Stately Ship Rock and the Navajo Volcanic Field

I took this photo of Ship Rock from Buffalo Pass 65 miles to the west, high in the Chuska Mountains

along the Arizona-New Mexico border. A fortuitous break in the clouds allowed the sun to illuminate the two black monoliths, reflecting back as an unearthly, metallic-white gleaming in the high desert. Typically, radial dikes can be seen to emanate outward from the base of Ship Rock on the desert floor. Dikes are magma-filled fractures in the Earth’s crust that serve as conduits for molten rock. There is also a smaller diatreme juxtaposed in the foreground called The Thumb.




Differential erosion has left Ship Rock and the Thumb towering above the surrounding plains. Interestingly, the Chuska Mountains are "held up" (and hence created) by a lava flow of the same geological ilk that created other volcanics within the region, but here, a trachybasalt which is the extrusive equivalent of minette (please read on). The resistant igneous rock prevented the Chuska's from eroding, while the neighboring regions of the plateau were unroofed. Seen in the photo, the pass was covered by the flow's underlying Chuska Sandstone. 

SHIP ROCK
Looking like a scene from a sci-fi movie, Ship Rock stands 1,583 feet above the high-desert plain in the northwest corner of New Mexico. It is the 30 million year old or so erosional remnant of the neck (or plug) of a volcano called a diatreme. Diatremes most likely form when rising magma in basic to ultrabasic volcanic fields creates a sudden gaseous explosion deep underground, when the magma comes in contact with subterranean water. The heated groundwater under pressure causes a hydro-volcanic (or phreatomagmatic) eruption, which results in the formation of a diatreme volcano. A series of explosive eruption excavates a shallow crater (or maar) at the surface, flanked by bedded pyroclastic ejecta. Over time, the diatremes have become exposed or exhumed by uplift of the Colorado Plateau. They typically survive erosion, since their composition is more resistant than the surrounding rock, which has worn away. This accounts for the Navajo (Dine') American Indian's reference to them as "black rocks protruding up" in their language.

Called Tsé Bitʼaʼí meaning "rock with wings", Ship Rock stands within the Navajo Tribal reservation and is of great religious and mythological significance to the tribe, being mentioned in many of their legends. According to one legend, after being transported from another place, the Navajos lived on the monolith, "coming down only to plant their fields and get water." One day, the peak was struck by lightning, obliterating the trail and leaving only a sheer cliff, and stranding the women and children on top to starve. The presence of people on the peak is forbidden "for fear they might stir up the chį́įdii (ghosts), or rob their corpses." Rock climbing on the monolith is strictly forbidden by the Navajos.

THE NAVAJO VOLCANIC FIELD
Ship Rock is one of over 80 late Oligocene to early Miocene-age (ca. 28-19 Ma) volcanoes and intrusive structures standing within the Navajo Volcanic Field. The Field covers roughly 20,000 square kilometers and is situated within a greater physiographic province called the Four Corners Platform, itself a member of the even greater Colorado Plateau. The platform includes the four corners states of Arizona, New Mexico, Colorado and Utah. The intrusive structures within the field include tuff pipes, dikes, intrusions and diatremes. Many of the diatremes are roughly clustered along Laramide-age monoclines such as the Comb Ridge, Defiance and Hogback monoclines. No faults are present at the surface, but magma ascent was probably facilitated by NE-SW trending Laramide fractures at depth.




Schematic diagram of the diatremes of the Navajo Volcanic Field clustered around the Four Corners region.

The letters identify diatremes: ME, Mules Ear; MR, Moses Rock; CV, Cane Valley; GR, Garnet Ridge;
RM, Red Mesa; GN, Green Knobs; BP, Buell Park; AG, Agathla; CR, Church Rock; SR, Ship Rock; MR, Mitten Rock;

RB, Roof Butte; WP, Washington Pass; FR, Fluted Rock; TB, Twin Buttes. Map from Roden and Smith (1979).

Map of the central Navajo Volcanic Field. Notice clustering of volcanic structures along Laramide-age monoclines.
 Dark circles indi­cate minettes; open triangles represent SUMs. Monoclines are indicated by heavy lines.
Abbreviations: AP, Agathla Peak; AZ, Arizona; BB, Boundary Butte; BP, Buell Park; CO, Colorado;
CRM, Comb Ridge Monocline; CV, Cane Valley; EDM, East Defiance Monocline; GN, Green Knobs;
GR, Garnet Ridge; ME, Mule Ear; MHM, Mesaverde Hogback Monocline, MR, Moses Rock;
NM, New Mexico; RM, Red Mesa; SR, Ship Rock; UT, Utah. Major communities are also shown. 
After Smith and Levy (1976) and McGetchin et al. (1977).

