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.
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!
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.
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.
