Lapilli

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Lapilli (singular: lapillus) are small rocks that fall from a volcanic eruption. The name means "little stones." They are tephra and measure about 2 to 64 mm across. If bigger than 64 mm, the piece is a volcanic bomb (molten) or a volcanic block (solid); if smaller than 2 mm, it’s ash.

Lapilli form when hot lava is blasted into the air and cools before reaching the ground. They can be fresh pumice or scoria, volcanic glass, chunks of rock, or ash that sticks together. They are usually jagged and irregular, but can be round, teardrop, dumbbell, or button-shaped if the lava was very fluid.

Some lapilli form from ash clumping together in moist volcanic clouds, creating accretionary lapilli with a central core and layered shells. Armoured lapilli form when there is a lot of moisture in the eruption column. In rocks, many lapilli together create lapilli tuffs. If the heat is high enough, the partially molten lapilli weld into a hard rock called welded tuff; non-welded lapilli tuffs are looser and easier to erode. Depending on the eruption, lapilli can lead to pumiceous lapilli tuff or ignimbrite.