Minette from the Oligocene of New Mexico, USA. (8.1 cm across at its widest)

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 shown above is an aphanitic, felsic minette lava sample from the Narbona Pass Maar Volcano in the Chuska Mountains of New Mexico. The gray material is minette - it has phenocrysts of phlogopite mica and diopside pyroxene in a trachytic groundmass of alkali feldspar laths. The greenish areas are mantle xenoliths of peridotite. The peridotite masses were originally olivine-rich, but the olivine component has been partially serpentinized. These mantle peridotites also have chrome diopside and spinel.

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

Locality: roadcut on northern side of Rt. 134, immediately east of Washington Pass/Narbona Pass, summit of Chuska Mountains, southwestern San Juan County, northwestern New Mexico, USA

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.