Friday, June 22, 2018

The Geologic Evolution of Iceland - Land of Hot Rocks and Water in All of Its Forms

"Fjarst í eilífðar útsæ vakir eylendan þín."
"Far in the eternal yonder sea your island wakes."
Icelandic poet and playwright Stefán Guðmundur Guðmundsson, 1853-1927.

Continental rifting of the Late Paleozoic supercontinent of Pangaea, its progressive break-up and initial spreading of the Atlantic Ocean was preceded by the intraplate emplacement of large igneous basalt provinces in the Mesozoic-Paleogene. Fragmented by seafloor spreading, eroded remnants of the dissociated continental flood basalt events are distributed on formerly conjugate margins of rifted and drifted landmasses across the domains of the North, Central and South Atlantic.



The View from North Rekyjavik
Separated by 30 km-long fjord Hvalfjordur, steep-sided table mountains Aktrafjall (left) and Esja (right) backdrop Faxaflói Bay on Reykjavik's North Shore. They originated some 35 km to the east in the West Volcanic rift zone and were carried on the migrating North American tectonic plate to their present location. They consist largely of hyaloclastites (a water-quenched, shattered basaltic magma) and pillow lavas (rounded masses erupted under water or subglacial) that formed when Pleistocene glaciers confined eruptive fissures on the landscape and subsequently capped by a succession of lava flows. Go there: 64°8'51.40"N, 21°55'20.78"W

The break-up of the boreal region of Pangaea between Greenland and Europe led to opening of the North Atlantic beginning in the Paleocene following the formation of the North Atlantic Igneous Province. It too was fragmented by seafloor spreading between the North American and Eurasian tectonic plates on opposing sides of the diverging Mid-Atlantic Ridge.



Krýsuvík-Seltún Geothermal Field
Subterranean groundwater in proximity to a volcanic system's magma chamber ascends fractures in the upper crust and reaches the surface superheated. Seltún is a high-temperature geothermal sub-field of five within the greater Krýsuvík field, one of five on the Reykjanes Peninsula of southwest Iceland. It's where the Reykjanes Ridge, an extension of the submarine Mid-Atlantic Ridge, comes ashore. Within Iceland's zones of volcanism and faulting, about 30 geothermal fields reach 200°C at a depth of one kilometer, while some 250 low-temp fields under one km flank the zones. High-temp fields are characterized by a number of geothermal features, sparse vegetation (mostly mosses, lichens and sedge) and light-colored hydrothermally-altered, acid-leached (metal-depleted) bedrock mainly of microporous hyaloclastites. Go there: 63°53'45.38"N, 22°3'9.07"W 

The entire northern Pangaea event and more - LIP emplacement, extension, rifting, break-up and seafloor spreading - is widely attributed to the buoyant arrival of the hot, deep-seated Iceland mantle plume. The interaction of its hotspot with the mid-ocean ridge produced persistent voluminous and effusive magmatism that culminated with the formation of the volcanic island of Iceland.



Fault-Controlled Lake Kleifarvatn in the Reykjanes Peninsula
Hyaloclastite ridge Sveifluháls (left) and table mountain Kistufell (right) formed subglacially during the Pleistocene, the former erupting from an elongate fissure (linear vent) and the latter from a central vent and acquired a lava cap supra-glacially. They and the intervening lake-filled, fault-bound topographic depression lie on NE-SW strike, en echelon (obliquely parallel) with the peninsula's four or five volcanic systems, which are subjected to shear stress along the Reykjanes' spreading axis. Precipitation-charged without stream input, the lake drains ~1 cm/day and diminished 20% after a year 2000 earthquake. Its formation and orientation are indicative of tectonic processes operational on the peninsula. Go there: 63°55'11.47"N, 21°59'9.75"W

The rate of magma production along the Mid-Atlantic Ridge has remained unusually high. Iceland, at the center of the northern North Atlantic Ocean, is the only part of the North Atlantic Igneous Province that remains magmatically active, albeit at a reduced flux rate. 



Kerið Crater
Originally thought to be a maar (formed by violent phreatomagmatic explosion when magma contacts water) or caldera (gravitationally-collapsed volcano), both common in Iceland, the 50 meter-deep pit crater is believed to have collapsed into a section of its lava conduit system during a monogenic (single eruption) fissure, evidenced by the shallow, elliptical shape. It's in the 8,-10,000 year old Grímsnes Volcanic Field of the Western Volcanic Zone. Normally dark gray to black, basalt is in the form of highly-vesicular (due to entrapped gas bubbles), brick-red scoria (frothy oxidized cinders of lava) and spatter (pancake-like clots of spray). Intense heat rewelded (agglutinated) the tephra (airborne pyroclasts) into pseudo-stratified rocks of the crater walls.

Go there: 64° 2'26.91"N, 20°53'5.37"W. Photo by Julia Share 

An alternative genetic origin to the plume hypothesis, one that is gaining ground, relies solely on shallow kinematic processes. In this scenario, plate tectonics, a nearly universally accepted process, drives lithospheric extension that permits the sub-lithospheric mantle to passively ascend and generate surface magmatism at a "meltspot."



Kirkjugólfið
As if man-made, the "Church Floor" is a protected national monument in the southern highlands. The outcrop is the uppermost surface of a glacially-eroded colonnade of porphyritic basalt (large, well-formed crystals surrounded by finer-grained ones of the same mineral). The characteristic polygonal morphology of lava flows, dikes and sills, which may also occur rhyolitic in composition, is due to rapid horizontal contractional cooling that produces columnar jointing (regular array of vertical rock columns). The feature is on the edge of the moss-covered Eldhraun lava field, the largest in the world. Photo by Julia Share

Regardless of the formative mechanism, the result is the elevated Iceland Basalt Plateau, the world's largest volcanic island. Young geologically, it's thought to have formed within the last 24 million years in spite of the fact that the oldest exposed rocks on the island are only 14 to 16 million years old.



The Amphitheater of Svartifoss
Surrounded by a colonnade of dark basalt that gave rise to its name, 12 meter-high Dark Falls is in the Skaftafell Wilderness region of Vatnajökull National Park in southeast Iceland. The falls is sourced by meltwater from Svinafellsjokull, which is a slender tongue of massive glacier Vatnajökull. Irregularly-jointed, curvy entablature basalt (upper left) forms by water-quenching, rapid cooling and stress distribution during emplacement. Effusive surface magmatism produced the layers of basalt in the falls and the entire island of Iceland.

Go there: 64°1'39.16"N, 16°58'31.09"W. Photo by Julia Share

It’s one of the most active and productive terrestrial regions on Earth. There are some 55,000 km of diverging plate boundaries on our planet, and along most of their length, they are submarine features. Iceland is one of only two places where a mid-ocean spreading center rises above the level of the sea. 



Reykjavik, Faxaflói Bay and Esja to the North
Penned "Smoky Bay" by first settlers for rising geothermal steam, the capital's flats and rolling hills are built on eroded bedrock of Late Pliocene to Early Pleistocene Grey Basalts and capped with thin Holocene basalts that extruded when the region was ice free. Bay islands are remnants of ~2 Ma caldera Viðey that provides the city's buildings with geothermal heat, hot water and snowless roads. Across the bay, Esja's lavas dip eastward toward the West Volcanic Zone, the major spreading axis at the time of Esja's emplacement. It replaced the westerly Snæfellsnes Zone some 6-7 million years ago, which hints at the transitional tectono-dynamics operative across the landscape of Iceland.

Iceland is a geological paradise with an on-land continuation of the Mid-Atlantic Ridge, volcanism that is subaerial, subglacial and submarine, at least 120 historical eruptions in the last 1,100 years that represent 60% of all recorded volcanic activity, ubiquitous Tertiary flood basalts and an eruption frequency about every five years.


Spouting Geyser Geysir
The "spouting hot spring" (gesya means "gush" in Icelandic) is the surface expression of a high-temperature hyrdothermal system, the first described in print to Europeans but on record in Iceland following s series of earthquakes in 1294. As first elucidated by Robert Bunsen (of chem-class burner fame) in the mid-19th century, water circulating in deep fissures is heated to 120°C at the margin of a shallow magma chamber. After hundreds to thousands of years, it reaches the surface and releases pressure and gas - the champagne cork effect - as gushing hot springs and geysers. Along with nearby geyser Strokkur, Geysir has been active for some 10,000 years. And along with waterfall Gulfoss and spreading center Þingvellir, they're the Golden Circle's major geological attractions. Photos by Julia Share

Although undeniable, Iceland borders on the inexplicable. Few places are as pristine, remote, otherworldly, serene and stunningly beautiful. It’s a sought after destination for its breathtaking landscapes and the unparalleled opportunity it affords to observe magmatism, active tectonic processes on land and their interaction with climate.



NNW Oblique View of Fracture Almannagjá
The east-facing normal fault marks the easternmost extent of the North American Plate and the westernmost boundary of Þingvellir graben. On the opposing side to the east is the more diminutive boundary fault Hrafnagjá. Both faults are surface expressions of deeply-rooted fractures that extend through the crust. Within the walls of the fault scarp, a sequence of ~9,100 year-old Þingvallahraun pahoehoe flood basalts is exposed that emplaced post-glacially during the Holocene. Evidence of rudimentary columnar jointing is evident but is less developed than those found in flow intrusions. A line of tension fractures can be seen in the middle distance on strike with the main fault. Snow-capped Upper Pleistocene hyaloclastite mountains Syðstasúla and Vestursúla í Botnssúlum bound the rift valley to the north-northwest.

If you seek to comprehend the fascinating history, culture and heritage of Iceland, then understanding its geology is essential. If you seek to better understand the natural forces that shape our planet or simply contemplate the enigma of mantle structure and geodynamics, it's the place to be. 



Steeple of Church Hallgrímskirkja in Upper Reykjavik
 Completed in 1986 but 45 years in the making, the Lutheran Church towers over Iceland's capital city of Reykjavik on its tallest hill. Although the church wasn't built of 'Iceland Stone", its striking exterior is clearly reminiscent of columnar basalt found throughout the country. The pahoehoe (ropy smooth) lava flows on which the city is built originated on the Reykjanes Peninsula immediately to the southwest between 100,000 and 200,000 years ago. Not-to-be-missed are circumferential views of the city, bay and mountains that are to be had from the church's observatory for a small fee. Go there: 64°8'30.33"N, 21°55'36.47"W

ABOUT THIS POST
The photographs in this post were taken by my daughter Julia Share (designated as such) and me on two separate visits to Iceland with the exception of a few contributions from a friend and colleague. Our excursions began in the capital district of Reykjavik (arrow) and included the Golden Circle region, the Reykjanes Peninsula, the South and Southeast Coastal Lowlands, and the Snæfellsnes Peninsula. Our plan is to return and negotiate the 1,332 km-long Ring Road that encircles the island and allows access to Iceland's rugged, isolated and largely uninhabited highlands and more remote geological features.

Relevant definitions in this post are italicized and important names are emphasized in boldface. Global coordinates of select locations are provided and can be pasted into a mapping program such as Google Earth. Click on images and maps for a larger view. Voluminous additional references beyond those provided are available upon request.



Boiling Mud Pots and Cooked Bedrock
Surface expressions of vented geothermal water include sinter (precipitated) deposits of travertine (calcium carbonate from mafic lava minerals), iron and sulfur oxides, geysers (far less common), boiling and steaming hot springs and boiling mudpots (with thermophilic and ethanol-fermenting microbes), fumeroles (steamy vents of H20, CO2, H2S and H2 gas) and occasional maar craters (when steam pressure in near-surface aquifers explosively exceeds overlying lithospheric continuity). Greenish to bluish-gray colors are due to palagonized hyaloclastite glass (hydrothermally- and chemically-altered primary minerals with precipitation of new ones) that form various clays such as smectites. Go there: 63° 53' 49.064" N, 22° 3' 17.117" W

OUR GEO-ITINERARY

The Golden Circle region is on everyone's must-see list. Northeast of Reykjavik, it includes Iceland's famous and celebrated rift valley, national park and UNESCO Site of Þingvellir, geothermal fields with hot springs, mud pots and geysers such as Geysir and Strokkur, the massive and famous waterfall Gulfoss and enigmatic Kerið crater. 

Southwest of the capital extends the elongated Reykjanes Peninsula. It's a landscape of volcanic fissures, lava fields, fault-bound lakes and geothermal activity. It includes Keflavik International Airport, the man-made Blue Lagoon spa, the Svartsengi power station and four volcanic systems arranged en echelon. The peninsula is a UNESCO Global Geopark with 55 listed sites, lying on strike with the Mid-Atlantic Ridge as it rises from the sea.



