Sunrise at Ship Rock: Part II - A Desert Landmark and Textbook Example of a Volcanic Neck with Radiating Dikes

 

On our geology-based excursion to the Colorado Rockies, Wayne Ranney and I took what was intended to be a quick shortcut through the northwestern corner of New Mexico from Flagstaff, Arizona. But upon seeing Ship Rock in the distance, we succumbed to its allure, and ended up experiencing the monolith both at sunset and sunrise on the following day.

My previous post "Ship Rock at Sunset" discusses our exploration of the south side and its elongate South dike. This post, "Sunrise at Ship Rock" investigates the geological surprises we discovered on its north side. For your convenience, here's a link to "yesterday's" post: http://written-in-stone-seen-through-my-lens.blogspot.com/2012_08_01_archive.html.    

Ship Rock gives the impression of having been volcanically thrust out from the sands of the Mancos desert, but this is not the case. Ship Rock is indeed a volcano but of a class called a diatreme, having formed explosively from gas-charged magma escaping at great velocity. It possessed a crater at the surface called a maar, but erosion has long since removed it and much of the sedimentary strata through which it erupted. What we see is the solidified plumbing that remains called a neck and its system of magma-radiating conduits called dikes. Thus, Ship Rock is a partially-exhumed and erosionally-sculpted diatreme. This is nicely portrayed in the following diagram. 

Ship Rock’s ancient crater and surrounding landscape are superimposed on the present-day neck and dikes

(Modified from oak.ucc.nau.edu/wittke/GLG101/5.pdf)  

Ship Rock resembles its “biligaana” namesake (Navajo for white man) of an enormous nineteenth-century clipper ship. With the neck coming to life in the vibrant colors of a New Mexican sunrise, two or three dike-remnants standout on its east profile. Ship Rock is largely composed of minette tuff-breccia, whereas the dikes are composed of hypabyssal minette.

The dark minette rock of the dikes cutting through the lighter tuff-breccia suggests that the dikes here were emplaced after the major eruption of the diatreme. The low-angled sun highlights the vertical cooling cracks in the magma and its irregular columnar jointing. Such surface lineations and morphological character are macroscopic indicators of magma-flow direction.

This lunar-esque photo was taken from the uplifted bedrock and apron of erosional debris that surrounds the base of Ship Rock. We’re facing southwest toward the eastern flank of the South dike that we explored on the previous day, one of three that radiate from Ship Rock (in addition to four minor dikes that do the same). The large boulders eroded from the diatreme’s neck and have come to rest on the desert’s bedrock of Late Cretaceous Mancos Shale. Deformation of strata during the emplacement of a diatreme may swell or even collapse the host rock.

Basking in solar warmth, this lizard displayed a wonderfully “tuff-brecciated” camouflage. Notice the small, varicolored, brecciated-xenoliths entrained within the matrix of Ship Rock's minette during its emplacement!

The entire Ship Rock volcanic complex emplaced between 28 and 19 million years ago during the Late Oligocene to Early Miocene. Its maar-crater is estimated to have been located 3,250 feet above the present day land surface of the Mancos Shale. That means that Ship Rock erupted through most if not all of the Western Interior Seaway’s sandstones and shales and even some Early overlying Tertiary sandstones. The tectonic forces that created the uplift of the Colorado Plateau were likely responsible for the diatreme’s emplacement within the Navajo Volcanic Field and its subsequent exhumation and erosion.

Ship Rock is on fire once again!

Found at the base of Ship Rock, this igneous rock appears to be an aplite, a fine-grained, light-colored granite, an intrusive rock in which quartz and feldspar are the dominant minerals. They often traverse granitic bodies as dikes and are the last part of magma to crystallize. It was brought to the surface from great depth as a xenolith and has since weathered out of its entombing matrix of minette.  

  

This southwest-facing Google earth image of Ship Rock shows three major and assorted minor dikes which form a radial pattern around the diatreme and thought to merge at depth. Yesterday’s post investigated Ship Rock’s south side and South dike, and this morning we are on the northeast side. You can make out the dirt road that we followed in the lower left corner. On the notheast side, we encountered a half-dozen cluster of small minette and breccia-bearing plugs and partially buried subsidiary dikes connecting them. The breccia is a mix of minette mixed with fractured and comminuted material derived from the host rocks during emplacement.

