Rock pedestal with exfoliating slabs in the background. Enchanted Rock, Texas. This granite is dated at 1.1 billion years (1.1Ga), likely the product of tectonic convergence between Laurentia (North America)and Gonawaland (South America+Africa), resulting in the super continent, Rondinia. The rock pedestal is the result of differential erosion: the base experiences greater chemical weathering than the upper part due to standing water at the base. Case-hardening on the surface of the top of the pedestal helps resist weathering. Exfoliation is explained as the reaction to the removal of overlying pressure (that is, rocks removed by erosion). Rocks exposed to such removal of overburden experience expansion: the result is exfoliation sheets. At Enchanted Rocks, one can see the exfoliated sheets sliding downslope. <br>

lin murphy > climbing geology

Rock pedestal with exfoliating slabs in the background. Enchanted Rock, Texas. This granite is dated at 1.1 billion years (1.1Ga), likely the product of tectonic convergence between Laurentia (North America)and Gonawaland (South America+Africa), resulting in the super continent, Rondinia. The rock pedestal is the result of differential erosion: the base experiences greater chemical weathering than the upper part due to standing water at the base. Case-hardening on the surface of the top of the pedestal helps resist weathering. Exfoliation is explained as the reaction to the removal of overlying pressure (that is, rocks removed by erosion). Rocks exposed to such removal of overburden experience expansion: the result is exfoliation sheets. At Enchanted Rocks, one can see the exfoliated sheets sliding downslope.<br>

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Fountain Formation in El Dorado Canyon, CO. This photo was taken through a polarizing petrographic microscope. The large bent crystal in the middle is mica, derived from Precambrian rock that cores the Rocky Mountains. The other grains include quartz and feldspar.

Chickenhead belay. 3rd pitch, Moby Dick. West Stronghold, AZ. These "heads" are the remnants of a once-larger area of case hardening. Nearby photos show other stages of case-hardening and weathering processes on this rock.

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lin murphy

Feb 21, 2009

“Rock pedestal with exfoliating slabs in the background. Enchanted Rock, Texas. This granite is dated at 1.1 billion years (1.1Ga), likely the product of tectonic convergence between Laurentia (North America)and Gonawaland (South America+Africa), resulting in the super continent, Rondinia. The rock pedestal is the result of differential erosion: the base experiences greater chemical weathering than the upper part due to standing water at the base. Case-hardening on the surface of the top of the pedestal helps resist weathering. Exfoliation is explained as the reaction to the removal of overlying pressure (that is, rocks removed by erosion). Rocks exposed to such removal of overburden experience expansion: the result is exfoliation sheets. At Enchanted Rocks, one can see the exfoliated sheets sliding downslope.
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Nearby Photos

Cavernous erosion in granite, Sardinia. Another name for this weathering is "tafoni," a Corsican word. Corsica is in sight from this climbing area. Tafoni is also common in sandstone, for example at Castle Rocks, south of San Francisco.

Crack in San Francisco street after Loma Prieta Earthquake. This apparent "climbing opportunity" was filled in by the streets department.

Mid-Atlantic rift. Iceland straddles the rift between the North American and European plates. As the volcanoes along the rift erupt more lava, Iceland grows wider.

Turtle Rock. Tiburon Peninsula, CA. This block, and nearby Split Rock, stand out of their soft serpentinite rock surroundings (Franciscan Complex). Called a "knocker" by some geologists, it is a high-pressure, low-temperature metamorphic rock, interpreted to have been formed in a subduction zone, where ocean crust of the proto-Pacific plate dived under the North American plate. Turtle and Split rocks are dated at ~ 150 million years, and are composed of the minerals glaucophane, lawsonite, jadeite, and chlorite. I have spent many wonderful times after work in SF, bouldering on this rock. California poppies grow in magnesium- and iron-rich soils, which are inhospitable to many plants.

Faulting in California. View from White Mountains (Nevada/CA) across Owens River Valley to Sierra Nevada. The Sierra Nevada rises abruptly from a normal fault on its east. The faulting is a result of the movement of the NW trending San Andreas fault near coastal California.

