Jonathan Procter

Quantifying the geomorphic impacts of a lake-breakout lahar, Mount Ruapehu, New Zealand

At 11:18 h (New Zealand time, GMT +12) on 18 March 2007 an impoundment of 0.01 × 10 6 m 3 of tephra collapsed, releasing 1.3 × 10 6 m 3 of water from Crater Lake at 2536 m elevation on Mount Ruapehu. The lahar traveled 200 km along the Whangaehu River. Aerial LiDAR surveys of the upper 62 km of flow path were made before and after the lahar. We present here the first large-scale quantification of the geomorphic impact of the dam-break flood along with the rates and controls on its sediment entrainment and deposition. The flood mobilized a net value of 2.5–3.1 × 10 6 m 3 of boulders, gravel, and sand over the first 5 km of travel to form a lahar of at least 4.4 × 10 6 m 3 passing instruments at 6.9 km. LiDAR volume-transfer calculations match dynamic measurements made. After a logarithmic increase in cumulative net sediment entrainment, the lahar appeared to reach its maximum sediment-carrying capacity at 22 km. Patterns of alternating sediment erosion and deposition occurred that dominantly reflect a combination of channel morphology and confinement on the local sediment-carrying capacity of the flow.

Energy growth in laharic mass flows

Lahars, debris flows, and sediment-rich floods are frequent and deadly hazards at all mountain-forming volcanoes. Their hazard potential is traditionally assessed through mass-conserving closed-system models, where peak conversion rates of potential energy to mechanical energy and hence maximum destruction potential are predicted to occur on the steepest volcano flanks. This belies evidence of extremely high-energy and deadly catastrophes caused by such flows at large distances from volcanoes. Here we use the first high-resolution record of a moving lahar to develop a new model of the temporally and spatially variable mass-flow structure. We show that bulk flow energy can grow dramatically in such systems over tens to hundreds of kilometers via momentum transfers from the lahar into water and particles along its path. We also demonstrate that dynamic transformations of such flows and their ultimate runout are primarily controlled by the mass flow front.

Explaining the extreme mobility of volcanic ice-slurry flows, Ruapehu volcano, New Zealand

The near-invisibility of ice-slurry flows in the geological record belies their signifi-cant hazard at snow-capped volcanoes. These four-phase flows exhibit extreme rates of volumetric bulking and unusually high mobility. Mechanisms of their motion are clarified through two examples generated on 25 September 2007 at Mount Ruapehu, New Zealand. Brief explosions through Crater Lake ejected 5700 m 3 of acidic water that entrained 60 times this volume of snow as it traveled over a snow-covered glacier. The resulting ice-slurry traveled up to 7.7 km (height/length [H/L] ratio of 0.16). A cogenerated second flow took a more tortuous initial path before riding over the already frozen deposits of the first unit and beyond (H/L 0.13). For the first time, downstream evolution of the kinematic properties of propagating ice-slurry fronts could be characterized as well as the longitudinal variation of the physical properties of their resulting deposits. The chemistry and composition of the deposits show that during flow, vertical percolation of water through the porous ice–particle–water–air mixture generated a basal zone of high internal pore pressure. This effect was particularly strong when a thick, high-density flow front formed, which raced ahead of the tail to control runout and consequent hazard.

Characterization and quantification of suspended sediment sources to the Manawatu River, New Zealand.

Knowledge of sediment movement throughout a catchment environment is essential due to its influence on the character and form of our landscape relating to agricultural productivity and ecological health. Sediment fingerprinting is a well-used tool for evaluating sediment sources within a fluvial catchment but still faces areas of uncertainty for applications to large catchments that have a complex arrangement of sources. Sediment fingerprinting was applied to the Manawatu River Catchment to differentiate 8 geological and geomorphological sources. The source categories were Mudstone, Hill Subsurface, Hill Surface, Channel Bank, Mountain Range, Gravel Terrace, Loess and Limestone. Geochemical analysis was conducted using XRF and LA-ICP-MS. Geochemical concentrations were analysed using Discriminant Function Analysis and sediment un-mixing models. Two mixing models were used in conjunction with GRG non-linear and Evolutionary optimization methods for comparison. Discriminant Function Analysis required 16 variables to correctly classify 92.6% of sediment sources. Geological explanations were achieved for some of the variables selected, although there is a need for mineralogical information to confirm causes for the geochemical signatures. Consistent source estimates were achieved between models with optimization techniques providing globally optimal solutions for sediment quantification. Sediment sources was attributed primarily to Mudstone, ≈38-46%; followed by the Mountain Range, ≈15-18%; Hill Surface, ≈12-16%; Hill Subsurface, ≈9-11%; Loess, ≈9-15%; Gravel Terrace, ≈0-4%; Channel Bank, ≈0-5%; and Limestone, ≈0%. Sediment source apportionment fits with the conceptual understanding of the catchment which has recognized soft sedimentary mudstone to be highly susceptible to erosion. Inference of the processes responsible for sediment generation can be made for processes where there is a clear relationship with the geomorphology, but is problematic for processes which occur within multiple terrains.

