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.

Wednesday, September 23, 2015

Flying the Geology of the Island of Hawai'i: Part I - Introduction to Volcano Country

'Eli 'eli kau mai. 
"Let awe possess me."
Ancient Hawaiian chant

According to legend, Pelehonuamea is the Goddess of Fire. She is one of a pantheon of gods, goddesses and guardians that Native Hawaiians recognize, respect and revere. Pele resides in the fire pit of Halema'uma'u within Kīlauea caldera, but her domain encompasses all volcanic activity on the island of Hawai'i. 

Referred to as Madam Pele or Tūtū Pele, as a sign of respect, she causes earthquakes by stamping her feet, hurls molten fountains into the air, governs great flows of lava and reveals herself throughout the island. Her divine strength is often misunderstood because of her fiery nature and is frequently mistaken for an angry deity. Instead, Pele uses fire to purify and rejuvenate, and is considered to be both creator and destroyer of the Hawaiian Islands. In regards to the latter, with all due respect to the Madam, geologists beg to differ.


Halema'uma'u pit crater within Kīlauea caldera is hallowed ground to Hawaiians.


Pele was born in Tahiti and was among the first voyagers to Hawai`i by canoe, while being pursued for seducing her husband by her angry older sister Namakaokaha'i, Goddess of the Sea. She first landed on Kauai`i, but every time she thrust her digging stick into the earth to excavate a crater, her sister would deluge it with water. Moving down the chain from one island to the next, she searched for a place to bury her sacred fire that was safe from her sister's wrath. Eventually, Pele landed on the last island and dug her home deep within Halema'uma'u where she remains to this day. 


Artist Arthur Johnsen's version of Pele from Wikipedia


Handed down through ancient songs, chants and dance, the legend of the early Hawaiians shows they recognized that the islands were progressively younger moving down the chain from the northwest to the southeast, which happens to coincide precisely with contemporary geological observations. Of course the mythological explanation for the chain's evolution differs from that of most geologists. They advocate that the chain is the consequence of an oceanic lithospheric plate drifting over a relatively fixed hotspot of volcanic activity fed by an ascending thermal plume originating from within the deep mantle. Let's investigate.


Yours truly with one of Sunshine Helicopter's Black Beauties


A VIEW OF VOLCANO COUNTRY LIKE NO OTHER
Undeniably, the most intimate geological character of the Hawaiian landscape is achieved on foot, but over 90% of the islands are only accessible from the air. What better way is there to gain a "big picture" perspective of the volcanic terrain! This post is the first of four on the geology of the island of Hawai'i - better known as the Big Island - and the evolution of the Hawaiian Island chain. It covers my arrival on a commercial flight and ends with a private, two and a half-hour geo-tour of the island by helicopter. Where contributory, I've interjected a few ground-level photos.

• Post I has some relevant Hawaiian geography and includes an introduction to two antithetical hypotheses regarding the evolution of the volcanic island chain. It ends with my arrival flight on the Big Island.
 Post II initiates my helicopter geo-tour of the island from the Waimea Plain above the Kohala Coast and heads south through the lofty Humu'ula saddle between volcanoes Mauna Kea and Mauna Loa. 
• Part III continues over Kilauea caldera, its active Halema'uma'u pit crater and nearby Kilauea Iki crater and then follows the East Rift Zone past Kilauea's active Puʻu ʻŌʻō cinder cone to the populated east coast town of Hilo.
• Part IV heads north along the east coast's sea cliffs to the dramatic gorges of the Kohala Mountains and finally back to the heli-pad.


Flight Paths
This oblique, north-facing view of the island of Hawai'i illustrates the flight path on the arrival of my passenger jet (Post I) and my helicopter geo-tour of the island (Posts II-IV). The Big Island's five volcanoes are labelled as such. Vertical exaggeration is increased and color-enhancement is utilized to better visualize the topography. Notice the the "Ring Road" (red) consisting of highways 19 and 11 that encircle the island. The Saddle Road between volcanoes Mauna Loa and Mauna Kea connects the coastal town of Hilo on the east with the Kona Coast on the west. The original computer-rendered Truflite image was generated by Martin Adamiker and used with Wikimedia Commons permission. 


SOME HAWAIIAN GEOGRAPHY
The linear chain of Hawaiian Islands pokes through the waters of the north Pacific Ocean basin almost 1,900 miles from the nearest continent and a good 2,400 miles from Los Angeles. That makes it the planet's most isolated population center. It's also the world's oldest and longest chain of volcanic islands at 1,523 miles from Kure Atoll in the northwest to the island of Hawaii in the southeast. An atoll is a ring-shaped coral reef that encircles a lagoon, and in the case of the Hawaiian Islands, sits atop the eroded and subsided rim of an extinct volcano called a seamount. Oceanic isolation makes the length of the Hawaiian Island chain deceiving. If superimposed on the continental U.S., the chain would extend from San Francisco to Houston. 




Geography aficionados also know that the entire archipelago includes 137 or so islands, smaller islets, atolls and seamounts. It's subdivided into "upwind" Southeastern or Windward Islands, which are the "main" Hawaiian Islands, and "downwind" Northwestern or Leeward Islands. The 'wind' refers to the Trade Winds that blow from the northeast, north of the equator, and the reverse from below. The Windwards are part of the State of Hawai'i and consist of a few unincorporated U.S. Minor Outlying Islands such as famous Midway Atoll, appropriately named for its oceanic locale and is the site of the decisive naval victory over the Japanese in World War II. 


The Hawaiian archipelago includes an oceanic mix of seamounts, atolls and islands. 
Numerous seafloor topographic features appear such as fracture zones, and the Hess and Shatsky Rises.
From soest.hawaii.edu


THE "MAIN" OR "PRINCIPAL" HAWAIIAN ISLANDS 
The Hawaiian Island chain was originally known to Europeans and Americans as the "Sandwich Islands", named by the first European to discover the islands, British Captain James Cook in 1778, after his sponsor the fourth Earl of Sandwich. In 1819, the chain was renamed the Kingdom of Hawai'i by King Kamehameha, who united the islands. China referred them as the "Sandalwood Islands" in the early 1800's from the importation of sandalwood, which was used for its fragrance in incense and medicinal purposes. In the 1840's, the name of the largest island of Hawai'i was adopted by the entire chain. In 1848, Hawai'i was annexed by the U.S. for geopolitical reasons and became the fiftieth state in 1959.


Map of the Sandwich Islands made by one of Cook's officers in 1785. Although some claim it was drawn by midshipman William Bligh, of Mutiny On The Bounty fame, the British Office credits Henry Roberts as the hydrocartographer. It was created on Cook's fateful third voyage to the South Pacific, when he was killed by angry Hawaiians at Karakakooa Bay (inset) on the Big Island's west coast. By the way, Cook was looking for the Northwest Passage, a sea route connecting the North Atlantic and Pacific Oceans, which exists through the Arctic. Notice the older spelling of the island of Hawai'i as "Owhyee."


The archipelago is ~1,600 miles long (2,400 km) and includes eight major islands on the southeast end of the chain: Ni'ihau (private), Kaua'i (the "Garden Isle"), Oahu (Honolulu, Diamond Head and Pearl Harbor), Molokai (highest sea cliffs in the world and former leper colony), Lana'i (think pineapples), Maui (Haleakala National Park), Kaho'olawe (uninhabited) and the volcanically active island of Hawai'i (think Kona coffee, macademia nuts and Volcanoes National Park). 

The eight islands comprise over 99% of the chain's 6,425 square miles of land, yet only 1.5% of the chain is actually above sea level - less than 6 square miles. The remainder represents an expansive, mountainous aseismic ridge (as opposed to a seismically-active mid-ocean ridge) of extinct undersea volcanoes. The amount of lava that erupted to form the ridge - about 186,000 cubic miles - is estimated to be more than enough to cover the State of California with a layer one mile thick. All the volcanoes in the Hawaiian chain, whether subaerial or submarine, are genetically related, are a shield type of volcano and grow progressively younger as one moves down the chain.


