Monday, February 3, 2020

The Geologic Evolution of Iceland: Part IV - The Northeast's Glacial River Jökulsá á Fjöllum and 'Glacial River Gorge' Jökulsárgljúfur

‘What were the gods enraged by
when the lava we are standing 
on here and now was burning?’

From the Kristni saga
The 10th Century Book of Iceland's Conversion to Christianity
 by Icelandic monk Gunnlaugr Leifsson

Halfway between Greenland and the British Isles in the Northeast Atlantic lies Iceland, the world's largest elevated basalt plateau. Following the break-up of supercontinent Pangaea, seafloor spreading at the Mid-Atlantic Ridge and interaction with the Icelandic mantle plume - the commonly held view - gave rise to the volcanic island via effusive and voluminous volcanism. It's a part of the oceanic crust that has risen some 3,000 m above the ocean floor starting around 24 million years ago.

Lower River Jökulsá á Fjöllum and Waterfall Selfoss of Canyon Jökulsárgljúfur

Iceland's unique landscape is a culmination of the opposing forces of construction via volcanism and sedimentation and destruction via wind, wave, water and glacial erosion. Working in concert, they have established it as one of the most geologically active and dynamic places on Earth where the intimate architecture of a mid-ocean ridge and its processes can be viewed and studied on land.

Mighty Dettifoss and its Perpetual Rainbow and Hordes of Tourists

It's about Iceland's second largest river and formation of its magnificent waterfall-endowed canyon. It's also the fourth of six posts on the "Geologic Evolution of Iceland" and a collaboration with my daughter and travel companion Julia Share. 

With a 4WD, high-clearance vehicle equipped with a roof tent, we traveled around the island on the Ring Road (Highway 1 or Þjóðvegur in Icelandic) and on F-Roads ('F' is for Fjallið' meaning mountain). The former is the 1,332 km-long, mostly two-lane, paved national highway, and the latter are backcountry, gravel-surfaced, variably maintained roads through the remote interior.

• In Part I (here), we explored the geology of Iceland's Golden Circle triumvirate - world-renowned Þingvellir National ParkGeysir geothermal area and iconic waterfall Gullfoss  - and southwest peninsula Reykjanes
• In Part II (here), we investigated volcanic systems Hengill and Hekla in the Southwest Highlands and the South Coast's waterfall-endowed escarpment above vegetated lowlands and glacial outwash plains along the sea.
• In Part III (here), we traveled in and out of East Iceland's fjords in the shadow of long-extinct, glaciated Tertiary-age volcanoes and turning inland, crossed the barren, windswept Northeast Highlands.

Herein, selected definitions are italicized, important names are highlighted in boldface at first mention and global coordinates of various locations are provided for you to visit. All photos were taken by my daughter and me unless otherwise credited.

Daughter Julia Meditates at Fjord Berufjörður of East Iceland
Geologists call this an "inverted topographic relief."

Having left East Iceland's fjord region and crossing the Northeast Highlands (post Part III), the landscape underwent a profound change. Turning inland to the west, the bedrock transitioned from topographically high, horizontally stacked lavas of the Tertiary Basalt Formation to more geologically recent lava fields and black sand deserts of the Pliocene and Pleistocene. 

An identical "forward-in-time" experience occurs in travelling west to east from the opposite side of the island. It's a clue as to the prevailing tectonics that formed Iceland. With the exception of a few outlier (independent) rift zones, it's the result of Iceland's central rift axes that generate new volcanic crust and tectonically transport it conveyor-belt fashion outward, away from the center of the island (details in posts Part I through III).

Gently Undulating, Majestic Highlands of the Northeast
It's a mix of wind-delivered tephra (fragmented rocks and particles ejected during a volcanic eruption) and pumice (light-colored, highly vesicular, ejected felsic volcanic rock) over a foundation of bedded lava flows bound by hyaloclastite (quench-fragmented volcanic rock) ridges. Where's Julia? Can you find her? Go there: 65°27'27.32"N,  15°48'23.41"W

With further northwestward travel, various volcaniforms of the North Volcanic Zone, our destination for the next few days, gradually came into view. In fact, the landforms of the region, in one form or another, have come under the influence of either volcanic or geothermal activity or extensional deformation of the landscape, in addition to glaciation during the Pleistocene.

Barren, Windswept and Starkly Beautiful
Go there: 65°28'51.35"N,  15°57'7.50"W

Heading due west (black arrows), the Ring Road crosses over the 'Glacial River of the Mountains' (dotted red arrows on map). It's Iceland's second longest river that signaled our approach to "canyon and national park country." The river's name refers to its glacio-fluvial classification and source from ice cap Vatnajökull, some 120 km to the south of the bridge. 

Not far to the north downriver is Jökulsárgljúfur National Park (official map here), established in 1973. It became incorporated as an independent section of ~13,500 sq km UNESCO World Heritage-listed Vatnajökull National Park in 2008. River Jökulsá á Fjöllum is the geographical connection between the two parks (outlined in purple) that flows across a vast expanse of the Northeast Highlands in between.

