Block-and-ash flow deposit is a pyroclastic flow deposit consisting of large fraction of juvenile volcanic blocks in a matrix of volcanic ash of the same composition1. Block-and-ash flow deposit is a type of ignimbrite as all deposits of pyroclastic flows are considered to be ignimbrites, regardless of whether they are welded or not2.
Pyroclastic flow is a very hot (up to 1000 °C) mixture of volcanic gases, ash, and blocks that runs rapidly downhill and spreads quickly under gravity. Pyroclastic flows are ground-hugging as they are denser than air. Pyroclastic flow is the deadliest expression of volcanism followed by lahars3.
Pyroclastic flow deposits show a considerable variation in texture and composition. They may be generated by several mechanisms. Block-and-ash flows are mostly the result of a collapse and fragmentation of volcanic domes3. Collapse of oversteepened dome may be either gravitational, explosive, or both at the same time. Well known volcanoes producing lava domes and block-and-ash flows are Unzen (Japan), Merapi (Indonesia), and Montserrat (British overseas territory in the Caribbean)1. Such collapses produce lots of angular juvenile volcanic blocks and are accompanied by a powerful volcanic blast generated by the release in pressure which produces volcanic ash and liberates lots of hot gas.
Pyroclastic flow deposits (sometimes also known as pyroclastic density current or PDC4) may cover huge areas (up to 50 000 km2 in extreme cases) but block-and-ash flows are confined to much smaller area because there is simply not enough blocks. Block-and-ash flows may extend up to 10 km from their source and they travel at speeds up to 100 km/h. Block-and-ash flows are usually 1-10 meters in thickness1. They may be either clast- or matrix-supported (clast-supported means that individual clasts are in contact).
Blocks stand out because the matrix is less resistant to erosion.
Most of the blocks are about 10-25 cm across but some may extend over one meter.
There is a sharp contact with the underlying darker layer of pyroclastic rocks.
Deposit in Tenerife which has lots of polymictic blocks embedded in ignimbrite. However, it is most likely lithic breccia, not block-and-ash flow deposit because it has lots of pumice clasts in it. True block-and-ash events are formed by non-explosive dome collapses and contain very little pumice. Read more about this outcrop: Block-and-ash flow deposit or lithic breccia? Width of view 0.8 meters.
Here are the coordinates of the exposure in Gran Canaria if you would like to visit it: 28° 08′ 26″ N 15° 29′ 33″ W. It is a middle section of a coastal cliff about 10 meters above sea level but it is accessible after some climbing on the rocks. It was unexpected find for me.
1. Freundt, A. & Wilson, C. J. N. & Carey, S. N. (1999). Ignimbrites and Block-And-Ash Flow Deposits. In: Encyclopedia of Volcanoes (Ed. Sigurdsson, H.). Academic Press. 581-599.
2. Tilling, Robert I. (2007). Ignimbrite. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 9. 20-21.
3. Schmincke, Hans-Ulrich (2005). Volcanism. Springer.
4. Francis, P. & Oppenheimer, C. (2003). Volcanoes, 2nd Edition. Oxford University Press.