The Southeastern, Windward or "main" Hawaiian Islands extend from Ni'hau to submerged Lo'ihi. Progressing from northwest to southeast, the island's of the chain become younger, larger, taller, less eroded, less subsided and more volcanically active. Historic (recorded) eruptions on the island of Hawaii are in red and emanate from Hualalai, Mauna Loa and Kilauea.
Modified from Wikipedia


WHAT'S A SHIELD VOLCANO?
Without exception, every island is comprised of either one primary volcano or is a composite of coalesced volcanoes. Fifteen volcanoes formed the eight major islands, five of which reside on the island of Hawai'i. Every volcaniform is a shield volcano - gently-sloping, massive and broad with a low profile ("shield-like"), generally effusive rather than explosive in eruptive behavior and composed almost exclusively of basalt, a mafic (high levels of magnesium and iron) magma.

In contrast, composite or stratovolcanoes, which are common at subduction zones (a convergent plate boundary), characteristically possess a steep profile, are generally explosive in behavior, emit lavas of high viscosity, form from felsic magmas (high levels of feldspar and silica), and are layered, as are shields, but with lava, tephra, pumice and volcanic ash. 

Idealized diagram of a Hawaiian eruption of a shield volcano.
Fed by a central conduit from a magma chamber and accessory dikes and sills, the summit crater holds a lava lake. Lava exudes from vents located along rift zones on the flanks of the volcano and is responsible for the edifice's layered architecture.
Modified image of Semhur from Wikipedia.


GOT BASALT?
Basalt (see chart below) is a hard, dark (due to high magnesium and iron and the crystalline structure of its mineral constituents of pyroxene, olivine and amphibole) igneous rock with a low silica (due to low quartz) content (less than 20%). That, along with its high temperature (1000 to 1200°C) and reduced gas content (mostly water vapor, some carbon dioxide), confers basalt with a low viscosity (high fluidity). The chemical composition and temperature of the magma in turn favors the construction of a "flow-built", lava-layered, shield-shaped, archetypal Hawaiian volcano. 

The lava that erupts tends to flow freely just beneath the surface and, upon eruption, exudes from vents on the shield's flanks or from the summit and flows with little resistance across the landscape. Thus, the eruptive style is characteristically "Hawaiian" - gentle, infrequent, weakly explosive or nonexplosive with the nonviolent release of volcanic gases (due to low viscosity), and relatively benign. It explains how you can "safely" get close to an erupting Hawaiian volcano such as Kilauea (not that it can't be violently explosive). Strombolian-style eruptions, marked by relatively mild, explosive bursts of gas and the emission of glowing-red ejecta (cinder, lapilli and bombs), occur at cinder cones that erupt on the flanks of Hawaiian volcanoes.

As mentioned, this is in contrast to viscous, gas-rich felsic magmas that suppress easy escape of volcanic gases causing pressure to buildup underground and erupt with an explosive release of gas and magma fragmentation such as Mount St. Helens. These two contrasting eruptive styles, respectively, are located at intraplate locales (at a distance from plate boundaries) versus volcanoes located along plate margins. A discussion of where and how basalt originates within the mantle to begin with will be deferred until post Part III.


In the early 1900's, geochemist N.L. Bowen determined that different minerals crystallize at different temperatures during the cooling of magma. This chart demonstrates the reaction rates and where basalt is positioned in the sequence in its evolution of the various igneous rock types. Basalt can be seen to be composed largely of the minerals olivine, pyroxene, amphibole and calcium-rich plagioclase feldspar.
Modified from nature.nps.gov


THE "BIG ISLAND"
The island of Hawai'i is the largest (4,021 sq.mi.) in the Hawaiian chain and the only island with a presently active volcano. It's also the largest island, about the size of Connecticut and twice the size of all the major islands combined - and still growing, magmatically! It's also the youngest and southernmost in the chain, which is in keeping with the formative geological history of the entire island chain. 

In order of eruptive age, the five shield volcanoes that form the island are: Kohala (extinct, oldest and northernmost), Mauna Kea (dormant and tallest with the world's largest collection of telescopes); Hualalai (third youngest and third most historically active); Mauna Loa (recently active, largest and tallest volcano on Earth measured from the seafloor and almost twice the height of Everest!) and Kilauea (most active, youngest and southernmost). 

The traditional nomenclature for volcanic activity is rather simplistic and nondescript: extinct (no eruptions for at least 10,000 years and not expected to erupt again); dormant (an active volcano that is currently not erupting); and active (since the last ice age). For Hawaiian volcanoes, geologists use a system of "life stages" that is temporally, structurally and geochemically more descriptive and relates to the distance from the Hawaiian hotspot. Life stages will be discussed in greater detail in my post Part II.


Map of the island of Hawaii and the surrounding ocean floor with water depths shown in colors. Gray areas are exposed land, while colors indicate water depth. The island includes five volcanic centers that have coalesced and two seamounts, inactive Makukona in the northwest and active Loihi in the southeast. Hilo and Puna Ridges, that extend some 50 km from the shoreline, have yielded the oldest known ages from pillow basalts for the island of Hawai'i at 1138 to 1159 Ka. They are interpreted as submarine rift zones from Kohala and Mauna Loa, one of three that typically radiate from Hawaii's basaltic volcanoes. The historical lava flows of Hualalai, Mauna Loa and Kilauea are highlighted in red. From hvo.wr.usgs.gov


VOLCANOES TRENDING
One can connect the Big Island's five volcanoes and those of the entire chain with two curvi-linear strands called trends. Hualalai and Mauna Loa lie along the Loa trend in the south, while Kohala, Mauna Kea and Kilauea lie along the Kea trend in the north. The trends likely extend beyond the islands beneath the sea and are geographically and geochemically distinct (such as lead and other isotopes), although some Loa compositions have erupted from Kea volcanoes and vice versa. 

The trends are important clues to the genesis of the volcanic chain and the construction of a plume - a trans-mantle, ascending conduit that is thought to feed the Hawaiian hotspot. Is the plume divided into halves that preserve differing compositions over extreme length and timescales? One explanation for the trends is proximity to the plume such that higher Kea melting temperatures are closer than Loa compositions that erupt from volcanoes on the plume margin.



In addition, both Mauna Loa and Kilauea, being the only active volcanoes on the south side of the island and one along each trend, rarely erupt simultaneously with few exceptions. The long intervals of repose for one volcano appear to correlate with increased activity of the other. Alternating activity might imply that both volcanoes may alternately tap the same deep magma source. This is in spite of physical evidence that each volcano possesses its own shallow magma reservoir that operates independently of the other, and that the lavas of the two volcanoes are chemically and isotopically distinct. There is much to be learned about the subterranean plumbing of the volcanoes and their relationships to the trends.

A SIXTH, SEVENTH AND "LOST" EIGHTH VOLCANO?
Although comprising five volcanoes, the island of Hawai'i actually began its formation with a sixth and will likely include a future seventh volcano. Mahukona (see map above) is an extinct seamount on the island's northwest flank that ended its building stage some 470,000 years ago. It rose 700 to 800 feet above the waves before eroding and isostatically sinking beneath the waves to 3,600 feet below sea level. Mahukona lies on the Loa trend.

Consistent with age-progression and lying along the Kea trend ~35 km off the island's southeast shore is intermittently-active Lōʻihi seamount in the youthful submarine stage of evolution. Discovered seismically in 1996, the "Long One" is possibly ~400,000 years old. Its height is a respectable ~3.5 km (2.2 mi) above the seafloor, and although hidden 3,100 feet beneath the waves, is comparable to Etna volcano in Italy and taller than Mount St. Helens prior to its 1980 cataclysmic demise. Although young geologically, Lōʻihi parallels the formative evolution of Mauna Loa and Kilauea with possibly pre-calderic summit pit craters and two radiating ridges that are interpreted as rift zones.


I captured this west-facing image of "long" Lōʻihi on the seafloor with Google Earth at a depth of some 16,000 feet below sea level, the average depth of the ocean around Hawaii. The submerged slope of the island of Hawaii can be faintly seen in the uppermost right corner.


Geologically, Lōʻihi possesses marine-induced pillow basalts and fresh glassy crusts (hyaloclastites) characteristic of its youth. Geochemically, it is transitioning from an alkalic basalt (from low degrees of partial melt) to a tholeitic basalt-generating seamount (from higher degrees of partial melt), thought due to its relationship to the Hawaiian plume. Should Lōʻihi emerge subaerially, estimated in a several tens of thousands of years, it will become the newest island in the chain and likely merge with the Big Island considering its proximity. By then, the existing volcanoes of the Big Island may be extinct or close to it, following the pattern that has progressed through the chain since the late Mesozoic and throughout the Cenozoic.

Computer generated, color enhanced, refined image of Lōʻihi
From soest.hawaii.edu


A "lost" eighth volcano? Isotopic findings by some researchers (Bilchert-Toft and Albarede, 2009) suggest the presence of lavas from more than one volcano in drill holes on the flanks of Mauna Kea. It stopped erupting some 550,000 ka., indicating that not every Hawaiian shield emerges from its submarine locale and how little we know about the evolution of the Hawaiian volcanic chain. 

TWO ISLAND CHAINS SEPARATED BY A BEND
The Hawaiian chain continues northward as the Emperor Seamounts, named mostly after Japanese emperors. The two chains, composed of over 80 volcanoes, form the remarkably long, 3,600 mile (5,800 km) Hawaiian Ridge-Emperor Seamount chain - another deception. It spans the distance from Los Angeles to Greenland! In the words of John McPhee in Annals of the Former World, the Emperor's volcanoes have been been "defeated by erosion" and "stand below the waves" as seamounts. And like the Hawaiian chain of volcanoes to the southeast, the Emperor chain continues the phenomenon of younging age-progression to the south and increasing subsidence and erosion to the north. I've inserted a few dates on the map for comparison from seamounts Lōʻihi to Meiji.

Curiously, the two linear chains are separated by a bend (below), variously referred to as a "kink" or "dogleg". Measuring some 130°, the bend has been used to calculate the velocity and direction of motion of the Pacific plate at ~7-11 cm/yr (~3-4 in/yr) along a WNW trend. The path is linear except at the kink in the chain, where the Emperor chain abruptly assumes a NNW trend. The bend has been dated too at~47 to 48 Ma. The successive dates of the volcanoes progressing down the chain give the illusion that they are moving, when of course it's the Pacific plate that is moving. Or is it? 

The bend in the Emperor-Hawaiian chain is the prototypical problem at the heart of the debate about the geological origin of the entire archipelago and is debated by two diametrical opposed groups of geoscientists - one popular and the other gaining ground. 


Seafloor map of the north Pacific basin with Kuril-Kamchatka and Aleutian trenches to the north and northwest, fracture zones that intersect with the Emperor-Hawaiian volcanic chains, the enigmatic "bend" and various dates of progressive volcanic activity. There are many topographic features of relevance to the Pacific plate and the genesis of the E-R chain, however, they are unlabeled and are beyond the scope of this post. Examples include the Shatsky and Hess Rises, the Musicians Seamount and the Mid-Pacific Mountains.


With the bend in between, the dog-legged chains are on the northern half of the Pacific plate, the world's largest at over 103 million square kilometers. It extends from the continental volcanic arc systems and divergent plate boundaries of North and South America to the Asian island arc subduction systems. In the south, it spans from the extensional mid-ocean ridge system of the Antarctic plate to the Okhotsk, North American and Eurasian plates in the north, where the oceanic plate is subducting into the extensive Kuril-Kamchatka and Aleutian trenches between Russia and Alaska (below). The northern boundary is a section of the circum-Pacific "Ring of Fire", the most seismically and volcanically active boundary zone in the world.  

WHAT'S DOWN THERE?
At the northern terminus of the Emperor chain, the ~85 million year-old Meiji seamount appears to be 'next' to subduct into the Aleutian trench, given the northwest drift of the Pacific plate. Might older seamounts have previously subducted into the trench's mantle abyss? And if they had, what might that tell us about the genetic evolution of the volcanic chain and even the structure of the Earth's mantle? 


And then, there's the strike of the two chains on either side of the bend. It implies that the Pacific plate changed its direction of motion some 50 million years ago at the time frame of the bend. Can a tectonic plate even do that, and so abruptly? Yet, linear fracture zones on the seafloor that are traceable from the western margin of North America fail to change their course in passing through the chains, the Mendocino F.Z. in particular that passes through the bend. Furthermore, the fracture zones, at the intersection with the chains, are Cretaceous in age....OLDER than the intersection! Possibly the Pacific plate didn't change course. If not, what's the explanation for the bend, and what does that tell us about the structure of the mantle? 

A TECTONICALLY-PUZZLING INTRAPLATE, LINEAR AGE-PROGRESSION OF VOLCANISM
More questions. Hawaii is almost in the geometric center of the Pacific plate and is about as far as you can get from any plate boundary. How did the Earth manufacture a melt that manifests itself on the surface in such an intraplate locale and along an age-progressive volcanic track? In the midst of the plate tectonic revolution, Canadian geophysicist and geologist J. Tuzo Wilson undoubtedly asked himself the same question when he climbed to the top of Mauna Loa in 1963. 

"It thus seems likely that the volcanoes of the Hawaiian chain had similar, 
rather than identical histories, and that each volcanic island in turn went 
through a similar cycle of volcanism and erosion, one after the other." 
J. Tuzo Wilson, 1963.

A major tenet of the plate tectonic theory is that the majority (about 90-95%) of volcanic activity on our planet occurs at plate boundary settings that are either constructive margins (mid-ocean ridges, back-arc spreading centers) or destructive margins (island arcs, active continental margins such as subduction zones). Whether on land or water, the remaining 5% or so (not a negligible amount by any means) is located at a considerable distance from plate margins in contradiction to tectonic theory. The Hawaiian Island chain, within an intra-plate oceanic setting, is our case in point. 


Locations of volcanism (from left to right) at convergent plate boundaries between oceanic plates, at an intraplate "hotspot" locale (encircled), at divergent plate boundaries (an oceanic spreading center), at convergent plate boundaries (oceanic-continental) and at an intraplate rift zone.
Modified from USGS image


Wilson wasn't the first to offer an explanation for Hawai'i's geological peculiarities. In 1849, James Dwight Dana, the foremost American geologist of the nineteenth century, recognized the age-progressive nature and suggested that "extrusions of lava" indicated eruptions along segments of a "great fissure" on the ocean floor that formed by thermal contraction "cracking." His theory was eventually discounted since older "ends ceased to move", but it became a working hypothesis for subsequent studies. Interestingly, it has gained resurgence recently with a newer hypothesis for the genesis of the chain. Another theory espoused that a section of the Hawaiian ridge was part of the Pacific-Farallon mid-ocean ridge spreading system, which turned out to be submarine landslides. Today, the mainstay of geophysics relies on a plumaceous idea that originated in part some 45 years ago during Wilson's ascent.

