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.[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]

Outcrop of a block and ash flow in the Tschicoma Formation, New Mexico, US

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]

References
  1. "Glossary - Block and ash flow". volcanoes.usgs.gov. United States Geological Survey. Retrieved 22 October 2018. Flow of ash and angular rock fragments larger than 26 cm or 10 in.
  2. 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.


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