Weathering in the Colorado Mineral Belt near Boulder. These rocks have been chemically changed by the circulation of hot fluids rising up along very old faults. While hydrothermal alteration has basically destroyed these rocks from a climbing point of view, other hot-fluid alteration (Adularia:potassium feldspar) accounts for the hard sandstone of the Fountain Formation in El Dorado Canyon. See scanning electron microscope pic of this adularia, infra. Fountain Formation rocks that are not near the Colorado Mineral Belt are poorly cemented and crumbly.

angular unconformity. Queen's Creek, AZ.<br>
paleozoic (?) limestone layers angle SW to NE. Originally deposited horizontally, the limestone layers have been faulted to SW/NE position. Volcanic rocks subsequently were deposited.

$Dihedrals caused by fractures and vertical uplift along fault during Laramide Orogeny. Cadillac Crag, Eldorado Canyon, CO. Fountain Formation here is well cemented by adularia cement because here the rock lies on the Colorado Mineral Belt.$

Needles, SD. These pinnacles are pegmatites, possibly coarse grained off-shoots of the 1.7+ billion year old Harney Peak Granite, which were injected into basement rock during the Trans-Hudson Orogeny. The basement rock has been eroded (away),leaving the pegmatites as pinnacles. One of the pegmatite minerals is mica, which appears as a squarish "book," of many sheets. I recall that the entire mica foothold bent as I stepped on it. See for comparison, the pegmatite spires of the Monastery, CO.

Cuernos del Paine, Chile. The black tops of these towers are Cretaceous shales, intruded by Miocene-age granitic rocks. Old rocks overlying younger granitic rocks are called "roof pendants." Another great example of a roof pendant is Mt Tom near Bishop, east of the Sierra Nevada. In Yosemite National Park, Red and Grey Peaks in the Clark Range are also roof pendants.

Dakota Hogback east of El Dorado Canyon, near Boulder, CO. This sandstone ridge (hogback) was deposited ~ 100 million years ago along the eastern edge of the north-trending Cretaceous Seaway, which divided North America. Dinosaur tracks are found in the Dakota Formation. The ridge was raised during the Laramide Orogeny (mountain building event) 63 million years ago. In this photo, the Dakota ridge is separated from the rocks to the left/west by a fault which extends up Shadow Canyon to Bear Peak in the background. Rocks in the distance are faulted blocks of the Fountain Formation--the right/east ridge forms the Flatirons; the left/west ridge forms the crags within El Dorado State Park.

Crevasses on Muldrow glacier. Denali. A glacier is a river of ice. The ice is somewhat elastic, but will crack as it moves downhill, much as a river produces cascades and falls as it descends more steeply to lower elevations.

Patagonia ice cap sends icebergs into a lake on the trek around the Torres del Pine in Chile

Alaska glacier, flying into Denali basecamp from Talkeetna. Colored ribbons of rock on the glacier indicate merging of several glaciers upstream.

Clear Creek Quartzite, CO. Stretched and sutured quartz grains and mica seen through a polarizing petrographic microscope. Quartz grains of the originally deposited sandstone have been pressed together in interlocking patterns by high temperature and regional stresses. Mud sections withing the original sandy deposits have been metamorphosed into micas. Dating of this rock is disputed: it is either older than the Boulder Creek Granodiorite (1700 My), or deposited upon the granodiorite surface. In either case the age likely exceeds 1.6 billion years.

Columnar jointing in basalt. North Table Mt, Golden, CO. Shown here are 2 flows of Shoshonite, a potassium-rich basalt, probably originating 5 miles north of Golden about 60 million years ago. One sees at Table Mt, spheroidal weathering, glassy-appearing cooling planes, and vesicles filled with white zeolite minerals.

