Volcanic block

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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 also occur due to the impact and breakage of volcanic bombs (a bomb is a block with streamlined appearance, often expelled in a molten state). 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.