The transition from endogenous to exogenous growth of lava domes with the development of shear bands
The transition from endogenous to exogenous growth of lava domes with the development of shear bands
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DOI：
10.1016/j.jvolgeores.2007.12.016
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发表时间：
2008-04-20
影响因子：
2.9
通讯作者：
Wadge, Geoff
中科院分区：
文献类型：
--
作者：
Hale, Alina J.;Wadge, Geoff
熔岩穹丘生长从内生状态向外生状态的转变必须通过穹丘内部的不连续性来实现。这种转变（反之亦然）是大多数长寿的熔岩穹丘的重要特征，通常与岩浆供应和熔岩穹丘坍塌事件的动态变化相吻合。为了本文的目的，根据最近的实验和观测证据，我们假设这样的转变发生时，剪切带的产生。剪切带形成的模型，因此在一个圆顶和耦合导管域的增长过渡。剪切带最有可能在管道和圆顶底部的交界处形成，在那里，进入圆顶的新熔岩和现有熔岩之间的剪切应力最大。剪切带内的应力积累可能导致脆性剪切，从而形成裂缝。熔岩流的有限元模拟表明，这种剪切带只发展为一定的挤出速率和熔岩粘度。类似地，熔岩穹丘的生长状态将取决于导管内的挤出速率和粘度，这在很大程度上受导管上部的挥发物损失和晶体生长控制。我们考虑一个简化的流变学在熔岩圆顶生长考虑等温条件下与晶体生长。导管中的剪切带的发展进行了探讨与数值模型参数化与适当的苏弗里埃山火山，蒙特塞拉特的值。在1996年10月至12月，这个熔岩圆顶形成喷发经历了从内源性到外源性增长的过渡，因为它的高度增长了约90英寸。模拟表明，由于穹丘压头的增加和粘度的演变，所观察到的岩浆挤出速率从约2.0 m（3）s（-1）下降到0.5 m（3）s（-1），可能随后由于剪切带的发展而改变了穹丘的生长状态。我们的模型提供了深入了解剪切应力场可能在管道和剪切带发展所需的剪切应力。(C)2008 Elsevier B. V.保留所有权利。
The transition from an endogenous to an exogenous regime of lava dome growth must be achieved by the fort-nation of discontinuities within the dome. Such transitions (and vice versa) are an important characteristic of most long-lived lava domes and often coincide with significant changes in the dynamics of magma supply and lava dome collapse events. For the purpose of this paper, following recent experimental and observational evidence, we assume that such a transition occurs when shear bands are generated. A model for the formation of shear bands, and therefore the growth transition within a dome and coupled conduit domain is presented. Shear bands are most likely to initiate at the junction of the conduit and base of the dome, where the shear stress experienced between new lava entering the dome and existing lava is greatest. Stress accumulation within shear bands is likely to lead to brittle shear, resulting in the fort-nation of fractures. Finite element modelling of lava flow shows that such shear bands only develop for certain extrusion rates and lava viscosities. Similarly, the growth regime of the lava dome will depend upon the extrusion rate and viscosity within the conduit, which is largely controlled by volatile loss and the growth of crystals in the upper part of the conduit. We consider a simplified rheology during lava dome growth considering isothermal conditions with crystal growth. The development of shear bands in the conduit is explored with a numerical model parameterized with values appropriate for Soufriere Hills Volcano, Montserrat. During October to December 1996 this lava dome forming eruption experienced a transition from endogenous to exogenous growth as it grew in height by about 90 in. Modelling indicates that the observed fall in magma extrusion rate from about 2.0 m(3) s(-1) to 0.5 m(3) s(-1), as a result of the increased pressure head from the dome and the evolution in viscosity, could have subsequently changed the dome growth regime due to the development of shear bands. Our models provide insight into the shear stress fields possible within the conduit and the shear stresses required for shear band development. (C) 2008 Elsevier B.V. All rights reserved.