Vatnajökull (blue jagged border) and Jökulsárgljúfur (purple) National Parks
Separated by over 150 to 200 km, the two are geographically unified by river Jökulsá á Fjöllum (red arrows) that flows from northern Vatnajökull (lower dotted circle) through canyon Jökulsárgljúfur and across a sandur-delta to the sea at fjord Öxarfjörður (upper dotted circle). The Ring Road suspension bridge (dotted arrow) and our trip route from the Southeast Coast are designated (black arrows). Click for larger view.

As for the park's eponymous 'Glacier of Lakes', at 8,100 sq km it's nearly twice the size of Rhode Island, covers ~13% of the island and is Europe's largest glacier outside the arctic. With ice up to 900 m-thick and about 30 outlet glaciers (radial glacial tongues) and proglacial lakes and outward-flowing rivers, beneath Vatnajökull there lurks a concealed landscape of mountains, valleys, plateaus and active and extinct volcanic centers. 

In addition, the ice cap's northwest corner contains the unique juncture of five major geothermal features (all discussed in detail in posts Part I and II):
• the submarine Mid-Atlantic Ridge
• the seismic Mid-Iceland Belt
• the active North and East East Volcanic Zones
• the upwelling Iceland mantle plume (Parts I and II).

Vatnajökull is also the largest glacial remnant of the island-blanketing Icelandic ice sheet that extended beyond the present-day coastline during the Pleistocene. Iceland was first glaciated 3 to 5 mya, but by the early Holocene, around 10,300 years ago, it began to retreat to topographic volcanic highs leaving a few isolated caps, such as Vatnajökull, by 8,700 BP. It's ultimate demise, given the rate of current climate warming, is in 150 to 200 years.

Icelandic Ice Sheet
Only a handful of isolated glaciers remain at volcanic topographic highs. Its greatest extent out to sea occurred during the Last Glacial Maximum (LGM) around 21 kya. Its climatologically and coevally linked with the Laurentide ice sheet of northern North America and Eurasian ice sheet of northern Europe and Asia.

The story of the two-lane bridge across Jökulsá á Fjöllum tells much about the geologic history of Northeast Iceland. Its single-lane, short-span predecessor, the only way across the river, was constantly assaulted by heavy trucks but more importantly not fully equipped to withstand powerful cold-weather ice-surges and jams that frequently occur on the river. An even greater potential hazard to the bridge is from extreme floods generated far upriver from eruptions of stratovolcano Bárðarbunga, one of three active volcanoes beneath Vatnajökull that has five volcanic systems in all. 

In this middle reach of the river at the bridge, the channel is broad and relatively shallow, as it meanders across the highland plain from its source, but its character will radically change about 30 km north of the bridge (right in the photo), our day's destination.

Sweeping Bend in the Middle Reach of Jökulsá á Fjöllum at the Ring Road Bridge
Solar-regulated, the river is at peak flow in the summer from glacial melt. In the distance to the west, volcanic mountain ridges are on NS-strike with the North Volcanic Zone from which they are derived. It's our destination in post Part V. Go there: 65°37' 22.13" N, 16°11' 26.03" W

They drain Iceland's melting glaciers and are its largest rivers, carrying one-third of its total runoff. They're typically milky-gray from a micro-fine suspension of glacially-eroded, crystalline basaltic rock, generally large and turbulent, and often exploited for their hydroelectric potential. G
enerally smaller Icelandic rivers are classified as direct-runoff from lakes and streams and slow-flowing spring-fed that are deep and cold.

By the way, naturally occurring Icelandic water is highly potable and regularly monitored. Basaltic bedrock provides excellent filtration, although there are a few caveats. Cloudy water is often from a glacial source and sediment-rich, and hot water from geothermal areas is likely high in iron, sulfur and other chemicals. Giardia (intestinal parasites from animals such as sheep) is rare in Iceland. 

Originating from groundwater (96%) and slightly alkaline and cold, fast-flowing springs and streams and even straight from the tap are the best sources of drinking water in Iceland. So, skip the market for bottled water, but use caution at natural sources! 

Glacial River Jökulsá á Fjöllum in the Afternoon
The northern margin of Ice cap Vatnajökull lies to the south-southeast about 100 km from the Ring Road bridge, well beyond the volcanic ridges. In this reach, the river is broad and shallow with a moderately slow current. Go there: 65°36'56.42"N, 16°11'19.12"W

Sediment-clogged glacial rivers, such as Jökulsá á Fjöllum, typically cross vast, barren expanses of sandurs, glacial outwash plains composed of volcaniclastic basaltic lavas and sediments. Being largely unvegetated, they lack a release of photosynthetic and decomposition-derived carbon dioxide - a plus in regards to climate warming. 

What's more, being a mafic igneous rock, basalt contains high mounts of calcium, magnesium and iron derived from constituent minerals such as pyroxene, olivine and amphibole that bind to CO2. 

