Block and ash flow explained

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. Block and ash flows are a type of pyroclastic flow and as such they form during volcanic eruptions.^[2] In contrast to other types of pyroclastic flows, block and ash flows do not contain pumice^[3] and the volume of block and ash flow deposits is usually small. 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.^[4] 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]

Notes and References

1. Web site: Glossary - Block and ash flow . . volcanoes.usgs.gov . . 22 October 2018 . Flow of ash and angular rock fragments larger than 26 cm or 10 in..
2. Ui . Tahide . Matsuwo . Norimichi. Sumita . Marit . Fujinawa. Akihiko . 1999 . Generation of block and ash flows during the 1990–1995 eruption of Unzen Volcano, Japan . . 89 . 1–4 . 123–137 . 10.1016/S0377-0273(98)00128-0 .
3. Web site: Block-and-ash flow deposit or lithic breccia? . . Sandsatlas . October 22, 2018 .
4. Book: Pyle, David M.. Encyclopedia of Volcanoes . Chapter 13 – Sizes of Volcanic Eruptions. Academic Press. 2015. 2nd. Amsterdam. 978-0-12-385938-9. 257–264. Haraldur. Sigurdsson. Haraldur Sigurdsson.
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 . . 146 . 1 . 113–120 . 10.1017/S0016756808005359.
6. Web site: Block-and-ash flow deposits . . Sandsatlas . October 22, 2018 .