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).
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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.
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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.
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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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