VERY HOT SPOTS TO THE RESCUE
Wilson thought that a possible origin of the Hawaiian Islands was that they arose from hot regions in the mantle. It came at a time when the then-new theory of plate tectonics could only explain magmatism at ocean ridges and subduction zones. He also felt the chain could not be explained by a shallow mantle process as required by plate tectonics; otherwise, the hotspot would migrate along with the Pacific plate and disallow a linear track from forming. In other words, the hotspot would no longer be fixed, relatively speaking (using global references to other hotspots, tectonic plates or geomagnetic poles).

Wilson's diagram of a convection cell beneath a chain of volcanoes.
(A) illustrates that if lava is generated in the stable core of a convection cell, and the surface is carried by the jet stream, then one source can give rise to a chain of extinct volcanoes even if the source is not over a rising current; (B) is the island chain of volcanoes.
From A Possible Origin of the Hawaiian Islands by J. Tuzo Wilson, 1963.

Building on the idea of "a system of convection currents in the earth", he proposed that the "source of lava is within a relatively stagnant center of a jet-stream type of cell, and if the surface layer (the Pacific seafloor) is moving past the source, then a chain of volcanoes could result. It is not necessary for the source to be immobile. It need only move more slowly than the near-surface current." Wilson's vision led to the concept of a "hotspot" - an area of anomalous and persistent volcanism.


As the Pacific plate drifts to the northwest over the stationary Hawaii hotspot, a line of volcanism forms on the ocean floor that reflects increasing age with distance from the hot spot. Thus, active volcanoes reside on the island of Hawaii, inactive volcanoes reside to the northwest, while Loihi seamount awaits emergence from the sea to the southeast. 


WHAT HEATS THE HOTSPOT?
Eight years later in 1971, geophysicist Jason Morgan proposed the Plume hypothesis as an explanation for hotspots. He postulated that they were surface manifestations of thermal plumes in the mantle that arose from the core-mantle boundary. Located at 1,798 mi (2,880 km), the 200 mile-thick boundary or D" layer is a seismic, thermal and chemically distinct region between the earth's hot, dense metallic outer core and the somewhat cooler, siliceous surrounding mantle. If you've seen a lava lamp, you grasp the concept.

Finger-like plumes in clusters rise through the mantle in a 3D numerical model (left). Another model shows a superplume (right) generated from the D'' layer that fuels upper mantle plumes from a second low-velocity zone (LVZ), partially melts, and then ascends through the asthenosphere. 
From G. Schubert, 2004 and Bres O'Hare (Wikipedia image)


Initially, Morgan envisioned about 20 stationary, long-lived, deep mantle plumes distributed around the planet, although his most recent list includes 69, while the world record is 5,200 (if you include a plume for every seamount). The sites that are underlain by plumes include Hawaii and others such as the Macdonald seamount (South Pacific), Easter Island (a Chilean island), the Galapagos islands, Yellowstone, Iceland, the Azores and the Canary islands (off the coasts of Portugal and Morocco, respectively). 




Morgan also suggested that plumes are the driving force of plate tectonics and that the material they transport from the deep mantle is primordial and compositionally different (such as helium isotopes from the deep mantle) from that derived from shallower mantle depths (such as mid-ocean ridges). His latter prediction of basalts that differ from their mantle source has been confirmed. Hawaiian lava from its hotspots is called Ocean Island Basalt or OIB, while mid-ocean ridge basalt from Iceland is called Mid-Ocean Ridge Basalt or MORB (please visit my upcoming post Part II on my discussion of basalt).

SURFACE VOLCANISM FROM A DEEP MANTLE SOURCE - THE PLUMACEOUS BIG PICTURE
Thermal conduction across the core-mantle boundary is thought to heat-nucleate a plume causing it to buoyantly rise through the viscous convecting shell of mantle. The ascending cylindrical diapir (a "thermal instability") is believed to be fixed in position with respect to one another and with a bulbous, mushroom-shaped head hundreds to possibly thousands of miles of kilometers across and a narrow, stem-like tail some tens to a hundred kilometers in diameter. As the head ascends through the mantle, it is inflated by injection from the faster-moving tail beneath it. 

Numerical simulation of a thermal plume. (Farnetani, 1997)

Upon arriving at the base of the lithosphere (oceanic or continental), the head ponds and spreads laterally causing a precursory domal uplift (500-1,000 m) on the surface that initiates lithospheric extension. Fed by the plume-tail acting as a feeder conduit, voluminous magma continues to penetrate through the crust to the surface, resulting in the extrusion of rapid flood basalt volcanism (large igneous province volcanism or an oceanic plateau) at the hotspot. Magma accumulates in a subsurface reservoir system which may rise and erupt at the summit or along the rift zone of a basaltic volcano. 

Three-dimensional simulation of plume head arriving beneath the Hawaiian hotspot.
Colors signify predicted mantle temperatures. (Maxim Ballmer,SOEST/UHM)  

On the surface, as the plate lithosphere (oceanic in the case of Hawai'i) continues to move, continuous volcanism from the relatively fixed plume-tail results in a volcanic trail. Thus, the youngest volcanism occurs above the present-day location of the plume and the oldest occurs progressively further along the trail. Voila, an age-progressive volcanic track! In a sense, plumes are the way the Earth's core gets rid of heat, while plate tectonics is the way the mantle gets rid of heat.


Halema'uma'u pit crater within the Kilauea caldera 
Does Kilauea sit atop a mantle plume or is there another explanation 
for the Hawaiian Island's intraplate, age-progressive volcanic track?


WHAT'S DOWN THERE IS A MATTER OF PERSPECTIVE
The Hotspot-Plume hypothesis provides an elegant explanation for volcanism at a distance from plate boundaries that is both time-progressive, relatively fixed and with distinct geochemical signatures. It's a picturesque and workable concept that was met with immediate praise and advocacy, and remains widely accepted. Most professional articles and textbooks expound, elaborate and embellish on the idea, but from the moment of its inception, the hypothesis was challenged by skeptics.

"The plume hypothesis has proven resistant to falsifications, 
because rationalizations have been adopted for all discrepant data." 
"It is the physics and the invalid assumptions that make the plume hypothesis untenable." 
G.R. Foulger, 2003. 

Opponents state that hotpsots are not fixed, that its predictions are not confirmed by first-level, field observations, that plumes have only been computer-modeled, have not been seismically-imaged and have "unobservable consequences." Plume devotees admit that mantle-plume behavior is not simple and that they don't necessarily ascend vertically straight, narrow and continuous, but swirl, plump-up, swell, thin-out, break-up, stagnate, pulsate and even shoot off laterally. Since many hotspots deviate from expected behavior, they began to modify the hypothesis with a multitude of variants that opponents say are untestable and amount to a "falsifiable hypothesis." 


Computer simulation of mantle convection surrounded by a latitiude-longitude grid. The hot upwelling plumes exhibit a range of characteristics including pulsations of varying intensity and those that merge, fade, cluster, split and are thin and ephemeral. From Davies and Davies, 2009


Examples of variants that have been employed by plume devotees to explain discrepancies in data sets include baby plumes, fossil plumes, stealth plumes, mini-plumes, dying plumes, head-free plumes, cold plumes, pulsating plumes, subduction fluid-fixed refractory plumes, plume clusters, superplumes, plume swells, plumelets (split-plumes to explain paired Loa and Kea trends) and cactoplumes ("quasi-horizontal chonoliths of anastomosing ductoliths"). 

"The plume hypothesis survived largely as a belief system and had to be extensively modified to account for unexpected observations."
G.R. Foulger and J.H. Natland, 2007.

Anti-plumists accuse plumists of dodging, weaving, stretching and over-rationalizations of the hypothesis. They state that the most serious problem with the plume idea is the lack of evidence for high magma temperatures or high heat flow around hotspots or for thermal uplift. They ask, "Are hotspots really hot?", "How many kinds of plumes are there?" and "Do they even exist?" It begs the question "What alternatives are there to the plume model in which volcanoes can erupt within plates at a distance from their boundaries?" 

