A short while ago, I moved from Abiquiu to Arroyo del Agua near Coyote, an area which long-time locals call northern New Mexico’s most beautiful spot. Almost 7,000 feet high, it is surrounded by the Santa Fe National Forest and boasts lush, alpine woodlands, pastoral mesas, and dark-red colored canyons and cliffs that are the signature signs of the region.
When I go on a walk with my dogs, both sides of the dirt road are strewn with agate. Many of them are covered with a thin, whitish film that hides the colors inside, but quite a number is cracked open, revealing browns, greys, or obsidian-black. My Rockhound’s Guide to New Mexico claims that there are also orange or red pieces, but I haven’t found one yet. I knew that these kinds of rock formations are of volcanic origin, but it surprised me to realize that they are part of a supervolcano that last erupted 1.2 million and 1.6 million years ago and is now known as the Valles Caldera National Preserve. We often went hiking near Valle Grande (the most famous section of Valles Caldera) in the Jemez Mountains, but that’s really far away from Coyote — how did these agates end up here? I had to do some research.
Strictly speaking, “supervolcano” isn’t a scientific term. Geologists and volcanologists refer to a “Volcanic Explosivity Index” (VEI) of 8 and 7 when they describe super-eruptions. An increase of 1 indicates a 10 times more powerful eruption. VEI-8 are colossal events with a volume of 1,000 km3 (240 cubic miles) erupted pyroclastic material (for example, ashfall, pyroclastic flows, and other ejecta), while VEI-7 volcanic events eject at least 100 km3 Dense Rock Equivalent (DRE). Valles Caldera belongs to the VEI-7 class of supermassive events (accounting for the countless agates in and around my backyard) and is situated within the Jemez Volcanic Field. The last eruption and collapse of the Valles Caldera occurred 1.2 million years ago, piling up 150 cubic miles of rock and blasting ash as far away as Iowa.
The name “caldera” comes from the Spanish word for “kettle”, “cooking pot”, or “cauldron”. Molten rock or magma begins to collect near the roof of a magma chamber bulging under older volcanic rocks. After an eruption begins and enough magma is ejaculated, the layer of rocks overlying the magma begins to collapse into the now emptied chamber because of the weight of the volcanic deposits. A roughly circular fracture develops around the edge of the chamber. In the case of Valles Caldera, the surrounding area continues to be shaped by ongoing volcanic activity, and an active geothermal system with hot springs and “fumaroles” (smoke plumes) exists even today.
Will Valles Caldera erupt again? While most of the media hype surrounding supervolcanoes focuses on Yellowstone where a VEI-8 event happened some 640,000 years ago which means that the next one could take place any moment or at least within the next 40,000 years, the Discovery Channel called Valles Caldera “a sleeping monster in the heart of New Mexico” but added in answer to the above question: No one knows. Duh.