Campo Volcánico de Durango

Northeast of the city of Durango, between the Guadiana and Nombre de Dios valleys, the plain turns into a crust of black, bubbly rock studded with cones. This is the Durango volcanic field: about a hundred volcanoes spread over some 2,100 km² along the eastern edge of the Sierra Madre Occidental. They erupted basanite, a low-silica lava that blankets the whole field in sheets around 10 meters thick, roughly 20 km³ of rock in all. The vents rose along northwest-trending faults tied to the San Luis–Tepehuanes fault system, at a time when the crust here was being pulled apart, and the lavas carry xenoliths, chunks of mantle rock brought up from deep below.

The field has never been fully dated. One cone gave an age of 0.8 million years, and from their erosion others look Pliocene to Pleistocene. The youngest are probably the La Breña and El Jagüey maars at the northern end, about 45 km north-northwest of the capital, whose lava flows are still almost bare; geologists suspect they are Holocene, only a few thousand years old. Maars are explosion craters, blasted out when rising magma met groundwater. La Breña is 1,400 to 1,500 meters across and holds several cinder cones inside; El Jagüey, about 700 meters wide, has a permanent lake on its floor. Both are ringed by tuff walls 30 to 70 meters high, at around 2,000 meters above sea level.

Much of the field is the malpaís of La Breña, a badland of vesicular basalt with little erosion, where prickly pear and yucca grow and cattle graze among the boulders. Named volcanoes include El Tepalcate and Puerto Colorado near the village of Tuitán, the latter known for reddish sand that makes an almost desert-like scene, and Maika in the Poanas valley, with the San Quintín ravine at its foot. The porous rock is thought to let rainwater seep down and warm up underground, feeding the hot-spring pools found around the region.