The Caja del Rio plateau is a monogenetic volcanic field, which includes approximately 60 cinder cones, spatter cones, and basalt outflows. The volcanism can be explained by the field's location, which is very close to the intersection of the Rio Grande Rift and the Jemez Lineament. The Rio Grande Rift is a result of extensional, (or divergent) tectonic forces exerted upon the American Southwest. This feature runs southward from the vicinity of Leadville, Colorado, through the entire state of New Mexico, through the vicinity of El Paso, Texas, and into Chihuahua, Mexico. The Rift began forming approximately 30 million years ago during the late Oligocene Epoch. The Jemez Lineament is another, older feature, which also represents a linear weakness in the crust. The Lineament trends southwest to northeast, and underlies a string of volcanic features across Arizona and New Mexico. Those features include the San Carlos volcanic field, Springerville volcanic field, Zuni-Bandera volcanic field, Mount Taylor volcanic field and the Raton-Clayton volcanic field. These structural weaknesses create a thinned crust, and pathways for intrusion by magma originating in the mantle. The result has been a series of intercontinental basaltic eruptions.