pyroclast

(geology) An individual pyroclastic fragment or clast.

A fragment of solidified lava, ejected during explosive volcanic eruptions. Classification of pyroclasts is by size. Fragments less than 4 mm across are ash; compacted ash is tuff. Material between 4 and 32 mm is lapili and fragments larger than 32 mm are blocks. Collectively, these fragments are tephra. Pyroclasts formed from lava produce volcanic bombs and volcanic breccia. Pyroclastic flows are also known as nuées ardentes. They result from the bursting of gas bubbles within the magma, which fragments the lava. Eventually, a dense cloud of fragments is thrown out to form a mixture of hot gases, volcanic fragments, crystals, ash, pumice, and shards of glass.