The Valles Caldera, 20-24 km wide., was formed following two large eruptions that created the plateau, called Bandelier Tuff. Before the collapse of the caldera, no great volcano was there ... only a few volcanoes dotted the region.
Digital 3D elevation of the Valles-Toledo complex, with boundaries of the calderas and internal post-caldera structures - Doc. New Mexico Museum of Natural History and Science
The Bandelier Tuff :
This rhyolite formation in Jemez mountain consists of deposits from two eruptions:
- The lower unit corresponds to the formation of the caldera Toledo, there are 1.45 Ma - volume 400 cubic km;
- the upper unit corresponds to the Valles Caldera, there are 1.1 Ma - volume 250 cubic km.
The formation begins when a mass of granitic magma rich in water about 1 million cubic kilometers rises to the surface. Arriving near the surface, it breaks the rocks which overcomes creating fractures in a circle.
When the eruption occurs, the water in the magma vaporizes and sprays the magma in thick clouds of ash and steam. These ignimbrites are deposited in all directions to form the Bandelier Tuff. The issuance of this huge volume empty the magma chamber, causing the collapse of a large caldera of 20-24 km. wide and 300 meters deep.
Above, the simplified geological evolution of the Valles Caldera - bottom, the volume compared of the Valles Caldera eruption compared to other major eruptions - one click to enlarge - diagrams New Mexico Museum of Natural History and Science
The Bandelier Ashflow tuff overcomes the red rocks of Permian - Larry Crumpler Photo / New Mexico Museum of Natural History and Science
Bandelier Tuff Ash Flow - wall in Rendija Canyon near Los Alamos. - Photo L.Crumpler / New Mexico Museum of Natural History and Science.
Rainwater and ice melt fill the caldera, to give a steaming sulphurous lake, because the eruption is still ongoing.
The post-caldera volcanism continues with the introduction of
- Lava domes located on the schematic map, from right to left anticlockwise, on circular fractures, and settled there 1Ma ( Cerro del abrigo and Media), 900,000 years (Cerro de Trasquilar), 800,000 years (Cerro Seco and San Louis) and 530,000 years (Mt. San Antonio)
- and a lifting of the floor of the caldera, with the formation of Redondo Peak, a resurgent dome. Crescent, the dome resurgence breaks and splits, forming cracks.
There are about 500,000 years, the caldera lake empties abruptly through the canyon of San Diego to the south, likely following the erosion of the ancestral Jemez River combined with the undermining due to hydrothermal phenomena. The dam failure decreases the amount of water available and hydrothermal events dimmed, especially as the magma chamber begins to cool.
Valles Caldera - Battleship Rock ignimbrite, structure left to SW by the draining of the caldera lake (see map above) - photo Ronda
The tuff, eroded by wind and rain, left cavities in the walls of canyons. These holes were used by the ancestral Pueblo people as living quarters and as building materials
... to see tomorrow.
- New Mexico Museum of Natural History and Science. - Valles caldeira / Jemez volcanic field