New Mexico's Valles Caldera
Caldera in the Spanish language means "kettle" or "cauldron"; in geological literature, it has come to be known as a volcanic collapse crater. Craters are the vents through which cinders, ash, lava and other ejected materials are erupted, and thus are related to cone building. Craters are usually formed by deposition around a volcanic vent and enlarged by the slumping of unstable deposits into the vent. Calderas, on the other hand, are formed when infrequent, truly large eruptions occur and the gas-rich magma is quickly erupted from depths of 4 to 6 kilometers (2.5 to 3.7 miles). The remaining void space from which the magma was drained results in a piston-like collapse of the overlying rocks. This, in turn, opens up many vents and the eruption continues as the piston collapses. The resulting collapse crater is mostly filled with its own volcanic ash and pumice. Calderas are known to have diameters ranging from less than 1.6 kilometers (1 mile) to more than 48 kilometers (30 miles). Large calderas are usually formed during a collapse that truncates an entire volcanic field, not just one large cone.
The Valles caldera of New Mexico is a beautiful example of a resurgent caldera located in the midst of the Jemez volcanic field. Scientists study this area to learn about the fundamental processes in magmatism, hydrothermal systems, and ore deposition. With nearly 40 deep wells which have resulted in extensive subsurface data, the Valles caldera is the best explored caldera complex in the United States. It is the youngest of two calderas in the region, having collapsed over and buried the Toledo caldera (which might have collapsed over older calderas.) These two large eruptions took place 1.4 million and 1 million years ago. The circular topographic expression of the Valles caldera measures 22 kilometers (14 miles) in diameter.The Jemez volcanic field, including the calderas, lies above the intersection of the Rio Grande rift, which runs north-south through New Mexico, and the Jemez lineament.