 The differences between these two deposits can be accounted for by a few factors , including time of eruption and placement . The material inside the caldera must be younger and erupted from a pool of magma that had larger crystals . Its extensive erosion can be explained by the presence of hydrothermal vents , and its high levels of lithics probably originate from either being exposed to the rocks after they were erupted , lying adjacent to them while they underwent subsidence , or is a result of long , slow formation . Any of these reasons would also effectively account for the poor mixing of the lavas . 
