 Taking place 0 @.@ 3 mya , the second eruption was the most extensive . It was probably between 250 cubic kilometres ( 60 cu mi ) and 300 cubic kilometres ( 72 cu mi ) in volume and extended past the reaches of the caldera , down the adjacent foothills . Beyond the caldera , the eruptive products are poor in phenocrysts ( unlike those of the first eruption ) . They are instead rich in lithic material , which makes up as much as 10 percent of the rock in parts ( 50 percent at the base of the caldera ) . As the first of the ash was deposited , it accumulated in layers that formed quickly and resisted erosion , but only partially melded together . However , except for these basal layers and thin zones where the ash was devitrified ( loses the properties of a glass and becomes brittle ) , the entire sheet is melded together . Inside Calabozos , the ash resisted welding and contains more phenocrysts . Instead , erosion ate away at it , in the form of acid leaching , and broke down much of its pumice content . Still , the rock layer here contains five to 30 percent phenocrysts , and has high levels of devitrification and lithic content . 
