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).
|My front walkway and the Alps of MIT|
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.
|South-facing view of the Niagara River, Gorge and Falls between New York State and Canada|
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
The Confluence of Two Rivers Named Colorado
|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
The Western Transverse Ranges, A Major Tectonic Anomaly
|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.
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.
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 boundary. Remaining 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.
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).
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.
"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
• 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.
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.
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
|Lichen-encrusted, Ordovician-age "turkey tracks" in Littleton schist of Mount Monadnock in the southern White Mountains of New Hampshire|
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
"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.
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.
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
• Atlantic Coastal Beaches by William J. Neal et al, 2007.
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.
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
• 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.