AN ALTERNATIVE GENESIS MODEL
The Plate hypothesis is a plumeless concept for mid-plate volcanism without changes in plate motion, without hotspots, but with melting anomalies on the surface that arise from shallow-based processes and with geochemistires that don't require a deep mantle source. Adherents prefer the term melting anomaly, which doesn't imply a process as do hotspots (that convey the assumption that volcanism is fed by an unusually hot, localized source). "Anomolous" term may not be entirely satisfactory term, because "what is an anomaly and what is merely a normal variation in a continuum is not easily decided" (Foulger). 

The continuum is part of a shallow, mantle-based process that relies on an existing hypothesis that geology already embraces - Plate Tectonic Theory"Simply put, it (referring to the Plate hypothesis) suggests that melting anomalies arise from permissive volcanism that occurs where the lithosphere is in extension" (Foulger)

"It is the plate-wide stress field that allows magma to rise. Then, the location of volcanism would be stable relative to the plate boundary system, which is what governs the pattern of stress in plates. As the seafloor moves, created at ridges and consumed at subduction zones, like an escalator, volcanic foci will remain roughly constant in position relative to the plate boundary system. So this theory predicts approximate fixity of volcanism in individual plates, just as the plume theory does. 
Personal communication with G.R. Foulger, July 3, 2015. 

"Tensional tectonics is essential for volcanism", and volcanism is controlled by lithospheric architecture and stress, not by narrow jets.  Extensional stresses and lithospheric fabric are the controlling influences on the timing and location of "midplate" volcanism. Island and seamount chains provide maps of stress and fabric, not plate motion.(Favela, 2000). 

Extension may be localized at continental rifts, mid-ocean ridges and plate triple junctions, or distributed in broad intraplate continental regions such as the Basin and Range province of western North America and oceanic regions - vis-à-vis Hawaii. Volcanism is thought to occur where magma 'escapes' from the asthenosphere to the surface as a result of extension of the lower lithosphere and migrates through the upper lithosphere through fractures created by flexure of the plate. 

The following image from Plates versus Plumes by G.L. Foulger graphically demonstrates the two models, one of sublithospheric melting anomalies (hotspots and plumes) and the other of propagating fractures induced by intraplate stress. 


The Plume versus the Plate Hypothesis
Schematic cross-section of the Earth showing the Plume model (left) and the Plate model (right). On the left, two proposed kinds of plumes are shown - narrow tubes and giant upwellings - that originate from the core-mantle boundary. Subducting slabs penetrate deep into the mantle with convection driven by plumes. On the right, volcanism is concentrated in extensional regions and depths of recycling are variable. In contrast to the left, the upper mantle is inhomogeneous and active, while the lower mantle is isolated, sluggish and inaccessible to surface volcanism. The locations of melting anomalies are governed by stress conditions and mantle infertility.
Text and diagram from Plates vs. Plumes by G.L. Foulger, 2010.

PLUMISTS VERSUS PLATISTS
Hawaii is the type locality for the Plume hypothesis. It was inspired by observations of the chain's linear geometrics, age progression, coincidence with the northward rate of Pacific plate motion, melting anomalies with implied fixity, volcanic tracks that lead away from them, and its restricted area of active, high rate volcanism. Advocates of the plate hypothesis concede that the Emperor-Hawaiian system superficially appears to fit a deep mantle plume process, but they find fault with the Plume theory's failure to predict observations, which have been modified for every new data twist and turn.



There are many points of contention between the Plume and Plate hypotheses. Here's an abbreviated and likely incomplete list: 
(1) Seafloor measurements have failed to detect high heat flow that the Plume hypothesis predicts. 
(2) The time-progression of the Hawaiian chain has varied more than a factor of three, challenging the convention of Pacific plate motion over a hotspot. 
(3) There's no correlation between the time progression of the Emperor chain and the rate of motion of the seafloor when it was emplaced. 
(4) The idea that hotspots are stationary or nearly so was simple and consistent with existing models. In light of recent observations, it appears that the melt extraction locus has not maintained fixity. It has geographically wandered to the south as the Emperor chain formed. Much of it has to do with the bend (see below). It is conceivable that a northwest trend of volcanoes could be produced if a southward drifting hotspot motion is combined with the westward movement of the Pacific plate.
(5) The average volume of lava that erupted at Kilauea since 1956 is between 110 and 130 million cubic yards per year. In contrast, the average rate of lava output for the entire Emperor-Hawaiian chain during its over 80 million year-life is only about 20 million per year. The contemporary surge of volcanism is without precedent, unlikely if one anticipates a consistent output from a plume source. 
(6) The Emperor-Hawaiian chain lacks a flood basalt at its Meiji terminus, assuming it is the original terminus. No evidence exists that it may have subducted, although it's not known whether oceanic plateaus can subduct, but they do obduct. 
(7) There's no evidence for precursory domal uplift above an ascending plume-head. The bathymetric high of Hawaii does not require high temperatures in the mantle. 
(8) An ascending conduit originating from the core-mantle boundary has never been observed beneath the Big Island, only computer modeled. 
(9) Plume fixity has not been observed using relative to the geomagnetic pole. 
(10) Other volcanic chains in the South Pacific, such as the Austral-Marshall seamount chain, that bear a geometric resemblance and orientation to the Emperor-Hawaiian chain with a similar bend are unlikely candidates for mantle plumes. Aside from being poorly dated, display intermittent volcanic activity, display shorter longevities than hotspot superposition implies, are not age-progressive and not timed with Pacific plate motion. In other words, they violate all assumptions of the classical Wilson-Morgan hotspot hypothesis. 
(11) Young volcanoes with alkalic melts change to tholeitic and, in the late stage, change back. Petrological observations suggest that the melt is derived from the asthenosphere and relates to pressure conditions. 
(12) Plumists argue that the hotspots are not located at shallow depths. If sub-lithospheric mantle is displaced in response to seaward movement of subduction zones, then the hotpsots would move with the surrounding mantle, negating fixity.
(13) Plumists interpret the bend as a kinematic feature subsequent to the Pacific plate changing direction. They also interpret that adjoining Pacific oceanic plates have changed their motion relative to the mantle. Seeking perhaps a transglobal explanation for the Pacific plate's change in course, some attribute the altered Pacific plate geometry and the bend to the collision of the Indian and Eurasian plates. Platists (Foulger) calls this a "perpetuated myth" based on global plate motion models and the fact that the collision has been ongoing for a much longer time than that taken for the bend to form. They also state that southward motion of the Emperor melt extraction locus makes it unlikely that the Emperor-Hawaiian chain fits the Plume hypothesis. In addition, there's the previously mentioned, earlier-forming fracture zones on the Pacific seafloor that failed to change direction along with the bend. Tectonics plates can change direction, but it's unlikely that they do so abruptly as is required at the bend.
(14) Plumists have noted the apparent passage of hotspots from one plate to another at locales other than the Hawaiian chain (such as the Great Meteor hotspot track in the Atlantic basin across the mid-ocean ridge) and suggest that hotspots are at least partially sub-lithospheric phenomena, and, therefore can't simply represent fractures propagated by intraplate stress fields.
(15) An obstacle exists in regards to paleolatitude. If the Hawaiian hot spot is fixed, then each volcano produced at the source should have formed at the same latitude as the present-day position of the hot spot, ~20° N. This is not the case. Paleolatitude data from the Emperor Seamounts indicate that the structures formed at paleolatitudes well north of the present-day hotspot position, and that it must have migrated southward between 81-47 Ma at a rate of 44 mm/year.

Let's turn our attention from the hypothetical to the real and visit volcano country.

