# Volcanic block

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A **volcanic block** is a fragment of [rock](/source/Rock_(geology)) 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](/source/Country_rock_(geology)) and are therefore mostly accessory or accidental in origin. Blocks also occur due to the impact and breakage of [volcanic bombs](/source/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.

## Features

Blocks are nearly always angular to sub-angular and roughly equidimensional. If the parent rock is flow-foliated lava, sedimentary material or [schistose](/source/Schistose) metamorphic rocks, the blocks may have a plate-like or slab-like form. 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](/source/Volcanic_vent). The 1924 eruption of [Kīlauea](/source/K%C4%ABlauea), [Hawaii](/source/Hawaii), expelled rocks weighing up to 14 tons;[1] and Mount [Vesuvius](/source/Vesuvius) in [Italy](/source/Italy) discharged blocks weighing 2-3 tons over distances of 100-200m.

## References

1. ["The May 1924 Explosive Eruption of Kīlauea | U.S. Geological Survey"](https://www.usgs.gov/volcanoes/kilauea/may-1924-explosive-eruption-kilauea). *www.usgs.gov*. Retrieved 2023-08-11.

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