Block and ash flow

A block and ash flow or block-and-ash flow is a flowing mixture of volcanic ash and large (>26 cm) angular blocks[1] commonly formed as a result of a gravitational collapse of a lava dome or lava flow.

[3] In contrast to other types of pyroclastic flows, block and ash flows do not contain pumice[4] and the volume of block and ash flow deposits is usually small.

[2] Block and ash flow deposits have densities in the range of 1600 to 2000 kg/m3, two to five times greater than ash fall deposits.

[2] Some blocks in block and ash flow deposits may have thin and shiny coatings of carbon derived from charcoal formed from vegetation trapped by the flow.

[5] Volcanoes known for their production of block and ash flows since the 1990s include Mount Unzen in Japan, Mount Merapi in Java and Soufrière Hills in the Lesser Antilles.