Accretionary Lapilli
Photographed by Michael P. Klimetz

Lapilli is a size classification term for tephra, which is material that descends through the atmosphere as a consequence of volcanic eruption. The term lapilli means "little stones" in Latin. By definition lapilli range in size from 2 mm to 64 mm in diameter. A particle greater than 64 mm in diameter at impact is correctly known as a volcanic bomb when molten, or a volcanic block when solid. Pyroclastic material with particles less than 2 mm in diameter is referred to as volcanic ash. Lapilli are spheroid, teardrop, dumbbell, or button shaped droplets of molten or semi-molten lava ejected from an erupting volcano which fall to earth while still partially molten. These granules are not accretionary, but are the direct result of liquid rock cooling as it travels through the air. Lapilli tuffs are a very common form of deposited volcanic rock [pyroclastic rock] typical of rhyolite, andesite and dacite pyroclastic eruptions. Here, large thicknesses of lapilli can be deposited during a basal surge eruption. Most lapilli tuffs which remain in ancient terrains are formed by the accumulation and welding of semi-molten lapilli into what is known as a welded tuff. The heat of the newly deposited volcanic pile tends to cause the semi-molten material to flatten out as they become welded. Welded tuff textures are eutaxitic, with flattened lapilli, fiamme, blocks and bombs forming oblate to discus-shaped forms within layers. These rocks are quite well-indurated and tough, as opposed to non-welded lapilli tuffs which are unconsolidated and friable. Rounded tephra balls are called accretionary lapilli if they consist of volcanic ash particles. Accretionary lapilli are formed in an eruption column or cloud by attraction due to moisture or electrostatic forces, with the volcanic ash nucleating on some object and then accreting in layers before the accretionary lapillus falls from the cloud. Accretionary lapilli are like volcanic hailstones that form by the addition of concentric layers of moist ash around a central nucleus.