The Touring Regions of Iceland
The volcanic island is typically divided into eight or nine touring regions. My daughter and I focused our efforts on exploring those on the West, South and Southeast in addition to the Golden Circle region. Many use Reykjavik as a base, but driving the the 1,332 km-long (828 mi) Ring Road that follows the perimeter of the country allows greater exposure and access to Iceland's geological features.

The South and Southeast Coasts are regions of coastal lowlands, sea cliffs, sea crags and caves, countless waterfalls and expansive glacial outwash plains that extend to the sea. They provide access to a number of active volcanoes, south highland glaciers and associated geomorphological features, the largest of which is glacier Vatnajökull and National Park, one of three in Iceland and Europe's second largest.

North of Reykjavik, after following zig-zagging fjords along the coastal plain, is 80 km-long, 10-30 km-wide Snæfellsnes Peninsula in West Iceland. "Iceland in miniature" has some of everything to offer geologically. Dominated by dormant stratovolcano Snæfellsjökull and its eponymous glacier and National Park, the diverse region includes Iceland's most photographed Kirkjufell mountain and scenic fishing villages that cling to the coast.



Palagonized Hyaloclastite Tuff on the Eastern Flank of Sveifluháls
The unvegetated, stratified and volcanic clast-sorted brown rock (foreground) is a palagonized hyaloclastite tuff. The ash formed when water (submarine or here subglacial) violently quenched and shattered molten lava into glassy shards and consolidated it into porous rock. Sveifluháls ridge formed in the Upper Quaternary during the region's last glaciations. The hyaloclastites are sorted into lithic fragments that are distributed into distinct horizons of breccia. The lowest slopes (far right) contain pillow lavas (rounded aggregations formed in water or subglacially here). A normal fault, scoriaceous material deposits and an erosion-resistant section of a NE-SW oriented dike swarm are visible on the ridge. Go there: 63°54'24.82"N, 22° 1'18.88"W



OUR EXPERIENCES
Our late May weather included everything from impenetrable fog and pelting, icy rain to cloudless, brilliantly blue skies, which is a brief commentary on Iceland's variable climate, travel preparedness and the never-ending photographic challenges one encounters. 



Our Intrepid Iceland Gang of Four
It might not look it from our dayglow rain suits, but this was one of the more pleasant weather days on our Icelandic geo-journey. Undaunted and prepared for whatever the sky has to deliver (as opposed to the day before), from left to right we are Diane, me, Robin and "Mountain Bike" Tony.

A few tips: Bring rain and wind-proof layers for blink-of-an-eye weather changes. Summers are beautiful but not hot. Bring a bathing suit for geothermal pools found everywhere. Look beyond the high-tech but very communal, touristy Blue Lagoon. Icelandic króna (ISK) is unnecessary except for small purchases and some rural areas, but bring those credit cards. Venture out of Reykjavik! Hire a guide with a SuperJeep or rent a car or 4X4 for backcountry F-roads (gravel roads with river crossings). Off-road driving is forbidden. Everyone speaks English. Campgrounds are plentiful. Bring a sleep mask for the midnight sun. The food is good but expensive. Mobile connections are spotty on the road.



Dyrhólaey from the Black Cobble Beach of Reynisfjara
 The "Door Hole" and sea crags was Iceland's southernmost peninsula until Katla erupted in 1918 beneath glacier Mýrdalsjökull to the north and formed Kötlutangi spit that extended the shoreline 3 km. Such is Iceland, forever changing. Dyrhólaey originated as a submarine Pleistocene volcano of the East Volcanic Zone when the coastal plains were submerged. Its core is a phreatomagmatic (steam-welded) hyaloclastite (angular glassy lava fragments) tuff (pyroclastic ash) and partially capped by erosion-resistant lava during the eruption's subaerial phase when cone growth isolated the vent from seawater. The eruption dynamics and rock sequences are Surtseyan, after the volcanic island that formed in 1963. Dotted at the base with sea caves, the peninsula is a nature preserve with puffin and eider ducks. 
Go there: 63°24'0.71"N, 19°7'36.27"W


WHERE IS ICELAND?
It's in the middle of the northern North Atlantic roughly between Greenland, Scandinavia and the British Isles. After Great Britain, it's Europe's second largest island, slightly larger than Ireland. With an area of about 103,000 sq km (40,000 sq mi) only 30% is above sea level. If you take the shallow, circumferential shelf into account, the dimensions swell from 300 x 500 km to 450 x 750. Owing to the large numbers of elongate fjords that were carved into its peripheral landscape, Iceland's coastline is an astounding 6,090 km-long.

Contrary to popular perception (or just geographic confusion), Iceland is part of Europe, even though it's divided between two tectonic plates. In reality, so is coastal California, part of North America but vertically sliced by the North American and Pacific plates.



Iceland in the North Atlantic
Iceland lies between eastern Greenland (~300 km), western Norway (~900 km), northwestern Scotland (~850 km) and the Faroe Islands (~450 km). Proximity to the surrounding landmasses has everything to do with Iceland's tectonic evolution.

HOW ÍSLAND GOT ITS NAME
It's a common myth that the "ice-land" was named to deter enemies in pursuit and encourage seafarers to continue on perhaps to a seemingly more hospitable, greener Greenland. It was not only discovered and settled after Iceland in 985 but was penned by Norwegian Viking Erik the Red, remembered in medieval and Icelandic saga sources, to actually encourage visitation and settlement (unless that too is a myth). 

Previous known names for Iceland were Snæland or "Land of Snow" by Naddoddur Ástvaldsson, the purported Norse discoverer of Iceland, and Garðarshólmi or "Isle of Garðar" by Garðar Svavarsson, a Swedish Viking who briefly resided there. So, who actually named the country and why?



Islandia Map ca 1590
Iceland is depicted with deeply incising fjords, fiery volcanoes, elongate mountain ranges, massive glaciers and fierce Arctic winds. Sea wrecks testify to ship-swallowing maelstroms, sea serpents, whales, driftwood and polar bears on ice flows. The map is by the Flemish geographer Abraham Ortelius. Based on their geometric similarity, he may have been the first to imagine the continents were once joined before drifting to their modern positions, a concept expanded by German geophysicist-meteorologist Alfred Wegener in 1912 that led to Plate Tectonic Theory with the contributions of many that followed. From Wikipedia Commons.

Icelandic sagas (ancient manuscripts written in 'Old Norse') tell of western Norwegian seafarer and explorer Flóki Vilgerðarson, who settled in Iceland in 865 during the Viking Age (800 to 1066 AD). Nicknamed Hrafna-Flóki, which means "Raven Floki" for the birds he released to find land, he cursed the country after a particularly harsh winter after failing to gather enough fodder (winter food) for his starving cattle. He was prevented from departing by an ice-choked fjord and therein gave the country the name that finally stuck.



Statue of Hrafna-Flóki with a Land-Locating Raven
His life story is documented in the Landnámabók manuscript, written in medieval Icelandic and describing the settlement of Iceland in the 9th and 10th centuries. It states that he was the first Norseman to deliberately sail to Iceland. The statue located at the Vikingaheimar Viking World Museum in Reykjanesbær. Go there: 63°58'33.64"N, 22°31'43.12"W  

First to briefly occupy Iceland were Gaelic monks seeking isolation and religious freedom. According to two ancient books - one by an Icelandic priest in the early 12th century and by Landnámabók, the Norse's Book of Settlements in the 9th and 10th centuries, settlement in earnest began in the second half of the 9th century when Norse settlers (Germanic inhabitants of Scandinavia) began migrating across the North Atlantic. The accursed name "Land of Ice" did little to deter Ingólfr Arnarson, Iceland's first known permanent settler and founder of Reykjavik in 874.



"Ingolf Tager Island i Besiddelse"
Danish painter Johan Peter Raadsig in 1849 glorified Ingolf Arnarson, who is "Taking Possession of Iceland" in Reykjavik by commanding the erection of a Nordic high-seat pillar (throne), where the head of the household sits. Notice the Viking sailing vessel and table mountain Esja across Flaxa Bay. From Wikimedia Commons Public Domain

HERITAGE, TRADITION AND LANGUAGE
The Icelandic language of Íslenska is derived from 'Old Norse', brought over by Norwegian Vikings and influenced by Scandinavian and northern European neighbors. Without dialects and having many runic alphabet letters from Germanic languages before the adoption of the Latin alphabet over a millennium ago, it's barely changed in some 800 years. What's more, spoken and written purity are protected by the Icelandic Language Institute that approves every new word such as tölva for computer, which means "number-prophetess".

"Icelandic Tongue" is celebrated on Language Day every November 16th. It's an example of nationalism and independence that allows sagas from the earliest days to be read in the language they were written to help preserve ancient culture and heritage. Íslenska is the most basic element of the national identity, but connectivity and globalization are taking their toll with words like "OK" that have crept into the lexicon.



Modern Day Vikings Parading through the Streets of Reykjavik
The word "Viking" is possibly derived from the Norwegian coastal region of Vik, from vikingr meaning "sea pirate" or the word vika for "sea mile", which is the distance between two shifts of rowers. It's embedded in place-names of Reykjavik, Húsavík and Vík í Mýrdal. Reminders of the heritage abound. Toy swords, axes and helmets along with souvenir trolls, elves and fairies - Huldufólk or the "hidden people" - are sold in stores and images are displayed on mugs, T-shirts and logos, while tales of their mischief and magic abound in folklore. 

GEOLOGICAL NAMES
Like other Germanic languages, Íslenska combines difficult to pronounce words into longer ones. Geologic and geographic prefixes and suffixes are perfect examples: -jökull meaning glacier; -foss, waterfall; -eldfjall, volcano; -jökulsá, large river; -höfn, harbor; -ey, island-fells, isolated mountain; -breidur, broad volcanic shield; -vatn, lake; -fjöll, mountain; -hraun, lava; -fjallgarður, mountain range; -öskjur, caldera; laugar-, hot spring; reyk-, steamy; -dalur, river valley; -vellir, field; -vegur, road; -falljöklar, icefall; -stapi, table mountain or tufa; -sandur, glacial outwash plain and -ur, for place names. 


As a result, Eyjafjallajökull is the "English Island-Mountain-Glacier"; Snæfellsjökull, the "Snow-Mountain-Glacier"; Dimmifjallagardur, the "Dark-Mountain Range"; Reykjavik, the "Smoky Bay", Laugarvatn, the "Hot Spring Lake", and Surtsey, the island volcano that is the genitive case of the mythological Norse giant of fire.



Lake Jökulsárlón Beneath Vatnajökull
Developing since 1934 in the trough beneath the retreating terminus of outlet glacier Breiðamerkurjökull, the proglacial lake is a product of climate change as is melting of parent glacier Vatnajökull. Since 1890, the outlet retreated a total of 5.6 km. Proglacial lakes 
can dam with ice, bedrock, moraine, landslide debris or a combination of materials. The glacial tongue is actually floating on the lake and calving into the water. Its highly photogenic scenery is ever-changing with clouds that often block the view of the domed glacier behind it. In spite of its lifeless appearance, a diverse aviary community of terns, gannets and skuas fill the skies, while herring, trout, salmon and seals ply its waters. Go there: 64° 4'54.87" N, 16°13'17.79" W

LAND OF FIRE, ICE, RAIN, SLEET AND SNOW
The Arctic Circle passes through the tiny Icelandic island of Grímsey, and yet, the mainland enjoys a moderate maritime climate. With small seasonal variations in temperature due to the north and west-flowing Irminger Current, a branch of the North Atlantic Current and northeast extension of the Gulf Stream, it delivers relatively warm, high-salinity Atlantic waters and plentiful moist air to the southern and western coasts. 

Persistent precipitation in the southern and central Highlands, where snow accumulation exceeds ablation (loss from melting, sublimation and evaporation) fuels glacier formation in a tundral climate. They're remnants of the vast Iceland Ice Sheet that blanketed the island to the shelf break and northern Eurasia in the Pleistocene. Today, volcanoes display two shapes depending on the age of generation: plateau-shaped formed during glacial periods and shield-shaped if formed in the post-glacial Holocene.



