A volcanic block is a fragment of rock

Rock (geology)

In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 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

Country rock (geology)

Country rock is a geological term meaning the rock native to an area. It is similar and in many cases interchangeable with the terms basement and wall rocks....

 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). Bombs 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 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. The 1924 eruption of Kīlauea

Kilauea

Kīlauea is a volcano in the Hawaiian Islands, and one of five shield volcanoes that together form the island of Hawaii. Kīlauea means "spewing" or "much spreading" in the Hawaiian language, referring to its frequent outpouring of lava. The Puu Ōō cone has been continuously erupting in the eastern...

, Hawaii

Hawaii

Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...

, expelled rocks weighing up to 14 tonnes; and Mount Vesuvius in Italy

Italy

Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

discharged blocks weighing 2-3 tonnes over distances of 100-200m.