Lapilli

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Lapilli are small spherical particles of solidified lava (tephra) which accumulate from phreatic or explosive volcanic eruptions.

Lapilli can range in size from 2 mm in diameter to 64 mm in diameter, above which they are more correctly known as volcanic bombs when molten, or volcanic blocks when solid.

Lapilli (singular: lapillus) means "little stones" in Latin. They are in some senses similar to ooids in calcareous sediments, or pisolites.

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Formation

Lapilli are spheroid, teardrop, dumbbell, or button shaped droplets of molten or semi-molten lava ejected from a volcanic eruption which fall to earth while still at least 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 volcanic 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 distinctive (termed eutaxitic), with flattened lapilli, fiamme, blocks and bombs forming oblate to discus-shaped forms within layers. These rocks are quite indurated and tough, as opposed to non-welded lapilli tuffs which are unconsilidated and easily eroded.

Accretionary Lapilli
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 moisture or electrostatic forces, with the volcanic ash nucleating on some object and then accreting to it 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.

This texture can be confused with spherulitic and axiolitic texture.

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