Hjörleifshöfði Bathed in a Sea of Purple Lupine
The inselberg (isolated outcrop) towers over outwash plain Mýrdalssandur of glacier Mýrdalsjökull. The mountain formed offshore submarine or subglacially when the coastal plain was submerged in the Pleistocene. Holocene streams and jökulhlaups (glacial outburst floods) from volcano Katla transported sediments that incorporated it within the lowland. Although beautiful in bloom and used to combat topsoil loss, Nootka lupine is an invasive species imported from North America in 1945. It suffocates more delicate indigenous flora such as Icelandic graymoss and is the focus of conservationists that desire to eradicate it. The "floral hazard" is part of a polarizied debate on invasive life and Icelandic values and morals. By the way, Hjörleifr was the brother-in-law of Iceland's first settler Arnarson, who was slain by his slaves and buried on the summit. Go there: 63°25'20.85"N, 18°45'14.75"W. Photo by Julia Share

During the Last Glacial Maximum, the last time ice sheets were at the greatest extent in Late Weichselian-time of the late Pleistocene and early Holocene. It's thickness exceeded 2,000 m and covered mountain summits. It extended as far as the shelf break doubling the size of the island, evidenced by ice-contact landforms such as 100 km-long and up to 50 m-high terminal moraines on the sea floor over 100 km around West, South and East Iceland.

Before glaciation blanketed the landscape of Iceland, Miocene through Pliocene lavas were the dominant landform. In the Pleistocene, major glaciation influenced landforms in volcanic zones across the landscape with hyaloclastite ridges and mountains (essentially melted cavities within ice) and steep-sided table mountains (the same with extruded lava on the surface). Both formed subglacially (or submarine), whereas the former emplaced from an elongate fissure and the latter from a central vent. 



Reynisdrangar
According to Icelandic folklore, trolls forcing a ship aground were turned to sentinels of stone by the morning sun just offshore on Reynisfjara black cobble beach. Of course, geology adheres to a volcanic origin for the basalt sea stacks, isolated crags and inselbergs that punctuate the broad sandur of coastal central South Iceland. As with various coastal table mountains, they formed subglacially or submarine offshore when sea level stood higher and the coastal plain hadn't yet formed during the Pleistocene. The spired sea crags lie on strike with hyaloclastite mountain Reynisfjall (extreme left), with which they are eruptively associated.

Go there: 63°23'57.81"N, 19°1'54.08"W

Spurned by climate deterioration and controlled by rising global seas, due in part to melting of ice sheets such as the Laurentide in North America and the Eurasian, rapid deglaciation began between 18.6 and 15 k before the present.
  


Map of the Iceland Ice Sheet during the Last Glacial Maxiumum
Easily extending twice the size of Iceland, the LGM ice sheet (red line via thermomechanical modeling and black lines via observed ice-contact landforms on the seafloor) covered the entire island as far as the shelf break (dotted line) in the late Pleistocene and early Holocene. 
Modified from Pétursson et al 
  
Iceland's glaciers are long-standing reservoirs of ice that turn to meltwater dependent on climate and locale. Released water enter the subsurface to feed aquifers and source rivers, the largest and most energetic of which are harnessed for hydropower. It accounts for more than ~75% of domestic energy production, while geothermal energy is used for heating and electricity.

Glacial runoff, which is frequently enhanced by jökulhlaups (catastrophic outbursts from subglacial eruptions, geothermal areas and sudden release of ice dammed lakes) may carve gorges into the successions of lava that built the volcanic island and blanket the landscape and spill over countless rapids and waterfalls leaving deposits across broad sandur outwash plains. 


River Hvítá and the Serene World of the Central Highlands
Named for its silica-rich, milky suspension, the White RIver originates from glacier Langjökull to the north (left) that also sources glacial lake Hvítárvatn in the Highlands. The sandur is a vast and barren desert, a majestic and subdued-hued expanse of erratics, cobbles, eskers, moraines and glaciofluvially transported volcaniclastic sediment on a bed of countless lava flows. Not far south, Hvítá spills over Gulfoss, one of Iceland's most iconic waterfalls. The region is bound by the active West and East Volcanic Zones, the latter possesses a chain of towering central volcanoes including Hekla, Iceland's largest active volcano. Go there: 64°27'15.07"N, 19°59'21.22"W

The Geometry of Gulfoss - A Study in Icelandic Lithology and Structure
In less than 10,700 years, Hvítá exploited lithologic zones of weakness in creating the Golden Falls, although a jökulhlaup flood-origin is a possibility. Its course is due to fractures of differing orientations that course through Iceland. They are strike-slip faults (compression of crustal blocks with horizontal, side by side movement) of the South Iceland Seismic Zone, whereas, the downfalls canyon is a normal fault (where extension pulls apart crustal blocks) typical of the forces yanking Iceland apart. The upper step's mini-cascades consist of erosion-resistant, interglacial lavas alternating with erodable glacial period sedimentary rocks, while columnar jointed basalts form the lower step and canyon walls. Go there: 64°19'37.46"N, 20°7'11.81"W 

"WARM" GLACIERS
Holocene warming caused rapid deglaciation, controlled by rising global sea level due to melting of ice sheets such as the Laurentide in North America, which began between 18.6 and 15 k before the present. Ever since, glaciers have been retreating and re-advancing (due to climate deterioration such as the Dryas) inside the coastline. Distinct shorelines formed, represented by sea cliffs along the central South Coast.

By 8.7 k, deglaciation had obliterated the Eurasian and Iceland Ice Sheet, and segregated Iceland's ice sheet it into some 269 named glaciers at higher elevations in the Highlands. Their location mirrors the maze of volcanic zones and belts that have been forming for over 16 million years. The result is a landscape that's about 11% glaciated, while overall, snow and sleet account for ~7% and ~35% of annual precipitation. Indeed, Ísland - EES-lahnd in Icelandic - is most deserving of its appellation.



Topography of Iceland with Glacier Distribution
Superimposed over Iceland's active volcanic zones and central volcanoes (inset) that controlled the dynamics of the ice sheet, 60% of Iceland's main ice caps are located within the zone in the south and central highlands and are bordered by smaller glaciers at high elevations. The regional distribution is indicative of the direction of precipitation arriving by prevailing southerly winds. They respond actively to climatic fluctuations, while acting as long-standing reservoirs of meltwater that feed Icelandic rivers, some of which have been harnessed for hydropower, and source glacier-related floods. From H. Björnsson and F. Pálsson, 2008.

BAROMETERS OF CLIMATE FLUCTUATION
Succumbing to climate change, Okjökull in West Iceland, which has been severely diminished by ablation, lacks the sheer mass to move under its own weight. In 1890, it was 16 sq km in size and is the first to no longer qualify as a glacier, simply known as “Ok” (rhymes with 'talk'). Future climate scenarios indicate even larger glaciers will follow suit in 150-200 years (some say as much as 500), perhaps with the exception of those on the highest peaks.

Melting of Icelandic ice's 3,600 cu km of water would raise global seas by 1 cm. Regardless of the time frame, it's the current trend seen in Iceland, as backwasting (peripherally), downwasting (on the surface), the terminus (toe-end of the glacier) and firm line (that separates the zone of accumulation and ablation) retreat upvalley. It is predicted that the Arctic Ocean could be ice-free mid-century as well. Prescient-thinking, lowly Reykjavik on the coastal southwest has even begun to reassess its harbor infrastructure.



Comparative Satellite Images of Glaciers Eyjafjallajökull and Mýrdalsjökull
The neighboring ice caps have dramatically changed in 28 years. Left, In 1986, smaller Eyjafjallajökull and Mýrdalsjökull (fourth largest ice cap that covers closely-monitored, hyperactive subglacial volcano Katla) were connected. Right, In 2014, the connection melted away and outlet glaciers, such as pendulous Sólheimajökull in the southwest, are greatly depleted, having retreated up to 50 m/yr. Other signs of deglaciation are sulfuric acid emissions in proglacial rivers that warn of impending outburst floods and ice cauldrons (surface depressions) formed from subglacial geothermal activity. Modified from NASA Earth Observatory

ISOSTATIC GLACIAL REBOUND

Counterintuitively, sea level at Iceland has dropped as its landlocked glaciers have melted. It's due to isostatic rebound of the landscape following glacial unloading of the lithosphere, a normal process of deglaciation. Parts of Iceland, especially on the southwest Reykjanes Peninsula, gradually relieved of the massive weight of ice some 2,000 meters thick and extending out to sea far enough to double the size of Iceland, are rising faster than any place on Earth - as much as 1.4 inches per year.

As the crust rebounded, it carried the landscape with it, exposing the volcanic shelf as the new coastline, while former sea cliffs became stranded inland from their original location and markedly uplifted, appearing as if formed by erosion or fault scarp uplift. Evidence is along Iceland's South Coast that occurred in the warmer Holocene beginning ~13,000 years ago when sea level rose some 100-150 m above the present level. The escarpments are known for its spectacular waterfalls that source from highland glaciers. 



View from behind Seljalandsfoss during a Relentless Driving-Rain
Towering over the coastal sandur that reaches a foggy sea, the 60 m-high waterfall is fed by meltwater from glacier Eyjafjallajökull that spills off the former sea cliff. Subsequent to isostatic emergence of the cliff within the last 13,000 years, Highland glacial meltwater spills off the escarpment in countless cascades and delivers basaltic sediment across the broad, flat sandur of the Markarfljót River of the South Coast. Sandur formation has extended the coastal plain and in so doing, isolated a variety of volcanic landforms from the sea in which they erupted or emplaced such as volcanic necks, stacks and table mountains.

Interestingly, crustal rebound appears to have triggered earthquakes and pulses of volcanic activity as subsurface pressures and stress adjustments act on magma chambers and melt generation, especially if shallow. It's a reminder of the interrelatedness of Earth's processes.



The Waterfall-Punctuated, Remnant Sea Cliff of the Central South Coast
The cliff was cut by wave erosion when sea level was much lower some 13,000 years ago. Although sea level rose following deglaciation, isostatic rebound of sea cliffs outpaced it. The rocks are a layered mix of hyaloclastites and successive lava flows. Below the cliffs, braided-streams and rivers course through a long and broad sandur. Many areas have been vegetated to reclaim and preserve soil for pastureland with lupine and crops such as barley, rutabaga and potatoes. Discharge across sandurs can fluctuate wildly, especially following thaws and heavy rains, causing them to flood with severe damage. Iceland is indeed a dynamic landscape in so many ways. 

GLACIERS GALORE
Almost every type is found in Iceland - ice sheets (continental-size masses not necessarily associated with mountains), ice caps (miniature sheets), ice fields (even smaller), ice streams (slowly-moving and ribbon-like), outlet glaciers (tongues that drain a sheet or cap), surge (fast-moving and short-lived), alpine (many over active volcanoes and geothermal areas), valley (originates from mountains), piedmont (spilling onto a plain), cirque (bowl-like) and tidewater (recurring advance and retreat with calving).



"Hollywood Glacier" Svínafellsjökull
The outlet glacier of Vatnajökull is energetically melting at 1m/yr. Its history is recorded in tephra that, since Iceland's settlement, records over 80 subglacial eruptions. Julia is clinging to a glacially-scoured lava wall while on a steep lateral moraine of till (unstratified, unconsolidated mix of glacial bedrock). Ash-stained crevasses (tension cracks) are products of internal deformation as the glacier creeps over bedrock. In response to gravity, movement is facilitated by a thin layer of water, plastic ice deformation, regelation (melting and refreezing under pressure) and bedrock deformation. The proglacial lake is a consequence of deglaciation and ice-front retreat, whose milky meltwater flows across Skeiðarársandur, the world's largest outwash plain. The glacier has appeared in numerous productions and advertisements, most recently Game of Thrones. Go there: 64°0'25.07" N, 16°52'23.52" W

By far, at 8,300 sq km, Vatnajökull in the southeast is not only Europe's largest ice cap - covering ~13% of Iceland's surface - but the largest outside the polar region. It has over 30 outlet glaciers that flow outward centrifugally, each constrained by valleys and troughs they've created. 

Hidden beneath is a diverse landscape of U-shaped valleys, canyons, rivers and lakes, and seven volcanoes. It's also the location of Europe's largest National Park. Some of Iceland's largest rivers originate in Vatnajökull such as Jökulsá á Fjöllum to the north that, on its way to the Greenland Sea, has carved Jökulsárgljúfur, the country's largest canyon with famous waterfalls Dettifoss and Selfoss.