LAND AHOY!
It's February in New England, and with over 100 inches of snow in my front yard excitement is running high for swaying palms, pounding surf, hula skirts, tropical fruit drinks and some world-class geology. My first sighting of Hawai'i from our passenger jet was the island of Hawai'i with four out of five shield volcanoes nudging above the cloud deck. You can just make out the outline of the island's rugged northeast coast (below). To not confuse the name of the island with the name of the state, everyone refers to it as "The Big Island." Each of its five volcanoes began their growth from the seafloor, emerged from the waves, and coalesced to form a single island over the span of some 800,000 years. 

Concealed by clouds, the east flank of Kohala (bottom right) faces the rainiest side of the island, which has carved it into dramatic, basalt-layered, near vertical-walled gorges with pendulous waterfalls at every turn. Kohala is the island's most eroded, most gorge-dissected, northernmost and oldest subaerial volcano, having emerged from the sea ~500,000 ka. It reached ~31 miles in width before erosion and subsidence took its toll. Its most recent eruption was ~120,000 ka. Plumists theorize that Kohala has migrated far enough from the hotspot that it is the least likely to re-erupt of the island's volcanoes.


We're facing the island of Hawai'i toward the southwest from about 20,000 feet. Poking above the clouds, four of its five volcanoes are in view. At the moment of this photo, the island of Maui could be be seen out of the right side of the plane.

Mauna Kea (center above) is appropriately called the "White Mountain" with remnant winter snows on the summit. You can ski, but there are no lifts or lodges. Someone will have to drive a 4WD vehicle to take you up. It's the highest mountain in the island chain at 13,979 feet but not the most massive. That honor goes to Mauna Loa, the "Long Mountain", the planet's largest volcano in mass and volume measured from the seafloor (right rear). In 1975, Mauna Loa awoke from dormancy with a single-day eruptive event and again in 1984 with a flank eruption that threatened the east coast city of Hilo. 

On the horizon (far right), lowly Hualalai clings to the west coast barely 11 miles from the Kona Airport, our immediate destination. The youngest and fifth volcano is Kilauea, hidden behind Mauna Kea and collapsed into a caldera. It's been erupting for 30 years in various forms with lava lake overflows, cinder cone eruptions and flank fissure eruptions emitting steam, gas plumes and an eerie red glow at night. That makes it the most active volcano in the world both temporally and volumetrically, and the only volcano in the entire island chain that is currently active subaerially. 

DESCENDING ON THE DRY, LEEWARD KOHALA COAST 
Turning south, our plane followed the Kohala Coast in the dry rainshadow of Kohala. On this western side of the island, the vegetation is noticeably brownish. Visible at this altitude and in stark color-contrast, a dozen or so upscale hotels have bulldozed 20 golf courses out of the barren lava flows that blanket the landscape.  



Following the island's northwest coast of Kohala, the harbor of Kawaihae is revealed. Further east, ritzy hotels and their golf courses stand in contrast to the lava fields from which they're cleared. That's Mauna Kea on the left and the long gradual slope of massive Mauna Loa on the right,

The aridity, sparse vegetation and desert-like climate is due to orographic precipitation, caused by the interaction of ocean temperatures, prevailing winds and lofty volcanic topography. Prevailing trade winds acquire moisture from the warm Pacific waters. Upon reaching the islands from the northeast, the moist air rises, cools, condenses and rains on east-facing, windward slopes of the volcanoes but not on their summits, which are mountain-top deserts. On west-facing leeward slopes, the warm, dry air descends creating an Arizona-like climate. The effect is most dramatic on the Big Island with the tallest volcanoes.   


Annual Precipitation on the Island of Hawai'i
The majority of precipitation is on the east and northeast sides of the island that faces the "trades", 
while the west and southeast sides enjoy a desert-like climate.
Modified from Hawaii-Guide.com

Thus, the leeward side of the island is sunny and dry, while the windward side of the island is rainy and wet. You can see the differences in the erosion of the landscape and the location of the waterfalls, the number of streams that reach the coast, the type of crops that are grown, the wildlife, cloud development, the location of the resort hotels and the natural vegetation - sparse, scrubby and brown on the west and lush, green tropical rainforests on the east. Our waitress at a restaurant in Hilo on the east coast said that when she wants to work on her tan, she drives an hour or so west. The following montage says it all. Each locale is separated by only 50 miles as the crow flies.



 


SACRED SUMMITS
Minutes from the airport, one million year-old Mauna Kea dominates the landscape. The volcano hasn't erupted in 4,500 years but is seismically active and capable of re-eruption. The arbitrary cut-off for volcanic extinction is 10,000 years, since the last ice age, even here in Hawaii. No visible caldera exists on the summit, but the ridge arrangement of cinder cones implies the presence of one, which was obliterated by the cones and their pyroclastic debris. It's past the active stage of edifice-building, typified by its over 300 cinder cones seen in profile. Each cone is asymmetric in keeping with the direction of the "trades" from the northeast. 



In the foreground, cinder cones on the northwest slope of Hualalai poke through the flat cloud-deck, induced by a temperature inversion in association with the trade winds. Photographed in February, there are only a few patches of snow on Mauna Kea's summit; however, earlier in the season it's very skiable, but there are no lifts (and no lift tickets!), no slope grooming or resorts. Skiers need a 4WD vehicle and driver to "get a lift" back up. 


Curiously, the 'trades' are also responsible for low clouds that blanket the landscape. Their formation is induced by a temperature inversion where a pronounced moisture discontinuity exists 50-70% of the time between 4,000 and 5,000 feet. The inversion embedded in the moving air suppresses upward flow, thereby restricting cloud development to the zone below the inversion. Towering clouds form along the mountains where the incoming trades converge as it moves up a valley and is forced up and over the mountains to heights of several thousand feet.


Orographic Lifting of Trade Wind Air
Source cited as requested: Giambelluca, T.W., Q. Chen, A.G. Frazier, J.P. Price, Y.-L. Chen, P.-S. Chu, J.K. Eischeid, and D.M. Delparte, 2013: Online Rainfall Atlas of Hawai‘i. Bull. Amer. Meteor. Soc. 94, 313-316, doi: 10.1175/BAMS-D-11-00228.1.

Mauna Kea's summit atmosphere is extremely dry and disturbance and cloud-free for optimal celestial viewing. Astronomers from eleven countries have assembled a 2 billion dollar-collection of 13 of the world's largest observatories on the summit for optical, infrared and sub-millimeter astronomy. Recently, there has been strong local opposition by Native Hawaiian, environmental and cultural groups to another observatory planned (a $1.3 billion Thirty Meter telescope, ten times more powerful than the Hubble) and an Army helicopter landing zone for high altitude training to be built on Mauna Kea and Mauna Loa's sacred summits.



The twin W.M. Keck Observatory domes atop Mauna Kea are among the largest optical telescopes in use with 33-foot primary mirrors.

SWAYING PALMS, POUNDING SURF, TURQUOISE WATERS AND MILES OF SAND
The prominent tongue of lava (below) originated from vents along the flanks of Kohala volcano and continues well beyond the water's edge. This region of the Kohala Coast is fronted by offshore fringing reefs and numerous pocket sandy beaches. A paucity of sediment-carrying perennial streams have conferred the west side with the cleanest and clearest water. Because of its clarity and the island's geological youth, it has the most live coral of the islands (57% or 29 sq. mi.). 

Terrigenous sediment run-off and deposition on reefs significantly affect their health by blocking light and inhibiting photosynthesis, smothering and abrading the coral, and triggering macro-algae growth. In Hawaiian mythology, corals were the first creature that came into being before any higher forms emerged. Their importance is also ecological and recreational. Warm, calm and clear waters have made fishing, snorkeling and scuba diving a multi-million dollar industry (~$385 million in 2002).


A dirt road cuts across a large platform of lava and connects remote and pristine Mahai'ula and Makalawena Beaches.

