Predicting the onset, style and duration of explosive volcanic eruptions remains a great challenge. While the fundamental underlying processes are thought to be known, a clear correlation between eruptive features observable above Earth’s surface and conditions and properties in the immediate subsurface is far from complete. Furthermore, the highly dynamic nature and inaccessibility of explosive events means that progress in the field investigation of such events remains slow. Scaled experimental investigations represent an opportunity to study individual volcanic processes separately and, despite their highly dynamic nature, to quantify them systematically. Here, impulsively generated vertical gas-particle jets were generated using rapid decompression shock-tube experiments. The angular deviation from the vertical, defined as the “spreading angle”, has been quantified for gas and particles on both sides of the jets at different time steps using high-speed video analysis. The experimental variables investigated are 1) vent geometry, 2) tube length, 3) particle load, 4) particle size, and 5) temperature. Immediately prior to the first above-vent observations, gas expansion accommodates the initial gas overpressure. All experimental jets inevitably start with a particle-free gas phase (gas-only), which is typically clearly visible due to expansion-induced cooling and condensation. We record that the gas spreading angle is directly influenced by 1) vent geometry and 2) the duration of the initial gas-only phase. After some delay, whose length depends on the experimental conditions, the jet incorporates particles becoming a gas-particle jet. Below we quantify how our experimental conditions affect the temporal evolution of these two phases (gas-only and gas-particle) of each jet. As expected, the gas spreading angle is always at least as large as the particle spreading angle. The latter is positively correlated with particle load and negatively correlated with particle size. Such empirical experimentally derived relationships between the observable features of the gas-particle jets and known initial conditions can serve as input for the parameterisation of equivalent observations at active volcanoes, alleviating the circumstances where an a priori knowledge of magma textures and ascent rate, temperature and gas overpressure and/or the geometry of the shallow plumbing system is typically chronically lacking. The generation of experimental parameterisations raises the possibility that detailed field investigations on gas-particle jets at frequently erupting volcanoes might be used for elucidating subsurface parameters and their temporal variability, with all the implications that may have for better defining hazard assessment.

预测爆发性火山喷发的开始、样式和持续时间仍然是一项巨大的挑战。虽然基本的潜在过程被认为是已知的，但在地球表面上方可观察到的喷发特征与直接地下的条件和性质之间的明确关联还远未完成。此外，爆炸性事件的高度动态性和不可接近性意味着对此类事件的实地调查进展仍然缓慢。规模化的实验研究代表了一个单独研究单个火山过程的机会，尽管它们具有高度动态性，但可以系统地量化它们。在这里，使用快速减压冲击管实验产生了脉冲产生的垂直气体粒子射流。与垂直方向的角度偏差，定义为“扩散角”，已使用高速视频分析在不同时间步长对射流两侧的气体和颗粒进行了量化。研究的实验变量是 1) 通风口几何形状、2) 管长度、3) 颗粒负载、4) 颗粒大小和 5) 温度。紧接在第一次以上通风口观察之前，气体膨胀适应初始气体超压。所有实验射流都不可避免地从无颗粒气相（仅气体）开始，由于膨胀引起的冷却和冷凝，这通常是清晰可见的。我们记录到，气体扩散角直接受 1）通风口几何形状和 2）初始纯气体阶段的持续时间的影响。经过一段延迟（其长度取决于实验条件）后，射流将粒子合并为气体-粒子射流。下面我们量化我们的实验条件如何影响每个射流的这两个阶段（仅气体和气体粒子）的时间演变。正如所料，气体扩散角总是至少与粒子扩散角一样大。后者与颗粒负荷呈正相关，与颗粒大小呈负相关。气体粒子射流的可观测特征与已知初始条件之间的这种经验实验得出的关系可以作为活火山等效观测参数化的输入，从而减轻先验知识的岩浆质地和上升速率、温度和气体超压和/或浅层管道系统的几何形状通常长期缺乏。