Fraser Goff

Picked up for a planned trip. A really nicely done little book, directed at an advanced lay audience. Author Fraser Goff worked at Los Alamos National Laboratory for 26 years, in the Geology/Geochemistry Group; I had no idea LANL had one of those, but I suppose it helps in figuring out where to direct the earthquake-inducing death rays.

A definition: Unfortunately, volcanologists use the term caldera to describe two rather different things. One is the crater at the top of a conventional volcano, especially if it is a large one and at least partially due to collapse after eruption. The other is a different animal altogether; huge circular volcanic features that are the result of collapse of a large magma chamber after a caldera eruption. Caldera eruptions do not come from anything a casual bystander would recognize as a volcano; there’s no central vent or mountain; instead the eruption comes from a ring fault surrounding the magma body. And if you are a casual bystander witnessing one of these things, you’re not going to be casual or standing by very long, because they are apocalyptic. Fortunately, they are also rare; the most recent one was the Lake Toba eruption in about 75000 BCE that came pretty close to wiping out humanity (I note there is some debate about whether Lake Toba is a true caldera or just a larger than ordinary volcano; not that it matters that much, it’s around 2800 km**3 of ash in either case).

The famous North American calderas are Yellowstone, Long Lake, and Valles. I was surprised to find Valles is far and away the most thoroughly studied; it was heavily drilled both for oil and geothermal energy, thus a lot is known about the subsurface. There are actually two caldera eruptions involved; the Valles eruption (around 1.2 Mya) partially destroyed the earlier Toledo caldera (around 1.6 Mya). Technically, Valles is still an active volcano, in the “Solfatara” stage, as there are still fumaroles producing steam and sulfur. There is, of course, no particular reason why it couldn’t erupt again as either a caldera or a lesser volcano; Yellowstone has been blowing up every 650 Kya or so.

After the caldera eruption, you end up with a negative terrain feature; and there is abundant sedimentological evidence of various size lakes in Valles. However, sooner or later the magma starts moving up again, resulting in a “resurgent caldera” which is more or less like a cork slowly being pushed out of a bottle by internal pressure. Since the “cork” has been pretty badly shattered there are often little (well, little relative to the caldera; not anything you’d want in your back yard) volcanoes scattered around the dome when magma has pushed up through faults, plus another batch (“moat volcanoes”) around the old ring fault; the most recent eruption of one of these was about 40 Kya.

Valles Caldera is roughly centered in another circular feature, the Jemez Mountains, which are also volcanic, going all the way back to the Miocene. There are no obvious signs of earlier caldera-type eruptions, but volcanoes have the geologically annoying habit of destroying evidence of their predecessors. Since the Jemez Mountains are mostly rhyolite, earlier caldera eruptions are certainly a possibility. You can draw a more or less straight line from the Raton volcanic fields around Raton, New Mexico, through the Mora volcanics around Taos, through the Jemez Mountains, Mt. Taylor, the Zuni-Bandera volcanics, the Springerville volcanics, the White Mountains in Arizona, and ending in the San Carlos mountains; the line is called the Jemez Lineament. Whether this is a real feature or wishful thinking like ley lines or canals on Mars is unclear; Goff allows that it might be related to an Proterozoic suture zone (apparently there’s some seismic reflection data that suggests something way down there) but is not convinced. All the mentioned fields are Miocene so it’s kind of a stretch to relate them to something Proterozoic. OTOH the Rio Grande rift started developing about the same time, so maybe something did get stirred up down there.

This is one of the best-illustrated popular geology books I’ve ever run across. All the diagrams and photographs are in color, and they all properly illustrate what they are supposed to show. There are no notes or direct references, but there is a suggested reading list. One thing that’s missing is any sort of field guide; although there are generalized highway maps there are no directions on how to get to specific points of interest. However, the reading list includes several of the excellent New Mexico Geological Society field guidebooks. Recommended for armchair geologists and excellent as a supplement to a good field guide.… (meer)