Impacts of catastrophic volcanic collapse on the erosion and morphology of a distal fluvial landscape: Hautapu River, Mount Ruapehu, New Zealand

Abstract

Debris avalanches caused by the collapse of volcanic fl anks pose a great risk to inhabited areas and may permanently change the surrounding landscape and its drainage systems. In this research, we explored the interplay between a debris avalanche and a tectonically uplifting surrounding landscape, providing insights into the long-term consequences of volcanic edifi ce failures . Exposures of coarse volcaniclastic sediments along the Hautapu River ~50 km southeast of Mount Ruapehu, New Zealand, show evidence of the largest known collapse event of the stratovolcano, which was followed by a vigorous regrowth phase that produced numerous pyroclastic eruptions and pumice-rich lahars. Similar diamicton deposits are exposed within the river catchment adjacent to the west. Coverbed stratigraphy and geochemical correlation of ande sitic lava blocks within the debris-avalanche deposit with dated lavas exposed on the cone indicate that deposition occurred between 125 and 150 ka. The collapse took place during the shift from a glacial to an interglacial climate, when glaciers on the cone were in retreat, and high porewater pressures combined with deep hydrothermal alteration weakened the cone. In addition, collapse may have been accompanied by magmatic unrest. The ~2-3 km 3 debris avalanche inundated an area of >260 km 2 and entered the proto-Hautapu catchment, where it was channelized within the deeply entrenched valley. Mass-wasting events associated with postcollapse volcanism continued to be channeled into the proto-Hautapu River for another ~10 k.y., producing longrunout lahars. Subsequently, the river catchment was isolated from the volcano by incision of the intervening Whangaehu River into the proximal volcaniclastic sediments, accompanied by regional faulting and graben deepening around Ruapehu. At present, the volcaniclastic deposits form a distinctive plateau on the highest topographic elevation within the Hautapu Valley, forming a reversed topography caused by preferred incision of the Hautapu River into softer Late Tertiary sediments concurrent with constant uplift.

Bridging Māori indigenous knowledge and western geosciences to reduce social vulnerability in active volcanic regions

“…Located at the southern end of the Taupo Volcanic Zone, it has erupted low-K andesitic-dacitic magmas in effusive and explosive (from phreatic to Plinian) eruptions since~270 ka (Donoghue et al 1995. The main edifice is asymmetrical; it comprises lava-flows, rare domes, and has been affected by events of sector collapse (cf., Palmer and Neall 1989, McClelland and Erwin 2003, Procter et al 2010, Tost et al 2014). The volcano is capped by small permanent glaciers and snowfields and comprises overlapping craters (Hackett andHoughton 1989, Cronin et al 1996).…”

Linking distal volcaniclastic sedimentation and stratigraphy with the development of Ruapehu volcano, New Zealand

“…Four of the six drainage systems currently originate from the steep volcanic flanks of Mt. Ruapehu, whereas the Hautapu River and the Turakina River rise from the distal Ruapehu ring plain (e.g., Tost et al 2015). The aim of this study is to improve the knowledge of the eruptive history of Mt.…”

“…3 and described in detail by Tost et al (2015). The volcaniclastic sequence overlies a massive conglomerate that, in-turn, unconformably overlies Miocene Taihape Mudstone.…”

“…3) also contains distinctive charcoal fragments (≤5 cm) in its upper third. The mass-flow sequence exposed at Hihitahi is overlain by four Last Glaciation loess units (each formed during a stadial period of the last glaciation; Milne 1973) and their associated paleosols implying a depositional age for the underlying volcaniclastic sequence of >125 ka (Tost et al 2015).…”

“…2b). Four Last Glaciation loess units and the corresponding paleosol horizons were identified on top of the Whangaehu Formation implying a depositional age of >125 ka (Hodgson 1993;Keigler et al 2011;Tost et al 2015).…”