The ignimbrite source problem: Significance of a co-ignimbrite lag-fall deposit

A co-ignimbrite lag-fall deposit is one that forms at or near the site of eruptive column collapse and consists mainly of pyroclasts that are too large and too heavy to be carried away in the resulting pumice flows. An example from Mexico is identified as such because it shows the same compositional zoning from rhyolite to andesite as the associated ignimbrite. It indicates that the ignimbrite originated by continuous collapse of the eruptive column and locates the ignimbrite source vent. In this example the vent is located on flat ground, apparently unrelated to any pre-existing volcano, and the ability of the ignimbrite to travel laterally some 20 km must be due to eruptive column height rather than ground slope.