Textural features as indicators of debris avalanche transport and emplacement, Taranaki volcano

The Pungarehu debris avalanche deposit was emplaced by the largest known collapse of the proto–Taranaki volcano, ca. 25,000 calibrated (cal.) years ago. This debris avalanche deposit displays a highly contrasting sedimentary character between its proximal and distal reaches. Examination of the deposit granulometry, sedimentary structures, and microscopic particle attributes provides new insights into debris avalanche transport and internal evolution processes. Initial collapse of the proto–Taranaki volcano during this event occurred near the Last Glacial Maximum, with snow and ice cover and substantial groundwater present. The collapsing, sliding large blocks of edifice material, “megaclasts,” were highly fractured by the landslide generation and the depressurization event, forming pervasive jigsaw textures. As the megaclasts moved, shear was focused in softer domains between the hardest, lava-dominated lithologies. These crush and shear zones developed a complex pattern of relative motion between horizontal and vertical parts of the landslide, rather than a simple basal shear zone that supported an upper pluglike mass. The sheared zones, concentrated in soft, pyroclastic lithologies, were areas of intense synflow fragmentation, producing a proto-interclast matrix between large blocks of coherent (albeit jigsaw fractured) lavas. Down flow, the interclast matrix component increased to become pervasive by ~23–25 km from the source, enveloping and preserving large megaclasts out to at least 30 km. The most distal exposures, limited by coastal erosion to ~25–27 km, show that the matrix was not completely water saturated, with only superficial penetration of the sand-dominated material into the margins of fractured lava domains, which still contained central void space. Evidence of multiple generations of particle fracturing is seen under scanning electron microscopy of sand-grade clasts, with initial decompression fractures crosscut by later cracks, pits, and scratches produced by collisional and frictional processes during transport. The findings from this study help to explain the formation of the highly irregular topography of debris avalanche deposits, with chaotically distributed (and probably temporary) zones of shear developing where softer lithologies occur in a collapsing mass, thus leading to differential velocity profiles of portions of the flowing mass in vertical and horizontal planes.

Forecasting catastrophic stratovolcano collapse: A model based on Mount Taranaki, New Zealand

Regular large-scale edifice collapse and regrowth is a common pattern during the long lifespans of andesitic stratovolcanoes worldwide. The >130 k.y. history of Mount Taranaki, New Zealand, is punctuated by at least 14 catastrophic collapses, producing debris avalanche deposits of 1 to >7.5 km 3 . The largest of these sudden events removed as much as one-third of the present-day equivalent cone. The resulting deposits show similar sedimentary and geomorphic features, suggesting similar proto-edifice characteristics, failure trigger mechanisms, and runout path conditions. Each collapse was followed by sustained renewed volcanism and cone regrowth, although there are no matching stepwise geochemical changes in the magma erupted; instead a stable, slowly evolving magmatic system has prevailed. Last Glacial climatic variations are also uncorrelated with the timing or magnitudes of edifice collapse. We demonstrate here that, if the magmatic composition erupted from stratovolcanoes is constant and basement geology conditions are stable, large-scale edifice collapse and the generation of catastrophic debris avalanches will be governed by the magma supply rate. Using a mass balance approach, a volume-frequency model can be applied to forecasting both the probable timing and volume of future edifice failure of such stratovolcanoes. In the Mount Taranaki case, the maximum potential size of a present collapse is estimated to be 7.9 km 3 , while the maximum interval before the next collapse is 3 ).

Earthquake history at the eastern boundary of the South Taupo Volcanic Zone, New Zealand

ABSTRACT At the eastern boundary of the south Taupo Rift, the NE-striking, rift-bounding Rangipo and the ENE-striking Wahianoa active normal faults intersect. We investigate their intersection at the Upper Waikato Stream to understand the kinematics of a rift termination in an active volcanic area. The Upper Waikato Stream Fault is a previously unrecognised seismogenic source also at the eastern boundary, capable of producing a MW6.5 and up to MW7.1 earthquake if it ruptures in conjunction with the Rangipo or Wahianoa faults. We found a minimum of 12 surface-rupturing earthquakes in the last 45.16 ka on the Upper Waikato Stream Fault (mean slip-rate c. 0.5 mm/yr), and a minimum of nine surface-rupturing earthquakes in the last 133 ka on the Wahianoa Fault (mean slip-rate c. 0.2 mm/yr). Periods of highest slip-rate on these faults may coincide in time with Taupo, Ruapehu or Tongariro eruptions, but, despite their intersection, movement was not coincident across all faults. The Upper Waikato Stream Fault responded to a major Taupo Volcano eruption, the Wahianoa to a major eruptive sequence from Mt Tongariro and the Rangipo to major explosive events from Mt Ruapehu.

