Chile's "Towers of Blue"; Rounding the Horn at the End of the World; A "Warm" Remnant of the Patagonian Ice Sheet; A Sure Sign of New England Spring; Iceland's "Golden" Waterfall; Hrafnabjörg - A Classic Table Mountain; Hutton's Section and "Deep Time"; Eroded "Bridal Path" Dike of the Franconia Range; A Sentinel Butte with an Uplifting Story to Tell; Roundup on the Coconino Plateau; Trekking the Geology of the Tonto Platform.
By the time the end of the year rolls around, there are always a few posts that were never written. And so, with this final post of the year – in what has become a tradition on my blog for six years running – here’s my end-of-the year post of some of the stuff that "never quite made it." Please visit the same for 2012 (here), 2013 (here), 2014 (here), 2015 (here) and 2016 (here). Coordinates have been provided that will take you there when pasted into an on-line mapping program such as Google Earth.
Chile's "Towers of Blue"
Torres del Paine National Park, Chile
Go to the summit of Cerro Paine Grande: 50°59′56″ S, 73°05′43″ W
|View of Cordillera del Paine and Valle Frances from Lake Pehoé Facing North|
Concomitant with the opening of the Atlantic Ocean, the Pacific initiated consumption along the Ring, as one global ocean widened at the expense of the other. As the North American and Eurasian and South American and African plates diverged, North and South American were driven westward, all as the Farallon plate initiated subduction beneath the North America plate. In South America, the Nazca plate - a Farallon fragment - descended beneath the South American plate, uplifting the Andes including the Paine Cordillera. Further south, the Antarctic plate was eventually involved in the process.
Cordillera del Paine's mafic and felsic magmas formed a laccolith at a shallow depth of 2-3 km some 12 million years ago in the Miocene. During ~90,000 years of emplacement, the mushroom-shaped molten mass intruded basin deposits of Cretaceous-age continental and shallow marine sandstones and mudstones, elevating the overburden into a dome. The Torres de Paine Intrusive Complex was exhumed in the Neogene and glacially carved in the Quaternary into the spectacular "Towers of Blue" we see today.
Rounding the Horn at the End of the World
Cape Horn and the Drake Passage
Southernmost Headland of the South America
Go there: 55°59′04.93" S, 67°16′22.87" W
"I am the albatross that waits for you at the end of the world.
I am the forgotten souls of dead mariners who passed Cape Horn."
English translation of first stanza of poem by Chilean Sara Vial on a marble plaque at Cape Horn
|With the ship's bell ringing loudly to celebrate our rounding of the Horn, this was our view from the Chilean Stella Australis.|
Contrary to what we learned in school, Cape Horn is not the southernmost point of South America. The geographical distinction belongs to Chile's Diego Ramirez islands. The Cape IS the southernmost headland of the continent. It's also the northern boundary of the Drake Passage between the South Shetland Islands of Antarctica some 500 miles south. The notoriously rough waters and westerly gales are where the Atlantic and Pacific meet. The "sailors' graveyard" has claimed more than 1,000 ships and over 10,000 lives. In March, we "rounded the Horn" on the Stella Australis on the roughest seas the captain had seen in 35 years of service.
The Drake Passage was closed some 41 million years ago when South America and Antarctica were unified and much warmer without an ice cap. That changed when the continents rifted apart and with the introduction of the Scotia tectonic plate that formed a collage of deep basins and a volcanic island arc. Once open, the Antarctic Circumpolar Current formed, the largest ocean current in the world with 600 times the flow of the Amazon. The result was Antarctic glaciation, triggering of Oligocene global cooling and the confirmation that ocean currents play a part in climate change. As for the Cape, it's part of the South Patagonian Batholith that formed from the amalgamation of subduction-related plutons during the Andean orogeny from the Late Jurassic to Neogene.
By the way, there are two famous interoceanic passages within the archipelago of Tierra del Fuego at the tip of South America. But, unlike the Drake that is "open water", the Beagle Channel and the Strait of Magellan are fjords that are too narrow for large ships and frequently icebound with headwinds often too great for sailing vessels. In common with the Cape, they were created during Andean orogenics and carved by repeated Pleistocene glaciations.
A "Warm" Remnant of the Southern Patagonian Ice Sheet
Agostini Fjord, Tierra del Fuego, Chile
Go there: 54°28′42.06" S, 70°26′38.88" W
The end result has been the retreat of the once expansive ice field. Vestiges remain in the form of the Northern and Southern Patagonian Ice Sheets and a large number of alpine (mountain) and outlet glaciers. The latter, in Chile, either reach the Pacific via glacially carved fjords or the Atlantic via Patagonian lakes and rivers that empty eastward. Águila is an example of the former that empties into the Agostini Fjord located within the maze of Tierra del Fuego islands in the Magallanes region of southern Chile.
