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.