Volcanic Ash and Tuff


Volcanic ash is a pyroclastic rock, composed of fine grains less than 2mm in diameter. Pyroclastic means ‘fire-broken’, a name for rocks that have been blown apart by exploding gases. Ash exploded out of a volcano, may fall through the air and settle in beds, called ash-falls when unconsolidated, or tuffs when consolidated. The ash can vary in composition, for example it may be made up entirely of rock fragments.

Colour: white or grey powder when newly fallen.
Composition: ash-fall tuffs composed of a mixture of rock fragments, crystals and glass shards.
Classification: pyroclastic igneous rock.
Occurrence: associated with explosive volcanic activity, particle size decreases away from the vent because lighter ash is carried further by the wind.
Texture: fine-grained, less than 2mm.
Structure: ash-fall tuffs are often well-bedded.