Currently extensive in Greenland and Antarctica, continental ice sheets have cold bottoms and remain frozen to the bedrock, whereas, alpine and outlet glaciers are on the move. Spawned by ice sheets such as the Patagonian, they are the predominant sculptors of the landscape. Although the Águila is retreating, it's fed above the equilibrium line where precipitation contributes to its growth. It's nestled on and confined by scoured late Paleozoic to early Mesozoic metamorphosed rocks overlain by Jurassic volcanic and Cretaceous clastic sedimentary rocks of the Cordillera Darwin range of the Austral Andes.
A Sure Sign of New England Spring
|The male robin takes his turn at tending to the brood of hatchlings.|
The female's plumage is subdued compared to the male's rich coloration. Darwin recognized that sexual dichromatism facilitates mate selection (and natural selection, if it includes something like horns) but also protects camouflaged females during incubation from predatory cats, squirrels, chipmunks, hawks, jays and owls. Why are the eggs blue? The pigment may confer a biophysical advantage to the eggs by striking a balance between harmful UV radiation and beneficial IR warmth during brooding.
Evolution has also adjusted the timing of the breeding season to maximize the number of young produced. In New England, robins mate and hatchlings emerge when food is abundant in May. Their appearance coincides perfectly with that of tasty lime green, foliage-eating Geometrid caterpillars that appear by the gazillions and dangle from the canopies of oaks and maples on long silken threads. Robins are an altricial species with unfeathered hatchlings that require intense care and feeding versus precocial chickens and ducks that are born feathered and soon on their own. Unfortunately, I missed the magical moment when the fledglings left the nest, which will likely remain empty since the weather-beaten, parasite-ridden, feces-laden nest won't be used again.
Gulfoss National Park
Go there: 64°19′34″ N, 20°07′16″ W
"Kemst þó hægt fari."
You will reach your destination even though you travel slowly.
The "White River" plummets 32 meters over Gulfoss's two main steps of erosion-resistant lava and continues within a 70 meter-high, columnar basalt-walled gorge called Hvítárgljúfur. The upper step is a thin flow that overlies a thick, conglomeratic sedimentary sequence, and the lower step is a thicker flow. They differ in orientation to each other and the trend of the gorge, related to two strike-slip (horizontal) faults typical of the South Iceland Seismic Zone. The gorge's linearity is on strike with the second step and then parallels the normal fault zone (vertical movement) of Þingvellir. The faults offer a path of weakness that Hvítá has exploited and account for the zig-zag geometry of the falls and gorge.
In the early 20th century, foreign investors wanted to harness Gulfoss within a dam and hydroelectric plant. Construction was prevented in part by Sigríður Tómasdóttir, the daughter of the land owner. To protest, she walked barefoot 75 miles on unpaved roads to Reykjavik on unpaved roads and threatened to throw herself over the brink if the project was initiated. Icelanders are a hardy breed, intensely dedicated to resource preservation. Fortunately, the falls is in possession of Iceland as Gulfoss National Park.
Hrafnabjörg - A Classic Table Mountain
Go there: 64°16′ 21″ N, 20°55′ 18″ W
Submarine eruptions may form islands, and subglacial ones may trigger catastrophic outburst floods and release great volumes of steam and water. Rocks are typically breccias (fragmented rock), hyaloclastites (aggregates of glassy fragments), rounded heaps of pillow lavas (rounded heaps) at the base and lava flows across the summit. Hrafnabjörg's source fissure is related to the developing Þingvellir graben and is on strike with it. Use the above coordinates to locate it, and you will see this.
Hutton's Section and "Deep Time"
Go there: 55°56′34.36″N, 3°09′59.77″ W
"The result, therefore, of our present inquiry is,
that we find no vestige of a beginning, no prospect of an end.“
James Hutton, 1788
In the Devonian, the Old Red Sandstone continent (British term for sedimentary rocks of North America's northeastern seaboard, Great Britain and Scandinavia) was built primarily via a collision of Laurentia (North America's Precambrian cratonic core) and the continent of Baltica (Europe, Scandinavia and Siberia). The event uplifted the Caledonides Mountains of northern Britain and Scandinavia. In the Permo-Carboniferous, the South Hemispheric megacontinent of Gondwana collided with Laurentia at the expense of the intervening Rheic Ocean. Oceanic plate subduction triggered back-arc extension within the overriding plate. The extensional, fault-controlled basin that formed had the structure of an ancient rift valley with sedimentation, basaltic magmatism and sill intrusion of sediments in the Midland Valley region of central Scotland. Arthur's Seat represents the time-eroded and Ice Age glaciated remnants of a volcanic plug where magma lithified within its vent.
|My Friend Tony and I Atop Arthur's Seat|
Magnificent views of the Midland Valley, farther Highland Mountains, the Firth of Forth and Edinburgh are to be had from the greatly eroded and glaciated summit of Arthur's Seat on Lion's Head.