Jökulsárlón in the Shadow of Massive Vatnajökull
Appearing motionless, glaciers are constantly on the move, advancing or retreating. Flow velocity is slowest along the lateral flanks and base due to friction with the bedrock. Variations are indicated by ogives (light banding in summer and dark in winter. Formed in the 1930's, the proglacial lake is an iceberg-choked, saltwater lagoon in tidal communication with the sea. It lies at the southern terminus of surge-type, outlet glacier Breiðamerkurjökull of parent Vatnajökull that towers above it. Its lateral moraines line the encompassing valley walls and medial moraines are fed debris from volcanic nunataks that project above the glacier. With a 300 meter-deep channel carved below sea level, the lagoon's dimensions have increased four-fold since the 1970's due to melting. Go there: 64° 4'54.87" N, 16°13'17.79" W

WHAT IS ICELAND GEOLOGICALLY?
It's the world's largest, solitary volcanic island, not to be confused with volcanic island chains along interplate subduction zones (eg. Antilles arc in the Caribbean and Aleutians in the North Pacific) or intraplate chains (eg. Hawaiian-Emperor Seamounts). It's a basalt plateauan elevated geological feature of relatively low relief but considerable overall elevation (over 4,000 m above the seafloor and emerging to an elevation of 1,775 m), crustal thickness (3 or 4 times thicker at 10 to 14 km) and area of 350,000 sq km.

It was built by vigorous and effusive magmatism that repeatedly flooded the landscape with largely basaltic lava flows that accumulated over time. A subject of considerable debate is how magmatic productivity allowed a mid-oceanic ridge on which Iceland resides to emerge from the seafloor and construct an elevated basalt plateau above the level of the sea. 



Landscape of the Southwest Reykjanes Peninsula
Keflavik Airport and nearby Reykjavik, in spite of the region's former level of volcanic activity, are built on 

"safe" bedrock outside of the peninsula's four active volcanic systems. Its roads rise and fall over an undulating, hummocky terrain built of multi-layered flows of ropy, smooth-surfaced pahoehoe and blocky, spiny a'a lava, Hawaiian names for the two types found globally. Notice tumuli, small domed-hillocks of lava that ponded in surface depressions and buckled beneath incoming flows. Formed recently, the lava field has an unglaciated, unaltered topography.


Iceland's sharply-delineated volcanic passive margin differs from the broad shelf of its Atlantic neighbors such as the North American East Coast that constitutes a passive continental margin, marked by thermal subsidence, massive sedimentation and seaward dipping lavas (flood basalts). Both types of margins are related to continental break-up, but in Iceland, it is commonly thought to occur over a hotter mantle with a high rate of lithospheric extension and is related to the development of a large igneous province. 

UBIQUITOUS "ICELAND STONE"
Iceland's rocks are less varied from those of other regions and consist mainly (>90%) of volcanic basalt. It's the building block of Iceland generated mainly from rift zones.  Sedimentary rock comprises only 5-10% and is greatest after the end of major glaciation. With the exception of rock formed at the margin of basaltic intrusions such as dikes, metamorphic rocks - formed under high pressure and temperature - do not occur.

Basalt typically has an aphanitic (fine-grained) texture due to rapid-cooling at the Earth's surface, enough to allow crystalline growth yet slow enough to form dramatic, vertical colonades that are mainly hexagonal in cross-section if the lava flow is sufficiently massive. Lava surface morphology is either pahoehoe (shiny, smooth, glassy, ropy and thinner), a'a (rubbly, slower and thicker) or blocky (even thicker). 



Assorted Common Icelandic Rocks of Mafic Origin
A, Basalt bomb, shaped in flight with aerodynamically-elongated vesicles from the force of ballistic ejection. B, Small multi-layered, ejected pyroclast from volcano or fissure with oxidized core and crust acquired in flight. Smaller tephra are lapilli (2-64 mm) and ash (<2mm). C, Chunk of razor sharp a'a lava. D, Highly-vesiculated scoria or cinder, a frothy pitch black tephra oxidized brick-red. Light-colored pumice is felsic tephra. E, Glaciofluvially-polished basalt cobble from the coast. F, Vesicular basalt with bubble-voids captured. G, Hyaloclastite, a common Ice Age Icelandic rock, which is an aggregate of glassy fragments that forms during quenching, instant cooling and shattering. Palagonization hydro-thermally and chemically-alters the rock turning it brown.

Basalt is a dark-colored, mafic igneous rock - a ferro-magnesian silicate rich in Mg, Fe, olivine, pyroxene and plagioclase. Its low-silica and low gas content (45-52% SiO2) in a partially molten state (984 to 1,260°C) results in low viscosity (fluidity). That allows basaltic lavas (extrusive magma) to be non-explosive and flow considerable distances (tens of km) at variable speeds (6 to 30 mph) from its source (a single vent or long fissure), all facilitated by a high rate of effusion (eruption rate) and steeper topography.

Gabbro is the intrusive (subsurface), chemically-equivocal, phaneritic (coarse-grained, slow-cooling) version found in shallow magma chambers and exposed dikes. Less common in Iceland are intermediate and explosive-erupting felsic igneous rocks (light colored, high SiO2, feldspar, quartz and muscovite), which is called a full spectrum or bimodal association when occurring together. 



Polygonal Stacks of Columnar Basalt
Slow-cooling of massive basaltic flow results in the formation of columnar-jointing. In cross-section, it typically forms five and six-sided polygons as downward-propagated cooling and horizontal mass contraction sets in. The sides of the columns often display a uniform corrugation of horizontal bands (striae) and inscribed circles on horizontal surfaces in positive or negative relief with plagioclase mineral laths (long and narrow) on microscopy. This display is along the black sand beach and sea cave of Reynisdrangar of central South Iceland.

Go there: 63°24'9.59"N, 19°2'23.76"W 

As a result of basalts physical and chemical properties, countless blanketing successions of basaltic lava are underfoot, cover the landscape and form hyaloclastite ridges and table mountains



Gerðuberg Cliffs of West Iceland's Snæfellsnes Peninsula
The 500 meter-long lava flow was, rather than confined subglacially, formed as a subaerial eruption spilled across the landscape. It's composed of dolerite, a dark coarse-grained basalt found in dikes and sills. The vertically-jointed, 5 to 7-sided colonade formed in a warmer period of the Quaternary within the last 500 ka. Although not fully understood, the formation of polygonal geometry in cross-section is related to the rate of thermal contraction, geologic setting, basalt chemistry and external factors. Sides of the columns possess horizontal banding, while superior surfaces have inscribed circles with radiating hackles related to cooling. Go there: 64°51'38.30" N, 22°21'21.91" W

Table mountains or tuya are basalt flow-constructed. Ubiquitous across the landscape, many formed as a submarine intrusion but most over an elongate volcanic fissure (móberg ridge) under glacial ice during the Pleistocene, when Iceland was covered by an ice sheet. As geothermal heat melts a confining ice cover, it forms a subglacial lake with signature pillow lavas that convert to brecciated hyaloclastites (glassy, quench-fragmented rock in a fine-grained matrix). When the eruption conduit reaches the surface, pahoehoe lava typically blankets the flat-topped, steep-sided edifice.



Tilted Strata of Table Mountain Ljósufjöll
The coastal drive north from Reykjavik to Snæfellsnes Peninsula in West Iceland is a jaw-dropping, perspective-humbling, camera-clutching experience. It travels across sandurs of glaciofluvially-delivered sediments. The Ljósufjöll fissure volcanic system erupted during the mid- to late Pleistocene ~700,000 years ago within the still-active Snæfellsnes Volcanic Belt with the last eruption about 1,000 years. The inclination of the strata is upward toward the Borgarfjörður anticline, indicative of the volcanic rift where it was generated rather than Iceland's central axis as one might expect. Magmatic overloading in active zones generates large-scale bending and synclinal landforms that reverse outside the rift zone. Go there: 64°49'22.01"N, 22°13'51.16"W

LARGE IGNEOUS PROVINCES
The Iceland Plateau is the mid-ocean centerpiece of a Large Igneous Province (LIP), a large accumulation of igneous rock, referred to as the North Atlantic Igneous Province (NAIP) that spanned the North Atlantic from Greenland to the northern British Isles (Morton and Parson, 1988). LIPs are intraplate magmatic events that formed relatively rapidly within oceanic and continental environments in a few million years, although some persisted for tens of millions extruding immense volumes (>100,000 cu km and often >1,000,000) of mainly mafic magma.

LIP emplacement is distinct and separate from magmatism associated with plate-boundary seafloor spreading and subduction events. They're an essential process in shaping our planet that precede continental rifting (extension, break-up and subsequent seafloor spreading). Beginning in the Proterozoic but likely in the Archean, Earth history has been punctuated by these crustal provinces (regions with common geomorphic, structural, temporal and genetic attributes) but are best preserved in the Mesozoic and Cenozoic. They include flood basalts, volcanic rifted margins, oceanic plateaus, giant dike swarms, etc. 



LIP Basalt Cliffs of Krýsuvíkurbjarg of Reykjanes
 Erosion and post-glacial rebound along the southern coast of the peninsula in southwest Iceland reveal multi-layered lava flows interbedded with sedimentary intervals that testify to the formation of Iceland as the centerpiece of the NAIP. This is the Reykjanes Volcanic Zone, one of four NE-SW trending volcanic systems, arranged en echelon due to extension-reducing compressive horizontal stress in the direction of plate separation. Just to the north is the active Krýsuvík volcanic system of crater rows, small shield volcanoes, hyaloclastite ridges, maar craters and geothermal fumaroles and mud pots. Go there: 63°50'8.31"N, 22° 5'58.80"W 

LARGE IGNEOUS PROVINCES OF THE ATLANTIC REALM
Three discrete Mesozoic-Paleogene LIPs emplaced before break-up of the late Paleozoic supercontinent of Pangaea and the subsequent opening of the Atlantic Ocean. On the present-day landscape, the once-unified LIPs exist as fragmented remnants vastly reduced in size by erosion and distributed across the ocean by tectonic fragmentation on pre-rift conjugate margins of the continents of the Atlantic domain:
• the North Atlantic Igneous Province (NAIP) that opened the North Atlantic ~55 Ma between Greenland and Europe with Iceland at the center; 
• the Central Atlantic Magmatic Province (CAMP) ~195 Ma between northeastern North America and northwest Africa; 
• the Paraná-Etendeka Igneous Province (PEIP) between South America and southwest Africa that opened the South Atlantic ~120 Ma. 



Remnants of Large Igneous Provinces of the Atlantic Realm
The emplacement of LIPs and continental rifting precede the break-up of continents and the opening of intervening seas. In regards to Pangaea, LIP remnants (red) remain along the borders of the new landmasses that border the newly-formed Atlantic realm. In the North Atlantic, they are the Eocene-Oligocene NAIP from eastern Greenland to the British Isles with Iceland (Ic) athwart the Mid-Atlantic Ridge (green), its only remaining active centerpiece; the Central Atlantic, the Late Triassic-Early Jurassic CAMP from the eastern North American margin to the northwest African margin; and in the South Atlantic, the Early Cretaceous PEIP from southeast South America (Pa) to the southwest margin of Africa (Ga and Et). From Lundin, 2005.


Related to the extrusion of massive quantities of flood basalts and associated gases, many LIPs have been associated with alteration of atmospheric and oceanic chemistries, rapid climate change and large-scale extinction events. Most notable in regards to the Atlantic domain is the end-Triassic CAMP extinction, one of the largest of the Phanerozoic. It enabled dinosaur domination of land and is attributed to climate change associated with voluminous degassing of basalt flows in addition to temperature, sea level change, marine anoxia, salinity and acidity, which may not be mutually exclusive. 

Similarly, NAIP volcanism and methane degassing of seafloor sediments into the atmosphere appears to be associated with short-term warming but an extinction event of uncertain magnitude, as is the PEIP. 

GALLERY OF NAIP, CAMP AND PEIP REMNANTS 
The following photos depict various remnants of the NAIP, CAMP and PEIP on conjugate continental margins that were separated by seafloor spreading and plate divergence. Again, Iceland forms the volcanically active centerpiece of the NAIP, while the other LIPs are magmatically extinct. What might account for this volcanological phenomenon?



