Dikes

After Mount Mazama ad grown to considerable height, its sides were split by more or less radial fissures, partly as a result of the weight of magma in the central conduits but mainly in response to a general doming of the entire volcano by increased pressure of magma in the underlying reservoir. The fillings of at least sixteen of these fissures are exposed as dikes on the caldera walls. Other radial fissures underlie the outer flanks of the volcano and the region beyond, where they served as feeders to many of the parasitic cinder cones and dacite domes. Considering the former size of Mount Mazama and the extent of the caldera walls, the number of exposed dikes is surprisingly small as compared with the number visible on many smaller volcanoes.

With the exception of two dikes of dacite, all are composed of andesite similar to that of the flows which they cut.