Weathering Reaction of Atmospheric Carbon Dioxide and Basaltic Mineral Constituents

They react with slightly acidic rainwater and drawdown carbon dioxide from the atmosphere, forming environmentally-friendly alkaline carbonates and bicarbonates in the process. That makes the river (and the basaltic volcanic soils and bedrock across which they flow) a carbon sink, consuming more than it releases into the atmosphere and the seas they flow. 

Glacial retreat, though a natural geological process, is often viewed as a negative consequence of climate change but ironically may possess surprising pollution-reducing climatological benefits.

The total drainage area of river Jökulsá is ~8,000 sq km, which is roughly equivalent to the size of Vatnajökull Park itself! The river emanates from subglacial ice caves at the termini of two outlet glaciers - Dyngjujökull and Brúarjökull - located on the northern margin of Vatnajökull and from geothermal areas (where superheated water at depth reaches the surface) in nearby Kverkfjöll geothermal mountains from which it acquires tributaries Kverká and Kreppa (map below).

Vatnajökull Ice Cave on its Northern Margin and a Source of Jökulsá á Fjöllum
From Greta Hoe Wells thesis 2016

More than just the intense heat applied subglacially, there exists a relationship between end-Ice Age melting and increasing volcanic activity. At the time of deglaciation, when the multitude of jökulhlaups (massive, sudden glacial outburst floods) were generated, volcanism in this north-central region of Iceland was about 20-30 times greater due to a pressure release of pooled magma through differential tectonic movements from ice unloading. 

In addition, in modern times active rifting in the North Volcanic Zone continues to exert controls on elevation change, base level lowering, tectonics and subglacial volcanism that affect the geodynamics of the river. More on that later. 

Source of Jökulsá á Fjöllum from Vatnajökull
So large is the ice cap's volume that Olfusa, the river with the greatest flow in Iceland, would need over 200 years to carry all its stored water to the sea, should it melt, which appears imminent in the current warming climate. Up to 1,000 meters of glacial ice covers Oraefajokull, Iceland's highest volcano. 
Major tributaries Kverká and Kreppa (arrows) feed Jökulsá á Fjöllum. Modified from

Driven north by the gentle seaward-tilt of the highland plateau, river Jökulsá is serpentine in form and braided with a classic floodplain morphology. It initially flows across the desolate post-glacial Holocene-age landscape of sandur Dyngjusandur. It then follows the eastern edge of Upper Pleistocene lava field Ódáðahraun, Iceland's most extensive at 5,000 sq km, passing east of tuya Herðubreið (flat-topped, steep-sided, subglacially-confined volcano). 

Rare worldwide but commonplace in Iceland, the table mountain is known as the "wedding cake" "Queen of the Mountains." The highly recognizable landmark rising above the desolate landscape, possesses lower and upper halves that emplaced during and after Ice Age glaciation. It provides geologists with the approximate date of deglaciation following the half-mile thick glacier that dictated its formation. 

Lava Field Ódáðahraun and Table Mountain Herðubreið
Seen from the Northeast, Jökulsá á Fjöllum flows south to north (left to right) between a cluster of eroded hyaloclastite ridges and 1,682 m-high Herðubreið. The lower aspect of the tuya formed when a Pleistocene-age fissure eruption was glacially confined resulting in a steep-sided, flat-topped morphology, while a Holocene post-glacial eruption coronated the summit with a pointed, shallow-sloped shield volcano.

The western portion of the Northeast Highlands that we crossed is within the region of the North Volcanic Zone (NVZ below), one of Iceland's most volcanically active areas. In fact, river Jökulsá á Fjöllum (dotted line) flows on NS-strike with it, as it exerts a tectonic control over the northerly direction of the river, facilitated by the subtle tilt of the highland plateau. 

Geologic Bedrock and Structural Map of Iceland
The NVZ is the northern component of a reverse h-shaped complex of Neovolcanic rift and fracture zones, where the lithosphere is actively diverging apart and magmatic and seismic activity is expressed on the surface. On-land, the major tectonic and volcanic feature correspond to a portion of the submarine Mid-Atlantic Ridge (Part II). Jökulsá á Fjöllum (dotted black line) originates from the northern aspect of ice cap Vatnajökull and empties into fjord Öxarfjörður at the Arctic Ocean. Modified from Stucky de Quay et al, 2019.

Jökulsá is typical of Vatnajökull's north-flowing rivers that course considerable distances to the Arctic Ocean across barren deserts and limitless expanses of lava. South-flowing ones descend from piedmont-style, climate-vulnerable outlet glaciers and run far shorter distances through sediment-filled, U-shaped glacial valleys or spill off fossil (former) marine escarpments to the coastal lowlands and sandurs with braided-stream morphologies to the Northeast Atlantic (post Part II). One way or other, all rivers reach the sea.

