Tuesday, December 22, 2015

2015 Geology Posts and Photos That Never Quite Made It

New England's "Mini-Ice Age" of 2015; Niagara on the Rocks; The Confluence of Two Rivers Named Colorado; The Western Transverse Ranges, A Major Tectonic Anomaly; "Frozen" Hawaiian Lavacicles in Subterranean Lava Tubes; Middle Devonian "Turkey Tracks"; A Race Against Tides and Time; A Geological Traverse of Franconia Ridge

Every geo-blogger confronts the challenge. What shall I post about next? Is the subject matter worthy of discussion? By the time the end of the year rolls around, there are often posts that never got written and images that never got uploaded. And so, with this final post of the year - in what has become a yearly tradition on my blog – here are a few in abbreviated form. Please visit the same for 2012 (here), 2013 (here) and 2014 (here).


January 
New England's "Mini-Ice Age" of 2015

My front walkway and the Alps of MIT

Bostonians will be talking about the winter of 2014-2015 for a long time. After a slow start, it didn’t quit. In January, New England was hit with six major storms, and in February, we got three more, while persistent cold weather prevented any meaningful snowmelt in between. These are the climatic conditions that led to glaciation during the Pleistocene. It's a misconception that temperatures need to plummet to create an "ice age." In reality, it's a period of climatic cooling where snow accumulation exceeds ablation. It only takes a few degrees. 

This is my walkway with over three feet of snow at the end of January. I thought that was deep, but by the time winter ended the total accumulation exceeded nine feet at 110.3 inches, surpassing the 125 year-old record of 107.6. Across the Charles River from Boston, students were skiing down a five-story mega-mound they dubbed the “Alps of MIT” that was heaped onto the school’s parking lot. The Farmers Almanac predicted that “the northeastern quarter of the country will have above-normal snowfall, although below normal in much of New England.” This winter, it forecasts a long, stormy, bitter cold one. Bostonians are praying for regional global warming.

February 
Niagara on the Rocks

South-facing view of the Niagara River, Gorge and Falls between New York State and Canada

While escaping to Hawai'i from Boston, I snapped this chilling photo of Niagara Falls looking south. It's comprised of three falls - American, Bridal Veil and Canadian Horseshoe - on the Niagara River, which flows 35 miles from south to north (top to bottom) between Great Lakes Erie and Ontario. It was named after Ongniaahra, an Iroquois village meaning "point of land cut in two”, which defines the international border between Canada (right) and the U.S. (left). The plunge pool below the falls is as deep as its height, which reaches 188 feet. While not exceptionally high, it is exceptionally wide at 3,950 feet. It’s the most powerful waterfall in North America with the highest flow rate in the world. 

Unlike the Grand Canyon, which may have been carved in as little as 6 million years, the falls was excavated from bedrock in a mere 12,000. All it took was a lot of water, a gradient (meaning enough change above sea level over a river's length to encourage degradation) and an erodable substrate, assisted in the case of Niagara Falls by glacially-induced isostatic rebound of the landscape (and it's still rebounding!). It may seem like an overly simplistic statement, but it explains why the Mississippi River has no waterfalls or gorges unlike the Colorado and Niagara Rivers. Niagara Falls is a knickpoint - a sharp change in channel slope reflecting different conditions and processes - formed by slower erosion above the falls than below. Changes in slope increases the shear stress at the base of the channel, which allows a stream to erode underlying substrate more readily than in non-knickpoint reaches. Over time, the knickpoint retreats upstream. 


Similar in perspective to my photo, here's a "Birdseye View of Niagara Falls and the Surrounding Country"
By James Hall, The Geology of New York, Part 4, 1843


Niagara's water came from the final melting of the continental Laurentide Ice Sheet in the Wisconsinan Stage of the Late Pleistocene, whereas the basin of the five Great Lakes that it drains was glacially-gouged from bedrock during its advance. Torrents of meltwater poured over five spillways that eventually consolidated into the three falls of Niagara. Interestingly, the Great Lakes contains over one-fifth of the world's fresh water, all of which cascades over the falls except some for hydroelectric diversion. In addition, it's "fossil water" left over from the Ice Ages with under 1% annual renewal by precipitation.

