Tephra

Tepra is fragmented material produced by a volcanic eruption, regardless of composition, fragment size or storage mechanism. Ash tephra (Greek for ash) is a generic term for any pyroclastic accumulation in the air. While tephra is not solidified, a pyroclastic rock is formed from the solidification of pyroclastic accumulations into a coherent rock type. In chemistry, tephra varies from basaltic (low-silica) to rhyolite (high-silica) compositions, reflecting the chemical variation typical of magmas. Another article discussing tephra: pyroclastic flow: