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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Dynamics of magma ascent and fragmentation in trachytic versus rhyolitic eruptions
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Dynamics of magma ascent and fragmentation in trachytic versus rhyolitic eruptions

机译：岩浆上升和破碎的动力学在流形和流纹岩的爆发中

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We have performed a parametric study on the dynamics of trachytic (alkaline) versus rhyolitic (calc-alkaline) eruptions by employing a steady, isothermal, multiphase non-equilibrium model of conduit flow and fragmentation. The employed compositions correspond to a typical rhyolite and to trachytic liquids from Phlegrean Fields eruptions, for which detailed viscosity measurements have been performed. The investigated conditions include conduit diameters in the range 30-90 m and total water contents from 2 to 6 wt%, corresponding to mass flow rates in the range 10~6-10~8 kg/s. The numerical results show that rhyolites fragment deep in the conduit and at a gas volume fraction ranging from 0.64 to 0.76, while for trachytes fragmentation is found to occur at much shallower levels and higher vesicularities (0.81-0.85). An unexpected result is that low-viscosity trachytes can be associated with lower mass flow rates with respect to more viscous rhyolites. This is due to the non-linear combined effects of viscosity and water solubility affecting the whole eruption dynamics. The lower viscosity of trachytes, together with higher water solubility, results in delayed fragmentation, or in a longer bubbly flow region within the conduit where viscous forces are dominant. Therefore, the total dissipation due to viscous forces can be higher for the less viscous trachytic magma, depending on the specific conditions and trachytic composition employed. The fragmentation conditions determined through the simulations agree with measured vesicularities in natural pumice clasts of both magma compositions. In fact, vesicularities average 0.80 in pumice from alkaline eruptions at Phlegrean Fields, while they tend to be lower in most calc-alkaline pumices. The results of numerical simulations suggest that higher vesicularities in alkaline products are related to, delayed fragmentation of magmas with this composition. Despite large differences in the distribution of flow variables which occur in the deep conduit region and at fragmentation, the flow dynamics of rhyolites and trachytes in the upper conduit and at the vent can be very similar, at equal conduit size and total water content. This is consistent with similar phenomenologies of eruptions associated with the two magma types.
机译：我们已经通过采用稳定，等温，多相导管流动和破碎的非平衡模型，对环流（碱性）与流纹（钙碱性）喷发的动力学进行了参数研究。所使用的组合物对应于典型的流纹岩和来自Phlegrean Fields喷发的松散液体，已经对其进行了详细的粘度测量。研究条件包括导管直径在30-90 m范围内，总水含量为2至6 wt％，对应于质量流量在10〜6-10〜8 kg / s范围内。数值结果表明，流纹岩在导管深处破碎，气体体积分数在0.64到0.76之间，而对于曲风石，碎裂发生在更浅的水平和更高的水泡度（0.81-0.85）。出乎意料的结果是，相对于粘性更高的流纹岩，低粘度的小粒状假粒可能与较低的质量流率相关。这是由于粘度和水溶性的非线性组合效应影响了整个喷发动力学。松露糖的较低粘度以及较高的水溶性会导致碎裂延迟，或在导管内以粘性力为主的较长气泡状流动区域。因此，对于粘性较小的特拉维克岩浆，由于粘性力而导致的总耗散会更高，这取决于所采用的具体条件和特拉维奇成分。通过模拟确定的破碎条件与两种岩浆成分的天然浮石碎屑中测得的囊泡相吻合。实际上，Phlegrean Fields碱性喷发产生的浮石的平均囊泡率为0.80，而在大多数钙碱性浮石中，它们的囊泡率往往较低。数值模拟的结果表明，碱性产物中较高的囊泡性与这种成分的岩浆延迟碎裂有关。尽管在深管道区域和破碎时发生的流量变量分布差异很大，但在相同的管道尺寸和总含水量的情况下，在上部管道中和在出口处的流纹岩和小摆线的流动动力学可以非常相似。这与与两种岩浆类型有关的喷发现象相似。

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