Volcanic blocks

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Volcanic blocks are fragments of rock which measure more than 64mm in size and are erupted in a solid condition.

Blocks are formed from previous eruptions or are country rocks and hence, are mostly accessory or accedental in origin. Blocks also occur due to the impact and breakage of volcanic bombs (remember that a bomb is the same as a block though has a streamlined appearance and is often expelled in a molten state). Bombs can also occur due to the distruption of the crust of a lava dome that has formed up or over a vent during an eruption.

[edit] Features

Blocks are nearly always angular to subangular and roughly equidimentional. Depending if the lava is flow foliated or sedimentary or metamorphic schistose rocks the blocks may have a plate-like or slab-like form. In other cases, blocks derived from great depths resemble polished waterworn pebbles and are cobbled due to fluidisation and upwards transport.

Blocks can be enormous and may be transported great distances from the volcanic vent. The 1924 eruption of Kīlauea, Hawaii, expelled rocks weighing up to 14 tonnes and Mount Vesuvius in Italy discharged blocks weighing 2-3 tonnes over distances of 100-200m.