Ascent and decompression of viscous vesicular magma in a volcanic conduit
Ascent and decompression of viscous vesicular magma in a volcanic conduit
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火山管道中粘性泡状岩浆的上升和减压
DOI：
10.1029/2001jb000385
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发表时间：
2001
影响因子：
--
通讯作者：
D. Pepper
中科院分区：
文献类型：
--
作者：
H. Massol;C. Jaupart;D. Pepper
在喷发期间，熔岩圆顶和熔岩流可能变得不稳定，并产生危险的爆炸。充满化石熔岩的喷发通道和古老的熔岩流通常具有气体含量复杂的内部变化的特征。这些意见表明，需要准确预测的气体含量和气泡压力的分布在喷发管道。气泡状岩浆表现为可压缩的粘性液体，涉及三种不同的压力：气相和岩浆相的压力，以及外部的压力。为了求解这三种不同的压力，必须考虑所有方向上的膨胀，从而考虑水平和垂直速度分量。本文提出了一个新的二维有限单元数值程序来求解气泡状岩浆的流动。即使溶解水浓度很小，火山口的气体超压也可能达到大于1 MPa的值。在粘度不变的情况下，气体超压的大小不依赖于岩浆粘度，而随通道半径和岩浆房压力的增大而增大。在管道中和排气口处，气体压力水平变化大，因此出溶水含量水平变化大。这种变化取决于减压率，并对流动的“出口”边界条件敏感。对于零水平切应力在通风口，有关熔岩流在表面水平传播，最大的气体超压，因此最小的exsolved气体含量，实现在导管壁。对于零水平速度在通风口，对应于通过预先存在的熔岩圆顶或脊柱生长的塞状喷发，气体超压是最大的中心的通风口。天然气超压的大小对脱气引起的岩浆粘度的变化和具有深度依赖性半径的管道中的浅层膨胀条件敏感。
During eruption, lava domes and flows may become unstable and generate dangerous explosions. Fossil lava-filled eruption conduits and ancient lava flows are often characterized by complex internal variations of gas content. These observations indicate a need for accurate predictions of the distribution of gas content and bubble pressure in an eruption conduit. Bubbly magma behaves as a compressible viscous liquid involving three different pressures: those of the gas and magma phases, and that of the exterior. To solve for these three different pressures, one must account for expansion in all directions and hence for both horizontal and vertical velocity components. We present a new two-dimensional finite element numerical code to solve for the flow of bubbly magma. Even with small dissolved water concentrations, gas overpressures may reach values larger than 1 MPa at a volcanic vent. For constant viscosity the magnitude of gas overpressure does not depend on magma viscosity and increases with the conduit radius and magma chamber pressure. In the conduit and at the vent, there are large horizontal variations of gas pressure and hence of exsolved water content. Such variations depend on decompression rate and are sensitive to the “exit” boundary conditions for the flow. For zero horizontal shear stress at the vent, relevant to lava flows spreading horizontally at the surface, the largest gas overpressures, and hence the smallest exsolved gas contents, are achieved at the conduit walls. For zero horizontal velocity at the vent, corresponding to a plug-like eruption through a preexisting lava dome or to spine growth, gas overpressures are largest at the center of the vent. The magnitude of gas overpressure is sensitive to changes of magma viscosity induced by degassing and to shallow expansion conditions in conduits with depth-dependent radii.