NAIP - Scotland's Waternish Peninsula on the Isle of Skye of Northwest Scotland's Inner Hebrides
The NAIP (aka Tertiary British Igneous Province in the northern British Isles) emplaced about 60 million years ago. Columnar basalts of the Giant's Causeway in Northern Ireland and Scotland's Staffa Island and Fingal's Cave on the Isle of Mull are familiar examples. The landmass across the Little Minch strait (above) is the Outer Hebrides of Scotland. Go there: 57°33'20.91"N, 6°38'29.08"W 


CAMP - The Majestic Palisades of the Hudson
The Early Jurassic diabase sill (horizontally-intrusive basalt) of the CAMP is interposed by thick, climate-forced, dinosaur track-containing sedimentary sequences. The exposed flank of the Palisades is the easternmost hinge-margin of the Newark Basin, which is an aborted rift basin (failed to the extent that the fault-bound Newark basin extended and subsided but never formed the rift-proper that opened the Atlantic). Conjugate trans-Atlantic remnants of the CAMP, its African continental margin equivalent, are located in Morocco. The Palisades is one of the most recognizable intrusive bodies in the world and a familiar fixture on the western skyline of Manhattan for New Yorkers. Go there: 40°51'53.51"N, 73°55'54.22"W

PEIP - Iguazú Falls of Brazil and Argentina
 Comprised of 275 individual falls at normal flow, the falls complex spans Rio Iguazú between southwestern Brazil and northeastern Argentina. Its waters spill off the uplifted, fault-segmented Paraná basin of the rift-separated Paraná-Etendeka Igneous Province in South America and Africa, the two fragmented when the South Atlantic opened in the Early Cretaceous. Read about it in my posts and here.

Go there: 25°41'4.52"S, 54°26'41.13"W

GEOLOGIC SETTING OF THE NAIP AND ICELAND
Iceland, representing the active center of the NAIP, is situated at the juncture of two elongate seafloor structures of volcanic origin: the Mid-Atlantic (MAR) and Greenland-Iceland-Faeroe Ridges (GIFR). The interpretation of the relationships of these features with Iceland is the crux of a longstanding debate not only about the formation of the volcanic island but the structure and behavior of the Earth's mantle, and not just in the North Atlantic but globally! 



Subaerial and Submarine Feature Map of the North Atlantic
The elevated plateau of Iceland lies in the northeast Atlantic Ocean roughly between Greenland, the Faeroe Islands, northern Great Britain and western Norway AND at the intersection of ridge segments Kolbeinsey and Reykjanes (orange line) and the Greenland-Iceland Ridge (red line) intersects Iceland with approximate positions (red dots) of the postulated Iceland Plume (discussion forthcoming). The NAIP includes subaerial (black blobs) and submarine components (light gray). Modified from Thordarson and Larsen, 2007.

THE GIFR AND MAR
The Greenland-Iceland-Faeroe Ridge is a roughly W-E trending submarine aseismic ridge or rise. With abnormally thick oceanic crust, it is a mountainous chain of seamount-volcanoes that extend from Greenland through Iceland (GIR segment) to the Faeroe Islands (IFR). It doesn't produce seafloor spreading or earthquakes except at its end beneath Iceland, where the theorized head of a mantle plume is centered. 

Straddling the Mid-Atlantic Ridge, Iceland is the longest area of subaerially exposed mid-ocean ridge on the planet. The spreading center is a NS-trending, 10,000 km-long, volcanic mountain chain from Bouvet Island near South Africa to 330 km short of the North Pole and bisects the ocean floor between the drifted continents of the Atlantic realm.

It's offset (laterally-interrupted) by relatively short transform faultsA consequence of plate tectonics on a sphere, they are strain-relieving fracture zones that convey mainly strike-slip motion (horizontal, side-to-side) that neither create nor destroy lithosphere. The Jan Mayen Fracture Zone is on the north and Bight FZ and Charlie-Gibbs FZ are on the south of Iceland. The former's history of eruptive magmatism is thought by some members of the scientific community to be part of the mantle plume that formed Iceland or a fragmented micro-continent beneath Iceland.



Bathymetry and Topography of the North Atlantic Region
The MAR separates the diverging North American and Eurasian plates with the elevated Iceland Plateau athwart the spreading center. To the north and south, the MAR intersects Iceland at the Kolbeinsey and Reykjanes Ridges, respectively, while the Iceland plume currently lies beneath glacier Vatnajökull in southeast Iceland. The Jan Mayen and Charlie-Gibbs Fracture Zones offset the Mar. The former may be related to the Jan Mayen hotspot and development of the northern North Atlantic before the formation of Iceland. Modified from ngdc.noaa.gov. 

The submarine MAR is a slow-spreading-type divergent plate boundary between the North American-Eurasian and South American-African plates that are separating at the rate of about 1.8 cm/yr. The mid-ocean spreading center is also a constructive plate boundary, where most of the Earth's crust is generated in diverging conveyor-belt fashion (red arrows). Over 70% of all spreading centers in the world's mid-ocean ridge systems are oriented obliquely to the direction of absolute plate motion.

To complete the concept, existing crust is removed via subduction at destructive margins, while plates that meet at a transform fault margin move parallel to each other called conservative margins.



Schematic of Mid-Ocean Seafloor Spreading and Volcanism
Modified from whoi.edu  

On a global scale, the MAR is part of a 80,000 km-long, mid-ocean seafloor-system that includes a volcanically-active 65,000 mile-long mountain range, the longest in the world. It encircles the planet like the raised seems on a baseball with crests that rise on average 1,000 to 3,000 meters above the adjacent ocean floor. Mid-ocean ridges are found in every major ocean in Earth. Normally, they don't build up above sea level with the exception of Iceland athwart the MAR in the North Atlantic. 
 
Bathymetric Map of the Global Mid-Ocean Ridge System
The diverging MAR separates the North American-Eurasian and South American-African plates (arrows) and is part of an elevated and continuous, world-girdling mid-ocean ridge system of predominately active normal faults (lithosphere extension perpendicular to the ridge and parallel to the direction of inferred plate motion). The system is both seismically and volcanically active. The Iceland Plateau (encircled) lies athwart the MAR in the northern North Atlantic. Modified from One Man's World.


The MAR and GIFR ridges, fracture zones and NAIP - played a role in the evolution of Iceland and the North Atlantic, but not everyone agrees on how. One widely accepted interpretation is challenged by several alternative hypotheses - one of which that is gaining ground. Let's briefly explore that notion and then return to a discussion of Iceland.

A NEW GEOLOGICAL PARADIGM
During the Plate Tectonic revolution of the 1960's that accounts for volcanism at 
interplate boundaries, geoscientists sought an explanation for exceptions such as 
intraplate volcanism of the Hawaiian-Emperor seamount chain (within the Pacific plate, thousands of miles from the nearest plate boundary of any kind, read about it here) and the persistent and voluminous interplate volcanism of Iceland. 

The solution was provided in 1963 by Canadian geophysicist-geologist J. Tuzo Wilson's Hotspot hypothesis. In it, the uindirectional, time-progressive volcanic island chain formed as the Pacific oceanic plate migrated over the relatively small, stationary melt anomaly of a hotspot (persistent hot region of the underlying Earth's mantle) where magma continuously breaks through the lithosphere. All that was needed was a mechanism of heating that came eight years later in 1971 when geophysicist Jason Morgan enumerated the Mantle Plume hypothesis.



Architecture of Columnar-Jointed Basalt with Two Facies
The rocky coastline around the fishing village of Anarstapi on Snaefellsnes Peninsula's is particularly scenic and popular. The exposed columnar jointing demonstrates two jointing facies: colonnades with regular columns and near-planar sides and an entablature sections with thinner and less regular columns with curved sides. The latter is often found independent of the colonnade or capped with an upper section. Based on grain-size, mesostasis (groundmass graininess) and rate of cooling is influenced by water immersion at the time of emplacement. Alternative explanations include mismatch of joints or different stress distributions where they meet. Small columnar diameters are possibly due to reduced isotherm velocity (rate of propagation) at the flow margin.

THE MANTLE PLUME HYPOTHESIS
As originally defined, there were about 20 mushroon-shaped plumes of long-lived, fixed (stationary relative to one another) diapirs (narrow, cylindrical heat-flow conduits), each with a pendulous tail and bulbous head. Less dense, solid but not molten, they are theorized to originate at great depth (~2,892 km) from the core-mantle boundary (the Mohorovicic discontinuity or Moho, a thermal boundary) to which they anchor ("deeply-rooted").

The unusually ("anomalously") hot, buoyantly upwelling material ascends through the uppermost mantle to the surface and, flattening against the more rigid lithosphere, fuels a region of hotspot volcanism via adabiatic melting (partial melting of a portion of solid mantle with a different resulting composition) from decompression (rapid decreased pressure like a champagne cork popping open).

The result is the extrusion of basaltic magma onto the surface. The process commonly occurs at divergent tectonic plate boundaries, such as at mid-ocean ridges but also explains volcanism in association with continental lithospheric extension (e.g. Basin and Range and Yellowstone). Of course, Iceland is unique in that a mantle plume is thought to be rising up through the Mid-Atlantic Ridge. In fact, the concept of a plume rooted in the deep mantle beneath Iceland dates back to the definition of Morgan's hypothesis.


Simplified Schematic of the Hotspot-Plume Concept
1.) Rising mantle plume anchored at the core-mantle boundary; 2.) Decompression melting and 3.) Surface extrusion of basaltic volcanic rocks. 
From wmblogs.wm.edu

Simple and elegant, it made sense and found near universal acceptance. It integrated well with Plate Tectonic theory while providing its driving mechanism, a process for cooling our planet, recycling of the crust and explaining hotspot melt anomalies. Replacing Hall and Dana's Geosynclinal Theory of 1859 - Earth cooling and contraction that ultimately builds mountains - it has served the scientific community well for over 50 years.

Unified with tectonic theory, the concept of hotspot-plumes explains a world of possibilities regarding crustal thinning and extension, flood basalts, rifting, seafloor spreading, large igneous provinces and their distinctive isotopic signatures. 

THE SAGA OF THE ICELAND PLUME - A PLUME-RIDGE INTERACTION
In the case of Hawaii, the magma source is from the top of a plume, fixed relative to the mantle below the migrating Pacific plate that produces a time-progressive track. In the North Atlantic, hotspot-plume advocates espouse Iceland's elevated and thickened oceanic crust and unusually high level of persistent volcanism is due to the interaction of the Mid-Atlantic Ridge spreading center and the Iceland plume that feeds the hotspot. 


Modeled Mantle Plume Beneath Iceland
About 24 million years ago, the center of the postulated Icelandic mantle plume was positioned at the intersection of the Reykjanes and Kolbeinsey Ridges. The present-day plume as a 200-300 km-wide and extending to a depth at least 400 km, cylindrical zone somewhere beneath the northwest corner of the Vatnajökull glacier. Revealed by anomalously low seismic wave velocities and computer modelling, it's described as a cylindrical, upwelling diapir with a higher temperature than that of the surrounding mantle. The plume accounts for the genetic origin of the Iceland Basalt Plateau as well as its earthquakes, volcanism and geothermal activity.

The plume's arrival beneath the migrating plate formed the GIFR hotspot track, the trail of volcanism indicative of where the plume had been (see dated track on map above). In essence, the Iceland hotspot was 'captured' in the MAR spreading system following NAIP break-up that formed as North America and Greenland drifted away from Europe following the rise of the plume.

A "PLUMELESS" ALTERNATIVE EXPLANATION
Plume skeptics or "platists" (Foulger, Natland, Anderson et al, 2001), using lines of argument that "plumists" use as confirmation, find fault with inconsistencies within the plume hypothesis. Contending that first-order observations fail to exist, only imaging, modeling and indirect chemical assays, proposed plumes don't meet the standard criteria to be called as such.

They say the extreme variability of hotspots and their tracks can't possibly fit into a "one-size-fits-all" model to explain them. Furthermore, they contend that hotspots aren't any hotter than normal and plume isotopes (such as helium) can be generated at shallow depths. As for Iceland's volcanism, it's remained entirely on the MAR since the opening of the Atlantic at ~54 Ma, not tracking along a GIFR hotspot time-track. 



Two Antithetical Views of the Earth's Mantle
Left, Mantle plumes from the core-mantle boundary ascend to sub-lithospheric depths, where partial melting occurs, and to the surface, where hotpsot lavas erupt forming features such as large igneous provinces. The plume head's arrival contributes to continental break-up and "punctuates plate tectonics by creating and modifying plate boundaries", while the tail forms a hotspot track beneath a migrating plate. Right, Without deep and shallow mantle communication and denying the existence of plumes, superficial hotspots (better called "meltspots") are the result of lithospheric tensile stress and decompression melting that permits anomaloulsy hot material to ascend to the surface. Modified from Torsvik et al, 2016

Platists believe Iceland is a "natural consequence of shallow processes related to plate tectonics" and that lithospheric spreading and thinning permit melted rock to escape to the surface. Thus, melting anomalies - hotspot volcanism - are the result of passive rather than active processes that occur where lithosphere is in extension along lines of structural weakness within the crust such as transform zones, old sutures and faults. 

