How did the Adirondack Mountains form? Please visit my post Part II here.
VESTIGES OF A SUPERCONTINENT
Virtually all of the bedrock in this Adirondack Mountain vista is Middle Proterozoic Grenville in origin. The last billion years were witness to the formation of the supercontinent-spanning Grenville mountain belt culminating with the assembly of Rodinia, to its fragmentation, to the Iapetus Ocean’s formation and eventual closure, to the supercontinent of Pangaea’s unification and rifting apart, and to the birth of the Atlantic Ocean. Blanketing Early Paleozoic marine assemblages have been unroofed by thermal doming of the Early Cretaceous. A hundred million years later, Pleistocene continental glaciation bulldozed the region at least four times, likely more, leaving its erosive signature everywhere. The story of the Adirondacks is indeed “Written in Stone.”
THE ADIRONDACK LOJ
In August, my daughter and I drove from Boston to the Adirondack Loj (correct spelling), a few miles south of Lake Placid, New York. The lodge is efficiently run by the Adirondack Mountain Club and served as our base for two days of geological exploration within the High Peaks region. The lodge is replete with home-cooked meals and bagged lunches for hikers. It is immaculately clean with private and family bunk-rooms, and a communal great room for relaxing beside a stone hearth. There’s even swimming and canoeing in crystal clear Heart Lake. Built in 1927, this idyllic “gem-in-the-woods” has it all: mountain hospitality, Wi-Fi access, education classes in geology, botany and mountain lore, and easy access to the high peaks. Go there (shameless plug)! For their website click here.
|My daughter (and climbing partner) enjoys the night air outside the lodge.|
And yes, that IS a moose head above the hearth!
GLACIAL HEART LAKE
The lodge is situated on the edge of most pristine Heart Lake in the shadow of Mount Jo at 2,340 feet. It’s diminutive by Adirondack standards, but after a short hike above the glacial talus that litters the region, anorthosite bedrock quickly crops out. Go a little further, and the gabbroic anorthosite becomes gneissic as its constituent labradorite feldspar crystals begin to align. Still further, the trail crosses a fine-grained, black camptonite dike. All that geology within a mile of the lodge!
|Taken from the summit of Mount Jo above Heart Lake with Mount Colden (left), the MacIntyre Range including the Peaks of Wright and Algonquin (center), and precipitous Wallface (right of center) are separated by the NE-SW fault valleys of Avalanche and Indian Pass, respectively. From a wonderful National Geographic article entitled “Adirondack Park-Forever Wild” at www.ngm.national geographic.com and photographer Michael Melford at www.michaelmelford.com)|
The geological verdict on the lake is still out. Some believe it's a kettle lake that formed when ice calved from the front of a receding glacier. In this scenario the lake would have become established in the glacial outwash when the ice melted. An alternative origin depicts its formation in a glacially-scoured basin replenished by melting glaciers and eventually mountain streams. That would lend credence to the thought that Heart Lake and the adjacent drybeds with unmistakable beaches were once one large glacial lake. The outlet of Heart Lake flows north into the lake basin of South Meadow. We’re looking south at the foothills of the MacIntyre range just before sunset, tomorrow’s destination.
Tranquility will have a new meaning!
ADIRONDACK MOUNTAIN HIGH
After a restful night in the lodge (2,174 feet), we began our sunrise-ascent to Wright Peak (4,580 feet) which was a warm up for Algonquin Peak (5,114 feet) to follow. Both mountains are within the MacIntyre Range, named after the owner of the Tahawus open pit, iron mining operation in the 1800’s and titanium dioxide in the early 1900’s.
The MacIntyre Range stands apart from the surrounding peaks and extends for eight miles running NE and SW along the trend of the faults that confine it. Its steep SW slope forms Indian Pass, while the NE side defines spectacular Avalanche Pass. Our two-day plan was to climb the range from Wright to Algonquin on the first day and investigate the system of lakes within the fault-valley to the east of the range on the second day.
