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If you've ever read older literature about geologic mapping in the Western United States, the term "ash-flow tuff" or even just "tuff" is a popular way to describe deposits created by pyroclastic flows. These deposits are composed of a matrix of volcanic ash carrying a load of larger clasts - usually pumice, chunks of local rock, and scoria. These flow into place in a violent cloud of hot gases, hence the name. In today's photo, you can see an example of a cliff mapped as tuff on a trail at Sonoma County's Mount St. Helena, which is carved from the pyroclastic deposits of a 12-million-year-old caldera eruption.

However, "Ignimbrite" is also used to describe the deposits from pyroclastic flows elsewhere in the world. Occasionally a modifier is included - ignimbrites refer only to welded deposits, or contain mostly pumice, or is applied to large-volume deposits from caldera eruptions, to name a few. However, in other places the two terms are used interchangeably, with a slight preference for "ignimbrite" among non-US scientists. (Even this isn't consistent, since the volcanic history of the Western US contains plenty of work on the 25-40 million year old "ignimbrite flare-up" that produced hundreds of "tuffs.")

So what does this mean? In the end, it's nothing more than a popularity contest, with "ash-flow tuff" being a bit more popular in US publications. A quick search in a text analysis tool reveals that ignimbrite has actually always held an edge over ash-flow tuff (or its variations), being several orders of magnitude more popular in books written since the 1930s. Both terms are correct - it's just a matter of preference!

To read more about the tuffs (or ignimbrites) of the Sonoma volcanics, check out https://pubs.usgs.gov/publication/70004605