsible for significant losses of life at many volcanoes worldwide. Other rapid phenomena such as pyroclastic flows, debris avalanches, and large lahars from intra- or extra-lake volcanoes are potentially tsunamigenic (Type I), and/or displacing, and can hence also lead to secondary (Type II) hazards, as can seismicity-producing volcano-tectonic movements. Slower processes including volcano-tectonic movements, subaqueous lava dome extrusion, cryptodome intrusion, and magmatic inflation can potentially produce Type II flooding through volumetric water displacement over the outlet. Erosion of the outlet can be catastrophic, magnifying the size of flood events. Damming of the outlet itself can result in backflooding of the basin. Type III hazards, i.e. volcanic lake break-out floods; result from breaching of the barrier constraining a volcanic lake as a result of passive overtopping, piping, mechanical failure, or headward erosion of the natural dam. Such events range in scale from relatively minor outflows triggered by failure of crater walls or the breaching of riverine dams composed of pyroclastic, volcaniclastic, or lava flow material to catastrophic floods generated by the breaching of caldera rims. Palaeohydraulic reconstructions of some of the latter indicate that they are amongst the largest post-glacial floods on Earth, being exceeded only by late Pleistocene deluges associated with breaching of ice-dammed lakes and pluvial basins.