什么是 Tephra and pyroclasts？

A pyroclast is a fragment produced by the explosive fragmentation of magma or of the rocks around it. Tephra is the collective name of all pyroclasts thrown into the air, whatever their size. By size, pyroclasts finer than $2\,\mathrm{mm}$ make volcanic ash; those of $2\text{ to }64\,\mathrm{mm}$ are lapilli; those larger than $64\,\mathrm{mm}$ are blocks if they were solid when ejected and angular, and a volcanic bomb if they were wholly or partly molten and took a rounded or spindle shape in flight.