10 June 2011
I meant to post this last year after my brief trip to Los Alamos, but now that I’m back on the Hill for the summer, it seems a shame not to show off the scenery!
The Jemez Volcanic Field in northern New Mexico – which includes the Valles Caldera – straddles the Rio Grande Rift in the east and the Colorado Plateau in the west. The Jemez contains volcanic rocks erupted from >13 to 0.13 million years ago, with compositions ranging from basalt (low silica content) to rhyolite (high silica). The best known of these is the Bandelier Tuff, a thick sequence of pyroclastic deposits which were erupted in several phases around 1.62 to 1.25 million years ago. The total volume of material in the Bandelier is around 300 cubic km (~75 cubic miles), and it covers much of the area in the Jemez Volcanic Field. (The Bandelier tends to be unwelded and relatively soft, and canyons have cut down through it in many places, creating wonderful vertical exposures as well as the mesas and plateaus that Los Alamos is built on.)
All that material being erupted from a magma chamber leaves a pretty big void, and the 19-km-wide Valles Caldera formed when the caldera floor (or magma chamber roof) collapsed into that space. (I blogged briefly about it in one of my short-lived Volcano Vocab features last year.)
Following the formation of the caldera, a resurgent dome (Redondo Peak) rose several thousand feet above the caldera floor. A resurgent dome is a structural feature rather than an eruptive one; it’s usually formed by magma rising under the caldera but not erupting. Here’s a closer view of Redondo Peak:
The caldera also contains six large rhyolite lava domes (which are eruptive features like the domes in my field area of Santiaguito); the first erupted ~1.23 million years ago and the last ~ 0.56 million years ago. (During this time there were apparently several lakes in the caldera – not an uncommon occurrence in this type of volcanic feature! – which left beach deposits and wave-cut benches behind.)
Rhyolite volcanism shifted to the south end of the caldera 0.52 million years ago, forming Cerro La Jara and South Mountain (as well as blocking part of the Jemez River and forming a deep lake that lasted 200,000 years before it drained).
The last rhyolites which erupted from the Valles were the El Cajete – Battleship Rock pyroclastic deposits (55 ka) and the Banco Bonito lava flow (40 ka). The Battleship Rock tuffs filled the old Jemez River Canyon (and were later eroded to form the Battleship Rock, below); and Banco Bonito lava flows are particularly known for their obsidian (meaning they were quenched quickly, without time for crystals to form).
The Valles is particularly dear to volcanologists because it is considered to be the type location (best example) of a resurgent caldera, and the Bandelier Tuff is one of the first (and best-studied) examples of an ash-flow tuff. Valles Caldera is where we get the classic figures of caldera formation stages (an example is shown below):
So, being in the Jemez Mountains (and really close to the classic caldera) is a bit of a pilgrimage for any volcanologist. And lucky me – I get to spend all summer here! Hopefully, that means lots more posts about the geology and scenery of the area.
Further Reading & Links:
Smith, R. L. and Bailey, R. A., 1968, Resurgent Cauldrons; in Coats, R. R., Hay, R. L., and Anderson, C. A. (eds. ), Studies in volcanology: Geological Society of America, Memoir 116: 613-662.
Goff, F., 2009, Valles Caldera: A Geologic History. University of New Mexico Press, Albuquerque, 114p.