As for the strata, the caprock consists of resistant carbonates of the Middle Silurian Lockport Formation (~420 Ma), lying over softer shales of the Lower Silurian Rochester Formation. They were deposited within a shallow, sub-tropical sea in a retro-arc basin during the Taconic Orogeny that ended some 440 million years ago, one of three or four mountain-building events that constructed North America’s eastern margin. Undercutting of the caprock has allowed the falls to retreat southward some seven miles in 12,500 years (~1.3m/yr), however, geologists speculate they could be replaced in a few thousand years by a series of rapids as climate change diminishes precipitation and retreat engages softer Salina shales.




Creationists use Niagara Falls as proof of a young Earth by arguing that if the planet were indeed billions of years old, the falls would have receded further. They also use the concept of a young falls to bolster their philosophy of catastrophism via a Biblical deluge. In defense of uniformitarianism - the geological doctrine of natural laws and processes operating now as they always have been - Charles Lyell, the famous nineteenth century Scottish geologist - calculated (albeit incorrectly) the age of the falls at 35,000 years, far in excess of Noah's Flood. Ironically, consistent with uniformitarianism, much of the fall's erosion has been in the last 5,500 years, although progressing catastrophically at times.

"Copy and Paste" the following co-ordinates into Google Earth and fly to Niagara Falls: 43°04'53.16"N 79°04'21.68"W

February 
The Confluence of Two Rivers Named Colorado

For John Wesley Powell and all others that followed, the Grand Canyon officially begins where the Marble Canyon ends at the Confluence of the Little Colorado and main Colorado Rivers in northern Arizona. Even at this altitude you can tell the river's direction by the downstream-V created by its rapids.

Still Hawai'i bound, I caught this lucky shot at the eastern edge of the Grand Canyon. Well beyond where tourists flock to peak into John Wesley Powell's "Great Unknown", two large rivers have joined forces - the Little Colorado (entering top left) and the main stem of the Colorado (entering bottom). At their confluence, the two carved a 3,400 feet trough into Middle Permian Kaibab Limestone down to Lower to Middle Cambrian Tapeats Sandstone. 


Everyone's highly anticipated meeting of the waters of the two Colorados.
Wayne Ranney river trip 2007.


To geographers and aficionados of the river, the Confluence has marked the end of Marble Canyon and the formal commencement of the Grand Canyon proper since Powell’s voyage of exploration in 1869. To river runners, it's a primary stopping point and highly anticipated destination at rivermile 61.5, measured from the put-in Lees Ferry. It's a place to relax and bodysurf in the warmer blue-green waters of the Little Colorado that often runs reddish-brown from upstream rain over iron-rich Early Triassic Moenkopi mud and siltstones. To the Navajo, Hopi and Zuni, the meeting of the rivers is a sacred place in their faith and traditions. To hikers on the Tanner Trail, it provides a majestic view of the Confluence from atop Cape Solitude and a much deserved reward after a four day scorching trek from the South Rim. To geologists, it holds vital secrets to the evolution of the Grand Canyon. 


Looking upstream at the Colorado River as the less turbulent Little Colorado enters from the right.
Wayne Ranney river trip 2007.

It's certain that the Colorado River or an ancestor is responsible for carving the Grand Canyon, but to what extent, how and when was it accomplished? Did an earlier river first head northeast? Did it bear any relationship to the modern drainage system in spite of its antithetical direction of flow? What effect was there on the northeast-flowing system, when its source area to the southwest began to subside? Why does the modern Colorado River below the confluence turn sharply from a southerly to a westerly direction into the heart of the Kaibab Upwarp, which would normally act as a barrier to a river’s course? Perhaps sinkhole-directed groundwater beneath the upwarp promoted its breach. Perhaps headward erosion into the upwarp from the west diverted flow by pirating the main river and the Little Colorado east of the upwarp, which reversed their directions, facilitated by a lowering of base level at the Gulf of California. 

Confused? There is no consensus, but several pieces of the puzzle are slowly coming together. You can read about it and more in Carving Grand Canyon, Second Edition (here) by Wayne Ranney.

"Copy and Paste" the following co-ordinates into Google Earth for the Confluence: 36°11'35.42"N 111°48'05.57"W

February
The Western Transverse Ranges, A Major Tectonic Anomaly


Marine sequence of uplifted and tilted conglomeritic strata with mixed gray shales of the Late Cretaceous Chico Formation (aka Tuna Canyon and Chatsworth Formations) in Temescal Canyon. The cobbles are granitic, metavolcanic and quartzitic in a sandstone matrix. These deposits are highly fossiliferous with mollusks, age-diagnostic ammonites and less common microfossils of foraminifera. As subduction of the Farallon plate progressed, sediment derived from the mountains was deposited in a marine setting of the developing forearc basin.


A few miles north of Los Angeles, where the Pacific Coast begins to bulge out and faces the south unlike the rest of the coast, is the Western Transverse Ranges. It acquired its distinctive name from its east-west orientation, which lies in sharp contrast to California's neighboring coastal ranges that are oriented parallel to the strike of the San Andreas Fault. In a sense, the Transverse Ranges even goes against the general grain of the lithotectonic fabric of most of North America (with one exception that comes to mind). Almost as if to accommodate the Ranges, the fault makes a swooping "Big Bend" (below) where it delineates its eastern extent. What can we make of these relationships?


The east-west oriented Transverse Ranges and many sub-ranges on the Pacific plate between the San Andreas Fault and the Pacific Ocean. Temescal Canyon is located at the arrow above Los Angeles.


The coastal "bulge" begins around the Pacific Palisades near Temescal Gateway Park (white arrow above and ellipse below) where we turned into Temescal Canyon and up into Topanga State Park. They're within the foothills of the 3,000 to 8,000 feet-high Western Santa Monica Mountains (below), wedged between the Pacific Ocean and the San Andreas Fault. The Santa Monica's are a sub-range along with others such as the San Gabriel, San Bernardino, Topa Topa and Santa Ynez Mountains. The Western Transverse Ranges is also a geomorphic province, a collection of mountain ranges and intervening valleys that share geologic attributes and evolutionary histories - a curiously "transverse" one. 

A closer look at the Santa Monica Mountains sub-range with Temescal Gateway Park and Canyon (ellipse). 
In contrast, the neighboring north Coastal and south Peninsular Ranges are oriented north to south.
Modified from nps.gov (here).

Sweeping views of LA (below) are available from the Temescal Ridge Trail, almost hidden in plain sight from the inhabitants of the city. Bound by mountains in the north, northeast and east, the city sprawls within a sediment-filled, lowland basin that hints at the common genesis it shares with other neighboring basins (such as the San Bernardino and Fernando Valleys) and neighboring crustal blocks (such as the Transverse Ranges and Continental Borderlands of the Channel Islands). They're all located on a narrow slice of the Pacific tectonic plate west of the serpentine line of the San Andreas Fault, drawn from Cape Mendocino, over 200 miles north of San Francisco, to a diffuse region of seafloor off the southeast tip of the Baja California Peninsula.



Tectonologists affirm that in 10 or 20 million years Los Angeles is destined to become an island suburb of San Francisco, as the Pacific plate pulls away from mainland California and subducts into the Aleutian Trench. The history is written in the active faults of the San Andreas Fault system that bound the region.


How did this major transverse anomaly evolve? The Transverse Ranges can be divided into three tectonic regimes that occurred as the Pacific-North American plate boundary and the San Andreas Fault system evolved: subduction (one plate descending beneath another) and two transform (strike-slip) processes of transtension (side-to-side motion with tension) and transpression (side-to-side with compression). Everyone knows how seismically destructive transform boundaries can be with the San Andreas Fault probably one of the best examples. But, they also have a constructive capability, not from the generation of crust but their "transformational" affect on the landscape.


Transform faults such as the San Andreas are responsible for the development of a host of complex structures with a varying geometry on the landscape. Transtension, where strike-slip motion is under tension, can produce rapidly-subsiding pull-apart basins fed by upland sediments (i.e. the Los Angeles Basin); transpression, producing compression, can result in reverse faulting and folding in adjacent crustal blocks (i.e. the mountains and intervening valleys of the Transverse Ranges).
Modified from Plate Tectonics - Continental Drift and Mountain Building by Wolfgang Frisch et al, 2011

In the latest Jurassic, the oceanic Farallon plate began to subduct eastward beneath the continental North-American plate's western rim. The Farallon was separated from the outlying Pacific plate by the East Pacific Rise spreading center, a divergent plate boundary. The region of the future Transverse Ranges was submerged and oriented north-south near the latitude of present day Anaheim and San Diego within the forearc region of the subduction zone, when it acquired the conglomeritic continental shelf sediments of the marine Chico Formation (see accompanying photos). 

By around 28 Ma in the mid-Cenozoic, the Pacific plate had made contact with North America. With the spreading center having entered the subduction zone, it "jumped" onto the continent of North America. That changed the continental margin from an east-northeast, oblique Farallon-North American plate subduction zone to a northwest Pacific-North American plate transtentional boundaryRemaining fragments of the consumed Farallon plate were captured by the Pacific plate and began to move with its motion to the northwest. 

