Strombolian and effusive activity as precursors to phreatomagmatism: eruptive sequence at maars of the Pinacate volcanic field, Sonora, Mexico

The Pinacate volcanic field, Sonora, Mexico, contains 400 cinder cones and eight maars. It is noteworthy that most of the maar-forming, phreatomagmatic eruptions were immediately preceded by effusive and Strombolian activity rather than occurring when magma first approached the surface. The Strombolian activity may have facilitated access of groundwater to the conduits in this arid region. The field evidence suggests that phreatomagmatism is inhibited unless the magma flux is low relative to the rate of water supply and unless the top of the magma column has subsided, probably below the water table. The latter condition is difficult to prove in the absence of direct observation, and alternative hypotheses involving disturbance of the conduit system are considered. The spatial distribution of maars in the Pinacate and the lithology of their associated tuffaceous ejecta both may reflect the course of an ancient river channel whose permeable gravels were pierced by the magmatic conduits.