The Gneiss is “nice” in Colorado

12 Sep

As a geologist living and working in the Front Range of Colorado’s Rocky Mountains for more than 17 years, I encountered a lot of interesting lithologies (“types of rock” in geo-jargon), but none were as photogenic as that displayed in an extraordinary outcrop (that is, where bedrock is exposed at the surface, where it “crops out”) in an obscure road-cut between Morrison and Aspen Park, Colorado.

Close-up shot of gneissic banding

Close-up of the intricate gneissic banding typical of this outcrop.
(Click on image to see a larger version.)

The Photos and the Locality

I found this unusual outcrop (see next photograph) one morning while riding my motorcycle on one of the many backroads that head westward out of Denver’s suburbs and into the heart of the Front Range. The road  traverses Precambrian Era metamorphic rocks (1.7 billion years) of the Early Proterozoic Gneiss Complex, a suite of lithologies seen throughout the Rocky Mountains in Colorado.

wide-angle view of the gneiss outcrop near Aspen Park

Wide-angle view of the gneiss outcrop near Aspen Park.
(Click on image to see a larger version.)

These rocks crop out all over the Front Range, but it’s unusual to see them in “fresh” (i.e., unweathered) outcrops such as this one.  In addition to their mint condition, the chaotic, seemingly random disconnected gneissic banding adds to the uniqueness of this outcrop. More about this (as well as some much-needed definitions of terminology) after the photographs.

Note the scale of the exposed rock surface in the next photographs. The number and complexity of the folding of the light and dark bands constrained within a relatively small surface area attests to the power of the compressional forces of metamorphism at work within the rock mass.

Portion of the gneiss outcrop

100 square-foot portion of the gneiss outcrop near Aspen Park.
(Click on the image to see a larger version.)

Gneissic banding and folding

Intricate small-scale gneissic banding and folding characteristic of regional metamorphism.
(Click on image to see a larger version.)

Close-up of small-scale folds in gneiss

Close-up of small-scale folds in gneiss (see previous photo).
(Click on image to see a larger version.)

Small-scale folds within larger-scale folds

Small-scale folds contained within larger-scale folds of gneissic banding.
(Click on image to see a larger version.)

I increased the contrast and saturation (using Photoshop) in the next photo to show in detail the hundreds of thin bands of light and dark minerals confined within a square foot of exposed rock surface.

Complex folding within minute gneissic banding

Complex folding within hundreds of minute gneissic bands.
(Click on image to see a larger version.)

The Rocks: What is Gneiss?

Granitic gneiss (pronounced “nice”), a metamorphic rock, is the dominant lithology of the Early Proterozoic Gneiss Complex of the Front Range. It was formed by high-grade regional metamorphism, a process whereby the pre-existing igneous rocks (granite, in this case) are subjected to high temperature and pressure conditions. This high temperature/pressure regime results in “metamorphic differentiation”, whereby chemical reactions within the rock mass cause segregation of light-colored minerals from dark-colored minerals (as evidenced by the characteristic gneissic banding of the rocks in my photos). This process currently is not fully understood, hence the vague description herein. Folding of the bands is caused by intense but very gradual compression of the rock mass well after mineral segregation has begun.

Brief Geologic History of the Area

Colorado’s geologic story begins about 2.5 billion years ago, after the oldest rocks (known as “cratons”) of the North American tectonic plate had already been deposited. The rocks of the Wyoming Province, the closest of these cratons, underlie what is today Wyoming, parts of Utah, Idaho, and Montana, and the extreme northwestern corner of Colorado. Three sets of volcanic island arcs (similar to the arc-shaped island chains seen in modern-day Indonesia) were accreted to the Wyoming Province as a result of three episodes of westward subduction of another tectonic plate beneath the craton. The second collision (1.7 to 1.6 billion years ago) is represented by the large igneous intrusive bodies of the Routt Plutonic Suite, which included the rocks seen in the outcrop I photographed.

4 Responses to “The Gneiss is “nice” in Colorado”

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  4. Snowball Solar System at 12:01 PM #

    Neptunism Reexamined:
    Mantled Gneiss Dome Problem, Migmatite Felsic/Mafic Differentiation and Metamorphic Folding:

    An alternative hypothesis suggests mantled gneiss domes are aqueously-differentiated 100 km and larger TNOs (trans-Neptunian objects) whose geochronology dates to the spiral in merger of their binary pairs in the inner Oort cloud (IOC).

    Dwarf planets typically arrive in the inner solar system in the form of larger core accretions of numerous TNOs and comets as they spiral in along the ecliptic due to galactic or other perturbations which erode their angular momentum, but they only have a good chance of catching up with the two inner solar-system planets with the most circular obits, Venus and Earth, since the almost-perfect hemicircular perihelia of objects in long-period orbits potentially overlap along a sizeable portion of Earth’s and Venus’ circular orbits, whereas, the more-eccentric orbits of Mercury and Mars at best only converge at two points.

    The K-Pg extinction event, 66 Ma, may have contributed the continental rock of far-East Russia, Alaska (minus the North Slope and the mountainous semicircle below Denali National Park with its contingent of terrestrial dinosaur fossil footprints washed down the Yukon River) and the balance of the North American Cordillera, complete with dwarf-planet aquatic fossils of the Burgess Shale. Cretaceous granite plutons and batholiths of the K-Pg dwarf planet core (granite plutons: aqueously-differentiated comet sedimentary cores followed by plutonic melting) as the perihelion of the dwarf planet spiraled in through the the densest concentration of comets at the inner edge of the IOC

    Felsic leucosome layers precipitate near the cold ice-water ceiling in core TNO salt-water oceans (eventually becoming ‘water logged’ and sinking onto the mafic core in leucosome layers) while mafic melanosome layers precipitate directly over the warmer growing sedimentary core, where the authigenic mineral-grain size is dependent on buoyancy of the micro-gravity oceans.

    ‘Circumferential folding’ occurs during diagenesis, as the core shrinks by expelling hydrothermal fluids, dramatically reducing the circumference and volume of the sedimentary core. Authigenic sandstone/quartzite, carbonate rock and schist mantling rock precipitates from the hydrothermal fluids, and thus the layering in the migmatite core and (mantled) gneiss dome covering rock is merely the result of sedimentary growth rings. Circumferential folding does not occur during diagenesis on Earth since Earth has such an enormous diameter, and thus the horizontal terrestrial sedimentary layers of the Grand Canyon following the Cambrian Period are not folded, but the tilted extraterrestrial layers of the Grand Canyon Supergroup exhibit circumferential folding.

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