The boundary is the 3,000 km-long San Andreas Fault, which is actually a complex, interlocking broad system of active faults rather than one big sliding margin. It defines the boundary between the North American and Pacific tectonic plates AND between the oceanic plate on the west and the continental plate on the east. With a displacement rate of 6 cm/yr, it's geologically categorized as a right-lateral (dextral) fault, since the block on either side of the fault moves to the right.


Tectonic history of the micro-blocks of continental crust of the Transverse Ranges (green TR) and Outer (OB) and Inner Borderlands (IB): 20 Ma, intitial collision of the Pacific and North American plates; 10 ma, transtension and rotation of the Transverse Ranges block, and Present, transpression and extrusion of the Transverse Ranges block around the larger transpressional bend of the San Andreas.
After Atwater, 1998 and  from Bartolomeo and Longinotti, 2010


So what about the Transverse Ranges block? The change in plate motion caused several blocks of continental crust to break off, including it. The other blocks were captured, but the Transverse Ranges became trapped at the north, causing it to rotate clockwise, ultimately 80-110°. If you've ever driven the bumper cars ride at the amusement park, you will know how your car rotates when you're hit obliquely from the side, if you're blocked on the front. The rotation also opened a slab window at the southern end with extension and thinning lithosphere, which was to evolve into the Los Angeles Basin (see the transform diagram above). 

As the captured microplates shifted to the northwest with the Pacific plate, tension captured Baja California in a similar manner, causing it to rift away from mainland Mexico, transport northwest and form the southern portion of the San Andreas system. The pressure of Baja pushing northwest against southern California created the two transpressional bends in the San Andreas at the Big Bend that trapped the Transverse Ranges block at the east against the larger of the two bends, extruding it westward while compressing it north-south. Compression created uplift and tilting in the range (in both photos). It's amazing how an anomalous transverse range and offset in the San Andreas Fault can be representative of a major tectonic process. 

Here's a Quicktime video by Tanya Atwater summarizing the later stages in the evolution of the Transverse Ranges block. It shows the growth of the Pacific-North American plate boundary from 20 Ma to the present and demonstrates the evolution of the San Andreas Fault system, emphasizing the rotation of the Transverse Ranges block within the plate boundary region. Credits to Tanya Atwater at http://emvc.geol.ucsb.edu (here).


video


The geologic history of the Transverse Ranges can be chronologically summarized as late Mesozoic Farallon plate subduction, Oligocene collision of the Pacific and North American plates with transition from subduction to a transtensional margin of the San Andreas Fault system, early Miocene microplate generation and capture, middle Miocene Western Transverse Ranges rotation and formation of the Los Angeles Basin and the Gulf of California, early Pliocene capture of Baja California with ongoing Ranges rotation and shifting to a transpressional tectonic regime, and finally Pleistocene transport of the Baja with Transverse Ranges ongoing rotation accompanied by compression, uplift and faulting.



Close look at the Cretaceous-deposited, Pleistocene-uplifted and tilted beds of conglomerate and interbedded shale of the Chico Formation on the Temescal Trail.

What's the other "transverse" range in the United States? It's the Uinta Mountains, a sub-range of the Rockies in northeastern Utah and a bit of southern Wyoming. Its genesis is also related to the geo-antics of the Farallon plate but in a different time frame and tectonic regime.


Quite by accident on our descent about 75 miles east of L.A. International Airport, I took this photo of the young and rapidly-rising San Bernardino Mountains, a sub-range of the Transverse Ranges. This is Mill Creek, a tributary of the Santa Ana River that follows the trace of the Mill Creek Fault, a now-inactive strand of the San Andreas Fault system. The narrow canyon is the result of erosion of the highly fractured rock around the linear fault, along which five miles of right-lateral strike-slip displacement occurred 500,000 to 250,000 years ago. Presently-active strands of the fault system lie to the south (top right) and lead to Palm Springs 35 miles to the southeast (upper left out of view).