The Role of Cultural and Indigenous Values in Geosite Evaluations on a Quaternary Monogenetic Volcanic Landscape at Ihumātao, Auckland Volcanic Field, New Zealand

The Ihumātao Peninsula is located within the wider Mangere region in South Auckland, New Zealand. Significant sites of geological, cultural and ecological value are a recorded feature of this area. One of the most significant sites in the region is the Otuataua Stonefields Historic Reserve (OSHR). Geologically, this area is a Quaternary lava flow field with tuff rings, scoria and spatter cones. It sits within the Auckland Volcanic Field (AVF), and the landscape of the OSHR and the wider Ihumātao Peninsula may be seen as the physical expression of a unique convergence of ecological, geological and cultural values. Geosite evaluation methods applied to the Ihumātao Peninsula, following two distinct methods, shed light on potential high geoheritage values the region holds. These values may be looked at as a good base to develop effective geoeducational, geoconservational and geotouristic programs. The study also showed that implementing management strategies to add and conserve geosite values in the region could provide positive outcomes; however, reduction of its main geosite values would be inevitable and irreversible should proposed urban development take place on a block of land immediately bordering the OSHR. The Ihumātao Peninsula is one of several areas of South Auckland where urbanization has left significant areas relatively untouched until the present, whereby they are now threatened by intense housing and industrial development. The geological features of these areas unquestionably hold geological heritage values, allowing understanding of the interplay between low coastal land and rising basaltic magma. This interplay has resulted in a landscape potentially featuring the greatest volcanic geodiversity of the entire AVF. In addition, this relatively undeveloped land provides an unbroken physical and cultural record dating to arrival of the first humans in New Zealand. Of particular note is the physical artefacts and archaeological sites telling a story of settlers attracted to, utilizing and shaping this discrete region of the AVF. Geosite evaluations demonstrate that high geoheritage values of regions like the Ihumātao Peninsula are influenced by the strong cultural link between the community (in particular the indigenous population) and the volcanic landscape. These cultural factors could be given more weight in currently used geosite evaluation methods, enabling such geoheritage values to be demonstrated in a more explicit and meaningful way and providing a basis for further community education and protection of specific sites within the geographical context of the Ihumātao Peninsula.

Integrating airborne hyperspectral imagery and LiDAR for volcano mapping and monitoring through image classification

Abstract Optical and laser remote sensing provide resources for monitoring volcanic activity and surface hydrothermal alteration. In particular, multispectral and hyperspectral imaging can be used for detecting lithologies and mineral alterations on the surface of actively degassing volcanoes. This paper proposes a novel workflow to integrate existing optical and laser remote sensing data for geological mapping after the 2012 Te Maari eruptions (Tongariro Volcanic Complex, New Zealand). The image classification is based on layer-stacking of image features (optical and textural) generated from high-resolution airborne hyperspectral imagery, Light Detection and Ranging data (LiDAR) derived terrain models, and aerial photography. The images were classified using a Random Forest algorithm where input images were added from multiple sensors. Maximum image classification accuracy (overall accuracy = 85%) was achieved by adding textural information (e.g. mean, homogeneity and entropy) to the hyperspectral and LiDAR data. This workflow returned a total surface alteration area of ∼0.4 km2 at Te Maari, which was confirmed by field work, lab-spectroscopy and backscatter electron imaging. Hydrothermal alteration on volcanoes forms precipitation crusts on the surface that can mislead image classification. Therefore, we also applied spectral matching algorithms to discriminate between fresh, crust altered, and completely altered volcanic rocks. This workflow confidently recognized areas with only surface alteration, establishing a new tool for mapping structurally controlled hydrothermal alteration, evolving debris flow and hydrothermal eruption hazards. We show that data fusion of remotely sensed data can be automated to map volcanoes and significantly benefit the understanding of volcanic processes and their hazards.

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

A new pedagogical methodology is proposed to reduce the social vulnerability of indigenous communities occupying areas subject to volcanic activity, as a potential interactive approach between those communities, scientists, and scientific institutions. The multidisciplinary methodology aims to increase scientist’s understanding of the relationship between native inhabitants and active volcanoes in indigenous territories, and to improve the effective dissemination of information. Also, the proposed methodology offers to the local community the scientific knowledge in an understandable and useful way, in order to maximize people’s awareness of their exposure to volcanic activity. The procedure starts with the recognition of the local ancestral comprehension of the volcano and the cultural, ecological, and economical bonds between humans and volcanic processes. Subsequently, the transmission of the indigenous knowledge to the scientific community and the appropriation of geological knowledge by the children and teachers in a specific Māori primary school in New Zealand, allowed: (1) the establishment of a common language, (2) enhanced communication and collaboration between the participants involved in understanding and living with an active volcano, and (3) increased awareness about the relationship between humans and active volcanoes.A permanent application (and site-specific adjustment) of this method, and the use of the resulting teaching tools, could reduce social vulnerability and empower indigenous communities in the development of volcanic risk mitigation strategies by revitalizing and sharing knowledge, rather than imposing one epistemological system onto the other.