Volcanic ash consists of very fine
rock and
mineral particles less than 2
mm in diameter that are ejected from a
volcanic vent. Ash is created when solid rock shatters and
magma separates into minute particles during explosive volcanic activity. The usually violent nature of an eruption involving steam (phreatic eruption) results in the
magma and perhaps solid rock surrounding the vent, being torn into particles of
clay to
sand size. The plume that is often seen above an erupting volcano is composed primarily of ash and steam. The very fine particles may be carried for many miles, settling out as a dust-like layer across the landscape. This is known as an "ash fall". The term for any material explosively thrown out from a vent is
tephra or
pyroclastic debris. If liquid
magma is ejected as a spray, the particles will solidify in the air to small fragments of volcanic glass. Tephra particles of
gravel size are termed
cinders.
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