The geological history and hazards of a long-lived stratovolcano, Mt. Taranaki, New Zealand

Mt. Taranaki is an andesitic stratovolcano in the western North Island of New Zealand. Its magmas show slab-dehydration signatures and over the last 200 kyr they show gradually increasing incompatible element concentrations. Source basaltic melts from the upper mantle lithosphere pond at the base of the crust (∼25 km), interacting with other stalled melts rich in amphibole. Evolved hydrous magmas rise and pause in the mid crust (14–6 km), before taking separate pathways to eruption. Over 228 tephras erupted over the last 30 kyr display a 1000–1500 yr-periodic cycle with a five-fold variation in eruption frequency. Magmatic supply and/or tectonic regime could control this rate-variability. The volcano has collapsed and re-grown 16 times, producing large (2 to >7.5 km3) debris avalanches. Magma intrusion along N-S striking faults below the edifice are the most likely trigger for its failure. The largest Mt. Taranaki Plinian eruption columns reach ∼27 km high, dispersing 0.1 to 0.6 km3 falls throughout the North Island. Smaller explosive eruptions, or dome-growth and collapse episodes were more frequent. Block-and-ash flows reached up to 13 km from the vent, while the largest pumice pyroclastic density currents travelled >23 km. Mt. Taranaki last erupted in AD1790 and the present annual probability of eruption is 1–1.3%.fals

Sedimentology of Volcanic Debris Avalanche Deposits

The deposits of volcanic debris avalanches (VDAs) contain diagnostic features that distinguish them from those of other landslides. In this chapter, we summarize the sedimentary characteristics and the different (litho-)facies described over the past four decades, and how findings from individual case studies can be adapted as globally applicable sedimentological tools. A plethora of descriptive terms and partially conflicting definitions emerged in the ever-growing literature on VDA deposits (VDADs). These we summarize and make recommendations for future use. Different facies models that were developed at different volcanoes might point to unique emplacement conditions (e.g. dry versus wet; confined versus unconfined) and, if confirmed, the apparent ‘conflict' of terminology might help identify the paleo-settings of ancient VDAs. General observations of large unsaturated landslides of different origin show that preservation of source stratigraphy, (mega-)clasts, jigsaw-fractured clasts, and incorporation of runout path material are common features. Their unique composition, grain sizes, and abundance of matrix sets VDADs apart from deposits of large rockslides and debris flows. The latter can be associated with VDAs, and whether they formed syn- or post-VDAD emplacement is reflected in forensic evidence within the depositional sequences. Recent case studies illustrate the advances in analytical techniques and in understanding the processes of debris avalanche transport and deposition forty years after the eruption and lateral collapse of Mount St. Helens volcano

Volcanic Debris Avalanche Transport and Emplacement Mechanisms

Field observations of volcanic debris avalanche (VDA) morphology, sedimentology, and structural features have inspired several hypotheses on their dynamic behaviour. These include plug flow, translational slide, and sliding along multiple shear zones, none of which involve large-scale turbulence during transport. The plug flow model shows normal gradation in the plug, and reverse grading in the laminar boundary layers. During translational sliding, spreading of the mass is accommodated by listric normal faults that flatten into a main sliding plane at the base of or within the avalanche body. Multiple shear zones include progressive fragmentation within the avalanching mass, resulting in pockets of shear and slip. We present case studies for each model and hypotheses for the formation of flowbands on the deposit surface. Processes involved during emplacement include disintegration, dynamic fragmentation, and matrix injection. Near the base, bulldozing and incorporation of substrata change the composition and behaviour of the VDA. In extreme cases, VDAs transform into lahars if sufficient water is available for entrainment. Post-emplacement, lahars can also happen, e.g., through debris dewatering, loading of saturated substrata or in the case of landslide dam failure. VDA also create secondary slides when deflected by topographic barriers or when the margins are oversteepened.</p

Factors Contributing to Volcano Lateral Collapse

Many factors can lead to volcano lateral collapse, which can produce devastating debris avalanches that travel up to several tens to over 100 km and cover hundreds to more than a thousand km2 with debris. Volcanic lateral collapses are severe hazards because of their destructive power and size, and sudden onset. Although their frequency of occurrence is not as high as those of smaller volcanic mass movements, such as rock falls and lahars, globally large collapses ≥0.1 km3 have occurred at least five times per century over the last 500 years. A large variety of destabilizing factors such as over-steepened slopes, magma intrusions, hydrothermal activity, climate fluctuations, deformation of the basement, and faulting can create the conditions for volcano collapse. Once a volcano reaches its critical point, a mechanism is necessary to trigger the failure event. We present the state-of-the-art of the knowledge acquired in the last few decades concerning the causes of large-scale volcanic failures to better understand the triggers, preparatory factors, and timing of volcano lateral collapse