Evolution of andesite magma systems; Egmont Volcano, New Zealand

A major issue in andesite magmas genesis is explaining disequilibrium crystal, matrix glass and whole rock compositions. Taranaki/Egmont is a high-K andesite volcano in the western North, Island, New Zealand, with a 200, 000 year eruption record. Thirteen recently identified and dated pre-7 ka debris avalanche deposits record the magmatic evolution of the Taranaki volcanic system. Clast compositions show a gradual enrichment in K (sub 2) O and LILE with time to high-K andesites in the Holocene. Pre-100 ka magmas include relatively primitive basalts and basaltic andesites and mineral chemistry indicates crystallisation within the lower crust or mantle. Modal rock compositions become more silicic in younger units, and the appearance of late-stage low-pressure mineral phases (high-Ti hornblende, biotite and Fe-rich orthopyroxene), suggests an increase in more evolved magmas with time. Six compositionally distinct Holocene magma batches erupted on 1500-2000 year timescales, synchronous with variations in eruptive frequency in which the largest volume (>0.5 km (super 3) ) events erupt the most evolved magmas. We suggest that andesite magmas were generated within a lower crustal 'hot zone' [1]. Matrix glasses in both xenoliths and lavas/tephras are mostly dacitic to rhyolitic in composition and, in younger lavas have a high K (sub 2) O content. These glasses may represent some of the partial melts from the 'hot zone'[2]. The disequilibrium observed in the andesites is due to the mixing of these diverse components. A complex and dispersed magma assembly and storage system developed in the upper crust where the magmas were further modified by fractional crystallisation and magma mixing and mingling [2].1 page(s

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 km³. 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 km³, while the maximum interval before the next collapse is <16.2 k.y. The current annual collapse probability is ∼0.00018, with the most likely collapse being a small one (<2 km³)

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%

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