The Rio Grande naturally sections the Cerros del Rio field creating White Rock Canyon. Like much of New Mexico's volcanic record, surface erosion is minimal, but vertical dissection can result in unusual natural cross sections. IN the case of White Rock Canyon ddown-cutting bt the Rio Grande exposes a variety of lava flow sections and phreatomagmatic deposits. View directed northward along White Rock Canyon. Buckman Mesa on the far right distance.
Montoso maar is an unusual exposure created when a side canyon off of White Rock Canyon emncroached through the center of a maar. As a result, one can stand on the deposits of blocks and ash forming the crater rim, yet descend into exposures of the upper one hundred meters of the same maar by walking down the canyons.
Section exposing blocks of sedimentary country rock caught in the debris of the lower inner maar walls.
The volcanic field represents the younger of a series of three extended time period over which mafic volcanism pre-date the silicic eruptions leading up to Valles Caldera formation. ranging from alkali basalt to andesite. Most of the eruptive volume occurred the 2.3-3.2 (Baldrige, 2004). Eruptions consisted of alakli basalts and transition high alkali intermediate or andesitic compositions. Andesitic eruptions are relatively low in volatiles such that flows are thick and originate from vents with very small pyroclastic centers. The volcanic rocks thicken towards the Rio Grande implying that the volcanism was contemporaneous with downcutting of the river through the accumulating volcanoclastic sediments. Maars, originally identified during mapping of the field, are common, including a half-sectioned example (Montoso maar).
Geologic Map of the Cerros del Rio field, J.C. Aubele, 1978
Frijoles Canyon on the west side of the Rio Grande at White Rock Canyon cuts through a sequence of massive phreatomagmatic and volcanoclastic deposits.
View Cerros del Rio volcanic field in a larger map
Aubele, J. C., 1978, Geology of Cerros del Rio volcanic field; in Hawley, J. W. (ed) Guidebook to Rio Grande rift in New Mexico and Colorado: New Mexico Bureau of Mines and Mineral Resources Circular 163, p. 198-201.
Aubele, J. C., 1979, The Cerros del Rio volcanic field: New Mexico Geological Society 30th Field Conference Guidebook, p. 243-252.
Aubele, J. C., 1999, Cerros del Rio volcanic field: New Mexico Geological Society 50th Field Conference Guidebook, Albuquerque Geology, p.13-14.
Dethier, D. P., 1989, Geology of White Rock quadrangle, Santa Fe and Los Alamos Counties, New Mexico: New Mexico Bureau of Mines and Mineral Resources Map, 1:24000 scale.
Duncker, K. E., 1988, Trace element geo-chemistry and stable isotope constraints on the petrogenesis of Cerros del Rio lavas, Jemez Mountains, New Mexico [M.S. thesis]: University of Texas, Arlington, 158 pp.
Manley, K., and Mehnert, H. H., 1981, New K- Ar ages for Miocene and Pliocene volcanic rocks in northwestern Espanola basin and their relationship to the history of the Rio Grande rift: Isochron/West, no. 30, p. 5-8.
Zimmerman, C., and Kudo, A. M., 1979, Geology and petrology of Tetilla Peak, Santa Fe County, New Mexico: New Mexico Geological Society 30th Field Conference Guidebook, p. 253-256.