They further contend that their plumeless model accounts for every major observation at Iceland - geophysical, geological, petrological, geochemical and seismological. They assert that the concept is consistent with plate tectonics and doesn't have to invoke an endless barrage of "ad hoc plume variants" of every imaginable shape, size and number. For example, some plume models - such as cactus and spaghetti plumes - have acquired an extremely complex geometry and internal composition to refine and accommodate any given situation.



Birch Forest along the Geothermal Field of the Laugarfjall Rhyolitic Dome
Most first-time visitors assume the treeless terrain is due to the inhospitable climate, but fossil evidence indicates heavy forestation with sequoia, magnolia and others during the warm and temperate middle to late Tertiary as recent as 5-15 million years ago. Late Pliocene cooler temps brought boreal pine, spruce, birch and alder, and Pleistocene glaciations made flora increasingly species-poor with dwarf and downy birch. At the time of settlement over 1,150 years ago when the climate was comparable to today, trees covered 25-40% of the landscape, negating the climate explanation. Man's need for fuel, building material, charcoal for smelting and tool making, livestock fodder and sheep that devoured birch seedlings devastated the trees down to 1% especially in the lowlands. A century ago, most locals had never seen a tree, but preservation, reforestation and afforestation (where tree-cover was nonexistent) now account for ~2% of the land.


THE ICELAND "PLATE MODEL"
Platists believe Iceland began to form where the MAR - the line of opening of the Atlantic Ocean that formed at ~54 Ma when Greenland and Eurasia rifted apart - crossed a transverse outer branch of the relict Caledonian suture on the Atlantic seafloor at shallow depth (lower right panel). The suture is the site of a ~440 year-old subduction zone created when what are now Greenland and Scandinavia collided as the intervening Iapetus Ocean closed during a phase of the multi-stage formation of Pangaea. 

Iapetus closure (part of the Wilson Cycle) occurred when three Paleozoic continental cratons (stable crustal blocks) collided - Laurentia (the Greenland region of ancestral North America), Baltica (ancestral Scandinavian micro-continent) and Avalonia (an elongate micro-terrane of volcanic islands that rifted from Gondwana as a prelude to the formation of Pangaea). For clarification, Rheic Ocean closure occurred when Gondwana 
(South Hemispheric Paleozoic mega-continent) collided with Laurasia (Laurentia + Baltica and Avalonia) to form Pangaea.



Closure of the Iapetus Ocean and Formation of the Caledonian Suture 
(a) Laurentia, Baltica and Avalonia were brought into convergence as oceanic Iapetus crust subducted beneath Greenland, Baltica and Britain. The red dashed-line is the inferred opening of the Mid-Atlantic Ridge ~54 Ma. Arrows are convergence directions. Thick lines are faults and orogenic fronts. Black triangles indicate thrust faults. (b) North Atlantic Bathymetry showing the GIFR Ridge, which is underlain by crust ~ 30 km thick. Other shallow areas are blocks of stretched continental crust. Thin purple line: MAR; thin dashed black lines: extinct ridges; thick lines: faults of the Caledonian suture; thick dashed line: inferred trend of suture crossing the Atlantic Ocean. Modified from Foulger et al

Awaiting formation of the MAR, lithospheric faults and old sutures such as the Caledonian provide pathways for the passive ascent of decompression-melted oceanic crust from the upper mantle. The event built the GIFR crustal band from Greenland to Britain over the suture and explains the persistently large melt volume, tholeiitic geochemistry (from partial melting), since the suture remnants are entrapped and can't migrate laterally.

ONE MANTLE WITH MULTIPLE INTERPRETATIONS
The formation of the North Atlantic provides clues to the origin of hotspot-meltspot globally. Where lithosphere is in extension, Platists contend that passive volcanism formed the Hawaiian-Emperor chain due to an intraplate extensional stress field within the Pacific plate. In contrast, the Plume model postulates the Hawaiian plume - where Tuzo's theory was first proposed - is responsible for the formation of the volcanic island chain as the Pacific plate migrates over the hotspot. 

To which mantle dynamic do you subscribe? Are both concepts mutually exclusive? Are we experiencing the beginning of a paradigm shift on a tectonic scale? Let's return to Iceland where the Mid-Atlantic Ridge is on land.



Volcano Eyjafjallajökull Erupting beneath Glacier Eyjafjallajökull
After melting overlying portions of the ice cap in 2010, the volcano sent a 5-6 km-high ash plume skyward that, carried by the jet stream with the explosive power of sudden subglacial water vaporization, interfered with European air travel for seven days. Volcanic lightning occurs when the separation of heat and movement-charged ash particles overcomes the capacity of air to insulate them resulting in the flow of electricity. The National Geographic 2010 "Picture of the Day" was posted with permission from Sigurdur Hrafn Stefnisson. Visit him here. Go there: 63°38'3.06"N, 19°37'10.16"W

THE MID-ATLANTIC RIDGE ON LAND
The submarine MAR spreading boundary between the North American and Eurasian plates, as it crosses Iceland subaerially, assumes a distinct appearance and structure. The on-land expression takes the form of a large (one-third of Iceland) interconnecting complex of roughly NS-trending volcanic zones and belts (solid red lines) and EW-trending fracture and seismic zones (dotted lines). 

Where the submarine MAR connects with Iceland, it forms the submarine Kolbeinsey Ridge (KR) in the Arctic Ocean in the north and rises on land as the Tjörnes Fracture Zone (TFZ), while the Reykjanes Ridge (RR) in the southwest rises as the Reykjanes Volcanic Belt (RVB). Their on-land extensions, which are postulated to have been continuous during Iceland's formative stages, converge eastward on a complex that has been reorganizing since the volcanic island emerged from the sea and, as one might expect, is interpreted antithetically by plumists and platists.



Volcanic Zones and Belts in Iceland
In addition to normal faulting which plays a major role in Iceland, strike-slip faulting is extremely common in all parts of the island, not only in the two transform zones. Offset of the East and North Volcanic Zones with respect to the Mid-Atlantic Ridge, Reykjanes and Kolbeinsey Ridges implies a shift or 'jump' eastward of the rift zone.  

A COMPLEX OF VOLCANIC AND SEISMIC ZONES
The complex possesses four neovolcanic rift zones or belts (solid red lines): the West (WVZ), East (EVZ) and North (NVZ) Volcanic Zones. They are the principal geologic structures where Iceland is moving apart - "seafloor spreading on land" - and where active volcanism, geothermal activity and faulting occurs. They cover 30,000 cu km in 15-50 km-wide belts with spreading rates that varies according to the specific rift zone but averages about 1.9 cm/yr.

The EVZ is the most active volcanically and is an axial rift in the making (more on that later) that will eventually transition from the WVZ, where there's no longer volcanic activity (the last eruption was about 1100 AD, although the time frame within the last 10,000 is considered dormant). There is, however, an abundance of spreading and faulting such as at Þingvellir and geothermal activity in the region. 



Principal Elements of Iceland's Geology
Main fault-seismic structures and volcanic zones and belts are delineated (solid and dotted red lines). Glaciers are located on the volcanic zones' highest mountains (white). The postulated Iceland plume lies in the northwest region of Vatnajökull (dotted circle). From the Miocene through Pleistocene, basaltic bedrock ages are colored chronologically and lie on opposite sides of the main spreading axes that traveled outward on diverging crust. Iceland is mainly formed by basalt flows younger than 17 Ma, grouped into four formations: Tertiary Basalt (16-3.3 my), Plio-Pleistocene (3.3-0.7 my), Upper Pleistocene (>0.7 my) and Holocene lavas and sandurs (>0.7 ky). The age differences directly correlate to the processes that built Iceland. Modified from Thordarson, 2012

Linked with the volcanic zones, seismic-fracture zones (dotted red lines) - the South Iceland Seismic Zone (SISZ), Mid-Iceland Belt (MIB) and Tjörnes Fracture Zone (TFZ) - are where mostly transform faults exhibit strike-slip (side-to-side) and some extensional motion with intense seismotectonic activity (earthquakes and crustal deformation). They formed as the complex evolved from divergent rifting to transform side-to-side motion.

TWO INDEPENDENT ZONES
In addition to the volcanic network within Iceland, two independent, flank zones - Snæfellsnes (SVB) in West Iceland and Öræfi (OVB) in East Iceland - account for ~1.5% of the island's verified eruptions. The former forms the backbone of the eponymous peninsula of West Iceland and consists of three volcanic systems. It's thought to be the product of volcanism that initiated about a million years ago along the tail of the Iceland plume and is the precursor to the West Volcanic Zone. Öræfi is east of the plume center currently beneath glacier Vatnajökull, in advance of the plume head.

Looking at the historical activity of the volcanic-seismic zone complex and dates and distribution of the formations tha comprise Iceland, the implication is that a dynamic transition exists in rifting and volcanic activity!


Djúpulón Lagoon in the Shadow of Snæfellsjökull
Shrouded in clouds, the 700,000 year-old, deeply-corrugated, dormant stratovolcano is capped by an eponymous glacier. It's one of at least four volcanic systems over time of the Snæfellsnes Volcanic Belt of West Iceland. Crowned by a summit caldera, upper flanks produced intermediate to felsic rock types, while lower ones are basaltic. The volcano is the main attraction on the Snæfellsnes Peninsula and centerpiece of Snæfellsnes National Park. The latest eruption was over 1,100 years ago, the time of the human settlement of Iceland. The Jules Verne classic "Journey to the Centre of the Earth" written in 1864 tells of ascending the volcano, venturing into its central conduit and riding a volcanic eruption out of Stromboli, off the coast of Sicily. What might that say about the structure of the mantle? By the way, in 2012 the summit was ice free for the first time in recorded history, a commentary on global warming. Go there: 64°48'35.83"N,  23°45'52.89"W


VOLCANIC SYSTEM - THE PRINCIPAL GEOLOGIC STRUCTURE IN ICELAND
Episodic crustal spreading allows swarms of vertical dikes to deliver magma to the surface and feed some 30 active volcanic systems within the volcanic zones. With a lifetime of about 0.5 to 1.5 myr, they are the principal geologic structure in Iceland. 

When present, eruptive activity within each system - called fires or "eldur" in Icelandic - is focused within a central volcano, often caldera-capped, glacier-covered and with more differentiated magmatic products (not just basalts but intermediate and felsic rocks), and/or fissure swarms (5 to 20 km-wide and 50 to 200 km-long) that consist of elongate, subparallel, deep tensional crustal cracks and normal faults. 



Main Structural Elements and Architecture of a Volcanic System
Each volcanic system of 30 in Iceland is defined by a particular architecture and distinct geochemistry. The system is the principal geologic structure in Iceland and contained within the volcanic zones or belts. Each system contains a central volcano, a fissure swarm or both and are surface expressions of a magma holding-chamber either at shallow or greater depth. Top, Magma reservoir feeding a fissure swarm in an extensional tectonic regime. Bottom, A magma chamber feeding a central volcano and fissurral eruption downflank.

From Thordarson et al, 2015.

With activity closely linked to plate movements, volcanoes and fissures are surface expressions of shallow or deep-seated crustal magma chambers. 



Hekla
In South Iceland, the 1,491 m-high stratovolcano, meaning 'short-hooded cloak' due to its persistent cloud cover, lies at the intersection of the South Iceland Fracture Zone and the East Volcanic Zone. It's one of Iceland's most active volcanoes and produces a bimodal distribution of igneous rock types from mafic (basalt) through felsic (rhyolite). During the Middle Ages, Europeans referred to Hekla as the "Gateway to Hell", alluding to its persistent explosivity. Tephra emissions carried aloft over time are used to date other volcanoes. The volcano's elongate ridge-shape is the result of polygenetic (repeated) eruptions over a single 5.5 km-long fissure.

Go there: 63°59'32.09"N, 19°39'58.37"W


UNSURPASSED VOLCANISM
Every eruption type occurs in Iceland determined by magma composition, temperature and locale of emplacement: Surtseyan (violent and water-contact explosive in shallow seas, Strombolian (gas-driven mild blasts), Hawaiian (effusive, voluminous, fountaining basalts) to Plinian (massive, gas-driven and stratospheric).

Depending on viscosity (fluidity), gas content, contact with water (submarine and subglacial) and delivery vehicle (conduit or fissure) to the surface, Iceland displays all known volcaniforms. Examples include: shield volcanoes (low profile, lava-built, symmetrical, long-lived, conduit-fed domes), lava domes (viscous mounds), stratovolcanoes (cone-shaped, conduit-fed, layered composite-built) and caldera (explosive summit collapse or extravasation) to spatter (gassy vent-ejected mound), scoria-cinder (steep-sided, tephra-built, short-lived, fissure-fed cone) and tuff cones and maars (hydrovolcanic fragmented-rock within circular depressions). 