The Adirondacks have a distinctive look and feel right down to the moss-covered, gnarled tree-roots that seem to imprison boulders of glacial talus.
The rough and rocky trail starts out in unconsolidated glacial talus and till, and transitions to anorthosite bedrock. The verdant slopes and valleys of the Adirondacks contain a deciduous mix of aspen, ash, cherry, beech, maple and birch at lower levels and hardy evergreens at higher elevations that includes pine, spruce, hemlock and cedar.
A TRAIL OF ANORTHOSITE
It wasn’t until about 2,340 feet that we encountered our first outcrop of anorthosite bedrock as the going steepened. From then on, the trail was entirely on exposures of metanorthosite and anorthositic gneiss requiring lots of scrambling and more planning for each step. We’re looking uptrail at one such steep exposure. The pitch is very deceiving at about 40-45º. My daughter is actually sitting upright. What a place to traverse in a downpour! The bedrock has been stripped of 30 km (give or take) of Grenville overburden by erosion, exhumation and uplift.
Notice the intrusion of a wide dike through the anorthosite with a small apophysis (offshoot) from the main channel mid-way up to the right. I suspect this dike to be of pyroxenite in composition. It lacks the chilled margin of fine crystalline growth indicative of most regional dikes which would indicate rapid cooling; therefore, the magma contacted the anorthosite while it was still hot. However, notice the cracks perpendicular to the path of dike-emplacement. The dike had already cooled enough to contract.
There are many dikes in the Adirondacks of various tectonic causations and time frames. Examples include: Late Proterozoic dikes of alkaline basalts (meta-diabasic) that intruded Grenvillian crust during orogenesis; late- to post-orogenic dikes associated with extensional collapse of the Grenville orogen; dikes associated with the rifting of Rodinia and the opening of the Iapetus Ocean in the latest Proterozoic and Early Cambrian; Mesozoic tholeiitic dikes associated with the rifting of Pangaea and the opening of the Atlantic Ocean; and dikes associated with passage over the Great Meteor hotspot (more so eastern Adirondacks). Dikes are of significance in studying such processes as continental breakup, and the composition of the lithosphere and asthenosphere.
Many of the waterfalls in the Adirondacks are associated with dikes that succumb more readily to erosion than the surrounding resistant anorthositic country rock. Such is the case with this waterfall of MacIntyre Brook associated with several diabase dikes that crosscut the bedrock. At an elevation of 3,255 feet, it only had a trickle of water. One can imagine the raging fury during a summer thunderstorm.
“Proterozoic massif-type anorthosites” (Ashwal, 1993) were emplaced along the southeastern aspect of the Canadian Shield within the Grenville Province during the waning stages of the Grenville Orogeny. The Adirondack Mountains of northern New York State represent a southern extension of the Grenville Province (visit my post Part II for details here). Separated by the Carthage-Colton Shear Zone, they are topographically divided into Central Highlands and Western Lowlands. Our climb in the High Peaks region of the Highlands was entirely within the Marcy massif (orange) and surrounded by associated granitoids of the AMCG suite (stripes), a tongue-twisting, felsic and intermediate complex of anorthosite, mangerite, charnockite and granite.
|Anorthosite and AMCG series distribution in the Central Highlands of the Adirondacks|
(Modified from Chiarenzelli and Valentino, 2008)
THE “ANORTHOSITE PROBLEM”
Anorthosite is the most difficult igneous rock to explain. Its unique geochemical nature and puzzling tectonogenesis have intrigued geologists for almost a hundred years. Enigmatic are its: near mono-mineralic composition and large crystals of over 90% plagioclase feldspar (fractional crystallization in Bowen’s Reaction Series is generally 40-50%); its gabbroic parental magma (the precursor of any igneous rock); its enigmatic association with bimodal granitoid-suites (the AMCG suite); its low (less than 10%) mafic to intermediate (diorite and gabbro) rock composition; its restrictive occurrence as plutonic rocks; its presence with layered mafic intrusions; its emplacement largely confined to the Middle Proterozoic; and its unique tectonic setting (“anorogenic”).
Many of these petrological problems have been resolved, but their genesis has remained elusive. Clearly, they formed by igneous processes, but they can not have formed from a magma of their own bulk composition. The problem with anorthosite is its geochemical composition and begins with the generation of magma, the necessary precursor of any igneous rock. Magma that is generated by small amounts of partial melting of the mantle is generally of basaltic composition, which has the opposite composition found in anorthosite, lower plagioclase and no ultramafic rocks.
BOWEN’S REACTION SERIES
The series (delineated by a petrologist in the early 1900’s) indicates the temperature at which minerals melt or crystallize in magma. It also explains why some minerals are always found together and why others are almost never associated. Magma generated by partial melting of the mantle is generally of basaltic composition. On the series under normal conditions, the composition of basaltic magma requires it to crystallize between 50 to 70% plagioclase with the bulk of the remaining magma crystallizing as mafic minerals such as pyroxene. Thus, basaltic magmas are typically plagioclase- AND pyroxene-rich. Basaltic magmas of anorthosite, however, are defined by a much higher plagioclase content and much lower mafic content. In petrology, this is known as the “anorthosite problem.”
For a more detailed explanation of the Bowen Reaction Series click here.
AN ANORTHOSITE (THEORETICAL) SOLUTION
Although controversial for many decades, a consensus has developed to provide an anorthosite solution. Simply stated, anorthosites are considered to be the product of basaltic magma and that the removal of mafic minerals has occurred at a deeper level. A key point is the ascending asthenosphere that provides thermal energy to melt gabbroic magma that has underplated the lower crust. And also uniquely Adirondack is the intense deformation during or after crystallization that occurred which generated the re-crystallized parent liquids of anorthosite.
The following is a chronological model of how anorthosite, plagioclase-rich and mafic-poor, may have formed along with its associated AMCG suite. Note that the process is “anorogenic” in that ponded magmas evolved in an extensional and regional event not directly derived from normal mantle melting rather than in an “orogenic” convergent tectonic event. Although the suite represents a small percentage of the Adirondacks, the AMCG's are crucial in understanding the petrogenesis of massif anorthosite. For clarification of events related to extension within the Grenville Orogeny, please visit my post Part II here.
A THEORETICAL MODEL
(A) After accretion of the Grenville Province in the late- to post-tectonic setting of the Grenville Orogeny, delamination of over-thickened lithosphere (from the Grenville contractional orogeny) and post-collisional extension (during orogen-collapse) promoted an influx of gabbroic magma from the asthenosphere yielded by decompression melting. Having left its mantle source, the picritic magma (olivine-rich and plagioclase-poor) underplated the crust, ponded there and differentiated into a magma chamber.
(B) Crystallization of olivine and pyroxene (aka Bowen) occurred with these dense mafic (ferro-magnesium) phases sinking back into the mantle.
(C) The remaining crystal mush became enriched in plagioclase, Al and Fe/Mg. This lower-density, buoyant basaltic melt (now a plagioclase-rich anorthosite) began to diapirically (hotspot plume-like) ascend into the crust.
(D) Anorthosite further ascended as plutons.
(E) The plutons coalesced to form massive anorthosite. The rising, hot asthenosphere (a key point) provides heat to partially melt the lower crust resulting in the formation of granitoids which, along with anorthosite magmas, formed the AMCG suites coevally (at the same time) but not co-magmatically (from separate magma chambers).
|Model of Anorthosite and AMCG Suite Petrogenesis|
(Modified from Ashwal, 1993)
The end result is our anorthosite, a phaneritic (coarse-grained), plutonic (magma chamber), intrusive (formed under the surface), mantle-derived (but not from mantle-melting), igneous rock that is enriched with plagioclase feldspar (usually labradorite, andesine or sometimes bytownite related to Bowen's Series) and depleted mafic derivatives (such as ilmenite, olivine, magnetite or pyroxene). The formation of anorthosite and associated granitoids are thought to have occurred late in the Shawinigan Orogeny and metamorphically imprinted during the Ottawan Orogeny (see Part II).
