HOW PRIMARY VOLCANIC EMPLACEMENT AND COOLING MECHANISMS INFLUENCE ROCKFALL HAZARD

How does a rock’s geologic origins contribute to its potential susceptibility to rockfall? This study seeks to understand the connection between the volcanic origins of rocks and their eventual rockfall hazard. In order to assess this connection we must first develop an understanding of the specific geologic histories. For volcanic rocks this is done through an understanding of volcanic emplacement mechanisms, followed by secondary processes, cooling mechanisms, effects of weathering, erosion, and landscape to draw their connection to rockfall hazard.

Vast variations exist within volcanic rocks, with outcrop and rock properties dependent on eruptive and emplacement process. To overcome this vastness this study categorized volcanics into lithofacies, and assigned field sites for each. Field sites are within the volcanics of Banks Peninsula, New Zealand, which have recently been influenced by proximal seismicity resulting in rockfall. Lithofacies can be broadly classified as: crater rim lava, lava dome, dike, seaward cliff lava, and valley lava. Field sites were studied in person, in addition to using past geological studies, and computer software. Field photos were annotated paying close attention to diagnostic features of the lithofacies and of the presence or lack of rockfall, including probable mechanism.

Lithofacies were classified using the following criteria: type of unit, type of material, presence of jointing, presence of fracturing, contacts between units, degree of weathering and erosion, presence or extent of detachment planes. For each lithofacies a geologic model was generated highlighting the relationship of the aforementioned criteria. Crater rim lava, lava dome, and dikes have distinct correlation between emplacement and eventual detachment, with inherited cooling properties and jointing playing an important role.

Processes involved in the emplacement of volcanic rocks, and how those rocks have been exposed over time provide distinct controls on how a lithofacies will manifest itself as a rockfall hazard. This study highlights the complexities of volcanics as a rockfall source, and the need to fully assess and understand exposures to determine rockfall hazard.