Ship Rock & the Chuska Volcanic Field

Location: Ship Rock, 36 degrees, 41 minutes N latitude, 108 degrees, 50 minutes W longitude; Chuska Volcanic Field, approx. 35.3 to 37 degrees NS and 108-109 degrees EW. Type: Group of intrusions and associated extrusive rocks Age: Middle Oligocene; fission track ages at Ship Rock (on southern dike) 27 ± 3 and 32 ± 3 million years; K-Ar, 30.6 million years Significance: The most famous example of a volcanic neck.

Ship Rock is a spectacular volcanic neck located within the Colorado Plateaus province in the northwestern ("Four Corners") region of New Mexico and in the middle of Navajo country. It is part of what is known as the Chuska volcanic field, a diffuse group of small intrusions, dikes, and some extrusive rocks scattered between Gallup, New Mexico and Farmington, New Mexico. Intrusive rocks in the field, like most Colorado Plateaus province intrusives, include some unusual petrologies: minette, vogesite, basaltic tuff, and tuff breccia. The Buell Park diatreme, which consists of kimberlite, is also part of the field.

Minette consists of alkali feldspar, biotite or phlogopite, and diopside. Diopside (pyroxene), phlogopitic biotite and olivine occur as phenocrysts in many hand samples. Edge-on view of the southern dike with Ship Rock in the distance.

Ship rock is thought to represent the interior of a maar-type eruption. The intrusion is 500 m in diameter at its greatest width, and rises 600 m above the surrounding plains. It consists of tuff breccia and fractured and comminuted host rocks wtih thin, sheet-like intrusions of minette locally cuttting through the entire mass. The intrusion is estimated to have been emplaced at a minimum of 750 and a maximum depth of 1000 m below the original surface. Host rocks at the current level of erosion are late Cretaceous, marine origin Mancos Shale.

Six dikes radiate from a point just west of the central intrusion, the three largest are 9, 4, and 3 km in length and trend S12 degrees E, N80 degrees W, and N55 degrees E respectively. The other three dikes are approximately parallel to the northern dike, trend N55 degrees E, and are less than 1 km in length. Several small plugs occur along the length of some of these dikes.

The northeast dike is actually segmented into 35 short dikes with intervening Mancos Shale. The dikes range from 0.6 to 4.6 m in width and 8 to 395 m long. Each dike segment is oriented somewhat differently from adjacent dike segments, and the pattern as a whole is not strictly en echélon. However, the trend of all the segments together is remarkably linear. This suggests that they are segmented from a larger dike at depth.

The central intrusion appears not to have strongly disturbed local sedimentary strata, which are flat-lying adjacent to the intrusion. Considerable amounts of the host rock have, however, slumped into the throat of the intrusion along the margins. Locally the breccias consists in large part of comminuted sedimentary materials with rare fragmens of minette and biotite mixed within the breccia. As is common with breccias within many volcanic necks, the mixture of comminuted sandy host material and juvenile material is some extreme in the breccias that it is often difficult to distinguish sedimentary and volcanic materials on a hand specimen basis.

Near the top and along some of the margins there is distinct inward-slumping and inward-dipping stratification of the tuff breccia. This configuration is typical of the structure seen the smaller Rio Puerco volcanc necks in the much younger Mount Taylor volcanic field farther east. It is largely on the basis of the inward-dipping stratification, that the current level of exposure is interpreted to be within several hundred meters below the original crater. Additional evidence is the observed maximum thickness of the Tertiary Chuska Sandstone In addition to sedimentary materials, some rounded cobbles of crystalline basement rocks occur within the tuff breccia. And some breccia bodies consist almost entirely of comminuted host rocks. Calcite and calcite veins are common throughout many of the breccia masses.

View west toward the southern end of the southernmost radiating dike. Arrows indicate windows in the dike.

• Appledorn, C.R., and H.W. Wright,Jr., Volcanic structures in the Chuska Mountains, Navajo Reservation, Arizona-New Mexico. Geol. Soc. America Bulletin, 68, 445-467, 1957.
  
  
• Beaumont, E.C., Preliminary geologic map of the Ship Rock and Hogback quadrangles, San Juan County, New Mexico. U. S. geol. Survey Coal Investigation Map C-29, scale 1:48,000, 1955.
  
  
• Ehrenberg, S.N., Petrology of potassic volcanic rocks and ultramafic xenoliths from the Navajo volcanic field, New Mexico and Arizona. Los Angeles, Univ. California. Ph.D. thesis, 259p., 1978.
  
  
• Gregory, H.E., Geology of the Navajo country. U. S. geol. Survey Professional Paper 93, 161p., 1917.
  
  
• Naeser, C.W., Geochronology of the Navajo-Hopi diatremes, Four Corners area [Colorado]. Jour. Geophys. Res., 76, 4978-4985, 1971.
  
  
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• Williams, H., Plocene volcanoes of the Navajo-Hopi country. Geol. Soc. America Bulletin, 47, 111-171, 1936.
  
  

• Delaney, P. T., and D. Pollard, Deformation of host rocks and flow of magma during growth of minette dikes and breccia-bearing intrusions near Ship Rock, New Mexico. U. S. Geol. Survey Professional Paper 1201, 61p., 1981.