The role of fluidization in the emplacement of pyroclastic flows, 2: Experimental results and their interpretation

Abstract

Abstract Although often quoted as being important in the emplacement of pyroclastic flows, little relevant experimental fluidization work has been done. Fluidization processes are first reviewed for a simple system, against which ignimbrite samples can be compared. Results from fluidization experiments on ignimbrite samples show that their behaviour differs radically from any simple system, principally because of their extremely poor sorting. At a certain gas velocity, U ie (whose value cannot be predicted), some ignimbrite samples begin to expand, whilst at a higher gas velocity, U mp , the samples begin to show segregation structures. The minimum fluidization velocity, U mf , in simple systems is replaced by U mp ; the value of the latter in pyroclastic materials cannot be easily or reliably predicted from published U mf correlations, and a new empirical method for determining U mp is presented from the experimental data. During fluidization, ignimbrite samples expand much less than more conventional materials due to the bypassing of gas through segregation channels. The total amount of expansion is also limited; e.g. a 100-m-thick pyroclastic flow will, from fairly high gas velocities, deflate to form an ignimbrite which is not less than 70–85 m thick. During fluidization, the poor sorting causes only part of the weight of the ignimbrite samples to be supported by the gas flow, the degree of support increasing with an improvement in sorting. Segregation structures, and grain-size and compositional variations within fluidized ignimbrite samples show varying behaviour, depending on the sorting of the sample. After subjecting a sample to high gas velocities, the net result is a grain-size- and compositionally-zoned bed which becomes richer upwards in pumice and fine material. Once formed, these features cannot be destroyed under laboratory conditions, implying that some segregation structures should be present in any ignimbrite where U mp was exceeded.