Spatter and welded air fall deposits generated by fire-fountaining eruptions: Cooling of pyroclasts during transport and deposition

According to the physico-mathematical model presented here, juvenile ballistic fragments can travel through the atmosphere during fire-fountaining eruptions preserving enough heat to agglutinate or to weld on impact with the ground’s surface, even a few kilometres from the source area. The range is mostly related to exit velocity and ejection angle as well as to such typical properties of the transported particles as size, density and shape. The selective nature of the transport system can give rise to inverse lateral size/density grading. The most important interdependent factors that control the degree of final agglutination or welding are accumulation rate, grain size and duration of deposition. In the investigated grain size range, an accumulation rate of 0.2 m/h appears as a minimum value to promote welding. This value cannot be considered as an absolute threshold because it can be significantly lowered by increasing the grain size. Duration of deposition appears as another critical factor because it controls the maximum temperature of the whole deposit and its final thickness, and therefore, the cooling rate of its main portion after the end of deposition. In the case of high accumulation rates of coarse particles but during a short period of time, primary agglutination may be the dominant factor in controlling the final sintering of particles. Conversely, high accumulation rates of coarse particles during a long period of time give rise to both primary agglutination and welding.