Friday, August 10, 2012

The Geology Train

A few weeks ago, my wife and I rode the Geology Train from Antonito, CO, to Chama, NM. The trip was 63 miles long, crossing the Colorado/New Mexico state line eleven times. The trip took about 8 hours and included a lunch stop at Osier, CO. The railroad was built in 1880 and only took 9 months to complete. The highest elevation during the trip is 10,015 feet at Cumbres Pass, the highest railroad pass in the United States.

This was the second annual Geology Train. The Cumbres & Toltec Scenic Railroad ( offers numerous tourist trips each day during the summer. Unlike the other train trips on this narrow gauge, steam-powered train, the Geology Train makes numerous stops along the way and one of the geologists on board spends several minutes explaining the geology we are passing through. The train also makes a couple of stops where the passengers were able to get off and look at the rocks more closely. At one of the stops, Toltec Gorge, we were able to walk through a tunnel and take pictures of the train as it came through.

The geology we observed started in the Rio Grande Rift valley that extends from Mexico, northward through central New Mexico, and into southern Colorado following  the San Luis valley as far north as Leadville. The Rio Grand Rift valley represents a spreading center that is still pulling the earth's crust apart. The Rio Grande Rift very slowly continues to widen today at a rate of between 0.5 and 2 mm/yr. Evidence of the rifting is expressed in volcanic cones and basalt flows along the margins of the valley that began forming about 5-million years old. Further south in New Mexico, more extensive volcanism associated with the rifting is evidence in the Valles Caldera near Los Alamos, New Mexico. The Valles Caldera, created about 1.2 million years ago, is one of the world's largest and youngest calderas1. Today geologists consider volcanic activity associated with the rift to be dormant, but not extinct.

Map of Rio Grande Rift region showing the primary basins of the rift. Image courtesy of the USGS.1
The graphic above depicts a cross-section of a rift. Image courtesy of the USGS.1
 As we traveled westward and started gaining elevation, we began to see lava flows and ashflow tuffs derived from the San Juan volcanic field to the north. Cataclysmic eruptions from volcanic centers in the San Juans began about 38 million years ago with mainly andesitic volcanics and ended with volcanics mainly of rhyolitic composition around 19 million years ago. The centers of these voluminous eruptions are marked by about 10 calderas that have been mapped in the San Juans. Each of these calderas formed when the overlying volcanics collapsed into voids left by the massive eruptions of ash-flow tuffs that cover much of southwestern Colorado. Major mining districts in the San Juans, associated with several of these calderas, produced rich gold, silver, lead, zinc, and copper ores. Only small-scale mining continues today in some of the historic mines of the area and a few of the mines offer underground tours.

Large-scale geologic features of the San Juan volcanic field in northwestern New Mexico and southwestern Colorado. 2

About 100 million years ago, the Farallon tectonic plate was subducting along the western margin of the North American plate at a fairly steep angle. Magmatism and crustal deformation associated with this subduction produced the Sierra Nevada intrusive granitic rock in California and mountain
Schematic cross-section of the western United States showing the changes in the geometry of the Farallon Plate through time. Top: Sevier Mountain Building Event 100 million years ago. Middle: Laramide Mountain Building Event 60 million years ago. Bottom: Voluminous volcanism in the San Juan volcanic field and extension in the Basin and Range Province since 35 million years ago.
building events included the Siever orogeny in Nevada. Compressional stresses generated across the subduction zone resulted in mountain building that progressively moved eastward over time. About 75 million years ago, the modern Rocky Mountains began to rise, reaching their peak elevations around 45 million years ago. The eastward migration of mountain building is thought to be the result of the flattening in the angle at which the Farallon plate was subducting. When the mid-ocean rift separating the Farallon plate from the Pacific plate reached the western margin of the North American plate, relative motion between the plates changed from subduction to the strike-slip faulting that is observed today along the San Andreas Fault. The compressional forces generated by subduction were relaxed as the Pacific plate moved northward relative to the North American plate. At the same time, the subducted Farallon plate began to sink into the mantle. This is thought to have set up conditions that generated the explosive and voluminous volcanism in the San Juan Mountains and elsewhere in Colorado,
My wife and I took this train trip about 10 years ago. However, having USGS geologists and geoologists from local colleges explaining the geology this time made the trip more interesting. The geologists kept their descriptions pretty simple so that the non-geologists on the trip were able to learn something. If you are in southwestern Colorado next summer around this time, I would recommend the trip. For trip schedules consult the Cumbres & Toltec Scenic Railroad website at

1 Rio Grande Rift FAQ, ____, Measuring Rio Grande Rift Crustal Deformation;, 2 pages.

2 _____, 2012, Geology Train Excursion on June 24th 2012 Aboard the Cumbres and Toltec Scenic Railroad; Cumbres and Toltec Scenic Railroad,  18pages.

Humphreys, E., Hessler, E., Dueker, K., Farmer, C., Erslev, E., and Atwater, T., 2003, How Laramide-age hydration of North American Lithosphere by the Farallon Slab Controlled Subsequent Activity in the Western United States; International Geology Review, v. 45, p. 575-595.

Kelly, Shari, 2012, Conceptual Models of the Rio Grande Rift; Lite Geology, Spring 2012, New Mexico Bureau of Geology & Mineral Resources, pp 2-5.
Lipman, P.W., 2006, Geologic Map of the Central San Juan Cluster, Southwestern Colorado; U.S. Geological Survey Map I-2799.

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