Traditionally, systematic temporal variations in lava or tephra compositions from a particular volcano have been explained by appealing to fractional crystallization. While this explanation may apply to compositionally continuous, long-term variations, fractional crystallization fails to explain wide ranges of compositions erupted during a single event, especially when the compositional range includes a gap.
Banded pumices are also common, but no homogeneously mixed pumice has been found.
Analyses of glass compositions across interfaces in banded pumices reveal gradients of 10 wt. The CFE dacite is the most silicic product to come from Aniakchak volcano. The clean separation of CFE pumice compositions suggests that the two magmas encountered each other just prior to the eruption, contrary to what is expected in a self-zoning magma model wherein the silicic cap is the distilled product of a crystallizing mafic magma. Prolonged contact between silicic and mafic layers would inevitably lead to the formation of a chemical and thermal boundary layer. Post-caldera volcanism at Aniakchak has produced a bimodal population of lavas and tephras.