The San Juan volcanic field, in southwestern Colorado, contains two phases of volcanism.
The earlier volcanism is Oligocene in age, and consists of largely intermediate composition lavas and breccias. A few ash flow tuffs also constitute this phase of volcanism. It probably was related to subduction along the western margin of North America.
The later volcanism is Miocene–Pliocene in age, and is basaltic in composition. It is best interpreted as a partial melt of the lower crust that was erupted onto the surface.