"Copy and Paste" the following co-ordinates into Google Earth to hike in Temescal Canyon: 34°03'32.49"N 118°31'47.94" W

References Cited: 
• Tectonic History of the Transverse Ranges by Eleanor S. Bartolomeo and Nicole Longinotti, 2010
• Microplate Capture, Rotation of the Western Transverse Ranges and Initiation of the San Andreas Transform Fault System by Craig Nicholson et al, 1994.
Plate Tectonic History of Southern California with Emphasis of the Western Transverse Ranges and Northern Channel Islands by Tanya Atwater, Dept. Geol. Services, UC, 1998.
Plate Tectonics - Continental Drift and Mountain Building by Wolfgang Frisch et al, 2011.

February
"Frozen" Hawaiian Lavacicles in Subterranean Lava Tubes

Less than one inch in length, thousand year-old lavacicles dangle from the ceiling of the Kula Kai Cavern beneath the southwest slope of Mauna Loa volcano on the Big Island of Hawai'i. Kula Kai is a segment of the Kipuka Kanohina Cave Preserve, currently the world's second-longest surveyed lava tube system with over 20 miles of anastomosing caverns. The ceilings are as high as 20 feet with towering cathedral vaults. Kula Kai is open to the public by reservation only.

Common in the Hawaiian Islands, lava tubes are subsurface conduits of hardened lava formed beneath surficial lava flows that emanated from a vent on the flanks of a shield volcano. Being basaltic in composition with low gas content and at high temperatures, lava flows downslope with relative fluidity. Initially, channels form within pahoehoe, a ropy and smooth form of lava, which may break down and form a master tube as they coalesce. Alternately, they form when a channel roofs crusts over. 

Tubes are excellent insulators, allowing lava to efficiently and quickly (up to 35 mph) travel many miles to the flow front. Temperature drops of only 15°C have been recorded over 15 km within lava tubes. They may be filled with flowing molten lava, reactivated if invaded by a subsequent eruption or abandoned when evacuated. A long cave-like subterranean "master" channel may develop complex anastomosing connections, multi-level branching and perched tributaries. Red Slope Cave in Kilauea Crater is at least 1,828 feet long. Aerial photographs suggest that over 80% of surface flows are fed by tubes with thousands of cave entrances. Once the lava supply has extinguished, the lava tube drains leaving an evacuated cave system. It's important not to underestimate the significance of these subterranean eruptions pumping lava downslope by adding 10-170 acres/year of land to the island.


Roofing of Lava Channels: A, Surface crust develops across a channel; B, Crust breaks into rafts that jam constrictions and are welded into a roof; C, Overflow builds a levee that arches over and joins as a roof.
Modified from Lava Tube Formation by Ken G. Grimes, 2005


Lava tubes typically have flat floors built up incrementally by successive flows and are littered with blocks that have fallen from the ceiling and welded to the floor. They have a rounded architecture often punctuated with 20 foot-high cathedral ceilings, cooling cracks, accretionary lava balls and curb-like benches with flow lines and levees that mark the level of previous flows. If near the surface, dangling tree roots such as ʻōhiʻa may penetrate the roof. 






"Frozen" lavacicles are often found suspended a few centimeters from the ceiling. Referred to as lava stalactites, they form as lava cools over the course of hours to weeks, which differs from stalactites in limestone caves formed from the evaporation of carbonate saturated water over millions of years. Similarly, lavacicles may drip onto the floor of the tube and create lava stalagmites.


Lavacicles in Kula Kai Caverns coated with secondary minerals.

Frequently, secondary mineralization in the walls and ceilings occurs from the leaching of trace minerals from infiltrating groundwater followed by deposition. As opposed to primary mineralization that occurs during the formation of the lava tube, secondary occurs after the cave formed or during its cooling process. Calcite is common, appearing in the form of whitish coralloids (nodules), crusts and coatings. Gypsum and other sulfate salts appear as crusts and puffballs, formed by the evaporation of seepage waters similar to speleothems ("cave deposits") found in limestone caves. Unlike calcium-rich waters in limestone caves, calcium in lava tubes likely comes from the breakdown of anorthite (calcium-rich) feldspar, one of the prominent mineral fractions contained within basalt and one of the least stable. Bright olive-green patches are a hydrated copper-vanadium silicate, likely deposited from fumarole gases at high temperatures. 




Where water is present and promoted by the cave's protection from harsh surface conditions such as ultraviolet light, growth of greenish algae-like microbial coatings are favored such as seen on this shark-tooth, lava dripstone on the cave wall. These form as lava drains from the tube and leaves linings on the walls that begin to drip. Microbiologists study these biomarkers in light of recent evidence from Mars and other bodies in our solar system that might potentially harbor life in volcanic caves.