Still aglow at sunrise, a remnant wall of bedded Mancos Shale bridges the gap between Ship Rock’s base on the left and a plug on the right, similar to the Mancos-preservation on the South Dike we saw the previous day.

Here are two more dark, knobby minette-plugs with their bases partly buried in talus that drape over dikes in the subsurface. The plugs are circular conduits thought to form subsequent to dike emplacement. Plugs have the potential to lead upward and become volcanic necks. All the intrusions that surround Ship Rock are marked by the presence of breccias that contain the major components of breccia, shale and sandstone with minor cobbles of crystalline basement rocks.

With the warm colors of the rising sun depleted, we’re looking back at a small plug against the backdrop of Ship Rock’s shear east face.

Another plug and buried dike.

Seen fully from the northeast, the plugs and dikes have come into view. The host bedrock rock remains the Mancos Shale. Notice Ship Rock’s tall “sand castle” spires.

We eventually reached Ship Rock’s 2 ½ mile-long Northeast dike. Standing on its crest highlighted its curvilinear shape, offset dike segments and staggered-path of emplacement. Numerous studies have confirmed what has been previously suspected, that dikes such as this were emplaced above the present-day land surface, that much of the dike has since eroded to the state we currently see and that minor dikes are shallowly-rooted.

The dike reminded me of a well-constructed, hand-hewn, old New England stone wall with rocks that fit precisely together. Obviously, the magma acquired this appearance after having been injected through the strata and subsequently fractured in situ. If interested, I wrote a post on the geology of New England’s stone walls at http://written-in-stone-seen-through-my-lens.blogspot.com/2011/01/writing-on-walls.html.



Making a rather noisy commotion, four inquisitive peregrine falcons descended from their lofty roost on Ship Rock when they saw us. Clearly concerned about our presence, they watched our every move from their perches on nearby boulders, undoubtedly intent on protecting their domain high atop the citadel. They remained totally transfixed until we drove away.

While two sentinels stood guard from a distance (above), two more shared a boulder nearby (below).

Peregrines are the fastest member of the animal kingdom reaching over 200 miles an hour in a high speed dive. They are bird-eating raptors which explains all the bird bones I found at the base of one of Ship Rock’s spires. Probably the peregrines hunted for waterfowl in the San Juan River wetlands 10-15 miles to the north.

The Dine’ or Navajo people call Ship Rock “Tse’ bit’a’i” (TSEH-bit-ahi) which means “rock with wings” in reference to its radial dikes. They hold Ship Rock to be sacred with great religious and mythological significance. Navajo teachings believe that geologic features are the result of interactions between the Earth and Sky. When you think about it, I suppose it’s not far from the truth.

With the clouds, sky, sun and moon playing on its surface, Ship Rock’s colors and temperament constantly changed. Its haunting beauty was overwhelming. It was difficult to take your eyes from it and even harder to leave. My visit was an unforgettable experience.



Ship Rock at Sunset: Part I - A Partially-Exhumed, Erosion-Sculpted Diatreme and Sacred Monolith of the Navajo People

On our drive from Arizona in July we planned a quick swing through northwestern New Mexico on our way to the Colorado Rockies. Upon seeing Ship Rock from a distance, its allure became too great to ignore. Overcome by its grandeur, we changed our itinerary to fully experience the stone edifice. It’s hard to believe that the ghostly-white Ship Rock in my masthead above is the same structure seen below. “Spectacular” falls short of describing it!

The masthead photo across the top of my blog was taken a few years ago from Buffalo Pass in the Chuska Mountains on the Arizona-New Mexico border when my good friend, geologist Wayne Ranney and I looked down on Ship Rock to the east. That trip also resulted in my a post on Ship Rock which I invite you to visit at: http://written-in-stone-seen-through-my-lens.blogspot.com/2011/01/ship-rock.html.

Taken with a long lens in the steamy haze of late day, this is how we saw the stone monolith from ten miles to the southeast in New Mexico. This time the Chuska’s were on the horizon with Ship Rock in the foreground, the reverse of my masthead. Note the long dike to the south (left), and a smaller dike and three volcanic plugs off to the north. 

