MODELS FOR THE ORIGIN OF ACCRETIONARY LAPILLI

Binding between initially cohesionless ash particles to form concentri c accretionary lapilli is provided primarily by the capillary forces o f liquid bridges from condensed moisture and by electrostatic attracti on. Capillary forces are strong bonds if the particles are in close co ntact, but they decrease rapidly with increasing particle spacing. Ele ctrostatic attraction between charged ash particles is much weaker but effective over larger distances, increasing the frequency of collisio n between them. Experimental results of liquid film binding of volcani c ash showed that agglomeration was most successful between 15 and 25 wt.%, defining the agglomeration window for the formation of accretion ary lapilli. Below 5-10 wt.% and above about 25-30 wt.% of water, conc entric agglomeration was inhibited. Particles < 350 mu m could be sele cted from a wider particle population in the experiments using only sm all amounts of water, which can explain the growth of accretionary lap illi in pyroclastic surges around agglomeration nuclei. Experiments te sting the behavior of volcanic ash in electric fields showed that ash clusters formed instantaneously when the ash entered the field between a corona discharge gun and a grounded metal plate. The maximum grain size incorporated into the artificial clusters was about 180 mu m but >90 wt.% of ash was <45 mu m. Accretionary lapilli form in turbulent a sh clouds when particles carrying liquid films of condensed moisture c ollide with each other and when the binding forces exceed the grain di spersive forces. Larger particles >500 mu m act as agglomeration nucle i in surges, accreting ash <350 mu m around them. In pyroclastic flows the aggregates are thought to originate from already size-sorted ash at the interface between the lower avalanche part of the flow and its overriding elutriation cloud. The fine-grained rims around accretionar y lapilli found close to source are interpreted to be accreted dominan tly by electrostatic attraction of very fine ash similar to clustering in elutriation clouds.