Abstract

Lava domes form when a highly viscous magma erupts on the surface. Several types of lava dome morphology can be distinguished depending on the flow rate and the rheology of magma: obelisks, lava lobes, and endogenic structures. The viscosity of magma nonlinearly depends on the volume fraction of crystals and temperature. Here we present an approach to magma viscosity estimation based on a comparison of observed and simulated morphological forms of lava domes. We consider a two-dimensional axisymmetric model of magma extrusion on the surface and lava dome evolution, and assume that the lava viscosity depends only on the volume fraction of crystals. The crystallization is associated with a growth of the liquidus temperature due to the volatile loss from the magma, and it is determined by the characteristic time of crystal content growth (CCGT) and the discharge rate. Lava domes are modeled using a finite-volume method implemented in Ansys Fluent software for various CCGTs and volcanic vent sizes. For a selected eruption duration a set of morphological shapes of domes (shapes of the interface between lava dome and air) is obtained. Lava dome shapes modeled this way are compared with the observed shape of the lava dome (synthesized in the study by a random modification of one of the calculated shapes). To estimate magma viscosity, the deviation between the observed dome shape and the simulated dome shapes is assessed by three functionals: the symmetric difference, the peak signal-to-noise ratio, and the structural similarity index measure. These functionals are often used in the computer vision and in image processing. Although each functional allows to determine the best fit between the modeled and observed shapes of lava dome, the functional based on the structural similarity index measure performs it better. The viscosity of the observed dome can be then approximated by the viscosity of the modeled dome, which shape fits best the shape of the observed dome. This approach can be extended to three-dimensional case studies to restore the conditions of natural lava dome growth.

中文翻译：

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熔岩穹窿形态学评估岩浆粘度的方法

摘要

当高粘性岩浆在地表喷发时，熔岩穹顶就会形成。根据岩浆的流速和流变性，可以区分几种类型的熔岩穹顶形态：方尖碑、熔岩裂片和内生结构。岩浆的粘度非线性地取决于晶体的体积分数和温度。在这里，我们提出了一种基于对熔岩穹顶的观察和模拟形态形式的比较来估计岩浆粘度的方法。我们考虑了地表岩浆挤压和熔岩穹窿演化的二维轴对称模型，并假设熔岩粘度仅取决于晶体的体积分数。由于岩浆的挥发损失，结晶与液相线温度的增长有关，它由晶体含量生长的特征时间（CCGT）和放电速率决定。熔岩穹顶使用 Ansys Fluent 软件中针对各种 CCGT 和火山喷口尺寸实施的有限体积方法进行建模。对于选定的喷发持续时间，获得一组圆顶的形态形状（熔岩圆顶和空气之间的界面形状）。以这种方式建模的熔岩穹顶形状与观察到的熔岩穹顶形状（在研究中通过对计算形状之一进行随机修改而合成）进行比较。为了估计岩浆粘度，观察到的圆顶形状与模拟圆顶形状之间的偏差通过三个函数进行评估：对称差异、峰值信噪比和结构相似性指数度量。这些泛函通常用于计算机视觉和图像处理。尽管每个函数都允许确定熔岩穹顶的建模和观察形状之间的最佳拟合，但基于结构相似性指数度量的函数执行得更好。观察到的圆顶的粘度然后可以通过建模的圆顶的粘度来近似，该形状最适合观察到的圆顶的形状。这种方法可以扩展到三维案例研究，以恢复自然熔岩穹顶生长的条件。哪种形状最适合观察到的圆顶形状。这种方法可以扩展到三维案例研究，以恢复自然熔岩穹顶生长的条件。哪种形状最适合观察到的圆顶形状。这种方法可以扩展到三维案例研究，以恢复自然熔岩穹顶生长的条件。