2012 Geology Posts That Never Quite Made It

Puddingstones in Brookline, Massachusetts; Pleistocene Coral in the Bahamas; Dinosaurs Tracks in Connecticut; Monster Sea Scorpions in Upstate NY; Diatreme Volcanoes in New Mexico and Deadly Poisonous Mushrooms in Chestnut Hill, Massachusetts

Every blogger knows the challenge. What shall I blog about next? What photos should I use? By the time the end of the year rolls around, there are always a few posts that never quite made it. And so, with this final post of the year, here they are from here and there.

January

This massive, foot-long clast of Westboro Formation quartzite is embedded within an arkosic sandstone matrix of the Late Proterozoic Roxbury Conglomerate, one of two surficial rock units that comprise the Boston Basin. The Roxbury arrived in (better stated to have participated in the formation of) New England within the terrane of Avalonia, having rifted from the supercontinent of Gondwana in the middle latitudes of the southern hemisphere. Avalonia and its accompanying Roxbury made the tectonic journey across the closing Iapetus and Rheic seas during the Early to Middle Paleozoic. This puddingstone initiated my personal geological journey some twenty years ago.

Brookline, Massachusetts

February
A paper-thin veneer of new ice supports a bevy of gulls.
Chestnut Hill Reservoir, Newton, Massachusetts

March
Evidence for changing sea levels exists around the world including the Bahamas.
Low tide has exposed "shore rocks" along the island's north coast which are in reality
150,000 year old fossilized star, starlet and brain coral. This former patch reef was once covered by water considerably deeper during the last interglacial period. During the ensuing glacial period, the sea floor became exposed on land and covered by a limestone-derived soil. The crusty soil is eroding and can be seen on the coral, that is if you can take your eyes off the Caribbean's incredibly blue-green water.
Cable Beach, New Providence Island, Bahamas

March
This is a positive (upper member) cast of a portion of a trackway of a bipedal theropod in shallow-water, arkosic sandstones of the Lower Jurassic Portland Formation. This brownstone, the building stone that shaped America during the late 1800's, was deposited in an aborted rift basin called the Hartford Basin in response to the opening of the Atlantic Ocean. The foot-long footprint is likely that of a Dilophosaurus or Coelophysis, early carnivors of the Mesozoic. Not too far from here in South Hadley, Massachusetts, in 1802 a farm boy named Pliny Moody discovered the first trackway in North America. That was in the Deerfield Basin, a failed rift basin almost identical stratigraphically to the Hartford. The local preacher, seeing the print's three-toed anatomy, called it Noah's Raven, a prophetic analysis considering the evolutionary relationship between reptiles and birds.
Meehan Quarry, Hartford Basin of the Connecticut Valley, Portland, Connecticut

March
This hexagonal tholeiitic basalt, with its characteristic geometry of extremely regular polygonal joints, formed as a consequence of its cooling history. These erratics fractured from a colonade of the Lower Jurassic Holyoke Basalt Flow, the middle of three flood basalts that were generated in 1,200 miles of Mesozoic rift basins along the eastern margin of North America (and across the Atlantic as well) during early rifting of the Atlantic Ocean. This trap rock, as it's called colloquially, has its name derived from the Swedish word for stairs ("trappa") referring to the step-like pattern the extrusive igneous rock assumes once cooled and contracted. Interestingly, the generation of massive volumes of this flood basalt is cited as a possible cause of the Permo-Triassic extinction event.
Tilcon Trap Rock Quarry, North Branford, Connecticut

  

April
Preserved in the famous Bertie Waterlimes of Central New York, these are exoskeletal molts of Eurypterus remipes, also known as a "sea scorpion," a necessity of growth for all body- and limb-jointed arthropods. Classified as a chelicerate (along with spiders and horseshoe crabs) based on the morphology of its anterior appendages, it was a marine creature actually related to a similarly marine scorpion. Both plied the hypersaline seas that formed cratonward within the foreland basin of the Taconic Orogeny during the Late Silurian. Eurypterids went extinct at the end of the Paleozoic during the end Permian extinction along with up to 96% of marine species. Scorpions survived the Great Dying and now enjoy a terrestrial existence.
Bertie Waterlimes, Lang’s Quarry, Passage Gulf, Ilion, NY

