Valle Grande (Va-lye Gra-n-de), also known as the Valles Caldera, is a pristine area in the Jemez Mountains of northern New Mexico in the United States. It is all that remains of a very large volcano, which erupted in ancient times in New Mexico, and helped shape the Jemez Mountains. Until recently, it was called Baca Ranch, and was private property; however, the owners sold the valley to the federal government, which is trying to find a way to let the public enjoy the land without spoiling it.
Scientists are studying this area to learn about the fundamental processes in magmatism, hydrothermal systems, and ore deposition. With nearly 40 deep cores examined, resulting in extensive subsurface data, the Valles Caldera is the best explored caldera complex in the United States. It is the younger of two calderas in the region, having collapsed over and buried the Toledo Caldera (which might have collapsed over yet 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, which extends from southeastern Arizona northeast to the westernmost part of Oklahoma. The volcanic activity is related to the tectonic movements along this intersection.
The lower Bandelier tuff which can be seen along canyon walls west of the Valles Caldera, including San Diego Canyon, is related to the eruption and collapse of the Toledo Caldera. The upper Bandalier tuff is believed to have been deposited during eruption and collapse of the Valles Caldera. The now eroded and exposed orange-tan, light colored Bandelier tuff from these events create the stunning mesas of the Pajarito Plateau.