Caja del Rio

Caja del Rio (Spanish: "box of the river") is a dissected plateau, of volcanic origin, which covers approximately 84,000 acres (34,000 ha) of land in northern Santa Fe County, New Mexico, United States. The region is also known as the Caja, Caja del Rio Plateau, and Cerros del Rio. The center of the area is approximately 15 miles (24 km) west of Santa Fe, New Mexico. Most of the Caja is owned by the United States Forest Service and managed by the Santa Fe National Forest. Access is through New Mexico Highway 599, Santa Fe County Road 62, and Forest Service Road 24.

The landscape of the Caja del Rio Plateau is generally flat to rolling terrain, with numerous steep cones rising up to 7,472 feet (2,277 m) above the plains. The highest points in the immediate area are the unnamed high point 800 feet (240 m), Cerro Micho (7,326 feet (2,233 m)), Montoso Peak (7,315 feet (2,230 m)), Ortiz Mountain (7,188 feet (2,191 m)), and Cerro Rito (7,296 feet (2,224 m)) The entire plateau is within the watershed of the Rio Grande; portions of the Caja drain into the larger river through two tributaries: the Santa Fe River and Cañada Ancha.

Virtually the entire perimeter of the Caja is ringed by cliffs or escarpments. White Rock Canyon forms the northwestern edge of the plateau. This canyon, through which the Rio Grande flows, has a maximum depth of over 1,000 feet (300 m). Bandelier National Monument lies directly across the river to the west of the plateau. Along the eastern edge is Canada Ancha, an ephemeral stream that flows northward into the Rio Grande near Buckman. Another deep canyon (approximately 400 feet (120 m) in depth) runs along the southern edge of the Caja, cut by the Santa Fe River. The canyon of the Santa Fe River separates the Caja land grant from another land grant to the south: the Mesita de Juana Lopez Grant.

At the southern part of the Caja is La Bajada Mesa, dominated by Tetilla Peak (7,203 feet (2,195 m)). The western edge of the mesa is a large escarpment known as La Bajada (Spanish: "the descent"), which is easily visible to highway traffic traveling north on Interstate 25 from Albuquerque to Santa Fe. After crossing the Santo Domingo Basin and the reservations of Cochiti and Santo Domingo Pueblo, the freeway makes a steep climb up the escarpment. Further north along La Bajada, at La Bajada Hill, is the site of the old route of El Camino Real de Tierra Adentro, the road from Mexico City to Santa Fe, which was also the original path of Route 66; it involves a tortuous series of 23 switchbacks up the bajada road, which is now closed. La Bajada traditionally forms the southern boundary of the río arriba or upper river area of New Mexico, also known as Northern New Mexico.

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. These structural weaknesses create a thinned crust, and pathways for intrusion by magma originating in the mantle. The result has been a series of intracontinental basaltic eruptions.

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 Rio Grande rift takes the form of a series of basins, each offset to the right from the previous basin as one travels along the rift. The Caja del Rio volcanic field lies almost on top of the Bajada Constriction Zone, which is the zone of offset between the Albuquerque Basin to the southwest and the Espanola Basin to the northeast.

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, the Jemez volcanic field, the Taos Plateau volcanic field, and the Raton-Clayton volcanic field. The Caja del Rio volcanic field lies in the southeastern part of the Jemez volcanic field.

The lavas erupted in the Caja are diverse in their characteristics, ranging from basalt with a silica content of 49% to dacite with a silica content of 63%. Most are alkali basalts and hawaiites, which are basaltic lavas low in silica and mildly enriched in alkali metal oxides. These originated in the Earth's mantle but mingled with melted crust on their way to the surface. The more silica-rich lavas incorporated more melted crust and are described as evolved magmas. The hawaiites have unusual trace element compositions, suggesting they originated in the remnants of the Farallon plate known to lie below western North America.