Abstract 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 km3 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 km3, 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 ~1 × 106 m3 and leading to the deposition of single bed ash units; (2) small-volume (5–10 × 106 m3) 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 × 106 m3), 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.

中文翻译：

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小体积、低强度喷发的岩石沉积学和火山灰地层特征：1800 年的 Tufa Trig 地层，Mt. 鲁阿佩胡（新西兰）

摘要 中低强度喷发（VEI ≤ 3）是历史时间尺度上最频繁的喷发，可持续数天到数年。对历史喷发的直接观察（例如 Ruapehu 1995-1996、Nakadake 2003-2005、埃特纳火山 2002-2003）突出了这些喷发的复杂性，它们通常涉及多个阶段和喷发行为的突然变化。与中低强度喷发相关的喷发产物的典型特征是喷发体积小 ≪ 1 km3 和大量灰烬大小的物质，这使得它们的保存受当地环境条件的影响。因此，长期（1000 年）的喷发记录往往偏向于通常不那么频繁但保存得更好、更强烈、VEI > 4 和体积 > 1 km3 的喷发，导致低估了小规模喷发的复杂性和频率。本研究为 Tufa Trig 组提供了高分辨率的火山灰岩地层框架，该组是在 2797 m 高的安山质复合火山 Mt. 过去 1800 年的活动中形成的。新西兰的鲁阿佩胡。宏观岩石沉积学特征和火山灰扩散的系统绘图和表征与对选定特征序列的进一步研究相结合，为新西兰历史上最活跃的火山之一的 1800 年火山灰记录增加了复杂性。Tephra 矿床显示出一系列矿床结构、成分和分布，喷发活动显示出随时间变化的分布。根据岩性沉积物特征，形成Tufa Trig组可以区分三种主要喷发类型：（1）以灰为主的低强度喷发，体积约1×106 m3，导致单层灰单元沉积；(2) 小体积（5-10 × 106 m3）中等强度的喷发，将火山岩大小的物质分布到 20 公里的距离；(3) 与多床灰层序相关的多相喷发，涉及一个数量级更大的火山灰体积（15.19-68.89 × 106 m3），其火山灰扩散在不同方向显示出多个厚度最大值。此处介绍的框架扩展了与中低强度喷发相关的火山灰沉积物复杂性的现有知识，并为 Mt. 的进一步、更详细的研究提供了基础。Ruapehu 和世界各地的类似火山。