Our sought after locality is Hutton's Section at the cliffbase of Salisbury Crags. The Section is composed of a granular limestone called cementstone (the granular carbonate equivalent of mudstone) that deposited within the aforementioned basin, while the Crag is a columnar-jointed, transgressive sill of teschenite (a type of igneous intrusive rock). In the mid-1700's, geologist James Hutton observed that the deposition of sedimentary rocks and the subsequent emplacement of the sill's molten igneous rocks must have occurred at different times and even in a different manner than geological wisdom at the time dictated. It was a ground breaking concept (pun intended) for a planet thought to be 7,000 years old - biblically speaking.
Eroded "Bridal Path" Dike of the Franconia Range
White Mountains of New Hampshire
Go there: 44°09'23.31"N, 71°39'49.15"W
The White Mountains reside in northern New Hampshire and a tad of western Maine. It's part of the Northern Appalachians and in turn, that of the 2,400 km-long Appalachian Mountain chain from the Canadian Maritimes to Alabama with disconnected portions in Arkansas and Oklahoma. The "Whites" includes the Presidential Range and five subranges such as Franconia, the second highest range in the system.
The Appalachians began to form 480 million years ago but continued to grow incrementally as fragments of disassembled Rodinia and Gondwana successively reassembled during the Paleozoic (which includes the accretion of intervening oceanic lithosphere). Five phases (up from the traditional three) are assigned to the protracted event: Ordovician Taconic; Silurian Salinic; Devonican Acadian; Devonian to Carboniferous Neoacadian; and Permian Alleghenian. Each added crust to and uplifted mountains along the eastern continental margin of North America. It culminated in the formation of Pangaea and the Appalachians on strike with the zone of tectonic convergence.
In New Hampshire's White Mountains, sedimentary and igneous rocks of the Taconic phase are found along the Vermont border to the east and are the oldest rocks. Progressing east across the orogen, Silurian marine and finally sedimentary Devonian rocks of the successive phases are encountered. The latter represents the Acadian orogenic collision of the Avalonian terrane that uplifted metamorphosed sands and muds of the Presidentials. So, how did the Franconia Range come to form later in the Mesozoic with different rocks types and to the west of the older Presidentials? What's this got to do with the "Bridal Path" dike?
|My son Will and I at Galehead Hut on the Appalachian Trail of the White Mountains|
Beginning in the early Mesozoic, Pangaea began to break apart. The event formed the Atlantic Ocean and present-day continents of the Atlantic realm. When Pangaea's lithosphere began to rift apart, rift-parallel grabens developed up and down the newly-active continental margin and the White Mountain Plutonic-Volcanic Suite emplaced in New England and Quebec fed by an extensive, subterranean plumbing system of dikes. As a result, most of the White Mountain's rocks are Early Jurassic granite plutons, the source of the Granite State's moniker. Many of the dikes fed the plutons or were part of ring-dike systems following the plutons' collapse into calderas.
The "Bridal Path" dike eroded out from Franconia's Conway granite, the host rock in which it and others emplaced. Pleistocene glaciation has since sculpted the region and excavated and polished a number of erosion-voided dikes turning them into slick sluiceways for runoff such as the one in the photo. Just think of all the large-scale global tectonics and geological processes that resulted in the formation of this seemingly insignificant and largely unnoticed landform!
|Will in the Bunkroom of the Galehead Hut|
Sometimes, after a long day of climbing, you're still not finished.
Coconino Plateau, Northern Arizona
Go there: 35°49′14″ N, 112°05′23″ W
Begging to be climbed for the great view from its 912 foot-high summit, the sombrero-shaped sentinel lies about half-way to the Grand Canyon from the San Francisco Volcanic Field north of Flagstaff. Just off the highway, it rests on Middle Permian Kaibab limestones that extend to the canyon's South Rim. Above the base, Red Butte consists of Mesozoic-age brick-red Early Triassic Moenkopi mudstones followed by Late Triassic basal Shinarump Member sandstones of the Chinle Formation.
The butte owes its existence to a remnant cap of dark gray, basaltic lava that protected it from erosion since its emplacement some 9 million years ago. The flow likely originated from a vent in the San Francisco Volcanic Field perhaps 50 miles or so to the south. The subtle tilt of the Colorado Plateau sent the low silica, low viscosity lava far from its source, but its trail across the landscape has long since eroded away along with undoubted other flows that never made it this far north.
