Heating of water under hot pyroclastic flow deposits can drive hydroeruptions, forming craters and aprons of secondary deposits. According to the established conceptual model, steam pressure builds until failure of the pyroclastic overburden and a relatively low permeability (fine-grained) cap promotes secondary explosions. We explore a complementary model where the stress from drag related to gas flow up through the particle interstices is comparable in magnitude to the static pressure difference between the base and the top of the pyroclastic flow deposit. The drag force supports (part of) the weight of the particles and so reduces inter-particle friction; in a mono-sized bed this friction is effectively eliminated at the “minimum fluidisation velocity,” which depends on the size and density of the particles. Through analogue experiments we show that violent outbursts can be generated when there are vertical variations in the minimum fluidisation velocities of granular materials. We ran experiments with layers of particles with different sizes or size distributions (bi-modal with different proportions of fine and coarse particles) in a tank with a porous base that allowed a distributed upward airflow through them. A finer-grained layer capping a coarser layer does not generate jets of particles or craters; rather, increased gas flux leads to fluidisation of first the fine and then the coarse (lower) layer. However, when the upper layer is coarser, the bed domes upward as a gas pocket grows within the finer layer for some combinations of layer thicknesses and grain sizes. When the gas pocket penetrates the top of the bed, it forms a crater and erupts particles. The gas velocity when doming initiates is greater than that calculated for the weight of the top layer to be balanced by drag and the pressure difference across that layer. This discrepancy is explained by the layers having a strength (from inter-particle friction), which is consistent with the observed dependence of the initiation velocity on the absolute thickness of the layer. Using data from Mt St Helens 1980 deposits, we show that the drag-related trigger observed in the laboratory is a feasible mechanism for secondary hydroeruptions through pyroclastic flow deposits.

在热火山碎屑流沉积物下加热水会驱使水力喷发，形成次生沉积物的火山口和围裙。根据已建立的概念模型，蒸汽压力不断增加，直到火山碎屑覆盖层破裂，相对较低的渗透率（细颗粒）顶盖促进了二次爆炸。我们探索了一种互补模型，其中与气体向上流过颗粒空隙有关的阻力所产生的应力在大小上可与热碎屑流沉积物的底部和顶部之间的静压差相媲美。拖曳力支撑（部分）颗粒的重量，因此减少了颗粒间的摩擦；在单一尺寸的床中，这种摩擦可以在“最小流化速度”下有效消除，该速度取决于颗粒的大小和密度。通过模拟实验，我们发现，当颗粒状物料的最小流化速度发生垂直变化时，会产生剧烈的爆发。我们在具有多孔基底的水箱中运行了具有不同大小或尺寸分布（双峰的细颗粒和粗颗粒比例不同）的颗粒层的实验，该颗粒允许向上分布的气流通过它们。覆盖较粗层的较细颗粒层不会产生颗粒或火山口的喷射；相反，增加的气体通量导致首先是细层然后是粗（下）层的流化。但是，当上层较粗糙时，对于某些层厚和晶粒尺寸的组合，随着气袋在较细层内的生长，床层将呈圆顶状上升。当气袋穿透床顶时，它形成一个火山口并喷发粒子。启动拱顶时的气体速度大于为顶层的重量（通过阻力和该层上的压差来平衡）所计算的气体速度。这种差异由具有强度（来自颗粒间摩擦）的层解释，该强度与观察到的起始速度对层的绝对厚度的依赖性一致。使用来自圣海伦斯山1980年沉积物的数据，我们表明，在实验室中观察到的与阻力有关的触发因素是通过火山碎屑流沉积物继发水力喷发的可行机制。这种差异由具有强度（来自颗粒间摩擦）的层解释，该强度与观察到的起始速度对层的绝对厚度的依赖性一致。使用来自圣海伦斯山1980年沉积物的数据，我们表明在实验室中观察到的与阻力有关的触发因素是通过火山碎屑流沉积物继发水力喷发的可行机制。这种差异由具有强度（来自颗粒间摩擦）的层解释，该强度与观察到的起始速度对层的绝对厚度的依赖性一致。使用来自圣海伦斯山1980年沉积物的数据，我们表明在实验室中观察到的与阻力有关的触发因素是通过火山碎屑流沉积物继发水力喷发的可行机制。