File:Rhyolitic pumice (El Cajete Pumice Bed, Upper Pleistocene, 55-60 ka; Rt. 4 roadcut, Valles Caldera, New Mexico, USA) 1.jpg

English: Rhyolitic pumice from the Pleistocene of New Mexico, USA. (field of view ~15.2 cm across)

The Valles Caldera (a.k.a. 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 just the latest caldera to form in this area. Volcanic activity at this locality has been ongoing since the Late Miocene, and has continued since the most recent caldera collapse.

Pumice is a light-colored, frothy-textured, felsic extrusive igneous rock. The pumice samples shown above are from an air-fall and/or pyroclastic surge pumice deposit. The eruption was a post-caldera collapse event from a former volcano located in the Valles Caldera.

Stratigraphy: El Cajete Pumice, Valles Rhyolite, Upper Pleistocene, 55 to 60 ka

Locality: loose pieces from a roadcut along the northern side of Rt. 4 (0.1 to 0.2 miles west of stop 5 of Goff et al., 1996), southern margin of the Valles Caldera, northeastern Sandoval County, northern New Mexico, USA (35° 48' 55.94" North latitude, 106° 32' 40.64" West longitude)

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Reference cited:

Goff et al. (1996) - The Jemez Mountains Region. New Mexico Geological Society Forty-Seventh Annual Field Conference, September 25-28, 1996. pp. 31-32.