Onset of caldera collapse during ignimbrite eruptions
Onset of caldera collapse during ignimbrite eruptions
复制标题
火山灰岩喷发期间火山口塌陷的开始
DOI：
10.1016/s0012-821x(01)00428-9
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发表时间：
2001
影响因子：
5.3
通讯作者：
T. Druitt
中科院分区：
文献类型：
--
作者：
O. Roche;T. Druitt
采用理论和实验相结合的方法研究了富含挥发性硅质岩浆室的快速减压和排放以及与火凝灰岩喷发相关的火山口塌陷的发生。我们首先对减压岩浆房顶部的力平衡进行比例分析。这为室顶提供了失效准则，并表明沿着垂直或陡峭向外倾斜的反向环断层触发相干（活塞）塌陷所需的室负压随着室顶纵横比（R=厚度/宽度）的增加而增加。破坏准则通过一系列实验进行了验证，这些实验揭示了随着顶板纵横比的增加，从活塞塌陷到 (1) 无塌陷（低储层欠压）或 (2) 非相干塌陷（高储层欠压）的转变。然后，考虑到通过缩放分析获得的失效准则，理论模型可以计算触发火山口塌陷所需的喷发室体积分数。我们考虑一个两阶段模型，其中一个最初超压的房间随着喷发的进行而逐渐变得低压。主要输入参数有洞室深度、俯视图垂直范围和椭圆度、岩浆含水量、顶板内聚力和内摩擦系数以及环断层倾角。在机械均匀顶板和单次、大且快速喷发的情况下，触发活塞塌陷所需的喷发体积分数随着顶板纵横比的增加而增加，直到室完全排空（在任何塌陷开始之前），R≈1-1.4，确切的值主要取决于环断层倾角，在较小程度上取决于顶板的内摩擦系数。实验表明，R>1-1.4 的储层塌陷可能无法像活塞一样塌陷，而且如果表面破火山口形成，塌陷更有可能发生非相干性。七次有据可查的大型火凝灰岩喷发期间火山口塌陷开始的数据与该模型基本一致。
The rapid depressurisation and discharge of volatile rich silicic magma chamber and the onset of caldera collapse associated with ignimbrite eruption is investigated using combined theoretical and experimental approaches. We first present a scaling analysis of the force balance on the roof of a depressurising magma chamber. This provides a failure criterion for the chamber roof and shows that the chamber underpressure required to trigger coherent (piston) collapse along a vertical or steeply outward dipping reverse ring fault increases with the roof aspect ratio (R=thickness/width). The failure criterion is validated by a series of experiments that reveal a transition, with increasing roof aspect ratio, from piston collapse to either (1) no collapse (low reservoir underpressure) or (2) non-coherent collapse (high reservoir underpressure). Then, taking into account the failure criterion obtained with the scaling analysis, a theoretical model allows calculation of the erupted chamber volume fraction required to trigger caldera collapse. We consider a two stage model, with an initially overpressured chamber that becomes progressively underpressured as the eruption proceeds. The main input parameters are the chamber depth, vertical extent and ellipticity in plan view, the magma water content, the cohesion and coefficient of internal friction of the roof, and the ring fault dip. In the case of a mechanically homogeneous roof and a single, large and rapid eruption, the erupted volume fraction required to trigger piston collapse increases with the roof aspect ratio until the chamber is totally emptied (before initiation of any collapse) at R≈1–1.4, the exact value depending mainly on the ring fault dip and to a lesser extent on the coefficient of internal friction of the roof. As shown by experiments, collapse into reservoirs with R>1–1.4 probably cannot collapse as a piston and, if a surface caldera forms at all, collapse is more likely to occur non-coherently. Data for the onset of caldera collapse during seven well documented major ignimbrite eruptions are in broad agreement with the model.