Stock photography by Siim+Sepp at Alamy


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Composition of the Crust
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What is black sand
What is black sand

Block-and-ash flow deposits

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.

6 comments to Block-and-ash flow deposits

  • Howard

    Siim–My experience is almost all in sedimentary geology, and at first glance I might mistake these deposits for poorly sorted, water-based debris flows (i.e. lahars). Are there clear criteria for distinguishing these unwelded block-and-ash flow deposits from lahar deposits? (Something like unburned pieces of wood entrained in the flow would be an obvious giveaway that it’s a lahar, but what other characteristics could be used?)

  • I was not even thinking about lahar when I was there. This is definitely a possibility. Especially if you say that it looks like lahar. However, I do believe that block-and-ash event is more likely. There are two main reasons that first cross my mind. 1. Ignimbrites of different types are very common in Gran Canaria. Most of the post-erosional sequence there is composed of ignimbrites. 2. Lahars are mudflows. They require lots of water to form. There is very little of it in Gran Canaria. No snow or ice whatsoever and low precipitation also.

  • Howard

    Siim–I have no reason to doubt that it’s a block-and-ash flow deposit, for the very reasons you give. I was just thinking that if I saw a road-cut somewhere else (say, in British Columbia, or the US Pacific Northwest, where there IS lots of water) that looked like this, I might have a hard time deciding if this was a lahar or a block-and-ash flow deposit, unless there are some diagnostic characteristics in favour of one or the other.

    I suppose a lahar would tend to have more rounded clasts, but not necessarily, if it was relatively proximal to its source volcano. I admit that, living in Alberta, I don’t see a whole lot of volcanic deposits, so I might be missing some lines of evidence that would be obvious to someone more familiar with these types of rocks.

    I’m really enjoying this series! Cheers.

  • I would also like to know how to tell them apart. Perhaps we should ask someone who knows more about it. I believe it is not very hard to decide which is which after lab analysis. Lahar probably contains clay which is lacking in most unweathered ignimbrites.

  • Brian Romans

    I haven’t spent much time studying volcanogenic sedimentary deposits, but I have looked at a lot of subaqueous debris-flow deposits. I agree that with these photos it’s tough to definitively distinguish. One thing to look at would be the matrix (sediment between the clasts/grains); if it’s mud/clay-rich then I’d be more likely to conclude it’s a lahar. If there’s no mud/clay in matrix it would be difficult to get that texture in a debris flow. But, I’m just thinking out loud here. Awesome photos!

  • Howard

    Brian–thanks for chiming in and “thinking out loud”. That’s what I love about geology: there’s so much you can think about, just looking at what most people would consider to be a pile of stupid rocks and dirt. :-)

    It seems to me that the clay/mud matrix content could be a bit tenuous, too, depending on the context of the deposit. A recent block-and-ash deposit that was dumped near the foot of a volcano, then 2 weeks after the eruption event, in a torrential rainstorm became remobilized and deposited another 300 m lower would technically be a lahar, but wouldn’t necessarily have any more clay/mud than its source block-and-ash deposit: all you’ve done is add water and move the same material downslope. The only difference is that one had water as a pore fluid and one had air. Once the water’s dried up, what’s left? Similarly, it seems likely that both types of deposit, when subjected to diagenesis, over a longer period of time, would end up with the same diagenetic matrix clays, given that both started with the same suite of precursor minerals.

    Maybe there’s something about the deposit architecture (paleo slope angles, thickness distribution?) that could point to one type of deposit over the other. Or maybe not! :-)

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