Variations in gas emissions of open-vent volcanoes are investigated using a model of magma convection in narrow conduits. Laboratory experiments with both vertical and inclined conduits and dimensional analysis show that for Grashof numbers lower than 100 the volumetric rate of magma ascent is a simple function of equivalent conduit radius, density difference between the magmas, and viscosity of the degassed magma that descends back to the reservoir. The rate of magma ascent depends on the flux coefficient, estimated as 0.1 and 0.2 for vertical and inclined conduits, respectively. The equivalent radius parameter accounts for the dimensions of the conduit(s) regardless of its geometry, thus extending the treatment by previous models that used flow in pipes. The volume flow rate of convection increases with higher density difference and conduit size, but is also highly influenced by the large variations in viscosity of the degassed magma as volatile content and crystallinity change. The model presented here can be used to constrain the degassing and ascent rates of volatile-rich magma when combined with petrologic data on magmatic volatile content. Application of the model to Villarrica volcano (Chile) reveals that the background degassing levels observed (∼3 kg s−1 SO2) are associated with convective ascent of a relatively degassed magma (0.04 wt% S, ∼0.5 wt% H2O), while episodes of higher SO2 emissions (measurements up to 15 kg s−1) can be explained by the ascent of magma with higher volatile content (up to 0.09 wt% S, ∼1.5 wt% H2O).
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机译：使用狭窄管道中的岩浆对流模型研究了敞开式火山的气体排放变化。垂直和倾斜管道的实验室实验以及尺寸分析表明，对于Grashof数小于100的岩浆上升速度，是等效管道半径，岩浆之间的密度差以及脱气岩浆粘度回落到的简单函数。水库。岩浆上升的速度取决于通量系数，垂直和倾斜导管的通量系数分别估计为0.1和0.2。等效半径参数说明了导管的尺寸，而不管其几何形状如何，因此可以通过以前使用管道中流动的模型来扩展处理。对流的体积流率随较高的密度差和导管尺寸而增加，但也受到脱气岩浆粘度随挥发物含量和结晶度变化而变化的很大影响。当与岩浆挥发物含量的岩石学数据结合使用时，此处介绍的模型可用于限制富含挥发物的岩浆的脱气率和上升率。该模型在比利亚里卡火山（智利）上的应用表明，观察到的背景脱气水平（约3 kg s-1 SO2）与相对脱气的岩浆（0.04 wt％S，〜0.5 wt％H2O）的对流上升有关，而较高的SO2排放量（测量值最高为15 kg s-1）可以用挥发物含量较高（最高为0.09 wt％S，〜1.5 wt％H2O）的岩浆上升来解释。
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