Lava dome collapses are a major threat to the population living near such volcanoes. However, it is not possible to forecast collapses reliably because the mechanisms are not clearly understood, due partly to the lack of continuous observations of such events. To address this need for field data, we have developed new monitoring stations, which are adapted to the volcanic environment. The stations tracked the complete evolution of the 2018–2019 lava dome of Merapi volcano (Indonesia) and the associated pyroclastic density currents. During the 14 months of activity, the stations acquired thermal, high-resolution visual images and movies in stereoscopic configurations. The dome developed on a plateau flanked by steep sides (~ 40°–50°) inside the crater, which was open to the SE. We observed that the dome behaved in a viscous manner (with a viscosity of 109 Pa s for the interior to 1013 Pa s for external parts of the dome) on gentle slopes, and in a brittle way (friction angle ~ 35°, cohesion < 100 kPa) on slopes steeper than 35°. Thus, the lava dome was unable to grow on the outer slopes of the plateau and a significant volume of lava (350–750 × 103 m3) accumulated and collapsed daily to the SE in relatively small volumes (< 10,000 m3), preventing the lava dome from reaching the critical volume necessary for pyroclastic density currents to form and threaten the surrounding population. The cause of the small and frequent collapses was purely gravitational during the dome activity. This suggests that relatively small differences in the summit morphology can control dome evolution, favouring either a lava dome restricted to a small volume and leading to only a minor crisis, or more voluminous dome growth and a catastrophic collapse.

中文翻译：

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默拉皮火山 2018-2019 年熔岩穹顶的生长和坍塌

熔岩穹顶坍塌是对居住在此类火山附近的居民的主要威胁。然而，不可能可靠地预测坍塌，因为其机制尚不清楚，部分原因是缺乏对此类事件的连续观察。为了满足对现场数据的这种需求，我们开发了新的监测站，以适应火山环境。这些站点跟踪了默拉皮火山（印度尼西亚）2018-2019 年熔岩穹顶的完整演化以及相关的火山碎屑密度流。在 14 个月的活动期间，这些站获得了立体配置的热、高分辨率视觉图像和电影。圆顶在火山口内的陡峭侧面（~40°–50°）两侧形成的高原上形成，该火山口向东南开放。我们观察到圆顶在缓坡上以粘性方式表现（圆顶内部的粘度为 109 Pa s，圆顶外部的粘度为 1013 Pa s），并且以脆性方式（摩擦角 ~ 35°，内聚力 < 100 kPa) 在坡度大于 35° 的斜坡上。因此，熔岩穹顶无法在高原的外坡生长，大量熔岩 (350–750 × 103 m3) 每天以相对较小的体积 (< 10,000 m3) 堆积和坍塌到东南部，阻止了熔岩圆顶以免达到火山碎屑密度流形成和威胁周围人口所需的临界体积。小而频繁的倒塌的原因纯粹是圆顶活动期间的重力。这表明顶峰形态上相对较小的差异可以控制穹顶的演化，