accretionary lapilli Pellets of ash, ranging in size from 2 mm to 64 mm, which commonly exhibit a concentric (‘onion skin’) internal structure. The lapilli are formed by the accretion of very fine ash around condensing water droplets or solid particles, particularly in steam-rich eruptive columns (see ERUPTION). Once formed they can be transported and deposited by pyroclastic fall, surge, or flow processes.
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