IGNIMBRITES1,2 are a characteristic component of the typically post-orogenic basalt–andesite–rhyolite volcanic association. These pyroclastic deposits are now generally accepted2 as having been produced by eruptions of nuees ardentes in which solid particles, both rock and crystal fragments, glass shards and pumice fragments are suspended by turbulent flow in gas, probably derived in part from the glass shard and pumice material itself3. Such a fluidized system4 forms the basal avalanche of a nuee ardente. While associated with several types of volcanic structure, most of those ignimbrite deposits with a volume greater than 10 km3, and the source of which is known, have been related either to calderas or, for the larger volume (> 1,000 km3) deposits, to volcano-tectonic depressions3. No intrusive igneous body has yet been recognized which, on clear-cut evidence, served as a feeder for extensive ignimbrite flows3.