Caja Del Rio - Geology

Geology

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.

The Caja is not the only volcanic feature in this part of New Mexico. Approximately 20 miles (32 km) to its northwest is the Valles Caldera, a spectacular caldera which lies at the heart of the Jemez Mountains. This mountain range has been created by a series of eruptions since the Miocene. The ages of the first eruptions are difficult to determine, since the older rocks have been almost entirely buried by the material from younger eruptions. The oldest exposed volcanic rocks in the vicinity are approximately 16 million years old. The Jemez area experienced an intense pulse of basalt volcanism between 9 and 11 million years ago. The lavas from this cycle had little silica, and originated in the mantle. Additional pulses of volcanism occurred between 7-10 million years ago, between 6-7 million years ago, between 3-6 million years ago, and between 2-3 million years ago. The last of these pulses is believed to have created the Cerros del Rio volcanic field, which covers the entire Caja del Rio Plateau.

Several of the later eruptions involved not only basalts, but lavas made of rhyolite or dacite. Such lavas contain greater amounts of silica, and are more viscous than basalt. These eruptions created most of the highest peaks in the Jemez Mountains, including Chicoma Mountain(11,561 feet), and Polvadera Peak and Pajarito Mountain. Weathering of the igneous rocks resulted in alluvial fans, which spread eastward into the Rio Grande Rift. By 2 million years ago, the sediments had displaced the Rio Grande eastward from an earlier location near Los Alamos to its current location in White Rock Canyon. Two more eruptive cycles took place approximately 1.6 and 1.25 million years ago. The first eruption created the Toledo Caldera, but this feature was largely obliterated by the later Valles eruption. Each eruption involved pyroclastic flows and produced some of the welded tuff now found west of the Rio Grande. There have been numerous eruptions of sticky lavas in the last 1.25 million years ago, but all have been within the Valles Caldera. The most recent eruptions took place about 40-50,000 years ago, and formed the El Cajete Crater. This crater lies at the foot of Redondo Peak, a resurgent dome that rises to 11,254 feet within the Valles Caldera. Lying between White Rock Canyon and the rim of the Valles Caldera is the Pajarito Plateau, the many canyons of which have been cut by relatively recent erosion.