Volcanic block

A volcanic block is a fragment of rock that measures more than 64 mm (2.5 in) in diameter and is erupted in a solid condition.

Blocks are formed from material from previous eruptions or from country rock and are therefore mostly accessory or accidental in origin.

Blocks can also occur due to the disruption of the crust of a lava dome that has formed up or over a vent during an eruption.

In other cases, blocks derived from great depths may resemble polished water-worn pebbles and are cobbled due to fluidisation and upwards transport.

Blocks can be enormous and may be transported great distances from the volcanic vent.