VALLES CALDERA
Northern New Mexico's Valles
Caldera (aka Jemez Caldera) formed 1.1 million years ago, during the
Early Pleistocene, after significant explosive ash & pyroclastic
eruptions. The large, ~empty magma chamber under the volcano collapsed,
resulting in a very large surface depression. The Valles Caldera is only
the latest caldera to form in this area. Volcanic activity here has been
ongoing since the Late Miocene, and has continued since caldera collapse.
The samples shown below were produced by post-caldera volcanic events during
the Middle and Late Pleistocene.
Valles Caldera (looking ~NNE)
Three Rhyolites Roadcut (above & below) along
Rt. 4 near the southern margin of the Valles Caldera, northern New Mexico, USA
(35° 49' 41.48" N, 106° 35' 32.07" W). The three volcanic units
exposed here have the same chemistry (rhyolitic), but are remarkably different
in terms of rock texture and origin (eruptive style). The blocky-weathering
lower unit is the South Mountain Rhyolite (521 k.y.). The
whitish-gray, layered ash-pumice unit above that is the El Cajete Pumice
(55-60 k.y.). The blackish, blocky-weathering unit at the top is the Banco
Bonito Obsidian (50-55 k.y.).
Rhyolitic volcanic breccia (6.2 cm across at its
widest) - this pyroclastic flow breccia is from the South Mountain Rhyolite.
It formed during the middle Middle Pleistocene, at about 521 k.y., from
post-caldera volcanic activity at Valles Caldera. This sample comes from
the basal unit exposed in the Three Rhyolites roadcut shown above.
Rhyolitic pumice (specimen at left is 3.4 cm
from top to bottom) - these pumice samples are from the El Cajete Pumice
(see whitish middle unit in roadcut photo above). Material from this unit
represents air-fall pumice deposits and pyroclastic surge deposits. The
El Cajete Pumice was deposited during the Late Pleistocene, at about 55-60
k.y., from a volcanic center in the Valles Caldera. This sample comes
from a roadcut along the northern side of Rt. 4, ~3.9 miles east of the Three
Rhyolites roadcut shown above (35° 48' 55.94" N, 106° 32' 40.64" W).
Porphyritic rhyolite
obsidian (6.9 cm across at its widest) - this glassy lava flow sample is from
the Banco Bonito Obsidian, one of several members of the Valles
Rhyolite, which represents a series of Late Pleistocene, post-caldera volcanic
eruptions. The Banco Bonito Obsidian dates to about 50-55 k.y. This
sample comes from the upper unit exposed in the Three Rhyolites roadcut shown
above.