Halfway to the sea on the highland plains, the middle reach of the river has remained relatively shallow, diffuse, serpentine and braided, but not for long. After one of many sweeping turns, it courses beneath the Ring Road bridge and continues another 30 km before funneling into its magnificent creation, Jökulsárgljúfur

One of Iceland's most breathtaking canyons ('gorge' is the preferred European geo-term), it contains three iconic waterfalls within 9 km of its canyon-like first third (furthest south) - Selfoss, Dettifoss and Hafragilsfoss - and a fourth - Réttarfoss - about 6 km downstream of the upper canyon. 

Aerial North View of Jökulsárgljúfur and Dettifoss
Graben Sveinar and crater row-eruptive fissure Rauðuborgir-Randhólar west of the canyon and cone Hljodaklettar on the east rim are linked to the North Volcanic Zone to the west (unseen left). Its emplacement preceded canyon formation 6,000 years ago but may have initiated its genesis. Photo with permission from Icelandic photographer, pilot, author ("Iceland from Above") and guide Bjorn Ruriksson. Visit him here.

Below Dettifoss, the river roughly follows the Sveinar graben and then cuts through the Rauðuborgir crater row and fissure swarms of the Fremrinámar Volcanic System, one of five active rift and volcanic zones within the North Volcanic Zone. It's emplacement preceded the formation of canyon Jökulsárgljúfur but likely tectonically dictated it's earliest genesis to some extent.

After 25 km of turbulent flow, Jökulsá emerges from the canyon-valley complex having lost energy and resumes a diffuse, braided course as it anastomoses another 18 km across and splits into two distrubutaries on an expansive depositional sandur plain. Ultimately, it empties into fjord Öxarfjörður of the Greenland Sea, the southern arm of the Arctic Ocean. From source to sea, it's a nearly two-day, 206 km journey. 

Glacial Canyon Ásbyrgi
Formed by repeated catastrophic flooding in the lower reaches of Jökulsá after the end of the last Ice Age, the horseshoe-shaped canyon is filled with indigenous birch and willow and non-indigenous planted arboreal species. Called Sleipnir's Footprint, Icelandic legend tells of Norse god Odin's eight-legged horse that touched one of its hooves to the ground here.

The lower course of Jökulsá á Fjöllum and its dramatic canyon are accessible from the Ring Road on either side of the suspension bridge. For our chosen views, we crossed over and headed north along the west bank on partially-paved Dettisforrvegur (Dettifoss Road or Route 862) rather than dirt-road Holsfjallavegur (Route 864) on the east. Turnoffs on both roads lead to the four thundering waterfalls, but the perspective differs.

Typical of glacial rivers, Jökulsá á Fjöllum's volumetric flow is solar-regulated that varies daily, seasonally and climatically as it affects Vatnajökull. Although vacillations in glacial advance and retreat occurred during Pleistocene interglacial intervals and various climatic perturbations in the Holocene, the current melt has been progressing at an unprecedented rate since the Last Glacial Maximum

Terminus of Outlet Glacier Skaftafellsjökull of Southern Vatnajökull
It's south-flowing proglacial river's journey to the North Atlantic is a comparatively short one compared to Jökulsá á Fjöllum's from the ice cap's northern edge to the Arctic Ocean. The terminus includes a proglacial lake and river that crosses a flat, broad black sand sandur.

The event occurred at the end of the Pleistocene, some 110,000 to perhaps 18,000 years ago. More event- than time-based, since spatial and time-variations differ by locale, it was the peak of the Ice Age when global ice and sequestered water reached its maximum extent and global seas were at the lowest.  

Sandur and Braided Stream on the South Coast
The anastomising network of ever-changing, interconnecting rivers and streams separated by temporary islands of volcaniclastic black sand and gravel belie the immense power of Iceland's jökulhlaups. The angular boulder erratics were most certainly delivered from the highlands under such megaflood conditions.

Formed where accumulation exceeds ablationglaciers are persistent bodies of ice on the move under their own weight. They're vast reservoirs of fresh water, the largest on the planet. Iceland's temperate glaciers (warm-based versus polar ones) exist near the melting point and are therefore subject to small changes in temperature and certainly climate. Perched at high elevations, 60% cover active volcanic centers in a potentially volatile situation should the sudden release of vast quantities of meltwater occur.

In addition to temperature, the generation of Jökulsá meltwater is dramatically triggered by the intermittent release of geothermal heat (600 to 5,000 cu m/sec) and violent subglacial volcanic eruptions (10,000 to 300,000) from one or more of Vatnajökull's subglacial volcanic centers (Kverkfjöll, Grimsvötn or Bárðarbunga).

The catastrophic release of voluminous meltwater also occurs from failed or over-topped moraine- or ice-dammed lakes that are precariously poised for escape. As many as six known subglacial lakes lie within the Grimsvötn caldera alone!