DECEPTIVELY LARGE
Ship Rock projects upward 1,583 feet from its base at an elevation of 5,494 feet. My fanciful juxtaposition provides a helpful perspective into its height which, as a solitary structure, somehow seems deceptively smaller. The Empire State Building’s spire tops out only at 1,545 feet.

THE NAVAJO VOLCANIC FIELD
Ship Rock erupted on the Navajo Volcanic Field of over 30,000 square kilometers in the Four Corners region of extreme northwestern New Mexico. Emplaced during the Late Oligocene to Early Miocene (28 to 19 Ma), the field contains over 100 diatremes with related dikes and plugs, and also preserves various flows and sills.



These are the volcaniforms of the central Navajo Volcanic Field: Minettes (dark circles); microbreccias (open triangles); AP, Agathla Peak; CRM, Comb Ridge Monocline; East Defiance Monocline; ME, Mule Ear; MHM, Mesaverde Hogback Monocline. The Laramide uplifts and monoclines (heavy lines) are labeled with Ship Rock (SR in red) situated on the Four Corners Platform.
(Modified from Smith and Levy, 1976 and McGetchin, 1977)

LARAMIDE LEGACIES
Diatremes are not unique to the Navajo Volcanic Field. Although diatremes on the Navajo field aren't visibly situated along exposed faults, as elsewhere on the Colorado Plateau, they are likely associated with NE-SW Laramide-age faults at depth that represent re-activated Precambrian fractures. These rents in the crust probably facilitated the ascent of magma in a tectonic regime that converted compression to extension fueled by changes in oceanic Farallon Plate subduction-geometries beneath the continental North American Plate. The result is potassium-rich mafic dikes and explosive diatreme volcanoes that erupted along the ancient faults and dot the Navajo Volcanic Field today.

On the map above and the cross-section below, Ship Rock and the Navajo Volcanic Field are situated on the physiographic province called the Four Corners Platform of the Colorado Plateau. The platform is bounded by the Laramide-age San Juan Basin to the east and the Defiance Uplift to the west. The northeast-trending Hogback monocline bounds the Four Corners Platform 25 km east of Ship Rock, while the north-south trending Mitten Rock monocline lies 30 km to the west. Created by Laramide compression, these uplifts and their monoclines are of Late Cretaceous and Early Tertiary age.  

A geologic cross-section through northwestern New Mexico.
(Modified from wrri.nmsu.edu/publish/watcon/proc41/Semken.pdf)

MEGA GEO-FART

Ship Rock (36°41’16”N and 108°50’12”W) is a diatreme, a volcanic vent or pipe that formed explosively from gas-charged magma escaping at great velocity. Geologists call it a “mantle blowout”, while Donald Baars graphically called it a “mega geo-fart.” The eruption is hydrovolcanic or phreatomagmatic, since the upward flow of magma encountered groundwater and heated it into steam under confining pressure.

 

ORIGINS AND EVOLUTION

Various emplacement mechanisms have been proposed by geologists, the events of which vary both temporally and sequentially. One model invokes the diatreme's minette magma, on its explosive journey to the surface, entraining shards of surrounding basement rock and overlying sedimentary layers. Additionally, xenoliths, the bodies of Proterozoic to Cretaceous host rock, were incorporated into the molten magma during its emplacement. Many of these “foreign rocks” are derived from crustal and upper-mantle sources that confirm the source rocks of Ship Rock are derived from Precambrian basement structures at great depth.

 

Near the surface, the minette contacted ground water or an aquifer which gas-charged the molten slurry. At the surface, a funnel-shaped crater called a maar blasted out ejecting ash and debris on the landscape. The chaotic mixture solidified into the magma and wallrock that cores Ship Rock.

Illustration of a diatreme such as Ship Rock with vertical-cone geometry
(Modified from cas.muohio.edu/~rakovajf/WTTW%20Diatreme.pdf)
 

 

Tertiary uplift and subsequent dissection of the Colorado Plateau removed Ship Rock's crater and moderately exhumed the diatreme from the weaker shales, sandstones and mudstones of the surrounding plateau. Erosion has sculpted the remnant landform, the diatreme's plumbing, into the monolith we see today towering over the desert-plain. It's estimated that the crater of Ship Rock was 3,250 feet above the present-day land surface!