After anorthosite crystallized, tectonic collisions toward the end of the multi-phasic Grenville event metamorphosed the rocks. This close-up of Marcy-type anorthositic gabbro shows metamorphic reaction-rims with coronas of garnet (C) surrounding mafic pyroxene megacrysts (B) within the plagioclase feldspar's interlocking-matrix (A). After initial metamorphism, an influx of fluids, garnet and hornblende growth, and textural modifications occurred. Garnets are indicative of the high temperature and pressure of granulite-facies metamorphism that occurred during the Ottawan Orogenic phase of the Grenville Orogeny. Garnets, whose formation is not completely understood, are useful in interpreting the genesis of many igneous and metamorphic rocks and in particular the temperature-time histories of the rocks in which they grew and in defining metamorphic facies of rocks.
By the way, garnet has been designated as the official New York State gemstone. It's used in coated abrasives, glass and metal grinding and polishing, and even to remove the red hulls of peanuts. The Barton mine in the Adirondacks sells up to 12,000 tons annually harvested from an amphibolite. Chances are if you're using red sandpaper, it's from the Barton mine.
Referring to the Bowen Reaction Series above, the plagioclase family of feldspars displays numerous mineral phases as it cools and migrates from calcium- to sodium-rich. One of the minerals, labradorite, is a principal constituent in anorthosite and is responsible for its blue-gray color, actually attributable to black ilmenite within its crystalline framework. Another interesting feature is labradorite’s blue-green iridescence (also called Schiller effect, labradorescence, opalescence and chatoyancy) especially under water. In fact, Opalescent River, that flows into the lake of Flowed Lands (see post Part IV coming next) contains a preponderance of iridescent anorthosite. The bluish optical phenomenon is related to light diffraction and reflection within submicroscopic layering or exsolution lamellae of the labradorite.
And lastly, the ‘zebra-stripes’ or ‘record-groove’ effect that plagioclase, particularly labradorite, exhibits is related to twinning during crystal growth. Symmetrical ingrowth of crystals enables plagioclase’s identification in the field.
|Photomicrograph of plagioclase crystal under cross-polarized light |
showing distinct banding effect called twinning
ASCENDING WRIGHT PEAK
The spectacular view from Wright’s treeless summit captivated my daughter’s attention with Pitchoff, Cascade and Porter Mountains off to the northeast. Cloaked in low, ominous, swirling, gray clouds, the temps plummeted 30 degrees with wind gusting 25-35 mph. Instantly cooling down, out came the fleece and windbreakers on this otherwise hot August day. The threatening skies had us wondering about the conditions on adjacent Algonquin and if there’d be a view at all. We would be duly surprised!
On Wright, two sets of prominent vertical joints in the anorthosite intersect at right angles. Jointing is actually widespread throughout the massif and is a manifestation of forces of compression that resulted in the NE-SW faults. In some cases jointing has slight offsets indicative of faulting. Faults are responsible for the formation of the NE-SW valleys, as well as the subordinate NW-SE valleys. We seldom see faults on the surface but are aware of their presence by the landforms they create: belts of high mountains separated by narrow, swamp or lake-filled valleys. Deformational folds exist in the anorthosite as well, but because of its nearly mono-mineralic composition, they are difficult to identify.
Notice the prominent vertical joints in the anorthosite that decorate the entire summit. Two sets of them intersect at right angles. Vertical jointing is common throughout the Adirondack massif and is a manifestation of the forces of compression that resulted in the NE-SW faults. In some cases the jointing has slight offsets indicative of faulting. Folds exist in the anorthosite as well, but because of its nearly mono-mineralic composition, they are difficult to identify.
