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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >The influence of conduit geometry on the dynamics of caldera-forming eruptions
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The influence of conduit geometry on the dynamics of caldera-forming eruptions

机译：导管几何形状对破火山口爆发动力学的影响

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摘要

During caldera collapse events, new conduits may open and dramatically modify the style of the associated eruption. In order to investigate the effect of conduit geometry on the dynamics of caldera-forming eruptions, we performed numerical simulations of magma ascent in single-vent, cylindrical conduits and ring-fissure conduits. The results show that, for a given volume of conduit, the discharge rate is an order of magnitude smaller in ring-fissure conduits due to the higher friction. Furthermore, the transition from a sustained Plinian column to a collapsing fountain feeding pyroclastic flows occurs at a discharge rate an order of magnitude higher for ring-fissure conduits, due to the higher rate of air entrainment in a curtain jet than in a cylindrical jet. The production of pyroclastic flows from ring-fissure conduits therefore requires a much larger conduit volume than from a cylindrical conduit. We argue that this should be correlated with a much larger volume of lithics in the deposit. We confront these theoretical considerations with two geological examples. The Taupo ignimbrite has been emplaced by a high-discharge rate pyroclastic flow and contains a small volume of lithics. This is consistent with its emplacement during a single-vent eruptive phase. The Bishop Tuff has been emplaced by a lower discharge rate pyroclastic flow and yet contains much more lithics. This is consistent with an eruption through successive vents migrating along a ring fissure. The lithic content of an ignimbrite and the eruption discharge rate can therefore give some insights into the mechanism of caldera collapse and the role of ring fissures as magma conduits. A conclusion of our analysis is that perfectly annular ring-fissure conduits do not favour the formation of pyroclastic flows. The occurrence of pyroclastic flows in virtually all caldera-forming eruptions suggests that magma ascent is mainly localised in more restricted conduits. A corollary conclusion is that widespread ignimbrites, which record high-discharge rate eruptions, are not necessarily the results of ring fissure opening during caldera collapse.
机译：在破火山口塌陷事件期间，新的导管可能会打开并显着改变相关喷发的样式。为了研究导管几何形状对破火山口爆发动力学的影响，我们在单通气，圆柱形导管和环裂导管中进行了岩浆上升的数值模拟。结果表明，对于给定体积的导管，由于较高的摩擦力，环形裂纹导管中的排放速率要小一个数量级。此外，由于帘式射流中夹带空气的速率比圆柱状射流中夹带的空气速率高，因此从持续的Plinian柱向塌陷的喷泉供给火山碎屑流的过渡发生在环形裂缝导管的排放速率高出一个数量级的情况下。因此，由环形裂缝导管产生的火山碎屑流需要比圆柱形导管大得多的导管体积。我们认为这应与沉积物中大量的石屑相关。我们通过两个地质实例来面对这些理论考虑。陶波的火成岩已被高排放速率的碎屑流所包围，并包含少量的石屑。这与其在单口喷发阶段的安置相一致。主教凝灰岩被较低的火山碎屑流速率所取代，但包含的岩屑更多。这与沿环形裂缝移动的连续喷口喷发是一致的。因此，火成岩的岩屑含量和喷发速率可以深入了解破火山口塌陷的机制以及环裂作为岩浆导管的作用。我们的分析结论是，完美的环形裂隙导管不利于形成火山碎屑流。火山碎屑流实际上发生在所有形成火山口的火山喷发中，表明岩浆上升主要局限在受限制的管道中。一个推论性的结论是，记录高放电速率爆发的大范围火成岩并不一定是破火山口塌陷过程中环形裂缝打开的结果。

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