The Great Desert

THE VALLES CALDERA - 2

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The Valles Caldera is the "type specimen" of a caldera complex volcano - a relatively unfamiliar type but very common through the world and through geologic history. As we approached the Valles Caldera (and left it), we saw tall cliffs of the ash-flow tuff deposited by the Valles eruptions -- the Bandelier Tuff.
 

Caldera on Skyline
 
Tuff and Pumice

The Jemez mountains, looking north from the site of the desert crusts. The hills on the skyline are all peaks of the Valles Caldera. They don't look much like a classic volcano, but more like the hills around a Meteor Crater. The low hills in the foreground expose gravels and sands of the Albuquerque basin, and the mesa in the mid-distance is capped by basalt lava flows of the San Filipe field. Credit.
 
Bandelier tuff, east of Los Alamos, seen from the Rio Grande River stop. The Bandelier ash-flow tuff holds up the flesh-colored cliffs here. Below the ash-flow tuff (i.e. earlier than it) is a thick white layer of pumice, also erupted from the Valles Caldera -- a prelude to the massive eruptions that produced the ash-flow tuffs. Beneath the pumice are black cliffs of basalt lava flows from the small volcanoes we saw across the Rio Grande. Credit.
     
Tuff above Jemez
 
Topographic Rim

Cliffs above the Jemez Pueblo with the Bandelier ash-flow tuff (buff layers at top) overlying red sandstones and shales of Permian age (~300 m.y. old), the Abo formation. There is nearly no pumice beneath the Bandelier ash-flows here on the south and west of the Valles caldera. Perhaps, during eruption, the winds had been blowing to the north and east! Image by Cliff Fenton.
 
Cliffs farther up San Diego Canyon (image taken by Treiman many years ago). The tall cliffs and bare slopes are in the Bandelier tuff, which overlies the Permian age Yeso formation (orange) and the Pennsylvanian age Madera Limestone (brown and gray). Note that the base of the Bandelier tuff is farther down slope in the middle of the image than to the left. This irregularity is all that remains of the hilly landscape before the Valles Caldera erupted. Credit.
     
Battleship Rock
 
Obsidian

Battleship Rock stands beside Jemez Creek in San Diego Canyon. It is made of ash-flow tuff, but not the Bandelier. It is probably from a small ash-flow eruption from the vent that later produced the El Cajete pumice and Banco Bonito obsidian (in the overlying Obsidian Cliffs). Photo by Rhonda Spidell.
 
The Obsidian Cliffs, upstream from Battleship Rock on the east side of Jemez Creek, are entirely of the Banco Bonito obsidian flow. It lies above the El Cajete pumice shown in the Valles 3 web page, which in turn lies above the Battleship Rock ash-flow tuff. These three eruptions probably came from the same vent, at nearly the same time. Image by Cliff Fenton.

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Updated 09/24/03.
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