May
I have been jogging around this reservoir for thirty-five years. It was constructed in 1870 to supply the fresh water demands of growing Boston and its environs but is now a haven of tranquility in the heart of the city. I’m continually astounded by the diversity of the wildlife that one finds here: geese, ducks, swans, gulls, hawks, falcons, turkeys, heron, egrets, fox, coyote, raccoons, muskrats, mice, snakes, frogs, fish, and the usual collection of squirrels, rabbits, dogs and humanoids. And it's decorated with fantastic ledges of the Roxbury Conglomerate!
Chestnut Hill Reservoir, Chestnut Hill, Massachusetts

May
...and even turtles.
Chestnut Hill Reservoir, Chestnut Hill, Massachusetts

June
It's the world's tallest freestanding stone structure, standing sentinel over our nation's capital since 1884. The Washington Monument is incredibly photogenic. It virtually begs to be photographed. The challenge is to capture it in a uniquely individual way. Architectural geology can be a lot of fun especially if you're familiar with the quarry of origination.  The obelisk's exterior is marble from Maryland, Texas and Massachusetts, while its interior backing is composed of sandstone and crystalline rocks (glassy intrusive igneous rocks) from Maryland. The Massachusett quarry is named the Lee Lime in my home state. Its carbonate rocks were part of a coastal shelf along the then, southern seaboard of the supercontinent of Rodinia over a billion years ago. They were subsequently metamorphosed into marble by the collisional events of the Taconic and Acadian orogenies during the Paleozoic. Knowing the geology seems to give greater depth (no pun intended) to any subject.
National Mall, Washington, District of Columbia 

July
My colleague and I, while traveling through northwestern New Mexico, spotted the stone edifice from a distance. Not intending to stop, we became overwhelmed by its mystical presence and stayed for a day. Unlike our conventional perception of volcanoes that exude lava and build up a conical, vertical structure, Ship Rock emplaced within the Earth's crust phreatomagmatically, gas-charging its magma when it hit the water table. Its maar-crater at the surface and over 3,000 feet of overburden have eroded away in the last 25 million years, give or take. That left the erosion-resistant diatreme as testimony to the fury, topping out at 1,583 feet. The wall-like linear structure off to the left is a radial dike, one of three major feeder-conduits that emanate from Ship Rock.
Ship Rock, San Juan County, New Mexico



 July

Between the San Juan Mountains on the west and the Sangre de Cristo Range on the east is an eight mile-long, 700 foot-high sand sea where you'd least expect it, in western Colorado. In fact, it's the tallest dune field in North America! Although its shifting sands rejuvenate with the whim of the wind, the erg remains in one place

in a perfect balance of sediment supply (from the only-true-desert-in-Colorado sands of the San Luis Valley), means of transport (wind and water) and accommodation space (embraced within the Sangre de Cristos). Although cast in the shadow of the late day sun, the dark color of the sand is due to quartz and the volcanic rocks of the San Juans. 

Wind-driven sand drifts up the windward slopes of the dunes and then cascades down the leeward slopes. The wind will sculpt the dunes until its windward side slopes gently and the leeward side is short and steep. Can you tell the direction of the prevailing wind?

  Great Sand Dunes National Park and Preserve, Colorado

July

I couldn't resist one more view.

Great Sand Dunes National Park and Preserve, Colorado

July
Volcanoes to the west in the Thirtynine Mile volcanic field and the Sawatch Range periodically filled the air 
with volcanic ash 35 million years ago. Carried by the wind, ash rained down on the region of ancient
Lake Florissant in Colorado, and along with mudflows, preserved a diverse Upper Eocene ecosystem of fish, insects, mammals and plant material. Silica derived from the ash, in a scenario remniscent of Pompeii, and its interaction
with planktonic blooms produced biofilms that retarded organic decomposition. Perhaps most remarkable
to be silicified are the VW-size tree stumps of Sequoia's, members of an ancient redwood forest
that blanketed the lake region. Notice the two, rusted ends of a saw embedded within the "Big Stump,"
a vestige of wanton and destructive fossil collecting in the late 1800's.
Florissant Fossil Beds National Monument, Florissant, Colorado

July

This amiable little fellow actually tried to sell me some auto insurance.