Eldborg á Mýrum
Ellipsoidal in shape in map view, 50 m-high and 200 m-wide Eldborg and its linear-array of subsidiary spatter cones are at the base of the Snæfellsnes Peninsula. The crater chain erupted along a 1 km-long fissure over 5,000 years ago. Corrugated remnants of fountain-fed lava streams still drape from the main crater's sparsely-vegetated rim, while within it, a solidified lava lake and subsurface lava tubes contributed to the bulk of the vegetated flow field. It's the largest lava field of six within the peninsula's Ljósufjöll volcano and fissure vent system. In the background begins the volcanic mountain chain that forms the spine of the peninsula that extends some 85 km to the west (left). Go there: 64°47'45.61"N, 22°19'19.46"W

SYNCLINES, ANTICLINES AND UNCONFORMITIES PROVIDE GENETIC CLUES
The Snæfellsnes Rift Zone in West Iceland was actively spreading until ~6 Ma when the plate boundary transitioned to the east. Partial proof resides on the landscape where Tertiary strata synclinally dip and strike (point at and tilt towards) in the direction of the older spreading center and then reverse direction and partially underlie strata (with unconformities) of the newly-forming West Volcanic Zone to the east. 

It can be seen along the SW-NE trending Borgarfjörður anticline, where strata (some of Iceland's oldest) are along the synclinal axis (downward linear fold). The identical landform (and similar transition process) is between the active West and East Volcanic Zones demonstrated by the Hreppar anticline between the West and East Volcanic Zones that follow the same axial trend as the rifts.



The Majestic Shore of Langaholt
On the base of the Snæfellsnes Peninsula and facing its southwest coast, the volcanic range on the horizon lies some 80 km to the west of the Borgarfjörður anticline. The strata tilt southward toward the extinct rift of SW-NE trending Snæfellsnes syncline (just beyond the range). Derived from shells brought ashore by ocean currents, the golden sand beach of Langaholt contrasts with Iceland's typically black basalt beaches. The terminus of the flow (right) is one of many that emanated centrifugally from volcano Snæfellsjökull (behind the viewer). Go there: 64°47'18.66"N, 23°38'28.81"W

JUMPING RIFTS AND PROPAGATING RIDGES
Loading by volcanism tilts the strata towards the magma-generating volcanic zones forming a shallow syncline centered on the spreading axis and a shallow anticline in the region between volcanic zones. The arrangement tells the story of a "rift jump", where plate boundaries shift from one volcanic zone to another.
 

W-E Cross-Section of Icelandic Crust from Snæfellsnes Peninsula through the West Volcanic Zone
The anticlines and synclines are associated with major unconformities at their interface (surfaces of contact between strata of different ages). The large-scale, anticlinal-synclinal folds form as the relative positions of spreading axes change with time. Alternative mechanisms include deep-mantle and shallow-mantle tectonic processes (explanations below). Modified from Thordarson, 2012.

It's unclear whether the western spreading center relocated eastward in single episodes or by gradual propagation of new rift branches from the MAR. What is apparent is that rift segments exist in various stages of activity from nascent to established to waning over time. Thus, the Öræfi Volcanic Belt in East Iceland Belt is volcanologically nascent, while dormant Snæfellsjökull volcano in West Iceland erupted recently enough to be considered active, although it is though to be subsiding.



'Bridge Between Two Continents'
In reality, the tourist attraction neither connects diverging plates nor continents. It does span a SW-NE striking tension fracture, one of many on the Reykjanes Peninsula in southwest Iceland, which is divided by an oblique axial rift. Holocene-age, Stampar pahoehoe lava flows are exposed in the walls of the extensional void, which is filled with a fine basaltic sand. Some of the island's youngest rocks outcrop here, where the submarine Reykjanes Ridge rises from the sea and joins the West Volcanic Zone via the Reykjanes Volcanic Belt. Within the deformation zone rifting structures include normal faults, crater rows and fissures. 

Go here: 63°52'5.85"N, 22°40'31.48"W


STRUCTURAL REORGANIZATION BEGETS TRANSITIONING VOLCANIC ACTIVITY
If we assume the plume that feeds the Iceland hotspot is a stationary structure, then the position of the spreading axis must have changed over time by migrating west-northwest, concurrent with North American-Eurasian plate divergence. Iceland's crust is in a constant state of transformation, as spreading centers and volcanic zones relocate progressively eastward in order to remain coupled (in proximity) to the Iceland hotspot. In the process, microplates (crustal blocks) transfer from one major plate to another. 

In contrast, platists look to the shallow mantle, plate tectonics and extensional lithospheric stress for an explanation of eastward rift reorganization. If Iceland's oldest rocks in the extreme northwest at ~16 Ma and in the extreme east at ~13Ma, were spreading apart at ~2 cm/yr, then they would be separated at ~290 km rather than 500 km as they now are. Their conclusion is that ~210 km-wide older crust (oceanic or continental in origin with the latter possibly a remnant of a Jan Mayen micro-continent) formed earlier and underlies Iceland, submerged beneath younger lavas (and accounts for Iceland thicker crust).



Tectonic Evolution of Iceland
For each time frame, red lines indicate currently active plate boundaries, while dashed red lines indicate imminent plate boundaries and dashed blue indicate. Evidence such as thicker crust than oceanic beneath Iceland has postulated (Foulger) that a microplate of continental crust may have been captured beneath Iceland. It might be a thin southerly extension of the Jan Mayen microcontinent or rifted microplate from Greenland during North Atlantic opening that lies submerged to the northeast of Iceland.
Modified from Foulger, 2010.

Regardless of the mechanism that facilitates the delivery of anomalously hot mantle material, as older formerly active rifts become extinct (recall the Snæfellsnes Zone), new ones initiate. This has occurred several times in Iceland at ~24, 17, 7 and 3 Ma. In fact, Iceland is currently experiencing a rift-transition from the West Volcanic Zone (where Þingvellir is located) to the East Volcanic Zone, a rift-in-the-making where 80% of the volcaic activity is occurring. And historically, the Reykjanes Ridge in the southwest and Kolbeinsey Ridge in the northwest are thought to have been continuous before detaching any evolving into the rift complex to the east.

ÞINGVELLIR - ASSEMBLY FIELDS AND WORLD-CLASS GEOLOGICAL SITE 
About 45 km northeast of Reykjavik is Iceland's most important and most visited site. The hallowed ground is the spiritual heart and soul of the nation and main attraction on the Golden Circle tour. It's where Icelanders celebrate their sovereignty and independence, where tourists flock by the car and busload, and where geologists pay tribute to Iceland's most dramatic geological feature. Þingvellir is a "protected national shrine for all Icelanders" according to documents that designated it a national park in 1928 and a UNESCO World Heritage Site in 2004.

Many central events in Iceland's history occurred at Þingvellir - notably the establishment in 930 of Alþingi (or "All-Men's Assembly") that became the National Parliament in Reykjavik in 1845, conversion to Christianity from the Norse pagan god Æsir in 1000 that ultimately provided national peace and unification, and in 1944, gaining independence from Denmark that resulted in the declaration of the new Icelandic Republic. 



Walking the Geology of Fault Almannagjá
 The normal fault (hanging wall drops relative to footwall) formed from extension and subsidence with a spreading rate of 0.6 cm/yr, it's the largest fracture in the Þingvellir graben and is a surface manifestation of the tectonic forces tearing the crust apart. East-sloping (11%), slumped lavas are Brunnar/Sko´garkot lavas. Vertical displacement of the more elevated western wall is secondary to subsidence of the eastern, lower wall. Although plate divergence is continuous, movement at the fracture occurs in discrete events accompanied by earthquakes. The last major one was in 1789 when the graben floor dropped 1-2m, although milder swarms (scores of tremors) are more frequent. Tearing of the crust occurs along a myriad of smaller segments, connected by offsets, that join as the main fault. Normal faulting at Almannagjá has exposed a section of the ~10,000 year old pahoehoe lava flows that blanket the valley.

Go there: 64°15'32.98"N, 21° 7'24.33"W

Although early Icelandic settlers most assuredly were unaware of the region's geological significance, they undoubtedly recognized something gomorphologically unique about it. It may seem off the beaten path from Reykjavik, but over a millennium ago its location was accessible and centralized from populated areas, its openness accommodated large numbers and its ledges afforded vantage points for regional chieftains to address the populous. 

The geology of the region made Þingvellir an ideal location to conduct important political and commercial business, recite and make the laws of the land, voice concerns, settle disputes and socialize. 



Artist Conception of Lögberg with Lake Þingvallavatn facing South
 The amphitheater of Law Rock was where the Lawmaker, the presiding official of the General Assembly, was seated and where Alþingi gathered. Its exact location is unknown but may have been on a flat ledge at the top of a slope named Hallurinn marked today by an Icelandic flag or somewhere within fault Almannagjá. The site was used in 1262, even when Iceland took allegiance to Norway, and moved to the capital city in 1798, where the Icelandic Parliament now convenes, the oldest in the world.
From Wikimedia Commons by W.G. Collingwood (1854–1932)

THE LAY OF THE RIFT
Þingvellir is a 4.7 km-wide graben (German for "grave"), a continuously (~4 mm/yr) subsiding (downdropping) tectonic depression, Iceland's deepest (70 m at the west) and widest (10 to 25 km). Subsidence is caused by compaction of volcanogenic material and lithostatic loading of erupted material.

Highly asymmetrical (deepest immediately to the east of east-facing faults along the western margin), southwest-sloping, NE-SW striking and partially filled by the waters of lake Þingvallavatn to the south, the graben or rift valley formed as rifting (lithospheric extension) - the consequence of tug of war between two tectonic plates - episodically stretched and opened the landscape some 5-10 mm/yr across the central axis of the West Volcanic Zone.

Today, it accounts for 20-30% of extension across South Iceland, the rest taken up by the East Volcanic Zone, in transition as previously discussed. And, as we know, the large scale deformation and thinned lithosphere produces seismic and magmatic activity manifested in the region's earthquakes and volcanic systems.



Cross-Section of Thingvellir Graben with Boundary Faults and Rift Valley
Along an EW transect just north of lake Þingvallavatn, the tectonic depression is bound on the west by the steep walls of 7.7 km-long normal faults Almannagjá and 11 km-long 
Hrafnagjá on the east. They form the boundaries of the diverging plates at the eastern extent of the North American plate and western extent of the Eurasian plate. A large number of subsidiary faults lie on strike on the valley floor. Masl is meters above sea level. Modified from Saemundsson , 1992.

As a whole Iceland is spreading at ~18.9 mm/yr at the plate boundary in the north (where there's the single rift of the NVZ) and ~20.2 in the southwest (more complicated with oblique spreading on the RVZ and two parallel WVZ AND EVZ but primarily across the latter). Again, it's assumed to be due to the presence of the Iceland mantle plume presently beneath Vatnajökull.



Þingvellir Graben and National Park
Subsiding at about 7 mm/yr, the graben is located within the main boundary faults of Almannagjá (with greater vertical displacement) and Hrafnagjá (with greater opening). Many subsidiary faults  and offsets exist both within the graben and subparalelel to the main faults. Flosagjá and Háagjá are offset, simple pull-apart tension fissures on the valley floor. They differ from the two main boundary normal fractures in that the fissure walls demonstrate no vertical displacement or subsidy. With individual names that denote their many sub-fissures, the southernmonst segments closest to the lake are water-filled. Surrounding the subsiding rift valley are hyaloclastite mountain Ármannsfell fault-dissected by those from the graben), monogenetic lava shield volcano Skjaldbreiður (distinguished from Hawaiian-style polygenetic shields), elongate Tindaskagi móberg ridge and steep-sided, lava-capped, hyaloclastite table mountain Hrafnabjörg.

At the northern end of the Hengill Volcanic System, the rift valley developed and subsided between the diverging North American and Eurasian tectonic plates. Within boundary faults on the west (Almannagjá) and east (Hrafnagjá), the extensive lava flow-blanketed, stream-dissected, grass-vegetated graben floor is scored with fissure swarms (sub-parallel tension gashes), bound on the south by lake Þingvallavatn, Iceland's largest (83 sq km) and deepest (114 m) lake, surrounded by four volcanic systems. In the last 9,000 years, the total amount of downward displacement is over 40 meters with 70 meters of extension. 