Upper Reach of Jökulsá á Fjöllum 
The largest jökulhlaup in the uppermost reaches of Jökulsá á Fjöllum (blue) with right and left banks (red lines) is thought to have inundated an area of ~1,390 sq km of the highlands just after early Holocene deglaciation between 9,000 and 7,100 BP. It may have been the largest known flood in Holocene Earth history. Further downstream (top), the flow was diverted into a system of channels across a scoured scabland and funneled into canyon Jökulsárgljúfur. Modified from Howard et al, 2012.


Called débâcles in the European Alps and aluviones in South America, jökulhlaups or "Glacial-Runs' are commonplace in Iceland, again due to the many ice caps that reside in active volcanic zones. Subglacially erupting lava can melt 14 times its volume of glacial ice. These high-magnitude events release tremendous quantities of water over short timeframes of several hours to days with the potential for sudden and catastrophic landscape change and destruction downriver. 

Fluvial Incision during Jökulhlaups
Deep and fast-flowing water erodes bedrock through several mechanisms such as cavitation (bursting water vapor bubbles that carve bedrock) and hydraulic quarrying (plucking where water vortices fragment weakened bedrock). Gorges are predominately formed via plucking and cause knickpoints to erode upstream. In Iceland, Holocene jökulhlaups carved Jökulsárgljúfur. Image source unknown.

A jökulhlaup's landscape altering ability is enhanced by Iceland's ubiquitous basalt bedrock (the most widely distributed rock in the world!). Its columnar joints (close-spaced vertical fractures in slowly-cooled lava, normal faults (in rift areas of crustal extension), open eruptive fissures and erosion-susceptible sedimentary interbeds facilitate rapid bedrock erosion and canyon excavation. In historic times, rural farms, fertile land, roads, bridges, power lines, hydroelectric plants and communication succumb readily.

The Incredible Power of Icelandic Megafloods
Against a spectacular backdrop of Vatnajökull's southern outlet glacier Skaftafellsjökull, decoratively painted twisted steel girders are all that remains of a 880 m-long Ring Road bridge that spanned river Skeiðará on outwash plain Skeiðarársandur. It succumbed to house-size icebergs delivered during a massive jökulhlaup that occurred in 1966. Go there: 63°59'4.74"N, 16°57'34.89"W

If you're desirous of a 'Grand Canyon' experience, 'Glacial River Gorge' won't disappoint. It has enthralled visitors for centuries and is Iceland's largest canyon by length and volume - 25 km-long, 500 m-wide and up to 100 to 120 m-deep. It was not created by erosional and depositional activity of Pleistocene glaciers, as was originally thought but, as mentioned, is the product of Holocene-age Vatnajökull deglaciation and subglacial magmatic activity.

The consensus is that Jökulsárgljúfur formed from three massive, canyon-carving jökulhlaups of glacio-volcanic origin and at least 16 ones of moderate size - the size, magnitude and timing of which remains controversial. The largest was ~2,500 years ago from a subglacial eruption near volcano Bárðarbunga. 

With a peak discharge of about 500,000 cu m/sec (2.5 times the average Amazon discharge), a total of 10 cu km carved the bulk of Jökulsárgljúfur and transported it to sandur and sea in addition to two previous mega-ones around 8,500 and 5,000 years ago. All occurred in the last 10,000 years as did Niagara Falls! 

Upstream View of Selfoss from the West Bank
Jökulsá á Fjöllum spills down an oversteepened stretch of channel above Selfoss into a large collection basin. The plume locates the apex of the horseshoe. On either side of the canyon, concordant dry, fossil fluvial terraces are littered with boulders on river-polished lava bedrock. Several undermined, collapsed columns of basalt litter the wide plungepool, while those from previous events have been fluvially transported away. Some years ago, plans were proposed to harness the hydroelectric potential of the canyon but were fortunately scrapped when the lava was found to be too porous for a reservoir. Go there: 65°47'56.58"N, 16°23'1.00"W 

Regionally, Holocene and Quaternary-age basalt lava flows (~6 to 8,000 and >800 k years old) occur stacked horizontally, one on top of the other. Unlike the Grand Canyon, there is no evidence of lava that flowed into canyon Jökulsargjlúfur and impounded meltwater for many kilometers upcanyon. The eruption age provides an independent constraint on the maximum age for the formation of the canyon upstream of the fissures that sourced the flows.

Canyon-carving can occur by incision (downcutting) from bedrock abrasion and widening by plucking and collapse as heavily jointed basalt becomes undermined (cavitation). As overhangs at knickpoints (abrupt changes in channel slope) become undermined and collapse, they retreat (advance or migrate) upstream (headward erosion). Knickpoint migration is common in flowing bodies of water over resistant rocks such as basalt. At Jökulsárgljúfur, it may have been as much as 2 km.

For over 100 years, geologists have debated the origin of Jökulsárgljúfur, which was initially thought to possess a glaciogenic origin. Modern cosmogenic dating of freshly exposed surfaces on bedrock and at knickpoints indicates a cluster of ages rather than a progressive aging downstream. It suggests rapid removal by plucking during violent, extreme flooding rather than gradual abrasion, although steady-state background erosion certainly does occur. 

