Deposit characteristics and dynamic processes of large volume pyroclastic density currents of the Taupo Volcanic Zone approaching and entering the sea

Pyroclastic density currents (PDCs) entering the sea are a rare but dynamic phenomenon that cause significant change to the topography and bathymetry of coastal and island arc settings. Very little is known about their complex behaviour and depositional processes, as observation of such an event is uncommon and conducting experiments that simulate PDCs is challenging. A better understanding of this phenomena can aid in future volcanic monitoring and hazard management around coastal and oceanic settings. This study combines both field observations and laboratory experiments of PDCs entering the sea. Case studies around North Island, New Zealand of pre-historic pyroclastic deposits found near the coast focused on, the Rotoiti Ignimbrite that originated from the Okataina Volcanic Centre (Pacific Coast Highway between Matata and Maketu; Mimiha Road near Matata), and the Ongatiti Ignimbrite (Glenbrook Beach, Waiuku; Kihi Road, inland of Kawhia Harbour) from the Mangakino Volcanic Centre. At each location, field observations were recorded, and samples collected, followed by grainsize and texture analysis in the laboratory (scanning electron microscopy, laser diffraction particle size analysis, dry sieving). Laboratory experiments that simulated small-scale PDCs generated by a column collapse mechanism and a dam-break mechanism into a flume were conducted. The Ongatiti Ignimbrite at Glenbrook Beach presented flow-water interaction features through soft sediment deformations such as dewatering structures, flame structures, and thin beds of convoluted laminae. The Rotoiti Ignimbrite at Mimiha Road showed distinctive crossbedding. Both examples highlighted PDC processes after entry into the water. The Rotoiti and Ongatiti ignimbrites along the Pacific Coast Highway and Kihi Road, respectively presented typical subaerial ignimbrites that had travelled long distance overland and were still undergoing significant deposition near the coastline. The flume experiment demonstrated that PDCs segregate into two parts when entering the water, (1) a dilute overflow cloud, and (2) a dense underwater current. The entrance of pyroclastic ash material into water also generated a wave. Underwater obstructions will affect a submergent current by significantly decreasing its height and runout distance