Since about 10 million years ago, the area has been subject to reverse faulting which has disrupted the basement along north-south lines, forming a rift valley that also stretches from north to south. The magma erupted by the Galán system was likewise channelled along such fault systems, and neighbouring volcanoes were similarly influenced by them; the fault systems at Galán proper are known as the Diablillos-Galán faults. Another major lineament in the area is the Archibarca lineament, which is formed by a strike-slip fault that extends from the northwest to the southeast in the region and which intersects the Diablillos-Galán faults at the location of the caldera.