Sketch illustrating Knickpoint and subsequent Waterfall Retreat
An oversteepened channel lies above the falls and a strata-undermining plungepool lies below. Overhang collapse from undermining and plucking contributes to knickpoint migration. Turbulent water in the plunge pool exerts pressure on the headwall behind it, weakening basalt columns to the point of collapse and shifts the knickpoint upstream. Modified from MediaWiki Commons image

The cumulative effect of the extreme floods at the head of the canyon at Selfoss is as much as 100 m of vertical erosion over the last 8,500 years, which is equivalent to an average vertical incision rate of ∼12 mm/yr. 

Additional canyon-carving factors at Jökulsárgljúfur include:
• a deglaciating climate
• drop in base level (sea level)
• isostatic (post-glacial crustal rebound) and tectonic uplift (rift-related)
• glacial proximity to volcanism at the source
• lithology and stratigraphy that forces Jökulsá to cascade from the top of one resistant flow to the top of the one below.
• tectono-structural controls such as NS-trending fissures and faults of the Krafla Volcanic System, discussed in upcoming post Part V). 

Simply stated, erosion occurs where floodwater velocity is high, facilitated by flow constriction, and deposition occurs where it's slow. Widespread and indisputable geomorphologic evidence of multiple jökulhlaups is found not only within and around canyon Jökulsárgljúfur but well upriver in the upper and middle reaches of Jökulsá on its broad floodplain as far as source Vatnajökull. 

Like large scars on the landscape, the region is a scabland, an expansive, scoured terrain of soil, traversed by deeply-carved post-glacial paleo-channels into bedrock. Megaflooding is not the only geologic process that's exerted an influence in the region. Past evidence of a compendium of glacial, fluvial, eolian and volcanic processes have acted over time that both compound and complicate interpretations of landscape evolution from Jökulsá's source all the way to the sea.

Uppermost River Jökulsá á Fjöllum near Source Vatnajökull
 Notice the flare of the deeply-eroded channel after it leaves the exit canyon it has excavated, the well-defined border-incisions of the floodplain and its multitude of braided, dry paleo-channels, all a clear and indisputable indication of extreme flooding. From with permission.

During peak flow during extreme flooding, a large braided river system with multiple channels likely flowed across the lava substrate as it does now. Swept clear of debris with edges carved into bordering flows, it deposited large erratics and sand and boulder bars in its wake. 

On Google Earth, a minor and major floodpath channel (dotted red) deflected around obstacles such as hyaloclastite (brecciated, water-quenched volcanic glass) mountain 
Upptyppinger that left boulders near the summit and trimline scars (highwater ersosional marks) on its flanks. 

North View of Upper Jökulsá near Source Vatnajökull
Dry paleo-channels (dotted red) were deflected around mount Upptyppingar of the North Volcanic Zone). Selfoss is where the river becomes encanyoned (white arrow). Present-day Jökulsá á Fjöllum was forced to the east (solid red) by obstructing lava flows and rejoins its deflected course downriver.

Downriver at the canyon (labelled Google Earth image), macro-scale erosional features are readily observable such as:
• flood-sculpted, multiple wide paleo-channels in a scabland landscape
• abandoned and concordant (cross-canyon and concurrent) flat river terraces that were elevated routeways (riverbed paths of flood waters) cut into bedrock
• relict, semi-circular amphitheaters, dry horseshoe-shaped cataracts (abandoned waterfalls) and plungepools (a deep depression at the base of a waterfall)
• notched overflow rims and spillways (flood-overspill drainage-saddles)
imbricated boulders (overlapping in the direction of flow)
• fluvially-plucked lava bedrock
lemniscate hillocks (hydrodynamically-streamlined and teardrop-shaped)

Large-scale depositional features include:
• boulder erratics (flood transported and ice-rafted) both isolated, clustered or in fields
• silt, sand, gravel and boulder bars 
• teardrop-shaped islands
• slackwater (low flow) deposits and silt and sandy dunes at river level. 

Upstream View of Jökulsárgljúfur and Jökulsá á Fjöllum
Wide paleo-terraces, dry overspill channels, amphitheaters and cataracts of the nascent abound. Their multitude likely precludes the occurrence of a single megaflood event. Waterfall Dettifoss lies oblique to the current flow on strike with a controlling fault. Notice the Sanddalur overspill channel with two dry cataracts to the west of the falls that rejoins the main channel downstream. Upriver, Selfoss is the first of the canyon's four waterfalls. Rutted Route 864 is on the east and cloud-cloaked, sprawling Vatnajökull lies on the horizon. Modified from Arctic Adventures.

Notice the size of the contemporary river compared to that of the canyon, the width-to-depth ratio. With the exception of the widened downstream section immediately below Dettifoss, Jökulsá doesn't 'fill' the canyon floor, even during multiple flood events of moderate size and duration. The underfit suggests that the canyon formed when river flow was significantly greater. 

