The effects of outgassing on the transition between effusive and explosive silicic eruptions
The effects of outgassing on the transition between effusive and explosive silicic eruptions
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DOI：
10.1016/j.epsl.2012.06.056
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发表时间：
2012-10-01
影响因子：
5.3
通讯作者：
Manga, M.
中科院分区：
文献类型：
--
作者：
Degruyter, W.;Bachmann, O.;Manga, M.
硅质岩浆的喷发方式受上升过程中气体损失（放气）的影响。我们利用数值模型研究了火山导管中一维、两相、稳定流动的放气问题。通过采用福希海默方程而不是达西方程来计算出气，我们能够研究达西渗透率和惯性渗透率对喷涌喷发和爆炸性喷发之间过渡的相对影响。这些渗透率由火山碎屑的结构分析得到的本构方程定义，并由气泡数密度、喉泡尺寸比、扭曲度和粗糙度决定。放气效率作为这些参数的函数可以用两个无维量来量化：斯托克斯数（岩浆响应时间与气体流动特征时间之比）和福奇海默数（气泡网络内粘性力与惯性力之比）。一个小的斯托克斯数表明气体和岩浆之间的强耦合，从而促进爆炸性喷发。较大的福希海默数表明，气体从气泡网络中逸出主要受惯性效应支配，从而导致爆炸行为。为了提供背景，我们将模型预测与1980年5月18日的圣海伦火山和1997年8月至9月的苏弗里埃尔火山喷发进行比较。我们证明了在喷涌和爆炸喷发过程中，惯性效应主导着放气，在这种情况下，喷发状态由Stokes数和Forchheimer数之比定义的一个新的无量纲量决定。在考虑的纹理参数中，气泡数密度对该量的影响最大。这一结果对渗透率研究和管道建模具有重要意义。(C) 2012 Elsevier B.V.版权所有
The eruption style of silicic magmas is affected by the loss of gas (outgassing) during ascent. We investigate outgassing using a numerical model for one-dimensional, two-phase, steady flow in a volcanic conduit. By implementing Forchheimer's equation rather than Darcy's equation for outgassing we are able to investigate the relative influence of Darcian and inertial permeability on the transition between effusive and explosive eruptions. These permeabilities are defined by constitutive equations obtained from textural analysis of pyroclasts and determined by bubble number density, throat-bubble size ratio, tortuosity, and roughness. The efficiency of outgassing as a function of these parameters can be quantified by two dimensionless quantities: the Stokes number, the ratio of the response time of the magma and the characteristic time of gas flow, and the Forchheimer number, the ratio of the viscous and inertial forces inside the bubble network. A small Stokes number indicates strong coupling between gas and magma and thus promotes explosive eruption. A large Forchheimer number signifies that gas escape from the bubble network is dominated by inertial effects, which leads to explosive behaviour. To provide context we compare model predictions to the May 18, 1980 Mount St. Helens and the August-September 1997 Soufriere Hills eruptions. We show that inertial effects dominate outgassing during both effusive and explosive eruptions, and that in this case the eruptive regime is determined by a new dimensionless quantity defined by the ratio of Stokes and Forchheimer number. Of the considered textural parameters, the bubble number density has the strongest influence on this quantity. This result has implications for permeability studies and conduit modelling. (C) 2012 Elsevier B.V. All rights reserved.