Mesozoic rocks once covered the Colorado Plateau and certainly the Grand Canyon since they were deposited at sea level. They are are found in southern Utah but are largely absent except in regions of northeast Arizona and isolated knobs like Red Butte. Under what geological circumstances caused the rocks to have been removed from the region?
The Laramide orogeny - the Late Cretaceous to mid-Paleogene tectonic mountain-building event related to progressive shallowing of subduction of the Farallon plate - is responsible for uplifting the Rocky Mountains and Colorado Plateau and creating its tilt. Uplift provided the impetus (in part) for the Colorado River system to carve the Grand Canyon and "unroof" most of the Mesozoic rocks from northern Arizona - of course with the exception of lone sentinels such as Red Butte with a great geological story to tell.
Go there: 35°35′24.27″ N, 111°34′10.66″ W
Just north of Flagstaff is the inwardly collapsed caldera and remnant peaks around the rim of the once-towering San Francisco stratovolcano. One of them is Mount Humphreys, Arizona's highest summit at 12,633 feet. The 1,800 square mile volcanic field extends many miles to the west, east and north of "The Peaks", almost half way to the South Rim of the Grand Canyon. You can't fully appreciate its size until you drive around and through it. During its six million year history, it produced over 600 volcanoes, mostly cinder cones, a few lava domes and layer after successive layer of basaltic lava flows. Its youngest volcaniform is Sunset Crater cinder cone. Its eruption date of 1066 AD tells us that the volcanic field is dormant and likely poised to awaken.
This October day some 25 miles north of Flagstaff, while exploring the field and cinder cones on the northern flank (and eventually climbing 71,000 year-old, 3,900 feet-high SP Mountain), I was four-wheeling my way across the Coconino Plateau on a dusty dirt road. The landscape gently undulates with Holocene to Middle Pliocene basaltic lava flows that variably blankets Early Triassic Moenkopi mudstones and Middle Permian Kaibab limestones.
I was forced off-road to skirt a large herd of Herefords on the expansive Babbitt Ranch. Around this time of year, cowboys trail cattle from their summer grazing lands on the north side of the Peaks to winter refuge on the east side. Looking back at the herd, I chanced this fortuitous shot. It's time like this that have taught me to never get caught without a camera on the Colorado Plateau (or anywhere for that matter).
"Although this trail constrains itself to one geologic layer for most of its length,
it is anything but monotonous."
From "Hiking the Grand Canyon's Geology" by Lon Abbott and Terri Cook, 2004.
With fits and starts, fluctuating Panthalassic (proto-Pacific) seas advanced ever-eastward, flooding Laurentia's western margin including the region of the future Grand Canyon. Reworking sediments derived from the land, the shores were blanketed with medium to coarse-grained Tapeats sands as deeper waters received Bright Angel Shale's fine-grained silts and muds, and even deeper, beyond the terrestrial sediments, Muav limestones were crystallized from the sea and built from shell fragments of newly-evolved marine organisms. These three lithologies form the Grand Canyon's Tonto Platform. It's a classic transgressive, onlapping and interfingering, sedimentary sequence that records the gradual migration of the shoreline as the sea advanced onto land.
The establishment of the Tonto Group as an east-west, river-paralleling, broad and relatively flat geological bench occurred during the carving of the Grand Canyon. As the Colorado River system eroded into the deepening abyss, it eventually reached soft deposits of the Bright Angel Shale. The river began to meander, gradually undermining the cliffs of stronger strata and causing them to collapse and retreat. A similar canyon-widening phenomenon happened previously during the formation of the more elevated Esplanade Platform, when the river reached erodable shales of the Hermit Formation.
The Tonto Platform formed when it became stranded as the river began to chisel into the erosion-resistant deposits of the Granite Gorge, Rodinia's basement suite of metamorphic Vishnu schist and igneous Zoroaster pegmatite granites. The platform, therefore, survives as a mid-canyon paleo-terrace some 4,000 feet below the rim and some 1,000 feet above the river (both on average).
|Yours Truly on the Tonto Trail at Sunrise|
The 95 mile-long Tonto Trail - one of the classics of the Southwest - follows the platform largely on the Bright Angel Shale and is anything but flat (and easy). It has seen use as a transcanyon route by everything from indigenous native Americans to miners, feral burros and adventurous, overnight backpackers where shade is rare and water is scarce (depending on the season). The trail rises and falls with the platform's dramatic undulations and swerves in and out to follow the innumerable massive drainages that repeatedly punctuate it from the rims. Photo by geologist, author and guide Wayne Ranney. Shameless plug - Wayne's and Ron Blakey's newest book is out "Ancient Landscapes of Western North America" by Springer Publishing (here).
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