Ignimbrite is a pyroclastic rock formed by very hot ground-hugging cloud of volcanic ash, blocks, and gases known as pyroclastic flow or pyroclastic density current. Ignimbrite is synonymous with flood tuff, welded tuff, ash-flow tuff and pyroclastic flow deposit1.
The term “ignimbrite” was coined by the New Zealand geologist Patrick Marshall in 1935. This term was originally used only to refer to welded tuffs. These are pyroclastic rocks that were so hot right after the deposition from the pyroclastic cloud that individual clasts adhered to each other. However, this restriction no longer applies. This term includes all pyroclastic flow deposits, no matter whether they are welded or not1.
Pyroclastic flows are the most deadly expressions of volcanism2. They are associated with explosive volcanism. Ignimbrite formations are highly variable in bulk volume (0.1 to over 1000 km3) and runout distance (1 to over 100 km)3. Their appearance is variable too. Many colors are possible and they may be unwelded which makes it hard to distinguish some ignimbrites from tuff. Its deposits are frequently hydrothermally altered. The water needed for that may come from former lakes and rivers that got buried beneath the fiery cloud. Hydrothermally altered ignimbrite is often beautifully colorful. Smaller deposits are associated with former river channels because pyroclastic flows are gravity-controlled and therefore tend to follow the valleys. Larger pyroclastic flows do not care much about topography. They just cover former valleys with thicker and highlands with thinner deposits.
Whether ignimbrite is welded or not depends mostly on the temperature in the deposit right after the deposition. Most ignimbrites tend to be felsic, although basaltic varieties are known as well. Felsic ignimbrites tend to weld if the temperature is at least 500 to 650°C. Temperature within pyroclastic flows may reach even 1000 degrees. It is no wonder that they are so deadly given also the speed at which they move (up to 700 km/h).
Well-known huge ignimbrite deposits are Bishop Tuff in western USA and Taupo ignimbrite in New Zealand. Large part of the post-erosional rocks in Gran Canaria are also ignimbrites.
Ignimbrites are composed of pumice and scoria (highly vesicular and often glassy volcanic rocks). Sometimes larger volcanic blocks are included, but most of the material ignimbrite is composed of are lapilli and ash.
References
1. Tilling, Robert I. (2007). Ignimbrite. In: McGraw Hill Encyclopedia of Science & Technology
, 10th Edition. McGraw-Hill. Volume 9. 20-21.
2. Schmincke, Hans-Ulrich (2005). Volcanism
. Springer.
3. 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.
Nice!
Do I see an angular unconformity in your #7 photo? (“Hydrothermally altered sequence…”). The beds in the lower 2/3 of the photo are dipping strongly to the right, and seem to be truncated by the golden brown bed, then everything above that is more-or-less flat lying. Or is it a trick of the perspective?
It is not a trick of perspective. The morphology of this exposure is indeed strange but I do not know anything certain about that. Maybe this ignimbrite sequence filled a former topographic low (river valley). That’s why it is tilted and seems to be thicker in the lower part and is also the reason why it has such a nice set of colors (evaporated river provided water for the alteration right after the deposition). There are no tectonic compressive forces that can cause angular unconformities. I would be careful with such explanations but again I am only theorizing. Such an outcrop would be a nice topic for someone’s MSc thesis.
Correction:
The term ignimbrite was coined by my grandfather New Zealand Geologist Dr PATRICK Marshall NOT Peter!