Lithosedimentological and tephrostratigraphical characterisation of small-volume, low-intensity eruptions: The 1800 years Tufa Trig Formation, Mt. Ruapehu (New Zealand)

摘要

Low to moderate-intensity eruptions (VEI = 3) constitute the most frequent eruptions on historical time scales and can last from days to years. Direct observations of historical eruptions (e.g. Ruapehu 1995-1996, Nakadake 2003-2005, Etna 2002-2003) have highlighted the complexity of these eruptions, which often involve multiple phases and sudden changes in eruption behaviour. Eruptive products associated with low to moderate-intensity eruptions are typically characterised by small erupted volumes 1 km(3) and a significant amount of ash-sized material, making their preservation subject to local environmental conditions. Accordingly, long-term (1000s of years) eruptive records tend to be biased towards the usually less frequent but better-preserved, more intense eruptions with VEI 4 and volumes 1 km(3), leading to an underestimation of the complexity and frequency of small-scale eruptions.This research presents a high-resolution tephrostratigraphic framework for the Tufa Trig Formation, formed during the last 1800 years of activity of the 2797 m high andesitic composite volcano Mt. Ruapehu in New Zealand. Systematic mapping and characterisation of macroscopic lithosedimentological features and tephra dispersal are combined with further studies on selected characteristic sequences to add complexity to the 1800 years tephra record of one of New Zealand's historically most active volcanoes. Tephra deposits display a range of deposit textures, componentry and dispersal, with eruptive activity showing a time-variant distribution. Based on lithosedimentological deposit features, three main eruption types can be distinguished to have formed the Tufa Trig Formation: (1) ash-dominated low-intensity eruptions having volumes of similar to 1 x 10(6) m(3) and leading to the deposition of single bed ash units; (2) small-volume (5-10 x 10(6) m(3)) moderate intensity eruptions, which distributed lapilli-sized material up to distances of 20 km; and (3) multi-phase eruptions that were associated with multi bed ash sequences, involved a magnitude larger tephra volumes ( 15.19-68.89 x 10(6) m(3)), and whose tephra dispersal shows multiple thickness maxima in different directions. The here presented framework extends existing knowledge on the complexity of tephra deposits associated with low to moderate-intensity eruptions and provides the fundamental base for further, more detailed studies at Mt. Ruapehu and similar volcanoes worldwide. To guide future studies on similar records of detailed VEI = 3 volcanic sequences elsewhere, several key lessons and challenges in the characterisation of such fall units are summarised also.