Pothole in Yosemite granitic rock. Sentinel Dome. Chemical and physical processes enlarge holes over time. Water is a polar molecule and acts as a chemical solvent, causing weathering of granite around the hole Standing water in pothole causes granite to disintegrate= auto cyclic reaction.<br>
<br>
This tree is the famous Ansel Adams photo: http://thetuncluer.files.wordpress.com/2007/07/ansel-adams-jeffrey-pine-sentinel-dome.jpg

Yellowstone hot spot. Geology theory says that there is a hot spot coming from deep in the Earth beneath Yellowstone National Park. This hot spot causes the hot springs and geysers for which the Park is known.<br>
There have been catastrophic eruptions at Yellowstone, that, if repeated, could wipe out a lot of us in the Rocky Mountain area.

Monastery, east of Estes Park, CO. Pegmatite sill intruded into "country rock," the metamorphic basement rock of Colorado. The basement rocks and sills were later faulted into horizontal positions. Pegmatites are coarse/large grained granitic masses with large crystals of quartz, feldspar, and tourmaline.

Case-hardening on desert granite. Cochise Srtonghold, AZ. Moby Dick. The case-hardened surface is breaking up into plates, which eventually will erode to become chickenheads.

Crevasse in the Kahiltna Glacier, Denali, AK. These crevasses begin to open in late June (or earlier).

Xenoliths in Clear Creek Canyon, CO. These angular pieces were broken off when hot, molten rock intruded preexisting, crystallized, (black) rock, a process known as brecciated intrusion. The angularity of the black pieces suggests that the intruding molten rock was not so hot--relative to the melting point of the dark rock-- otherwise the black pieces would have melted or at least become rounded. Many black chunks in lighter colored granitic rock, for example, chicken heads in Yosemite granodiorite are rounded, indicating that they were melted somewhat by the enclosing magma.

Two mountain-building events in El Dorado State Park. Fountain Formation (red foreground)was raised during Laramide Orogeny (63 million years ago). Clear Creek Quartzite (white rock) was faulted up, perhaps, during the Paleozoic, ~300 million years ago, when the Ancestral Rockies were raised. We are looking at the east side of these ridges, called the "dip." Geology also describes a ridge like by its "strike," the direction in which it is trending. The strike of the Fountain Formation is mostly N. The strike of the Clear Creek Quartzite is more to the NE. This difference in strike indicates that these ridges were not raised to their present position in the same mountain building event.

Flatirons. NW CO, near Dinosaur National Monument. Flatirons are produced by erosion of an uplifted rock. Compare the Flatirons near Boulder, CO.

Chickenheads on Moby Dick. The heads are protected by a surface layer known as case-hardening of the granite. The heads become increasingly isolated and smaller over time until they break off, producing shallow "heucos."

Flow bands in volcanic rock. Viscous (very thick) lava (rhyolite) flow. New Jack City, CA

Adularia, a potassium-rich feldspar that forms from hot fluids circulating in the rock. Professor Ted Walker of CU, Boulder, took this scanning electron microscope pic of adularia cementing the particles of Fountain Formation. This adularia cement has made Eldorado Canyon State Park a super climbing area. Elsewhere, for example, Garden of the Gods near Colorado Springs--away from the Colorado Mineral Belt--where this cement has not penetrated, Fountain rock cannot be climbed because it is poorly cemented and very crumbly. At Shelf Road, CO, there is a faulted hill of Fountain that is very crumbly, another example of the absence of adularia cement.

Weathering patterns in desert granite. Heuco Tanks, Texas. Heucos and "plates" are common in the granitic rocks of the arid American SW. See Alabama Hills east of Whitney for classic "plates." Also known as "case hardening," these crusts are formed by the movement of ions in the rock by hydrological processes. The crusts are often oxides of iron and magnesium deposited on the surface by freezing and thawing cycles. These crusts are hard in dry periods and softer when it is wet--so watch out or you will make a heuco by pulling off a platy crust.

Rumney, NH. Devonian Littleton Formation is intruded. engulfed by granite.

Faulted surface. Supremacy Rock. El Dorado Canyon, CO. Coal Creek Quartzite. Another nearby faulted climbing surface is Red Slab, in Clear Creek Canyon, Golden, CO.<br>
The polished surface is the result of 2 blocks of rock moving across each other.

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