The Hawaiian Islands might be the best place to study lava tubes. On the Big Island, the only one with active volcanoes, you can watch them form on the surface, look into them via skylights from roof collapse and explore empty tubes in all stages of degradation. They are of interest to volcanologists (who study the process of volcano formation), biologists (who study obligatory cavelife called troglobites), chiropterologists (who study the endangered Hawaiian Hoary bat), microbiologists (who study microbial communities), archaeologists (who study early Hawaiians who used the tubes for shelter, burial chambers, petroglyphs, refuge during war and possibly rain catchment), vulcanospeleologists (who seek the thrill and challenge of exploration), tourists (out to have a good time) and geologists (who take it all in). 

"Copy and Paste" the following co-ordinates into Google Earth for Kula Kai Caverns: 19°04'00.50"N 155°47'57.92"W

July
Middle Devonian "Turkey Tracks"

Lichen-encrusted, Ordovician-age "turkey tracks" in Littleton schist of Mount Monadnock in the southern White Mountains of New Hampshire

In 1802, a central Massachusetts farm boy named Pliny Moody was plowing the family fields in the rural hamlet of South Hadley. By chance, he uncovered a slab of rock that contained a series of three-toed footprints set in mudstone from the Late Triassic strata of the Deerfield aborted rift basin. It was the first recorded discovery of dinosaurs in North America, but the definitive connection wasn't made until 1824 in England. At the time, Pliny's colossal discovery was identified as the footprints of Noah's raven from the Biblical flood. According to the story, the specimen became the door stop to Pliny's home, which was later substantiated as non-Biblical but definitely avian by Professor Hitchcock of Amherst College, Massachusetts, a leading vertebrate ichnologist. 

It wasn't so far fetched that a similar find should also have a similar ornithological explanation, only not related to Noah's Flood. On the upper flanks of nearby Mount Monadnock in the southern White Mountains of New Hampshire, stampedes of four-inch long "turkey tracks" abound, called as such for over 100 years. Only there, they're in much earlier, Ordovician-age metamorphosed rock of the Littleton Formation, a gray-weathering pelitic schist and micaceous quartzite. From a taphonomous (fossil preservation) standpoint, rocks such as these that have been submitted to considerable heat and pressure at great depth - which were deposited and later deformed in the Middle Devonian Acadian foreland basin - rarely preserve fossils.



Mount Monadnock in southern New Hampshire from the west

Pliny's ichnofossils are traces or tracks of lifeforms rather than preserved organic remains, whereas Monadnock's turkey tracks are pseudomorphs or "false forms". They are crystals consisting of one mineral but having the form of another which it has replaced. Thus, sillimanite pseudomorphs regionally metamorphosed from andalusite are found within the schist that preserve chiastolite cross-shaped inclusion patterns - our turkey tracks.

"Copy and Paste" the following co-ordinates into Google Earth to visit Mount Monadnock: 42°51'40.22"N 72°06'30.36"W
   

September
A Race Against Tides and Time

Sunrise on the southern Jersey Shore

In September, I spent a week on the "Shore", Jersey that is, downbeach from Atlantic City. No gambling. Off season. Just R&R with pleasant morning strolls on the beach (barefoot of course), while contemplating life's infinite possibilities and the geological evolution of the Atlantic coastline. 

"AC" and its abutting three towns are delicately perched only a few feet above sea level on a nine mile-long and barely one mile-wide barrier island called Absecon in southern New Jersey. By definition, it's a long, narrow and extremely flat, offshore deposit of shifting sand, unconsolidated sediments that lie parallel to the coastline. Typically, Absecon Island's sandy beaches and "world famous" boardwalk face the open sea, while escalloped "washouts" and "washovers" face a shallow tidal bay and the mainland. 

From global studies of beach morphodynamics, there are many different kinds of beaches from both a morphological and processes perspective. Beaches also differ in terms of composition and grain size. Suffice it to say that the main types are Arctic, bay mouth, sandur, composite, accidental, man-made and lagoonal barrier islands that occur in a wide range of environments.



Densely populated Long Beach Island is two barrier islands north of Atlantic City's Absecon Island. Typically, a beach faces the sea, while escalloped "washouts" and "washovers" face the mainland. Between the two is the open water of a shallow tidal bay, sound or lagoon, which provides a rich habitat for wildlife. Natural environments and habitat zones vary from island to island and generally include dunes, swales, maritime forests, marshes and tidal flats.

