THE ORIGIN OF ACCRETIONARY LAPILLI

Experimental investigations in a recirculating wind tunnel of the mech anisms of formation of accretionary lapilli have demonstrated that gro wt' is controlled by collision of liquid-coated particles, due to diff erences in fall velocities, and binding as a result of surface tension forces and secondary mineral growth. The liquids present on particle surfaces in eruption plumes are acid solutions stable at much-less-tha n-or-equal-to 100% relative humidity, from which secondary minerals, e .g. calcium sulphate and sodium chloride, precipitate prior to impact of accretionary lapilli with the ground. Concentric grain-size zones w ithin accretionary lapilli build up due to differences in the supply o f particular particle sizes during aggregate growth. Accretionary lapi lli do not evolve by scavenging of particles by liquid drops followed by evaporation - a process which, in wind tunnel experiments, generate s horizontally layered hemispherical aggregates. Size analysis of part icles in the wind tunnel air stream and particles adhering to growing aggregates demonstrate that the aggregation coefficient is highly grai n-size dependent. Theoretical simulation of accretionary lapilli growt h in eruption plumes predicts maximum sizes in the range 0.7-20 mm for ash cloud thicknesses of 0.5-10 km respectively.