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首页> 外文期刊>Journal of Volcanology and Geothermal Research >The nature, origins and distribution of ash aggregates in a large-scale wet eruption deposit: Oruanui, New Zealand
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The nature, origins and distribution of ash aggregates in a large-scale wet eruption deposit: Oruanui, New Zealand

机译：大规模湿喷沉积床中灰烬聚集体的性质，来源和分布：新西兰奥拉努伊

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摘要

This study documents the processes and products of volcanic ash aggregation in phreatomagmatic phases of the 25.4 ka Oruanui supereruption from Taupo volcano. New Zealand. Detailed textural and stratigraphic relationships of aggregates are examined in six of the ten erupted units, which range from relatively dry styles of eruption and deposition (units 2,5) to mixed (units 6, 7,8) and dominantly wet (unit 3). Aggregate structures and grain size distributions shift abruptly over vertical scales of cm to dm, providing diagnostic features to identify deposits emplaced primarily as vertical fallout or pyroclastic density currents (PDCs). The six categories of ash aggregates documented here are used to infer distinct volcanic and meteorological interactions in the eruption cloud related to dispersal characteristics and mode of emplacement. Our field observations support the notion of Brown et al. (2010, Origin of accretionary lapilli within ground-hugging density currents: evidence from pyroclastic couplets on Tenerife. Geol. Soc. Am. Bull. 122, 305-320) that deposits bearing matrix-supported accretionary lapilli with concentric internal structure and abundant rim fragments are associated with emplacement of PDCs. However, on the basis of grain size distributions and field relationships, it is inferred that these types of ash aggregates formed their ultrafine ash (dominantly <10 urn) outer layers in the buoyant plumes of fine ash lofted from PDCs, rather than during lateral transport in ground-hugging density currents. The propagation of voluminous PDCs beneath an overriding buoyant cloud - whether coignimbrite or vent-derived in origin - is proposed to generate the observed, concentrically structured accretionary lapilli by producing multiple updrafts of convectively unstable, ash-laden air. The apparent coarsening of mean grain size with distance from source, which is observed in aggregate-bearing fall fades, reflects a combination of multi-level plume transport and enhanced proximal fallout of fine ash (<250 μm) by aggregation. Gravitational fallout and melting of abundant ice in the clouds was likely to have contributed a key source of liquid water for wet aggregation in near-source areas. In contrast, deposits from relatively drier eruption phases are aggregate-poor in proximal areas, yet develop loosely-bound particle clusters and mm-scale massive ash pellets > 100 km from vent. It is inferred that ambient meteorological conditions play a more important role in ash fallout in these cases. Entrainment of moist air, and distal subsidence and melting of ice carried by the plume, are both likely to have contributed to the observed features of late-stage aggregation in the drier phases of eruption. These observations suggest that proximal, column-influenced aggregation processes, which weaken with distance from source, are overprinted by secondary, later-stage aggregation mechanisms farther downwind.
机译：这项研究记录了陶波火山25.4 ka Oruanui火山喷发的岩浆岩相中火山灰聚集的过程和产物。新西兰。在十个喷发单元中的六个中检查了骨料的详细纹理和地层关系，范围从相对干燥的喷发和沉积样式（第2，5个单元）到混合（第6、7、8个单元）和主要是湿润（第3单元） 。聚集体结构和晶粒尺寸分布在厘米到dm的垂直尺度上突然偏移，提供了诊断功能，可识别主要是垂直沉降或火山碎屑密度流（PDC）沉积的沉积物。此处记录的六类灰烬聚集体用于推断喷发云中与散布特征和进位方式有关的独特的火山和气象相互作用。我们的现场观察支持了Brown等人的观点。 （2010年，在拥抱地面的密度流中增生性lapilli的起源：来自特内里费岛上的火山碎屑对的证据。Geol。Soc。Am。Bull。122，305-320）沉积了具有同心内部结构且边缘丰富的基质支持的增生性lapilli碎片与PDC的放置有关。但是，根据粒度分布和田间关系，可以推断出这些类型的灰渣聚集体是在从PDC散发出来的细灰的浮羽中形成了其超细灰（主要为<10 um）外层，而不是在横向运输过程中在地面密集的电流中。提议通过覆盖浮力云下大量的PDC传播（无论是共生的还是起源于通风的），通过产生多个对流不稳定的，充满灰分的空气上升气流来产生观察到的同心结构增生性lapilli。在含骨料的秋天衰落中观察到平均晶粒尺寸随距源的距离而出现的明显变粗，反映了多级羽流运输和聚集导致的细灰（<250μm）近端沉降增强。引力的沉降和云层中大量冰的融化可能是造成近源地区湿性聚集的关键液态水来源。相反，来自相对较干燥喷发期的沉积物在近端区域聚集较差，但会形成松散结合的颗粒簇和毫米毫米的块状灰渣颗粒，距离发泄口> 100 km。据推测，在这些情况下，周围的气象条件在灰分沉降中起着更重要的作用。湿空气的夹带，烟羽携带的远侧沉降和冰的融化，都可能导致了喷发干燥期后期聚集的观测特征。这些观察结果表明，随着距源的距离变弱，近端受柱影响的聚集过程被顺风向后的次要后期聚集机制所覆盖。

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