The influence of conduit geometry on the dynamics of caldera-forming eruptions
The influence of conduit geometry on the dynamics of caldera-forming eruptions
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管道几何形状对火山口形成喷发动力学的影响
DOI：
10.1016/s0012-821x(00)00109-6
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发表时间：
2000
影响因子：
5.3
通讯作者：
J. Martí
中科院分区：
文献类型：
--
作者：
F. Legros;K. Kelfoun;J. Martí
在火山口崩塌事件中，新的管道可能会打开，并戏剧性地改变相关喷发的风格。为了研究管道几何形状对火山口形成喷发动力学的影响，我们对岩浆在单喷口、圆柱形管道和环形裂缝管道中的上升进行了数值模拟。结果表明，对于相同体积的导管，环形裂缝导管由于摩擦力较大，放电速率要小一个数量级。此外，从一个持续的普林尼柱到一个崩塌的喷泉的过渡，在环形裂缝管道中发生的流量要高一个数量级，这是由于幕状射流中的空气夹带率比圆柱形射流高。因此，从环形裂缝导管中产生火山碎屑流需要比从圆柱形导管中产生大得多的导管体积。我们认为，这应该与矿床中更大体积的岩屑有关。我们用两个地质实例来面对这些理论问题。陶坡烟灰岩为高流量火山碎屑流侵位，含少量岩屑。这与它在单喷口喷发阶段的就位相一致。毕晓普凝灰岩是由流速较低的火山碎屑流形成的，但含有较多的岩屑。这与通过沿环形裂缝迁移的连续喷口喷发相一致。因此，火成岩的岩屑含量和喷发速率可以对破火山口崩塌的机制和环形裂缝作为岩浆通道的作用提供一些见解。我们分析的一个结论是，完全环形的环形裂缝导管不利于火山碎屑流的形成。在几乎所有形成火山口的喷发中都出现了火山碎屑流，这表明岩浆上升主要集中在较为狭窄的管道中。一个必然的结论是，广泛分布的火成岩，记录了高喷发率的喷发，不一定是火山口崩塌时环形裂缝打开的结果。
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 result of ring fissure opening during caldera collapse.