PETROLOGY OF THE ROCKS
Typical of the magma of many diatremes within the Navajo Volcanic Field, Ship Rock is composed of a tuff-breccia or technically, a serpentized ultramafic microbreccia (or SUM). In fact two principal rock types are found within the Navajo Volcanic Field, the other being a "minette" which is a greenish-gray, ultramafic, highly potassic, orthoclase biotite lamprophyre. After the intermediate, basic and ultrabasic scoria cones, monogenetic maar-diatreme volcanoes are actually the second most common volcano type on continents and islands. The majority of the maar-diatreme volcanoes represent the phreatomagmatic (referring to the interaction of water and magma) equivalent of the magmatic scoria cones and their associated lava flows.

ANATOMY OF A DIATREME
A diatreme volcano generally consists of a maar-crater at the surface with a tephra-ring surrounding the crater. The more-or-less cone-shaped diatreme underlies the maar, and the irregular-shaped root zone beneath that. At the very bottom enters the feeder dike, the source of the magma. In the photo of Ship Rock, the maar-crater has been removed during the process of exhumation that uplifted the surrounding plateau, resulting in the exposure of a portion of the previously-bedded diatreme.

From "Maar-Diatreme Volcanoes..." by Lorenz

The common chronological-thread amongst the many diatremes is end-Laramide emplacement in association with faults at depth, exhumation in association with post-eruptive uplift and dissection of the Colorado Plateau (the regional bedrock is eroded away in addition to the maar-crater which is composed of unconsolidated material) , and finally, erosive sculpting of the exposed necks. 

Diagram of a diatreme with its maar-crater illustrating its exhumed position (land surface today) 
as a volcanic neck following its erosive exposure.
Source unknown.

TECTONIC ORIGINS
The origin, unique chemistry and emplacement of the minettes is likely related to the underlying Farallon Plate, which during the Oligocene, increased its angle of subduction beneath the North American Plate at the end of Laramide-time. This possibly created a pulse of volcanic activity as the plate sank into hotter regions of the mantle and subsequently melted.The region of the Four Corners began to experience extensional forces. Volcanic activity also accompanied the opening of the Rio Grand rift as magmas penetrated the thinning, subsiding crust. Mafic minette-magma may have been derived from fractional crystallization within the upper mantle.

Baars (Colorado Plateau, 1972) theorizes that the most abundant volcanoes are found on the margins of the Colorado Plateau prior to its uplift. The plateau at that time, still a basin, received Rocky Mountain sediments causing it to futher depress. Baars goes on to explain the origin of the diatremes to have occurred largely at the folded-edges of the various basins as they sank.  

OTHER DIATREMES WITHIN THE NAVAJO VOLCANIC FIELD



Somewhat north of Monument Valley, is this small, possibly unnamed diatreme. Notice the remnants of its dike running off to the right projecting vertically through the strata that it dissected.

Agathla Peak, also known as El Capitan in Spanish (named by Kit Carson), stands over 1,500 feet south of Monument Valley. Situated in Arizona, it is about 85 miles west of Ship Rock, in New Mexico. It too is a diatreme within the Navajo Volcanic Field. Notice the Navajo dwellings in the foreground for scale.

Looking south-southeast from Goosenecks State Park near Mexican Hat, New Mexico

, this exhumed diatreme is called Alhambra Rock. Typical of other Navajo Volcanic Field diatremes, erosion has left the resistant neck standing as a lone sentinel. In the foreground, erosion has exposed the Pennsylvanian rocks of the Hermosa Group's Honaker Trail Formation within the Goosenecks of the San Juan River. Alhambra is framed against Comb Ridge in the distance, the eroded, upturned eastern limb of the Monument Upwarp.

From the crest of Comb Ridge looking south into Comb Wash and the San Juan River (11 miles to the east of Gooseneck State Park), the Mule Ear, a resistance flank of Comb Ridge, points skyward with the eroded remnants of the Mule Ear diatreme situated to the right in the strike valley. The diatreme is positioned within the eroded eastern flank of the Laramide-age Monument Upwarp. Interestingly, the Mule Ear's prominence above Comb Ridge, of identical sedimentary composition, may be related to it having been subjected to low grade metamorphism from the neighboring diatreme which conferred a resistance to erosion.

This is a closer view of the Mule Ear (left) and the Mule Ear diatreme (center and right) taken from the San Juan River as it heads south just before turning to the west and heading into Lime Ridge and the Raplee Anticline.

The effects of Farallon Plate subduction and its consumption beneath North America has been manifested with transformational faulting in California, extension through the Basin and Range Province in Nevada and Arizona, with volcanic activity within the fields of the Colorado Plateau, and extension further east into New Mexico with the graben of the Rio Grande rift and horst of the Sandia Mountains. The result of crustal thinning in the Four Corners has allowed the ascension of magma through the crust and the emplacement of the many diatremes within the Navajo Volcanic Field. The uplift and unroofing of the Colorado Plateau has exhumed the diatremes and exposed them to the effects of erosion.