WHITE, RED, GREEN AND BLACK SAND BEACHES
The varied colors of Hawai'i's beaches convey the geological story of the islands. Coral reefs flourish along older more stable, volcanically-quiescent coasts on the Big Island and the older islands of the chain progressing to the north, where water is sediment-clear and shallow to the sun. 

Atoll formation was described as early as 1836 in the writings of Charles Darwin about the islands of Tahiti. As the Hawaiian islands age, erode and subside beneath the sea, they provide an environment for the formation of an encircling-ring of coral. Waves that pound offshore reefs and pulverize shells on the seafloor provide beaches with a steady supply of fine-grained, beige to yellowish calcareous sand. The small cobbles and smooth pebbles of black vesicular lava pleasantly clink underfoot while strolling on the beach.



Have a stroll on volcanic lithic fragments and calacreous bioclasts.
Along this section of the Kohala Coast (below), beach composition is slightly more volcanically than biologically-derived but is clearly a mix of vesicular basalt cobbles and pebbles, pulverized bivalves and fragmented reef material. The island of Hawaii has 428 miles of coastline but less actual beaches than the other islands in the Hawaiian chain because of its geological youth.

Over time, surf and currents erode fresh beds of volcanic ash and separate their component minerals by chemical erosion and mechanical action into greenish grains of olivine from lighter (specific gravity) grains of black pyroxene by their differential densities. Most of these minerals don't survive long as sand grains (olivine for example is highly susceptible to weathering), but they dominate because there is so much in basaltic magma (see Bowen's chart above) and there is simply no "durable" quartz available (such as on continental beaches). The calcium plagioclase in basalt weathers quickly. Besides green and black sand beaches, red sand beaches form on somewhat older shores via streams that carry black basalt and red oxidized cinder to the coast. 

FLOWS OF LAVA TO THE SEA AND BEYOND
The thick bench of lava beyond the palms traveled some 35 miles from vents on the flanks of distant Mauna Loa in 1859. In the haze, the sloping west flank of Kohala is far to the right. With the exception of infrequent tsunamis, this side of the island is protected from large waves by its shape and the direction of prevailing ocean currents from the northeast. But it's not immune, as indicated by buried sediments containing seafloor coral fragments, mollusk shells and outer shelf deposits indicative of ancient inland surges. 


A dramatic pyroxene-rich black sand beach lies at pristine and isolated Kiholo Bay on the Kohalo Coast. The lava in the foreground emanated from Haulalai in 1800-1801, while the thick bench of lava was derived from Mauna Kai in 1859. The flow partially destroyed the historic site of a tidal fishpond built by King Kamehameha the Great in 1820. It was two miles in circumference with rocks walls 6 feet and and 20 feet wide. With turquoise waters, a serene lagoon, swaying palms, green sea turtles and humpback whales offshore, this area is one of the island's best kept secrets.


Distant earthquakes and seafloor volcanic eruptions have and will continue to generate massive, fast-travelling walls of water. Even local flank landslides can displace water that "comes back" as a tsunamis. In a period of 157 years, a damaging or destructive tsunamis struck the islands every 12 years on average. All coastal regions post evacuation routes and have warning sirens that direct everyone to higher ground if a tsunami landfall is imminent. 

Hilo is the Big Island's largest city and the tsunami capital of the world, facilitated by its funnel-shaped bay. It was devastated in 1946 (by a 7.8 magnitude earthquake near the Aleutian Islands) and 1960 (from a 9.5 magnitude quake off the coast of Chile). It's also the only city in the U.S. threatened by a lava flow (from Mauna Loa in 1984).

TOUCHDOWN IMMINENT 
Seconds from Kona Airport, I caught a glimpse of Hualalai, shrouded in haze at 8,275 feet (below). The summit possessed a caldera, which has been obliterated by cinder and spatter cones typical of the volcano's late stage of eruption. Directly facing the viewer is Hualalai's Northwest Rift Zone with cinder and spatter cones along with faults, cracks and grabens formed from extension. It's one of three fissures that typically radiate from the flanks of Hawaiian basaltic volcanoes. Formed as the edifice settles under its own weight, the rift facilitates the lateral extrusion of lava rather than having to build sufficient pressure for a summit eruption, which occurs as well.

Hualalai emerged from the sea prior to 300,000 years and is considered potentially dangerous having erupted in 1801. In fact, the airport and neighboring coastal communities are built on the most recent and underlying flows from 1,500 to 3,000 years ago. South of the airport begins the Kona Coast in the heart of the leeward side of the island in the rainshadow of Hualalai, Mauna Kea and Mauna Loa, which even further insulate the coast from moisture-bearing winds. 



East-facing view of the summit of Kohala and the patchwork of interbedded lava flows that have emanated from flank vents as recent as 1801. Highway 19 (from left to right) encircles the island along with Highway 11 in the south. The island's third main road is Highway 130 or Saddle Road that slices through the lofty saddle from west to east between Mauna Kea and Mauna Loa. 

What I thought was atmospheric haze on the Kohala Coast turned out to be vog, an odorless mix of volcanic gases, largely sulfur dioxide and water vapor carried some 35 miles by the wind from distant Kilauea to the south. The entire island is monitored for air quality by the state, and health advisories are issued if a plume of gas reaches dangerous levels. Persistent, gas concentrated plumes generated by Halema'uma'u pit crater at Kilauea and its cinder cone Pu'u 'O'o have resulted in the closure of downwind roads in Volcanoes National Park.

TROUBLE IN PARADISE
Clumps of straw-colored fountain grass provide a striking color-contrast on the overlapping patchwork of pre-historic reddish-brown and relatively fresh, black lava flows that blanket Hualalai (below), but its an unwanted, invasive species. Introduced by man in the 1920's from Africa as an ornamental plant and still sold in nurseries, its has "escaped" to wilderness areas such as Hawai'i but also Arizona, Nevada and California. It outcompetes indigenous plants for water and space in pasture lands and is a fire threat.

The story of fountain grass typifies Hawaii's ongoing struggle to prevent the introduction of non-indigenous species, prevent the extinction of native species and reverse the island's declining biodiversity. Prior to human intervention, Hawaii's geographical isolation and varied topography have been the source of evolution and adaptation amongst the many lifeforms that reached the islands via the wind, ocean currents and attached to migratory birds. Unique birds and plants became perfectly suited to its environment and highly dependent on a fragile ecological balance to survive. 

With the arrival of man, both intentional and accidental introduction of new species have upset that balance. The rate at which new species is introduced is estimated to be 2 million times more rapid than the natural rate. Thus, it is more crucial than ever that invasive, unwanted species be kept off the island, which accounts for the rigorous screening we experienced at the airport, even on domestic arrivals. Volcanoes National Park on the Big Island is a highly protected environment of preservation.



Facing northwest from Highway 19, Kiholo Bay, Park and lagoon are nestled in the embayment. The lava in this section of the Kohala Coast are a mix of superficial flows from both Hualalai (1800-1801) and in the distance from Mauna Loa (1859). Older Mauna Loa flows between 1,500 and 10,000 years are underlying. 

Another non-indigenous example is the weasel-like Asian mongoose, introduced to sugar plantations in 1872 to control the destructive rat population that likely arrived on Polynesian canoes and later on European and American sailing vessels. Unfortunately, mongoose are diurnal and rats are nocturnal. They both have no natural predators in Hawaii and subsequently have overrun the island. I was amazed to spot a mongoose in downtown Hilo at noon scurrying across the main street between cars. Unfortunately, both have a taste for the eggs and hatchlings of native birds and endangered sea turtles. They also carry leptospirosis and other disease-producing bacteria in their droppings, which has entered some freshwater streams. And so it goes.

