File:Fault in volcanic breccia (Upper Oligocene, 25 Ma; Narbona Pass Maar Volcano, near the summit of the Chuska Mountains, Navajo Volcanic Field, New Mexico, USA).jpg

English: Fault in volcanic breccia from the Oligocene of New Mexico, USA.

The Navajo Volcanic Field in the Four Corners area of the American southwest has about 80 old, eroded volcanic centers (volcanic necks/diatremes) of Oligocene to Miocene age. The most famous and visually distinctive volcanic neck in this area is Ship Rock, in far-northwestern New Mexico.

Navajo Volcanic Field eruptive centers are dominated by the scarce rock minette, a crystalline-textured, potassium-rich igneous rock. Minette can also be referred to as a potassic mica lamprophyre. Minette is usually finely-crystalline (aphanitic) with some phenocrysts. Contained minerals include alkali feldspar (K-Na feldspar - often sanidine), phlogopite and/or biotite mica (which glitters nicely in the light), diopside pyroxene, some olivine, plus other minor minerals.

Published studies of the eruptive centers in the Navajo Volcanic Field indicate that the original volcanoes erupted violently. This happened when rising magmas came in contact with groundwater - the water boiled to steam while confined underground. The steam pressure increased until it overcame the strength of the overlying rocks, resulting in an explosion and the creation of a surface crater (maar). Volcanologists call these events phreatomagmatic eruptions (a.k.a. hydrovolcanic eruptions).

The rock seen here is a volcanic breccia, a poorly-sorted, clastic-textured, extrusive igneous rock with numerous large, angular grains. Clasts in this material include minette, sedimentary rocks, and mantle peridotite. The rock represents a tephra fallout or tephra flow deposit, erupted from the long-extinct Narbona Pass Maar Volcano in the Chuska Mountains of New Mexico.

The lineations that extend from upper left to lower right are "slickensides", which were formed by fault movement.

Stratigraphy: unit Tt of Brand et al. (2009), eastern side of Narbona Pass Maar Volcano, Navajo Volcanic Field, Upper Oligocene, 25 Ma

Locality: roadcut on the southern side of Rt. 134 (apparently = "Inner Crater east section" of Brand et al., 2009), east of Washington Pass/Narbona Pass, near the summit of the Chuska Mountains, southwestern San Juan County, northwestern New Mexico, USA (36º 05.875' North latitude, 108º 51.377' West longitude)

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Mostly synthesized from:

Semken, S. 2003. Black rocks protruding up: the Navajo Volcanic Field. New Mexico Geological Society Guidebook, 54th Field Conference, September 24-27, 2003: 133-138.

Brand, B.D., A.B. Clarke & S. Semken. 2009. Eruptive conditions and depositional processes of Narbona Pass Maar Volcano, Navajo Volcanic Field, Navajo Nation, New Mexico (USA). Bulletin of Volcanology 71: 49-77.