DIATREME GEO-CHEMISTRY
Ship Rock is composed of two rock types: a minette and a tuff breccia (specifically a "serpentized ultramafic microbreccia"). Minette, an older term but still used, is a lamprophyre, a dark igneous rock lacking plagioclase that's common in dikes, laccoliths, stocks and small volcanic intrusions. They are greenish-gray to black in color with a low silica-content. Minettes erupted at high temperatures from a source deep in the mantle accounting for their high potassium content (ultrapotassic). In contrast, the olive-green to brown breccia, being a solid-gas mixture, is thought to have been generated during intrusion of minette magma into cooler, hydrated mantle at lower temperatures. Both rock types are xenolith-bearing as we shall see.

GEO-MORPHOLOGY
The geometry of the volcanic neck-radial dike system on the landscape is exquisitely revealed from the air. Ship Rock has three major dikes of mafic minette known as the South, West and Northeast Dikes, and four minor dikes. They are similar in composition being minette, are of varying length, demonstrate varying degrees of exhumation and project outward from the neck roughly at 120°. The overall divergence pattern suggests that the dikes merge at depth, and the consensus is that the root zone remains deeply buried.

This dramatic southeast aerial view captures Ship Rock and its dike system at sunset.
(Photo courtesy of Alex S. MacLean, Landslides Aerial Photography www.Landslides.com and www.AlexMacLean.com)

This view looks northeast from Red Rock Highway (Indian Service Route 13) which passes through a break in the South dike. Ship Rock is hiding at the left near the dike’s beginning about four miles away. The heavily-eroded dike has an en echelon, segmented appearance which gives it a faulted look. It runs almost 6 miles to the south before either terminating or diving into the bedrock. Subsurface imaging has confirmed that much of the dike remains buried beneath the outcropped portion.

 

Many aspects of diatremes remain unclear particularly timing and emplacement sequencing. Debated in particular is whether radial dikes form after diatreme emplacement or whether ascending magma first propagates through dikes, the central pipes of which become the diatreme. The orientation and spatial association of the dikes and plugs represent discrete events during their formation. Although the south dike radiates from Ship Rock’s base, notice that a linear projection does not directly point to it. You can also see this on the aerial photo above.

In some regions along the surfaces of dike margins linear to curvilinear grooves called slickenlines indicate the initial direction of magma flow. Typically, hot slickenlines are shallowly inclined toward the diatreme. Similar lines are found on the diatreme’s neck indicative of vertical flow during the ascent of tuff breccia. Notice the broad flanks of erosional debris extending from the dike.

The South dike has the appearance of a long, fortress-wall (below) and is not perfectly linear but has slight curves and offsets, and in places is discontinuous. Notice the undulation of the solidified magma stream of minette. At the time of emplacement, the dike was confined by sedimentary host rocks, here, the flat-lying Late Cretaceous Mancos Shale. The Mancos (pronounced MAN-cuss "like a cowboy") is a marine mudstone deposited some 60 million years ago during the first transgression of the Western Interior Seaway. Below it is the seaway's nearshore Dakota Sandstone and above it, the seaway’s Mesaverde Group, which in the region of Ship Rock has been completely unroofed by erosion.

Dike emplacement is governed by many factors such as regional host rock density, stress levels and orientations, magma pressurization, temperature, gas content and exosolution patterns. Although dike-emplacement can intersect the surface if shallow enough and result in extrusion of magma from a vent, this is not the case at Ship Rock where emplacement occurred at depth (estimated at over 1,000 m) with solidification well-below the subsurface. Exposure only occurred after erosional exhumation. Studies of Ship Rock's dike system have proven invaluable in the emerging science of planetary geology such as on the giant dike systems of Mars.

The stagnant, muddy environment during the deposition of the Mancos Shale contained a surprisingly rich marine fauna of the Western Interior Seaway. I stumbled on this tightly-coiled, fossil marine gastropod (sea snail) of the Genus Turritella while walking along the south dike near the base of Ship Rock, 500 miles from the nearest present-day ocean!

