Linking distal volcaniclastic sedimentation and stratigraphy with the growth and development of stratovolcanoes, Ruapehu volcano, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of PhD in Earth Sciences at Massey University, New Zealand

Abstract

Large, long-lived stratovolcanoes are inherently unstable, and commonly experience large -scale flank collapse. The resulting debris avalanches permanently alter the edifice and the valleys they impact. New mapping reveals that at least six hitherto unknown debris avalanches occurred from Mt. Ruapehu, New Zealand. They collectively inundated >1,200 km2 and ranged between 1.3 and >3 km3 in volume, the latter being the largest debris avalanche known from the volcano. Constriction of the sliding debris avalanches into deep river valleys enhanced basal erosion, incorporation of water-saturated substrate and formation of a basal lubrication zone. This led to runouts of up to 100 km, 2 - 3 times longer than expected for equivalent unconfined dry landslides. Two of the seven river catchments affected by debris avalanches were truncated from the volcano by proximal debris choking. The debris avalanches commonly coincided with warming from glacial into interglacial periods and rapid deglaciation of Mt. Ruapehu. A loss of ice-armouring of the slopes and increased water saturation likely weakened the edifice. At least two of the debris avalanches were triggered by intrusion of new magma into the mountain. The highly resistant debris-avalanche deposits form distinctive plateaus at the highest topographic elevations along present eroding river valleys, in places reflecting earlier drainage pathways. Deposit ages and those from lower climate-controlled (non-volcanic) fluvial aggradational terraces allowed calculation of regional uplift rates, which varied between 1.3 ± 0.5 mm yr-1 to 5 ± 1.3 mm yr-1 over the last c. 125 ka. Each major flank failure led to decompression of the Mt. Ruapehu magmatic system, triggering pulses of numerous large -scale eruptions and syn-eruptive lahars. Ar- Ar dating of lava clasts within the debris avalanche deposits provided evidence of volcanic episodes that are not exposed on the present edifice. The oldest deposits from Mt. Ruapehu are now identified at =340,000 ka and show that a complex multi -stage storage magma system was operating, similar to that of the present day. Hornblende -bearing xenoliths from these lavas show that a magmatic crustal underplate at >40 km depth existed beneath the volcano by ~486.5 ± 37.6 ka. Combined, samples from the mass -flow deposits and the cone lavas show more complex variation over time than previously thought, but generally reflect a progressively increasing heat flux and a shift of the magma -storage system from the lower crust to mid- and upper -crustal levels.