English: Bennett Peak volcanic neck in 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/volcanic plugs/diatremes) of Oligocene to Miocene age. The most famous and visually distinctive volcanic neck in the area is Ship Rock.

Ship Rock and the dikes radiating from it are principally composed of the scarce igneous rock minette (= potassic mica lamprophyre, the intrusive equivalent of trachybasalt lava) and volcanic breccias. Typical Ship Rock minettes are dense, non-vesicular, finely crystalline, and are composed of alkali feldspar (K-Na feldspar), phlogopite 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 typically happens if the magmas are rich in dissolved gases (water, carbon dioxide, sulfur dioxide, etc.). Minette magmas were not rich in dissolved gases. So why the violent eruptions? The magmas came in contact with groundwater, and 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 phreatomagmatic eruptions (a.k.a. hydrovolcanic eruptions).

Locality: Bennett Peak, western side of Rt. 491/Rt. 666, near the town of Tocito, San Juan County, northwestern New Mexico, USA

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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. (semken.asu.edu/pubs/semken03_nvf.pdf)