 The formation is fairly heterogeneous, with some ignimbrites separated by sharp contacts and the degree of welding and crystal content of pumices varies from one ignimbrite to the other.Generally the ignimbrites are rich in crystals and pumice, are unwelded and contain few flow structures, with the exception of the welded Cueva Negra ignimbrite.Some ignimbrite eruptions were preceded by the formation of Plinian eruption columns that generated ash fallout, and there is evidence for pulsating flow in the ignimbrites.