On January 16, 1962, a jet-powered strategic bomber, 30 miles off course in bad weather, clipped the top of Wright during a training mission killing four men on board. Parts of the plane still litter the crash site. Coincidentally, earlier this summer I climbed Mount Humphreys, the tallest peak in Arizona. It too was struck by a bomber on September 15, 1944 killing 8 airmen. A bronze plaque on Wright memorializes the airmen who lost their lives in service to their country.
THE ARCTIC-ALPINE ZONE
The Adirondack timberline is about 4,000 feet, where the sub-alpine forest transitions into treeless alpine tundra. Timberline is not simply a matter of elevation. After all, timberline in the Rockies is nearly 12,000 feet. Even elevation and latitude together do not tell the entire story. In fact, timberline can be substantially lower on a cooler north-facing slope versus a sun-exposed southern slope. Timberline is determined by a combination of conditions that include low temperatures, frequent frosts, high winds, thick snow pack, inadequate precipitation and poor soils, all of which diminish seed production and viability.
The Arctic-Alpine Plant Zone is the rarest habitat in New York State on 11 of the highest peaks of only 85 acres in the entire state! Its plants are identical to those found in tundra arctic regions at high latitudes, being equivocal to extreme elevation. Alpine low mean annual temperatures, frost-free periods (only two months a year), exposure to wind and ultraviolet radiation, lack of sufficient and nutritious soils, and wind speeds are comparable to that of the arctic. The Alpine Zone in the High Peaks Region is restricted to the meadows of 14 summits and are relics of the Ice Age, common throughout the region as the last glaciers made their retreat about 12,000 years ago. The plant communities were forced upslope by warming trends and the expansion of the forests in order to sustain their optimal growing conditions. The vegetation faces extinction similar to the threats facing arctic plants as the climate slowly warms.
The tundra vegetation is very fragile and slow-growing confined to isolated patches on thin remnants of soil that tenuously cling to the anorthosite. This Deer’s Hair Sedge is a densely tufted grass-like perennial that grows in large, windswept patches. The vegetated region seen here is on the leeward side of the summit from the wind. Can you tell the direction of the prevailing winds from the twisted balsam fir? Small stones were brought to the summit (over four tons!) by hikers and placed as barriers to protect the plants from inadvertent human trampling. For the last twenty years, many of the higher peaks have Summit Stewards that camp down below and spend their days educating the public about everything Adirondack especially the rare and fragile alpine ecosystems.
Compared to the windy, cold and overcast summit of Wright, Algonquin, 536 feet higher, was semi-tropical in the upper 70’s with bright sun and a gentle breeze. It’s a lesson in Adirondack weather on the summits. Even in summer conditions can change in a flash. Being prepared is essential to survival.
Our view to the east takes in massive Mount Colden (4,714 feet), scarred with landslides that look like huge vertical stripes. A veneer of thin soil, often less than a meter thick, tentatively mantles the slopes of many of the high peaks. Held in place by tangles of trees, shrubs, grassy roots and the coarse texture of anorthosite, soils on steep slopes can easily be destabilized by heavy, saturating rains.
Such was the case with Mount Colden during Hurricane Floyd in 1999 that delivered 10% of the annual regional precipitation in one day. In fact, Floyd’s was the single largest precipitation event recorded in the previous 71 years. The slide completely blocked Avalanche Pass with rock debris and a tangled mass of vegetation. More recently, Hurricane Irene in 2011 created the highly noticeable clean white slide. In all, I counted over 15 separate slides on Colden’s western face! Snow avalanches are a major threat to skiers and winter hikers as well in the pass. Mount Marcy is in the background to the left. At the base of Colden and out of view is a magnificent faulted-valley that contains a string of glacially-derived spillover lakes. We’ll visit those lakes tomorrow.
My daughter took this panoramic video with her iPhone. It begins and ends facing to the west.