Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

New Mexico is a volcanic wonderland, home to numerous types of volcanic features like lava flows similar to those in Hawaiʻi and eroded stratovolcanoes that once resembled the giants of the Cascade Range.  The youngest eruption in the state occurred about 3,900 years ago near Grants—the McCarty’s lava flow.

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One of the most noteworthy volcanic areas in New Mexico is located in the Jemez Mountains, in the north-central part of the state.  That’s where you’ll find Valles Caldera, a volcanic system that is similar to Yellowstone Caldera in many ways.

The Jemez Mountains are located at the intersection of two important geologic structures.  First, the Jemez Lineament angles across the state along a southwest-northeast trend, from southern Arizona through northeastern New Mexico.  There are numerous volcanic fields along its length, including the San Carlos and Springerville volcanic fields in eastern Arizona and the Zuni-Bandera, Mount Taylor, and Raton-Clayton volcanic field in central and northeastern New Mexico.  Second, running from south to north through the center of New Mexico and into Colorado is the Rio Grande Rift, an area of crustal extension that also hosts several volcanic vents, like those near Albuquerque and Carrizozo.  It should come as no surprise that where these two “leaky” structures come together, there is a lot of volcanism.

Volcanic eruptions in the Jemez Mountains began about 14 million years ago.  There was never a main central volcano that developed, but instead volcanic vents were scattered throughout the region, with eruptions building up a high plateau.  About 1.61 million years ago, a major eruption occurred—the first caldera-forming eruption in the Jemez Mountains.  The collapse scar that resulted is called Toledo Caldera, and the associated deposit makes up the Otowi Member of the Bandelier Tuff.  But that wasn’t the end of caldera-forming eruptions.

About 1.25 million years ago, a second caldera formed—Valles Caldera.  The eruption was about the same size as that which formed the Toledo Caldera, and it deposited the Tshirege Member of the Bandelier Tuff.  The Valles Caldera cut into the Toledo Caldera, partially destroying that earlier structure.  Although huge in volume, each of the caldera-forming eruptions in the Jemez Mountains was less than half the size of that which formed Yellowstone Caldera 631,000 years ago.

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After the formation of Valles Caldera, uplift occurred as magma reoccupied the collapsed reservoir beneath the surface.  This uplift built a “resurgent dome” in the caldera center.  In fact Valles Caldera was one of the first locations worldwide where this process was studied and understood.  Following the resurgence, eruptions of thick, pasty rhyolite lava occurred in the caldera’s “moat”—the flattish area between the caldera wall and the resurgent dome.  Each lava dome was built by a series of successive eruptions over the course of thousands to tens of thousands of years before activity ceased.  After pauses of tens to hundreds of thousands of years, a new lava dome formed.  Interestingly, these domes erupted in a sequence, with activity migrating in a counterclockwise direction around the caldera.

The most recent eruptions included an explosive event about 74,000 years ago that deposited the El Cajete pumice and Battleship Rock ignimbrite, with the Banco Bonito rhyolite lava flow erupting from roughly the same area about 68,000 years ago.  Since that sequence of eruptions, Valles Caldera has been dormant.

The general sequence of activity—resurgence followed by emplacement of lava flows during episodic eruptions—is similar to that experienced by Yellowstone following its caldera-forming eruption 631,000 years ago, although the Yellowstone eruptions (both the explosive events and lava flows) were much larger in size.

Another similarity between the Yellowstone and Valles Calderas is hydrothermal activity, although again the scales differ.  Yellowstone National Park is well known for its geysers and hot springs.  While such activity is not as prominent at Valles Caldera, there is a noteworthy hydrothermal area, called Sulphur Springs, that is similar to the acid-sulfate thermal areas in Yellowstone, like the Mud Volcano region.

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And still another similarity is the presence of a partially molten region about 5–20 kilometers (3–12 miles) beneath the surface of both caldera systems.  These magma reservoirs are mostly solid, but still hot.  A major difference between the two locations, however, is the activity recorded by monitoring networks.  While Yellowstone is a site of thousand of earthquakes every year and both uplift and subsidence of the ground, seismicity at Valles Caldera is rare, and there is no evidence for significant ground deformation.

Given its long geologic history, there is no reason to think that Valles is “dead.”  It may very well erupt again, but like Yellowstone, the most likely form of future eruptive activity is a lava flow, and the event would be preceded by significant changes in monitoring data.

Much of what was learned at Valles Caldera about how calderas formed and evolved over time was applied to other caldera systems in the USA and around the world.  We know more about how Yellowstone works thanks to studies of its older sibling in the southwest USA.