Barrier islands are found along 13% of the world's coastlines and are a characteristic of the Atlantic Seaboard's relatively flat Coastal Plain physiographic region. The geologic province extends some 2,200 miles from Long Island to Florida and west to the sea from the 900-mile long, fall line escarpment of the Piedmont region of the Appalachian Mountain Range. Barrier islands generally lack bedrock, although underlying structures may have a profound influence on their geomorphology. Why are they largely found on North America's east coast? Why not the Pacific Coast? Why not the coast of New England?


Schematic of a barrier island system from the seaward beach to the landward marsh with components labelled
Modified from Reinson, 1992


Barrier islands are generally viewed as static landforms, unless a storm rolls in with flooding and high winds that rearranges the beaches. The truth is that the entirety of barrier islands are dynamic places at ALL times. They're the buffers between land and sea. Like organisms, they're evolving entities, absorbing energy and changing their shapes in reaction to changing circumstances. They're in constant motion from wind-driven, microscopic sand-transport via saltation, constant waves and diurnal tidal cycles, and long-term global changes in the level of the sea. How did these coastal geological "lifeforms" evolve?

They're features of passive rifted continental margins in contrast to active margins that border the Pacific Ocean, which are plate boundaries between continents and oceans that are either subducting or slip-sliding along the infamous Ring of Fire. Active margins typically exhibit volcanism, mountain-building and seismic activity; whereas, passive margins, especially more mature ones, are typified by subsidence and sedimentation. 




North America's Atlantic shores, fronted by barrier islands, are products of the fragmentation of the supercontinent of Pangaea in the Mesozoic. Throughout the Cenozoic, an abundance of clastic sediments were largely derived from erosion of the Appalachian highlands and delivered to the coast by large river systems that were reworked by tides and fluctuating levels of the sea. 

Offshore, the ocean bottom is bordered by a broad, gentle continental shelf that played a crucial role in the origin and distribution of the Atlantic's barrier islands and their beaches. The wide shelf dissipates wave energy moving sand and acts as a repository for coastal landform replenishment. As sea levels rose and fell in response to tectonic processes and orbital parameters, the position of the shoreline transgressed landward and regressed seaward. Glacial cycles of the Pleistocene, that either sequestered or delivered water to the seas, are responsible for the appearance of the barrier islands we see today, a landscape that's only 8 to 10,000 or so years old.






Franklin the Border Collie (above) is demonstrating a few features of the beach, another ever-changing entity of barrier islands. He's standing on a berm of the backshore part of the beach that extends landward from the sloping, wet foreshore. It's a nearly horizontal terrace formed by the deposition of sediment by receding waves. The line of vegetation and extraneous debris is a drift or wrack line and is a good indicator of the high tide or storm wave limit. Lesser lines can be found, the lowest of which marks the normal high-tide line. Storms dramatically widen and flatten the above-water beach, which serves to dissipate wave energy. Below Frankie's droopy tongue is a storm scarp formed by wave undercutting. 

In reality, this is a daily-groomed, artificial beach that has been extensively and repetitively "renourished" for decades by the U.S. Army Corps of Engineers at great expense and is ultimately ineffective against rising sea levels. Tidal gauges at Atlantic City show the sea is rising at 3.8 millimeters per year or over a meter a century. That's over twice the global average eustatic rate of 1.7. That will force barrier islands to migrate landward, such as Absecon Island, up and down the east coast, as they have done repetitively throughout the Cenozoic. 





Why aren't barrier islands generally found off the coast of New England? Although the entire Atlantic coastline of North America share's a similar tectonic history, the north coast, above Long Island, New York possesses a glacial heritage that stripped the Coastal Plain sediments and deposited them south of the extent of glaciation, while bringing rocks of all sizes down to the coast from the oldest and first to form section of the Appalachians. There are regions of Coastal Plain sediments offshore in the northeast such as the Georges Bank 120 km off the coast of New England and the Grand Bank of the Canadian Maritimes.

"Copy and Paste" the following co-ordinates into Google Earth to stroll on the beach where the photo of Frankie was taken: 39°19'57.74"N 74°29'06.57"W

Reference Cited:
Atlantic Coastal Beaches by William J. Neal et al, 2007.

October
A Geological Traverse of Franconia Ridge

Perched at treeline and facing east from the Appalachian Mountain Club's Greenleaf Hut, before you lies the Franconia Ridge Traverse. In the foreground is proglacial Eagle Lake tarn. From right to left (south to north) and connected by cols are landslide-scarred Little Haystack, Mount Lincoln and Mount Lafayette at 5,249 feet. The bedrock is Jurassic Conway granite.