A large car park is located just off Route 862 that directs throngs of onlookers (and geologists) to Selfoss just upcanyon to the south and Dettifoss to the north ('foss' means waterfall in Icelandic). Not to be confused with the small town in the south (place names can be repetitious in Iceland), the horseshoe-shaped waterfall is 13 meters tall and 387 meters wide, likely the widest in Iceland. 

As mentioned, Selfoss formed at the first step or knickpoint in the canyon that is gradually migrating upstream. It's the first of four falls that lie within it and the beginning of the 9 km-long, true-canyon section of Jökulsárgljúfur, although the canyon and channel dramatically narrows below Dettifoss, which is the official beginning of Jökulsárgljúfur National Park, 
Vatnajökull's Nantional Park's northern outlier section.

South-Facing View of Selfoss
Braided and oversteepened on the plateau (red circle), Jökulsá funnels into a horseshoe-notched Selfoss with some flow on concordant paleo-channels (dotted red). Earlier channels lie laterally (black arrows). Canyon forms by upstream knickpoint retreat at the head, while maintaining uniform width (dotted white) versus erosion of sidewalls. Notice detached blocks of basalt about to be toppled (dotted arrows). Once initiated, the embayment captures the majority of the flow and initiates upstream retreat of the headwall. Horseshoe-shaped waterfalls and canyons modify upstream flow by accelerating water from steady, uniform flow conditions. Modified from Reddit.

A short trail leads to the brink of Selfoss. It's safe to say that onlookers are oblivious not only that the platform on which they're hiking traverses across one of the early paleo-terraces that formed at the canyon and that evidence of fluvial-sculpting abounds both macro and micro. 

Paleo-Terrace Leading to Selfoss
To the right, as across the canyon, a river-plucked, fluted wall of lava borders the platform. Flood-delivered boulders are scattered across the fluvially-polished bedrock surface. A large boulder-field lies crosscanyon. A fine-grained, dusty andisol (volcanic soil) intermittently blankets the terrace's landscape.

Here and there in cryptic refugia sheltered from the wind, well-adapted vegetation thrives in Iceland's harsh and long winters in shallow, isolated pockets of andisols (soils formed from the weathering of basalt). Paleo-biologists claim that many lifeforms (such as crustacea and plant pollen in lake sediments) survived the Ice Age in subglacial refugia and some coastal ice-free areas.

This far north in Northeast Iceland, plants such as the Dwarf Fireweed or Arctic Riverbeauty (Epilobium latifolium or 'Eyrarrós' in Icelandic), a member of the willow family, must be hardy to survive. The relatively warm Irminger Current, a climate-moderating, northward-flowing, hyper-saline branch of the tropical North Atlantic Drift, has a diminished affect on Iceland's north side. Instead, the polar East Greenland Current affects this side of the island.

Dwarf Fireweed clings to an Exposed Lava Flow
Estimated to have been 60% vegetated at the time of Viking settlement in 874, Iceland has become highly depleted (under 25%) mostly due to human activity, as opposed to climate which most assume. At 300 m above sea level here at the canyon's rim, flora is visibly sparse, whereas above 700 m, Icelandic landscapes are mostly non-vegetated. Plants are low-growing, largely non-vascular and well-adapted for long winters, thin volcanic soils and challenging growing conditions.

Historically at Jökulsárgljúfur, distinguishing between glacial and extreme flood-related morphology has been both challenging and controversial, in spite of rather obvious present-day interpretations. The multitude of findings over such a large-scale lndscpe eventually became irrefutable.

Micro-scale erosional evidence of flooding in the direction of paleo-flow is preserved in the bedrock:
• longitudinal flutes (scalloped and open downcurrent), grooves and scours
• roughly circular potholes
• obstacle marks (upstream crescent-shaped scours with depositional feature downstream)
• stoss-and-lee bedrock forms (smooth-abraded downslope and upslope "bumps" versus glacially plucked-upslopes).

Concordant Fluvio-Sculpted Paleo-Terraces below Selfoss
Nearest the falls, the semi-flooded platform is rife with plucked potholes, scouring, stoss and lee forms and polished bedrock. Across the canyon an earlier elevated terrace preserves sculpted walls with spillover notches distributed across the top. Out of view (seen from aerial views), are even earlier broad, shallower flood terraces. Boulder depositional terraces well above the modern river channel extend over some distance on both sides of the canyon indicate the direction of paleo-flow as do strath terraces that correspond to the tops of lava flows. Polished and abraded surface rock and plungepools, potholes, flutes and furrows are additional proof of plaeo-fluvial presence. Go there: 65°48'51.86"N, 16°23'0.97"W

Extensive depositional evidence is also preserved beyond the canyon's outlet on the north at the sandur-delta, where layered sediments of sand, silt and tephra on the river delta allow the dating and distribution of megaflood events via tephrochronology (such as the Helka H5 tephra ~7,125 yrs/BP) and the reconstruction of hydraulic conditions.

