Block and ash flow

From Wikipedia, the free encyclopedia
Outcrop of a block and ash flow in the Tschicoma Formation, New Mexico, US

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.[2] Block and ash flows are a type of pyroclastic flow and as such they form during volcanic eruptions.[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.[6]


  1. ^ "Glossary - Block and ash flow". United States Geological Survey. Retrieved 22 October 2018. Flow of ash and angular rock fragments larger than 26 cm or 10 in.
  2. ^ a b c Pyle, David M. (2015). "Chapter 13 – Sizes of Volcanic Eruptions". In Sigurdsson, Haraldur (ed.). Encyclopedia of Volcanoes (2nd ed.). Amsterdam: Academic Press. pp. 257–264. ISBN 978-0-12-385938-9.
  3. ^ Tahide, Ui; Matsuwo, Norimichi; Sumita, Marit; Fujinawa, Akihiko (1999). "Generation of block and ash flows during the 1990–1995 eruption of Unzen Volcano, Japan". Journal of Volcanology and Geothermal Research. 89 (1–4): 123–137. doi:10.1016/S0377-0273(98)00128-0.
  4. ^ "Block-and-ash flow deposit or lithic breccia?". Sandsatlas. Retrieved October 22, 2018.
  5. ^ Donoghue, E.; Troll, V.R.; Schwarzkopf, L.M.; Clayton, G.; Goodhue, R. (January 1, 2009). "Organic block coatings in block-and-ash flow deposits at Merapi Volcano, central Java". Geological Magazine. 146 (1): 113–120. doi:10.1017/S0016756808005359.
  6. ^ "Block-and-ash flow deposits". Sandsatlas. Retrieved October 22, 2018.