Acid volcanoes of San Miguel,Azores

The trachytic volcanoes of San Miguel Island are emplaced on either side of a young basaltic area, which could correspond to some active branch of the mid-Atlantic rift. Geological and geomagnetic surveys suggest crustal drift of about 1 cm/year on either direction. From the seismic anomaly data, shallow magma chambers have been inferred for those acid volcanoes, where owing to the crustal drift the main vents appear to have been shifted relatively to the fractures feeding the chambers from the deep upper mantle supply. The primitive magma is certainly basaltic; when the surface vents are directly above the deep feeding fractures, only basaltic lavas of some primitive composition can be extruded; but, when the upper vents are laterally displaced, the magma is delayed in the chambers and will differentiate, eventually into trachytic material capable of producing huge pumice explosions.