Florissant Fossil Beds National Monument, Florissant, Colorado

August
Minutes from Lake Placid in northern New York State, we're viewing the High Peaks Region
across a dry, pro-glacial lakebed drained by an active Holocene stream. Both formed 
after the retreat of the Laurentide Continental Ice Sheet at the end of the Pleistocene.
The bedrock throughout the region, unless buried below glacial erratics, till and outwash,
is Middle Proterozoic Grenville metanorthosite, final vestiges of the supercontinent of Rodinia.
North Elba, Adirondack State Park and Reserve, New York State

September

This over three-inch monster was spinning its web on my patio. Its the largest spider I've seen outside of the zoo. I've found the web-sheathed dens of tarantulas in the Grand Canyon but never any inhabitants. Taken at night, I illuminated the critter with a flash light to try and photograph its web.

August
For the second consecutive year, this brightly-colored, orange-yellow cluster of mushrooms arose from exactly the same location and at precisely the same time of year in my neighbor’s yard. They fruited on the stump of an aging Maple tree following a week of humid, soaking rains. Their scientific name is Omphalotus but are commonly known as the Jack O’Lantern mushroom. Under suitable conditions of day length, heat,
humidity and nutrition, spores in the soil germinate to produce hyphae. When hyphae of the opposite mating type meet (a romantic love affair made in the soil rather than in heaven), a fruitbody is produced, in this case a mushroom. Mushrooms possess the spore-shedding organs of a new generation. The mushroom and its spores is analogous to an apple and its seeds. The hidden mycelium beneath the soil is the "tree" (sort of). Mushrooms are fungi, nature’s morticians in the natural environment, beneficially biodegrading and nutrient-recycling. As we all know, not all of them are edible. These delectable-looking delicacies are deadly poisonous (as in difficulty breathing, drop in blood pressure, irregular heartbeat and respiratory failure). They also exhibit bioluminescence by glowing in the dark. I returned the following day to harvest a few and observe that peculiar property in a dark room, but my neighbor unfortunately excavated his crop before I could. Based on my calculations, next August there’ll be new specimens to collect. Lesson learned? Don't eat mushrooms that glow in the dark, and you never know what’s growing in your neighbor's yard.

November

Back in D.C. again, I couldn't resist one more shot of the Monument illuminated by the setting sun.

National Mall, Washington, District of Columbia

November

This was my very first try at High Dynamic Range (HDR) photography.
Taken at sunrise, the autumnal colors are totally natural.
This pond is in the heart of town next to a parking lot at the back of a shopping center.

Hammond Pond, Chestnut Hill, Massachusetts



 December

The last snow storm of 2012 was a mild nor'easter in Boston. It gets its name from the direction the wind is coming from. Regardless of the site of origin of the storm, the nor'easter has a low pressure area whose center of rotation is just off the east coast of New England and Atlantic Canada. Its counter-clockwise rotation produces leading winds in the left-forward quadrant onto land from the northeast. That usually translates into heavy snow or rain depending on the time of the year along with high winds, pounding surf and coastal flooding. By the way, "down east" refers to coastal New England and has its origins as a Maine term for sailing down wind to the east. Can you tell which direction is northeast from the accumulation of snow on the trees?
Chestnut Hill, Massachusetts

 That's it for 2012. Happy New Year!

From Doctor Jack (and Franklin the Border Collie)

A Shiprock-Monument Valley Geological Juxtaposition

 

I was surfing the web this morning and somehow ended up in YouTube, the universe’s online repository for all things video. I stumbled on a trailer for the upcoming movie of the Lone Ranger set for a 2013 release. Check out the image that appears at about 9 seconds.

Notice anything strange about this photo capture? It’s the diatreme of Shiprock in New Mexico sharing the Colorado Plateau with the buttes of Monument Valley on the Arizona-Utah line. They almost look like they belong together.

Only in Hollywood!

Here’s the link to the trailer: http://www.youtube.com/watch?v=nlrQD8Kvk6M

For a bigger thrill (for all you Baby Boomer’s out there), here’s the original 1950's intro of the Lone Ranger: http://www.youtube.com/watch?v=tXRjuaEVK78  

Want to learn more about Shiprock, go here: http://written-in-stone-seen-through-my-lens.blogspot.com/2012_08_01_archive.html.

How about Monument Valley and the Gouldings, go here: http://written-in-stone-seen-through-my-lens.blogspot.com/2011/01/hollywoods-monument-valley.html.

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.