The Great Desert



Continuation of the geology of the Valles Caldera in northern New Mexico. The geology of Yellowstone is similar to this, but less obvious from the air (or ground). There is some controversy about when the crater lake broke through - the version given here has the breakthrough late. However, there is evidence that a caldera-wide lake broke through during or just after formation of the resurgent dome, and that smaller lakes formed and broke through to San Diego canyon several times since then. Also, the age of the most recent lava flows is in some question -- they may be as old as 130,000 years, rather than 57,000 years. No volcanic activity more recent than that. All that's left are hot springs, both silica+sulfur and carbonate, and hot rock. However, volcanos could arise here again. The Valles caldera is built on the remains of an older caldera volcano, just as Yellowstone is on the remnants of older calderas.

No Place Like Dome

More Eruptions and Resurgent Dome. The Bandelier tuff was followed by smaller ash eruptions and obsidian lavas in the caldera. As the granite magma beneath the caldera continued ascending, it pumped out small eruptions of ash and obsidian (gray overlays), and pushed the floor of the caldera upward to form a dome (the black oval). This 'resurgent dome' rose up nearly a kilometer in ~20,000 years, and tilted the earlier ash beds and obsidian flows. As the dome grew, its top split and broke, forming a fault-bounded trough (faults are black lines, ball on downthrown side).  
Intra-Caldera Lavas I. Small ash eruptions and obsidian lavas continued to leak into the caldera from the granite magma beneath. These erupted into the caldera's lake, along the ring fracture in several 'pulses': ~1,000,000 years ago (in violet), ~900,000 years ago (in blue), ~800,000 years age (in red), and ~530,000 years ago (in orange). The lake water circulated deep into the earth along cracks, and fed a large system of hot springs and fumaroles, both in the caldera and outside. Probably there were geysers also, although no humans were there to appreciate them.
When the Levee Breaks
The Last Gasp?

The Dam Breaks. Approximately 500,000 years ago, the Valles caldera lake drained catastrophically through San Diego canyon to the south (arrow). Probably, the ancestral Jemez river slowly eroded back toward the caldera rim, aided by hot springs and groundwater leaking through the caldera rim. Once the rim was breached, more water came out, which caused more erosion, which let more water out, etc. The ensuing flood was catastrophic, and carved out a wide deep valley (arrow) through the volcano's ash-flow tuff and into the rocks beneath. Without the lake as an abundant source of groundwater, the hydrothermal spring system "dried up" to have fewer hot springs and more fumaroles. Hot spring activity was diminishing anyway as the magma blob underground crystallized and cooled.
Intra-Caldera Lavas II: The Last Gasp? After the caldera lake drained, there have been only a few notable, and probably related eruptions at ~57,000 years ago (in yellow). These include the Battleship Rock ash-flow tuff, the Banco Bonito obsidian flow that we saw in the road cut and in the obsidian cliffs above Battleship rock, and several pumice layers above and below the Banco Bonito flow. Since then - nothing, and no hint that the Valles caldera will erupt again.

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Copyright Allan Treiman, LPI.
Updated 09/06/03.
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