Large part of southern Tenerife is covered with ignimbrites and pumice fall deposits but these pyroclastic sediments contain only a small amount of lithic material (except pumice lapilli). However, in one short section of a roadcut I noticed something very different — many dark fragments embedded in usual cream-colored ignimbrite.

Not long ago I wrote about a block-and-ash flow deposit in Gran Canaria. This outcrop in Tenerife is somewhat similar. Clasts (blocks) do not stand out as they do in the outcrop in Gran Canaria and they seem to be more versatile (polymictic) but I believe that most likely it is yet another example of block-and-ash flow deposit because the clasts are clearly surrounded by ignimbrite (read also the update below which contradicts this hypothesis).

Block-and-ash flow deposits are not very extensive. They are usually 1-10 meters in thickness¹ which seems to be the case here as well. I have a longer description of these interesting and fiery rocks in the article describing the Gran Canaria outcrop.

Width of view 1.6 meters.

Width of view 0.8 meters.

Here are the coordinates: 28° 13′ 59″ N 16° 26′ 12″ W. Altitude 370 meters. These coordinates refer to a parking place. The actual roadcut is few hundred meters down the road towards the coast. Be careful if you plan to visit the place because it is a roadcut and unfortunately there is not much room left for geologists.

It is a common problem in Tenerife. Roads are narrow and winding. It is very difficult to find a place to park a car. In this particular case I was actually lucky because the first opportunity to pull off was only few hundred meters away. Tenerife is a wonderful island geologically but it is densely populated. There are almost one million inhabitants living in a relatively small island plus hordes of tourists. Hence, most roads are quite busy.

Update

Volcanologist Paul Cole pointed my attention to the fact that there are pumice clasts in it which makes it unlikely to be a block-and-ash flow deposit. The problem is that true block-and-ash flows are formed by non-explosive dome collapses and contain very little pumice. Domes are not composed of pumice. Pumice is thrown out of volcanoes. Hence, it is more likely lithic breccia deposited from a pyroclastic density current.

Another aspect that makes this hypothesis more likely is the polymictic assemblage of the clasts. It is hard to explain how can one volcanic dome yield such a variety of clasts. It actually bothered me while I was inspecting the outcrop but because of my limited experience I could not think of any better explanation than a block-ash-flow event. So these rocks we see in the outcrop were most likely picked up by a ground-hugging pyroclastic flow while it was cascading down the flanks of a volcano.

References

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.