PLEASE STAY TUNED FOR POST PART II
The Hawaiian Island chain inspired the theory of hotspots and mantle plumes. The ease of access and frequency of volcanic activity on the island of Hawai'i have established it as a type locality for basaltic volcanism; however, much is still unclear and unknown such as the fundamentals of how Hawaiian volcanoes actually work, the structure of the mantle and the functionality of thermal plumes, if they really exist.

In posts Part II-IV, I'll cover my geo-heli-tour of the island of Hawaii. Here's a small sample of a video I took of a vegetated cinder cone as we climbed into the lofty Humuʻula Saddle between Mauna Kea and Mauna Loa. That's Mauna Loa in the distance. 



video


Aloha! 

RECOMMENDED PRINTED SOURCES OF INFORMATION ON THE HAWAIIAN ISLANDS
This rather lengthy list includes material on Hawaiian shield volcanoes, Pacific plate tectonics, hotspots, mantle plumes, theories on melting anomalies, mantle dynamics, Hawaiian glaciation, and basalt geochemistry and geophysics. The scientific articles, special papers, books, field trip guides and maps were used as reference information in the writing of this post. Have fun!

•  A Brief History of the Plume Hypothesis and its Competitors: Concept and Controversy by Don L. Anderson and James Natland, GSA, Special Paper, 2005.
A New Insight into the Hawaiian Plume by Jianshe Lei and Dapeng Zhao, Earth and Planetary Science Letters, 2006.
Annals of the Former World by John McPhee, 1998.
A Possible Origin of the Hawaiian Islands by J. Tuzo Wilson, Canadian Journal of Physics 41, 1963.
Archipelago - The Origin and Discovery of the Hawaiian Islands by Richard W. Grigg, 2014.
Convection Plumes in the Lower Mantle by W.J. Morgan, Nature 230, 1971.
Deep Mantle Convection Plumes and Plate Motions by W.J. Morgan, Bull. Am. Assoc. Pet. Geol. 56, 1972. 
Did the Atlantic Close and Then Reopen? by J. Tuzo Wilson, Nature, v. 211, 1966.
• Divergence Between Paleomagnetic and Hotspot Model Predicted Polar Wander for the Pacific Plate with Implications for Hotspot Fixity by William W. Sager, Texas A&M University, Revised Draft 23, 2006.
Eruptions of Hawaiian Volcanoes - Past,Present and Future, USGS, General Information Product 117, 2014.
Evidence From Islands on the Spreading of Ocean Floors by J. Tuzo Wilson, Nature Publishing Group 197, 1963.
• Explore the Geology of Kilauea Volcano by Richard Hazlett, 2014.
Extensional Tectonics and Global Volcanism by J. Favela, Javier and D.L. Anderson, in Problems in Geophysics for the New Mellenium, 2000.
• Fast Paleogene Motion of the Pacific Hotspots from Revised Global Plate Circuit Constraints by C.A. Raymond et al, History and Dynamics of Plate Motions, edited by M.A. Richards, R.G. Gordon, and R.D. van der Hilst, pp. 359-375, 2000.
Geologic Map of the State of Hawaii by David R. Sherrod, John M. Sinton, Sarah E. Watkins and Kelly M. Blunt, USGS, Open File Report 2007-1089.
Hawaiian Volcanoes - From Source to Surface by Rebecca Carey et al, AGU, 2015.
Hawaii Volcanoes National Park - Geologic Resources Inventory Report, NPS, 2009.
Hawaiian Xenolith Populations , Magma Supply Rates and Development of Magma Chambers by D.A. Clague, Bulletin of Vulcanology, 1987. 
How Many Plumes Are There? by Bruce D. Malamud and Donald L. Turcotte, Earth and Planetary science Letters, 1999.
Geochemistry of Lavas from the Emperor Seamounts, and the Geochemical Evolution Hawaiian Magmatism from 85 to 42 Ma by M. Regelous et al, Journal of Petrology, Vol. 44, 2003.
Geology of Hawaii - Hofstra University Field Trip Guidebook by Charles Merguerian and Steven Okulewicz, 2007.
Hotspots and Melting Anomalies by Garrett Ito and Peter E. van Keken, Treatise on Geophysics, 2015.
Illustrated Geological Guide to the Island of Hawaii by Richard C. Robinson, 2010. 
• Is Hotspot Volcanism a Consequence of Plate Tectonics? by G.R.Foulger and J.H. Natland, Science, Vol. 300, 2003.
• New Evidence for the Hawaiian Hotspot Plume Motion Since the Eocene by Josep M. Pares and Ted C. Moore, Earth and Planetary Science Letters, 2005.
• Oceanic Island Basalts and Mantle Plumes: The Geochemical Perspective by William M. White, Department of Earth and Atmospheric Sciences, Cornell University, Reviews in Advance, 2010.
On the Motion of Hawaii and other Mantle Plumes by John A. Tarduno, Chemical Geology, 2007.
Plate Tectonics by Wolfgang Frisch, Martin Meschede and Ronald Blakey, 2011.
Plates vs Plumes - a Geological Controversy by G.R. Foulger, Wiley-Blackwell, 2010.
Pleistocene Snowlines and Glaciation of the Hawaiian Islands by Stephen C. Porter, Department of Earth and Space Sciences, 2005.
Plumes, or Plate Tectonic Processes by G.R. Foulger, Astronomy and Geophysics 43, 2002.
Revision of Paleogene Plate Motions in the Pacific and Implications for the Hawaiian-Emperor Bend by Nicky M. Wright, GSA, Geology, 2014.
• Roadside Geology of Hawai'i by Richard W. Hazlett and Donald W. Hyndman, Mountain Press Publishing Company, 1966.
Superplumes or Plume Clusters by G. Schubert et al, Physics of the Earth and Planetary Science Interiors, 2004.
The Evolution of Mauna Kea Volcano, Hawaii: Petrogenesis of Tholeiitic and Alkalic Basalts by F.A. Frey et al, Journal of Geophysical Research, 1991.
• The Hawaiian-Emperor Volcanic Chain. Part I. Geologic Evolution by D.A. Clague and G.B. Dalrymple, Volcanism in Hawaii, Geological Survey Professional Paper 1350, 1987.
The Mantle Plume Debate in Undergraduate Geoscience Education: Pverview, History and Recommendations by Brennan T. Jordan, Department of Earth Sciences, University of South Dakota, in Mantleplume.org. 
The Plate Model for the Genesis of Melting Anomalies by Gillian R. Foulger, Mantleplumes.org, 2006. 
Tectonics - Continental Drift and Mountain Building by Eldridge M. Moores and Robert J. Twiss, University of California at Davis, 1995. 
The Bend: Origin and Significance by Rex H. Pilger, GSA Bulletin, 2007.
The Plate Model for the Genesis of Melting Anomalies - Chapter 1 by G.R. Foulger, GSA, 2007.
Three Distinct Types of Hotspots in the Earth's Mantle by Vincent Courtillot et al, Earth and Planetary Science Letters 205, 2003.
Through Thick and Thin by Neil M. Riber, Nature, Vol. 427, Barberry 2004.

NOTEWORTHY REFERENCES ON THE WEB
There's a ton of stuff on the web, but somehow I always ended up at these sites.

• The Hawaiian Plume Project: http://igppweb.ucsd.edu/~gabi/plume.html
The USGS Hawaiian Sites: http://search.usa.gov/search?affiliate=usgs&utf8=%E2%9C%93&query=hawaii&commit=Search
• Mantle Plumes from the Platist's perspective: http://www.mantleplumes.org
National Park Service site: http://www.nps.gov/havo/index.htm
USGS Hawaiian Volcano Observatory: http://hvo.wr.usgs.gov
On Wayne Ranney's blog, his well photo-documented field excursions always make you feel like you are right there: http://earthly-musings.blogspot.com/2011/06/hawaiian-geology-at-haleakala-crater.html and http://earthly-musings.blogspot.com/2011/06/trip-around-island-of-oahu.html