A block and ash flow or block-and-ash flow is a flowing mixture of volcanic ash and large (>26 cm) angular blocks commonly formed as 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. In difference to other types of pyroclastic flows block and ash fows do no contain pumice and the volume of block and ash flow deposits is usually small. Block and ash flows deposits have densities in the range of 1600 to 2000 kg/m3, two to five time greater than ash fall deposits. 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.
- "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.
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- 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.
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