It's late October and close to freezing in the landslide-scarred Franconia Range of the White Mountains of north-central New Hampshire. An "ever-green" blanket of black spruce and balsam fir is at high elevations lies in contrast to glorious, deciduous fall colors down below. The range is a tiny section of the 2,160 mile-long Appalachian Trail that runs from Georgia to Maine along the spine of the Appalachian Mountains. Directly behind me (below), is a spectacular glacially-carved mountain pass in Jurassic Conway granite and a sediment-filled, U-shaped glacial-valley called Franconia Notch. It's one of dozens in the "Whites."  



Facing west from the summit of Mount Lincoln at 5,089 feet, the mountain pass of Franconia Notch is on full display. Cannon Mountain presents classic exfoliated granite with reverse steps on the cliff-face and a massive talus heap at the base. The now-collapsed, facial profile of New Hampsire's famous Old Man of the Mountain rock formation was on the far right of Cannon Cliff. Off to the south (left), the Cannon Range is juxtaposed to the Kinsman Range. In the foreground Agony Ridge, which defines the east side of the mountain pass, descends in a narrow landslide-prone spine from Mount Lafayette. Facing the Green Mountains of Vermont, the Bethlehem-Littleton lowlands to the north are outside the White Mountains proper and possess classic moraines and other glacial and postglacial features.

My son and I drove up from Boston to hike the famous Franconia Ridge Traverse (aka "The Loop"), which generally encompasses Little Haystack, Lincoln and Lafayette Mountains. National Geographic extols the 8.9-mile, 7-8 hour trek as "The World's Best Hike," but its not to be taken lightly. It's difficult, unforgiving and relentless, rising 3,480 feet in the first four miles! But once above the fall line, you remain there with stunning open views of Franconia Notch to the west, and to the east, the Pemigewasset Basin, and beyond, the high peaks of the Whites.



From above treeline and looking north from Little Haystack, the saddles connect the Mounts of Lincoln and Lafayette. The Franconia Range is second in height to the high peaks of the Presidential Range in the White Mountains of New Hampshire, a sub-range itself of the northern section of the Appalachians.

Tectonic cognoscenti know the Appalachians are the North American portion of a transglobal chain of mountains that formed during the collision of the minor supercontinents of Laurussia and Gondwana that led to the formation of Pangaea by the end of the Paleozoic. When Pangaea broke apart in the early Mesozoic, it divided the Central Pangaean Range into fragments that drifted across the Atlantic on the backs of the continents of the modern world. That event left the Appalachians in residence along North America's eastern margin. 

Most of the White Mountains consist of highly metamorphosized schists and gneisses formed during the Acadian Orogeny, which began in the middle Devonian. The majority of New Hampshire's Mesozoic rocks, such as the Early Jurassic Conway Granite of Franconia, belong to the White Mountain Plutonic-Volcanic Suite and are related to Pangaea's rifting, drifting, and the opening of the ocean. The Franconia Range forms a massive ring dike in the western half of the White Mountain batholith, a large composite of several bodies of magma.

Followed by 100 million years of erosion and exhumation, the icing on the cake occurred during the Pleistocene and Holocene, when the Laurentide Ice Sheet from Canada bulldozed across New Hampshire and left a myriad of erosional and depositional features on the landscape.


My son Will and Flash the Husky on the summit of Mount Lincoln in the Franconia Range of the White Mountains of New Hampshire.

"Copy and Paste" the following co-ordinates into Google Earth to visit Mount Lafayette: 44°09'39.11"N 71°38'40.36"W

References Cited: 
• The Geology of New Hampshire's White Mountains by J. Dykstra et al, 2013. 

That's it for 2015. Thanks for following and contributing to my blog. 
I'm humbled by your comments and most appreciative of your visits. 
Have a Happy and Healthy New Year! See you in 2016.

3 comments:

  1. Thank you, Dr. Share, for these fascinating posts! Since moving to San Diego, I always wondered why the San Gabriel and San Bernardino ranges were transverse as opposed to peninsular, like the ranges in San Diego County are. Now I know!

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  2. There's an explanation for everything. Unfortunately, I only have a few. Thanks for visiting and the comment! Jack

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  3. Looks like you had a great year, Jack! Best wishes for another one in 2016.

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