Some segments of the Mancos that are in contact with the dike have the original stratification preserved and appear to encase the dike. Proximity to the dike has sheltered these sections from erosion. From a distance, I suspected that the dike might have regionally metamorphosed these sections of Mancos thereby conferring a resistance to erosion. This theory was proposed for resistant sections of Navajo Sandstone in proximity to the Mule Ear diatreme at Comb Ridge, but I saw no metamorphic alteration in the Mancos on close inspection.

This close-up of the South dike exhibits its heavily-eroded superior surface and its fenestrated breadth. One can sense the direction of flow lineation, tremendous horizontal injection-energy and turbulence of magma flow where minette appears in elongated, parallel vesicles as if tubular. Portions of the dike also exhibited bulging at the expense of the host rock in order to accept a greater volume of magma. It's dynamic geology frozen in time!

With afternoon shadows beginning to grow, the south face displays its eroded and fractured surface with large patches of dark minette cutting through lighter-colored tuff breccia. Notice the uplifted bedrock at its base and the scattered talus of boulders. The minette does not contain vesicles indicative of gas content, implying that the magma was gas-poor and likely cooled underground. Ship Rock is composed of tuff breccia at mid-diatreme and shot through at the base with small branching dikes. I'm standing just west of the radial South dike's crest. The dirt road on which we travelled passes through the visible break in the dike. Seen previously, the strike of the dike is offset from the direction of the neck. The beginning of the West dike is seen at the far left.

As previously mentioned, both the minette of Ship Rock's neck and dikes contain xenoliths of diverse geochemical populations. They provide clues to the diatreme's depth of origin and information about the otherwise inaccessible mantle and crust such as the location of major lithospheric boundaries and their tectonic histories. These magmas were the only ones that penetrated the Colorado Plateau at the end of the Laramide Orogeny around 30 million years ago when border magmatism was voluminous. The subject of ultramafic magma generation on the Colorado Plateau is complex and likely related to the angle of Farallon Plate subduction beneath North America (amongst other theories) and fractional crystallization.

Inclusions also are good indicators of magma flow direction when they become synonymously-oriented and imbricated. The xenolith to the right was embedded in the South dike and appears to be Precambrian granite derived from the mantle which itself formed over a billion years ago! Ship Rock's magma source emplaced through an amazing thickness of basement rocks…well over 20 miles. 

A few brecciated lithic-fragments of Mancos Shale were incorporated into the molten matrix of the dike. I spotted these nearest to the South dike’s connection with the base of Ship Rock, undoubtedly related to the high injection force and velocity of the magma when it hit the Mancos.

Xenoliths are also incorporated into the neck of Ship Rock, here a granitoid likely of Late Precambrian affinity. Could this be a derivative of Zoroaster Pegmatite such as found at the bottom of the Grand Canyon? What does that say about the depth of genesis of the diatreme?

 

 

This sedimentary xenolith, also from the neck, is a red, iron-rich, fine-grained sandstone likely of Mesozoic origin. The base of Ship Rock rests on predominantly clastic Mesozoic strata while beneath are largely Paleozoic calcareous rocks and at depth, a largely metamorphic Proterozoic basement. The diatreme likely emplaced through all these strata on its journey to the surface. Could this be a sandstone such as the Entrada, Moenkopi, De Chelly or Organ Rock?

The setting sun begins to cast a magical glow. In the foreground, the Mancos Shale slopes upward to meet the south dike with blocks of car-sized minette talus scattered about. Dikes are found in association with every diatreme within the Navajo Volcanic field. A small diatreme known as The Thumb (seen on my masthead above) has no dikes visible surficially, but they have been mapped via imaging of gravity anomalies within the subsurface volcanic system. Undoubtedly, they someday will be exhumed. An inspection of Ship Rock (here at its south face), indicates patches of dark minette rock cutting through the lighter tuff-breccia suggesting that the dikes here were emplaced after the major eruption of the diatreme.

Nightfall. Ship Rock seems illuminated by its own aura.

We returned to Ship Rock the following morning to experience sunrise. Please visit my next post to see what geological surprises we found on the north side! Also, check out Wayne Ranney's blog for his posts on our Colorado Rockies excursion at http://earthly-musings.blogspot.com.

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.