SHIP ROCK

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.

Ship Rock is a brownish-colored, craggy, vertical-sided mountain in far northwestern New Mexico, USA. It represents rocks that filled up the subsurface vent complex of an ancient volcano. The original volcano and its surrounding landscape were approximately 1 to 2.5 kilometers above the present surface, according to published estimates.

Radiating from the central volcanic neck are three, miles-long, sublinear, vertical dikes. One extends to the northeast, one extends to the west, and one extends to the south. Several shorter radiating dikes extending in other directions occur near Ship Rock itself. The southern major dike is the most easily accessible, as Red Rock Highway was constructed through it.

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).

Age of Ship Rock: Oligocene, 27-32 million years.

Ship Rock Volcanic Neck (diatreme), Navajo Volcanic Field, northwestern New Mexico, USA (looking NW). The mountain consists of minettes and volcanic breccias.

Ship Rock Volcanic Neck (diatreme) (looking NW) - the long linear feature on the left is the southern dike, composed of finely crystalline minette rocks. The surrounding plains are fine-grained siliciclastic sedimentary rocks of the Mancos Shale (Upper Cretaceous).

Ship Rock (looking ~NNE) - this eroded wall is the southern dike, composed of finely crystalline, dark-colored minette. Host rocks to the dike are fine-grained siliciclastics of the Upper Cretaceous Mancos Shale.

Ship Rock & eroded wall of the southern dike (looking ~N), composed of dark-colored, finely-crystalline minette.

Minette - this is the rock making up much of Ship Rock and the surrounding dikes. This exposure is from the southern dike, along the northern side of Red Rock Highway. The dominant minerals are alkali feldspar, phlogopite mica, and diopside pyroxene.

NAVAJO VOLCANIC FIELD

(other volcanic necks/diatremes)

Bennett Peak volcanic neck, western side of Rt. 491/Rt. 666, near Tocito, San Juan County, northwestern New Mexico, USA.

Ford Butte volcanic neck, eastern side of Rt. 491/Rt. 666, near Tocito, San Juan County, northwestern New Mexico, USA.

Cathedral Cliff volcanic neck (a.k.a. Table Mesa Plug), western side of Rt. 491/Rt. 666, adjacent to Table Mesa, between Newcomb and Shiprock, San Juan County, northwestern New Mexico, USA.

Cathedral Cliff volcanic neck (a.k.a. Table Mesa Plug), western side of Rt. 491/Rt. 666, adjacent to Table Mesa, between Newcomb and Shiprock, San Juan County, northwestern New Mexico, USA. The structure shown above is a dike, called the Chinese Wall, that extends south from the Cathedral Cliff eroded volcanic neck.

Barber Peak volcanic neck, eastern side of Rt. 491/Rt. 666, between Newcomb and Shiprock, San Juan County, northwestern New Mexico, USA.

Mitten Rock volcanic neck (diatreme), northern side of Red Rock Highway, southwest of Ship Rock, San Juan County, northwestern New Mexico, USA.

The Thumb volcanic neck, southern side of Red Rock Highway, near Red Rock, San Juan County, northwestern New Mexico, USA.

Black Rock volcanic neck, Redrock Valley, Apache County, northeastern Arizona, USA. The surrounding rocks are Triassic- to Jurassic-aged redbeds (= hematite-rich siliciclastic sedimentary rocks).

Alhambra Rock volcanic neck, northern side of Rt. 163, west of Mexican Hat, San Juan County, southeastern Utah, USA.

Agathla Peak volcanic neck (diatreme), eastern side of Rt. 163, north-northeast of Kayenta, Navajo County, northeastern Arizona, USA. Agathla Peak has volcanic breccia and minette dikes.

Chaistla Butte volcanic neck, eastern side of Rt. 163, northeast of Kayenta, Navajo County, northeastern Arizona, USA.

Church Rock volcanic neck (diatreme), northern side of Rt. 160, east of Kayenta, Navajo County, northeastern Arizona, USA.

Info. principally synthesized from:

Delaney (1987) - Ship Rock, New Mexico: the vent of a violent volcanic eruption. Rocky Mountain Section of the Geological Society of America, Centennial Field Guide 2: 411-415.

Semken (2003) - Black rocks protruding up: the Navajo Volcanic Field. in Geology of the Zuni Plateau. New Mexico Geological Society Guidebook, 54th Field Conference, September 24-27, 2003: 133-138.

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