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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 >Pressure evolution during explosive caldera-forming eruptions
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Pressure evolution during explosive caldera-forming eruptions

机译：爆炸性火山口形成爆发期间的压力演化

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摘要

Caldera-forming eruptions of silicic magmas result from a complex coupling of the mechanics and fluid dynamics of the associated magma chamber. Field studies of caldera-forming eruption products suggest that great pressure variations occur inside the magma chamber and associated conduits during these eruptions. Pressure evolution during explosive caldera-forming eruptions is investigated through a simple model that describes the first-order quantitative behaviour of the chamber. We consider a piston-like model that assumes a coherent block subsiding along circular, sub-vertical, ring faults into the magma chamber. This subsidence occurs after significant decompression of the chamber by an initial central vent eruption. We assume that the initial pressure distribution in the chamber is magmastatic. Once collapse has begun the chamber roof is supported by the magma, so that magma pressure at the chamber roof increases to lithostatic. We suggest that pressure variations during caldera-forming eruptions are mainly controlled by variations in magma volatile content. Regardless of what induces the formation of ring faults, the model suggests that the occurrence of explosive caldera-forming events depends on the strength of the chamber walls, and the depth, water content and aspect ratio of the magma chamber. No significant differences exist between model results for a cylindrical, or a more realistic elliptical magma chamber geometry of comparable aspect ratio. Assuming a constant strength of the host rock, the mass fraction of magma that must be erupted during the central vent phase in order to trigger caldera collapse ranges, for deep, gas-poor chambers, from a few percent up to 40% for shallow, gas-rich chambers. The model suggests that zoned chambers tend to collapse earlier than homogeneous chambers. Dike-shaped chambers will erupt less magma than sill-like chambers before caldera collapse initiates, although dike-like geometries are not associated with stress fields appropriate to create ring faults. The model suggests that once initiated, caldera collapse will tend to force out most or all of the volatile-rich magma from the chamber. For volatile-rich magma chambers, the total volume of erupted magma during caldera-forming event is of the same order as the chamber volume. The model also explains the variation in the erupted mass during the different phases of explosive caldera-forming eruptions, and is in good agreement with natural examples.
机译：硅质岩浆的火山口形成喷发是由于相关岩浆腔的力学和流体动力学复杂耦合造成的。破火山口形成喷发产物的现场研究表明，在这些喷发过程中，岩浆室内及相关导管内部发生了很大的压力变化。通过描述腔室一阶定量行为的简单模型研究了爆炸性火山口形成爆发期间的压力演化。我们考虑一个类似活塞的模型，该模型假设一个相干的块体沿着圆形的，次垂直的环形断层陷落到岩浆室内。这种沉陷发生在腔室因初始中央通风孔喷出而显着减压之后。我们假设腔室内的初始压力分布是静磁的。一旦开始坍塌，岩浆室的顶板便受到岩浆的支撑，因此岩浆室顶板的岩浆压力会增加至岩性。我们建议火山口形成爆发期间的压力变化主要受岩浆挥发物含量的变化控制。无论导致环断层形成的原因如何，该模型都表明爆炸性破火山口形成事件的发生取决于室壁的强度以及岩浆室的深度，含水量和纵横比。在具有类似纵横比的圆柱形或更实际的椭圆形岩浆腔室几何模型的模型结果之间，没有显着差异。假设基质岩石的强度恒定，则在深部气少的气室中，为了触发破火山口坍塌，必须在中央通风阶段喷出的岩浆质量分数从浅到深的几个百分点到40％不等，富气室。该模型表明分区腔室倾向于比均质腔室更早坍塌。尽管破火山口的几何形状与产生环断层的应力场无关，但在破火山口坍塌开始之前，堤防室比岩浆室的岩浆喷出的量少。该模型表明，一旦破火山口坍塌，就会趋向于将大部分或全部富含挥发物的岩浆从室内驱除。对于富含挥发物的岩浆室，在破火山口形成过程中喷出的岩浆总体积与该室体积相同。该模型还解释了爆炸性破火山口形成喷发不同阶段喷发质量的变化，并且与自然实例非常吻合。

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