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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Interaction between particles transported by fallout and surge in the deposits of the Agnano-Monte Spina eruption (Campi Flegrei, Southern Italy)
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Interaction between particles transported by fallout and surge in the deposits of the Agnano-Monte Spina eruption (Campi Flegrei, Southern Italy)

机译：尘埃运移带来的粒子之间的相互作用以及阿格纳诺-蒙特斯皮纳火山爆发的沉积物激增之间的相互作用（意大利南部坎皮弗莱格里）

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摘要

The Agnano-Monte Spina Tephra (AMST) is a complex sequence of beds generated by. contrasting fragmentation and transportation dynamics. The 4.1 ka eruption was accompanied by a volcano-tectonic collapse, part of which is the present Agnano plain. The pyroclastic sequence is subdivided into members and submembers, each characterized by different lithological and sedimentological features. Plinian/subplinian fallout deposits frequently alternate with base-surge beds of phreatomagmatic origin. Analysis of lateral facies variations and vertical facies associations of correlated layers of submembers B2, D2 and E2 reveals that during some eruption phases the contrasting eruptive dynamics were almost contemporaneous. Base-surge deposits of submember B2 formed during the declining stage of a plinian column. They resulted from highly energetic and steady pyroclastic density currents that traveled long distances from the vent area and surmounted topographic obstacles such as the Posillipo hill (～150 m) and the northern sector of the Camaldoli hill (～250 m). Submember D2 pyroclastic density currents formed when contemporaneous fallout from an eruptive column was present. Coarse particles from the column settled throughout the pyroclastic density current, determining a significant increase of the solid load of the base surge. The consequent increase of supply rate from the transportation to the depositional zone of the base surge led to the formation of unsteady flows that could not efficiently transport the solid load and did not have the ability to travel long distances. Base-surge deposits of submember E2 were fed by a pulsating phreatomagmatic activity, which was punctuated by a short-lived fallout phase. Fallout material, which was incorporated as an additional load to the base surges, was partially transported by low-energy, steady pyroclastic density currents that traveled over the Agnano plain but did not surmount either the Camaldoli or the Posillipo hills. Only the very fine material, in continuous suspension in the upper, no-shearing part of the base-surge cloud, was dispersed higher in the atmosphere and quietly settled over a large area outside the caldera rim. The phreatomagmatic origin of base surges, which contrasts with the magmatic origin of fallout activity, suggests that the pyroclastic density currents of the Agnano-Monte Spina eruption did not result from eruption column collapses. They were most likely related to radially spreading clouds which were contemporaneous with fallout activity but issued from distinct zones in the vent area. The turbulent nature and the high expansion of base surges made them capable, under certain conditions, of passing over high topographic obstacles, with hazardous effects in distal areas.
机译：Agnano-Monte Spina Tephra（AMST）是由一系列复杂的床层组成。相反的破碎和运输动力学。 4.1 ka火山喷发伴有火山-构造塌陷，其中一部分是目前的阿格纳诺平原。火山碎屑岩层序可分为成员和亚成员，每一个都具有不同的岩性和沉积学特征。 Plinian / subplinian沉积物沉积经常与岩浆岩起源的底涌床交替出现。对子成员B2，D2和E2的相关层的侧相变化和垂直相联系的分析表明，在某些喷发阶段，对比的爆发动力几乎是同时期的。子构件B2的基极喘振沉积在柱状柱下降阶段形成。它们是由高能和稳定的火山碎屑密度流产生的，这些电流从喷口处经过很长一段距离，并克服了诸如Posillipo丘陵（〜150 m）和Camaldoli丘陵的北部（〜250 m）之类的地形障碍。当存在喷发柱同时沉降时，会形成子成员D2火山碎屑密度流。在整个火山碎屑密度流中，来自色谱柱的粗颗粒沉降，从而确定了基础电涌的固体负荷的显着增加。结果是，从基本波涌的运输到沉积区的供应速率增加，导致形成不稳定流，这种不稳定流不能有效地运输固体负荷，也不能长距离行驶。 E2子群的基础涌动沉积物是通过脉动的岩浆岩活动来补充的，该活动被短暂的沉降期所打断。辐射物质作为基础浪涌的附加载荷而被吸收，一部分是通过低能量，稳定的火山碎屑密度流传输的，这些电流流过了阿格纳诺平原，但没有超过卡马尔多利山或波西利波利山。只有极细的物质连续悬浮在基波涌动云的上部无剪切部分中，才能在大气中较高的位置散布，并在火山口边缘以外的大面积区域内安静地沉降。基涌的吞噬岩浆起源，与沉降活动的岩浆起源相反，表明阿格纳诺-蒙特斯皮纳火山喷发的火山碎屑密度流不是由喷发柱塌陷引起的。它们最有可能与放射状云层有关，这些云层与降尘活动同时发生，但发源于通风区的不同区域。湍流的性质和基础浪涌的高扩展性使其在一定条件下能够越过高地形障碍，并在远端区域产生危险影响。

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