Northeast View of Fault Almannagjá and Rift Valley
Þingvellir is the main attraction on the Golden Circle tour and on every geologist's wish list of places to visit. It affords the opportunity to study the crest of the Mid-Atlantic Ridge where seafloor spreading is uniquely occurring on land. The region is completley circumscribed on all sides by volcanoes that belong to four active volcanic systems of the West Volcanic Zone: Prestahnúkur and Hrafnabjörg (far right) on the north and Hengill and Hrómundartindur to the south. In the center, shrouded in clouds and a blanket snow, is massive Skjaldbreiður that gave all shield volcanoes their name. Its nearly perfect conical contours capture the uniformity of its 8° slope. The northern shore of lake Þingvallavatn can be seen to the right.

LAKE ÞINGVALLAVATN
An old Icelandic proverb states, "Frjósöm er vatn sem liggur undir hrauni" or "Fertile is water that runs under lava." As such, 83 sq km and 144 m-deep, lake Þingvallavatn is Iceland's largest. Residing in the southern part of the Þingvellir graben, its basin formed by tectonic rifting, faulting, subsidence and glacial erosion and was modified by volcanic activity. Its waters are exceptionally clear, clean and icy cold (3-5°C). Although the nutrient concentration is low, it provides fertility for many plants and fish such as trout and char and exceptional visibility for divers and photographers (below).

The lake is sourced from groundwater springs, lava-filtered aquifers fed by precipitation and river Öxará (the Axe River, possibly called as such since weapons were ritually thrown in during the Alþingi) that originates from glaciers as far as Langjökull 50 km to the north.


Diving the Geology of Silfra
Silfra is one of many subparallel tension fissures (fractures) on the northern shore of lake Þingvallavatn. It's the southernmost, submarine segment of Flosagjá, one of a number of boundary faults within the Þingvellir graben. They're surface expressions of deep-rooted normal faults within the crust that formed from seafloor spreading. The exceptional clarity of Iceland's waters and accessibility of Silfra make it a popular diving locale. With the exception of geothermal areas, Iceland's running waters from mountains and glaciers are safe to drink almost anywhere with minimal chemicals or bacteria. Iceland has the most available freshwater than any European country. 95% is untreated and extracted from boreholes and wells. In fact, most bottled water for purchase is straight from the tap. 
Photo by friend and diver Joel Feingold.
Go there: 64°15′ 18″ N, 21° 7′ 22.8″ W

TRANSITION OF THE WVZ TO THE EVZ
Þingvellir is located at the Hengill triple junction, a three plate intersection of two rift zones and a transform zone - the Reykjanes Ridge oblique rift, the South Icelandic Seismic Zone and the Western Volcanic Zone. Three plates meet there: the North American, Eurasian and the Southern Iceland or Hreppar microplate (crustal block named after the central volcano), between the WVZ and EVZ.  Þingvellir records rifting along the WVZ, which was the primary zone of spreading between ~6 Ma and 2 Ma when the EVZ formed from propagation from the NZV. 


Today, only 20-30% of spreading in southern Iceland is accommodated by rifting on the WVZ, while the remainder is on the EVZ. The implication is that plate spreading at Þingvellir is transitioning eastward, rift jumping through the volcanic/seismic zone complex that has developed. The phenomenon clearly exists, but interpretations of the process responsible for it are the subject of debate among geoscientists and are related to mantle and plate geo-dynamics.

Not only does rifting along the West Volcanic Zone demonstrate a decline of volcanism, but the ultra-slow spreading rate of the Þingvellir graben, its fast subsidence and extraordinary depth have led many to suggest that the WVZ is a failed or dying rift. It fuels the implication for some is that the rift is magma-starved and that all motion will eventually be transferred to the EVZ as volcanism is converting to stretching and normal faulting.



Geothermal Heat in Reykjavik
In my naiveté on my first trip to Iceland years ago, I asked the hotel clerk where was the thermostat in my oppressively hot room. He answered there was none and just open the window. That was my introduction to Iceland's bountiful and renewable geothermal resources. Hydrothermal waters are piped in from geothermal areas to Reykjavik and the north coastal city of Akureyri for space heating, pools (every Iceland town has one no matter how small), greenhouses and snow melting of pavements and car parks. First used in 1908, geothermal heating in Reykjavik began on a small scale in 1930 and today serves 90% of the capital and more than half of Iceland's population. In addition, electricity is also co-generated with clean air as a major benefit.

THE GENETIC BIG PICTURE (OR HOW OLD IS ICELAND?)
Without collecting field data, drawing geological maps or conducting tectonic analyses, a simple mathematical calculation will provide the answer to the question of Iceland's age. If measured along its west-east spreading-direction from shelf to shelf (not shore to shore), Iceland is about 500 km-wide. Since the spreading rate is about 2cm/yr, Iceland must have taken around 25 million years to form in the Early Miocene.

This is in spite of the fact that the oldest exposed crustal rocks are Middle Miocene in age. They appear as fine-grained Tertiay Basalts (as previously discussed) in the extreme northwest (~16 Ma) and east (~13 or 14 Ma). They are located furthest from the volcanic zones that formed them, while progressively younger rocks are found as one moves toward the central spreading axis. And (as also discussed), more than one age-progression set exists, since it appears that at one time there was more than one spreading axis that transitioned eastward. The age distribution of Iceland's formations tells us something about the genesis of Iceland, axial divergence and jumping rifts.



Interaction of the Iceland Hotspot Plume and the Mid-Atlantic Ridge
A, Some 54 Ma, the Iceland plume is responsible for emplacement of the NAIP, the fragmentation of boreal Pangaea and the first phase of NAIP emplacement between ancestral North America and Greenland; B, At 30 Ma, the northern North Atlantic opens, the second phase, concomitant with the development of the MAR. The path of the plume follows a hotspot track that intersects the MAR at Iceland on the verge of formation; C, As the Atlantic continues to open between Greenland and Eurasia, Iceland remains athwart the MAR and the active Iceland plume. Modified from Jones and Bartlett Learning 2015.

In the big picture, the formation of Iceland and the North Atlantic is the consequence of:
• the emplacement of the North Atlantic Igneous Province, which appears to have formed in two phases: a failed rift in the Labrador Sea in the "Middle" Paleocene (~62-58 Ma) between between Greenland and North America proper and in the latest Paleocene to earliest Eocene (~56-53 Ma);
• the extension and rifting apart of boreal Pangaea beginning around 200 Ma between Greenland and Eurasia beginning ~90 to 150 Ma;
• the opening of the North Atlantic in the Early Eocene (initiated ~54 Ma) and syntectonic development of the Mid-Atlantic spreading center.


Paleotectonic Development of the North Atlantic Ocean
The tectonic evolution of the Arctic, North Atlantic and associated seas are integrally related. Left Panel (Paleocene): The Thulean plateau (a term for the unified lava plain across Greenland, red blobs) is emplacing in East Greenland, the Faeroes and northern British Isles (the British Volcanic Province). Iceland has not yet begun to form athwart the MAR still in infancy. The LIP in western Greenland is thought to be either a pre-NAIP LIP associated with opening of the Labrador Sea and Baffin Bay or an extension of the NAIP to the west. Right Panel, (Late Oligocene): Following a second phase of volcanism, the MAR, associated fracture zones (lateral strike-slip offsets) and NAIP are well developed. Iceland is forming astride the MAR at ~24 Ma. Modified from AAPG Memoir 43, Peter A. Ziegler

IN CONCLUSION
Somewhere around 24 to 25 Ma, plumists believe the Iceland plume rooted in the deep mantle was the driving force in the process that formed Iceland when it contacted thinned lithosphere of the Mid-Atlantic Ridge. For them, the Greenland-Iceland Ridge on the Atlantic floor is the trace of the plume head.

In contrast, anti-plume platists, that deny the existence of a plume and presence of a hotspot track, adhere to a tectonic causation in the shallow mantle when the fossil Caledonian suture intersected the MAR. That event re-activated and melted a slab of Iapetus oceanic crust trapped in the suture from closure of Laurentia and Avalonia. They espouse that the GIR suggests in situ development along the buried orogenic plate boundary. In either case, effusive and voluminous magmatism constructed the elevated basalt plateau of Iceland that persists as the centerpiece of the NAIP. 

ON THE LIGHTER SIDE
The streets and alleyways of Reykjavik are home to a thriving and creative art scene that brings color to otherwise dull parts of town and its often gray skies. Many are murals that adorn walls, underpasses and the sides of entire buildings. All whimsical, here are a few with geological inclinations. 



Left, Swiss street artists Wes21 and Onur, inspired by a song from the Icelandic band Monsters and Men, created this huge mural entitled "Heavy Stones Fear No Weather". It depicts a stone fist rising as a sign of persistence and endurance, all Icelandic traits. Right, Created by sculptor Magnús Tómasson, "Monument to the Unknown Bureaucrat" depicts a thankless, everyday worker in a crumpled suit heading to the office for another day on the job. My first impression was of course geological and that it represented the inseparable, inescapable and intimate connection that all Icelanders have with their landscape - basalt of course!

The real robotic source of Iceland's anomalous volcanism

SPECIAL THANKS
Sincere appreciation is extended to Gillian R. Foulger, PhD., British geologist, author of "Plates, Plumes and Paradigms" and Professor of Physics at Durham University, England for her patient emails that helped clarify issues regarding Iceland, the North Atlantic and mantle dynamics. As always, geologist, lecturer, author and Smithsonian Expert Wayne Ranney (here) provided invaluable geological assistance. I am also grateful to Sonia Didriksson, my local Icelandic connection for all things geographical and unpronounceable and for the excellent photographic contributions of Joel Feingold and renown Icelandic photographer Sigurður Stefnisson (here).

SUGGESTED READING
A Cool Model for the Iceland Hotspot by G.R. Foulger and Don Anderson, Journal of Volcanology and Geothermal Research 141, 2005.
An Alternative Model for Iceland and the North Atlantic Igneous Province by G. R. Foulger et al, On-line. No date provided.
An Iceland Hotspot Saga by Ingi Þorleifur Bjarnason, Jökull 58, 2008.
• Earth Evolution and Dynamics – A Tribute to Kevin Burke by Trond Helge Torsvik et al, Canadian Journal of Earth Sciences, 2016. 
Fixity of the Iceland “Hotspot” on the Mid-Atlantic Ridge: Observational Evidence, Mechanisms, and Implications for Atlantic Volcanic Margins by Erik R. Lundin et al, GSA Special Paper 388, 2005.
Frontiers in Large Igneous Province Research by Richard E. Ernst et al, Lithos 79, 2005.
Iceland - Classic Geology in Europe 3 by Thor Thordarson and Ármann Höskuldsson, 2015. 
Iceland is Fertile: The Geochemistry of Icelandic Lavas Indicates Extensive Melting of Subducted Iapetus Crust in the Caledonian Suture by G. R. Foulger et al, Source On-line.
Icelandic Glaciers by Helgi Björnsson and Finnur Pálsson, Jökull 58, 2008.
Icelandic Rocks and Minerals by Kristján Sæmundsson and Einar Gunnlaugsson, 2002.
Late Weichselian History of Relative Sea Level Changes in Iceland During a Collapse and Subsequent Retreat of Marine Based Ice Sheet by Halldór G. Pétursson et al, Cuadernos de Investigacion Geografica, 2015
Mid-Ocean Ridge Jumps Associated with Hotspot Magmatism by Eric Mittelstaedt et al, Earth and Planetary Science Letters 206, 2008.
• NE Atlantic Break-up: A Re-examination of the Iceland Mantle Plume Model and the Atlantic–Arctic Linkage by E.R. Lundini and A.G. Gore, Petroleum Geology: North-West Europe and Global Perspectives - Proceedings of the 6th Petroleum Geology Conference, 2005.
North Atlantic Igneous Province: A Review of Models for its Formation by Romain Meyer et al, On-line, No date provided.
Plate Boundaries, Rifts and Transforms in Iceland by Páll Einarsson, Jökull 58, 2008. 
Postglacial Volcanism in Iceland by Thorvaldur Thordarson and Ármann Höskuldsson, 
Jökull 58, 2008.
The Glorious Geology of Iceland's Golden Circle by Agust Gudmundsson, 2017.
The Greenland–Iceland–Faroe Ridge Complex by Arni Hjartarson et al, Geological Society of London, 2017.
• The “Plate” Model for the Genesis of Melting Anomalies by Gillian R. Foulger, GSA Special Paper 430, 2007.
Volcanism in Iceland in Historical Time: Volcano Types, Eruption Styles and Eruptive History by T. Thordarson and G. Larsen, Journal of Geodynamics 43, 2007.