Crosscanyon View below Selfoss
A small boulder bar and bedded sand-silt dunes testify to past flooding, all below a dry cascade at the terminus of the east-side Selfoss paleo-terrace. It appears that terrace abandonment has occurred as knickpoints have retreated upstream. On the platforms, features such as scours, flutes, potholes, polish indicate that abrasion is important in the process in addition primarily to block toppling. Note person for scale.

Immediately downstream from Selfoss, the canyon-confined river is moderately broad. Both banks consist of broad, elevated fossil terraces littered with boulder-size erratics delivered during flooding and early stages of canyon formation.

Geological mapping and research along river Jökulsá á Fjöllum and Jökulsárgljúfur was initiated in earnest when hydroelectric development was first considered. Analyses indicated that the surrounding basalt bedrock was too porous to retain a reservoir in addition to proximity to the active North Volcanic Zone to the west, our next destination in post Part V.

Downstream View below Selfoss

In the walls of the canyon, erosion exposed six lava flows and sedimentary redbeds between flows 3 and 4 and at the base at the canyon beneath two thick flows. The flows emplaced post-glacially some 9,000 years ago with oldest flows progressively exposed with travel downstream but dramatically below each successive knickpoint. 

Widely found in the Miocene-Pliocene-age Tertiary Formation, the oldest stratigraphical formation in Iceland on the west and east extremes of the island, in the canyon the rust-colored redbeds are latest Upper Pleistocene and volcanogenic in origin. They consist of iron-oxidized eolian (wind-delivered) tephra (ash), lahars (mudflows), tuffs (cemented ejecta) and breccias (angular fragments in a binding matrix). 

The concoction is compacted into a lateric topsoil (Fe and Al-rich formed in warmer climates) that formed on and overun by lava during ice-free interglacial, quiescent volcanic periods. Some redbeds are flora-fossiliferous and good paleo-climate indicators. 

Heavily Jointed East Wall of Canyon Jökulsárgljúfur below Selfoss
Spillover notches on the rims are the result of early megaflood activity as are the erratics distributed across flat paleo-channels (people for scale). Intermittent springs that emanate between flows and interbeds (note one at river level) contribute to undermining. In the early days of analysis, it was thought that glaciation was responsible for the abundant landscape features of the region before megaflood theories were introduced and substantiated. Recently exposed surfaces are cosmogenically datable and provide a history of canyon carving during multiple megafloods

The redbeds contribute to canyon formation and knickpoint migration, being erosion-susceptible with springs that emanate from exposed faces. At river level below Selfoss, freshly-toppled, jointed-columns above the lowest redbed litter the banks of the canyon and patiently await removal. Polygonal (5-7 sided) columns separated by vertical joints with evenly-spaced subhorizontal striae (surface-banding) on faces are characteristics of slow-contraction, uniform conductive-cooling of the homogeneous basaltic lava flow.

No entablature forms of lava (irregular, curved columns) are to be found here, that are 
thought to form in a cooling-enhanced environment such as flooding or where opposing joint-sets meet that induces complicated internal stress and distortion. These freshly-exposed surfaces are cosmogenically datable, but dates are generally calibrated from more readily accessible bedrock on paleo-terraces and knickpoints.

Toppled Columns Awaiting Removal and Freshly-Exposed Surfaces at River Level in Jökulsárgljúfur
They demonstrate typical cooling and solidification features of basalt - a polygonal shape in cross-section, nearly equal sized vertically-stacked columns and concave-convex horizontal banding. The glacially sediment-rich river is clearly evident. The toppling and removal of heavily-jointed basaltic bedrock has dramatically contributed to the formation of the canyon. Bedrock erosion during the high-magnitude, extreme floods is dominated by plucking rather than abrasion and has resulted in the upstream propagation of large vertical knickpoints. 

Today, the scabland surrounding Jökulsá á Fjöllum from its upper to lowest reaches is attributed to the megafloods that ravaged the landscape seems more than apparent, but it wasn't always the case. The concept was rejected in the early 20th century and considered to have been a radical departure from the concept of uniformitarianism, the theory that natural processes worked in the past as they do in the present. 

It represented a Dark Ages geological return to catastrophism, the idea that the Earth was formed by cataclysmic events such as a Biblical flood. The extreme flooding concept that was advocated by pioneer USGS geologist J. Harlen Bretz in 1923 was ultimately accepted in the mid-1960s as a result of his decades-long geomorphological analysis of the Channeled Scabland of eastern Washington.

Downstream View of the Brink of Dettifoss
The rising mist and background thunder is a dead giveaway that Iceland's largest waterfall is close-by. Again, observe the fluvially-polished bedrock, undermined colonnades of basalt, boulder-littered elevated paleo-terraces and toppled columns at river level.

About 1 km downstream from Selfoss is Iceland's most voluminous waterfall and first or second (depending on the source) most powerful in Europe, purportedly after the Rhine Falls in Switzerland. Its ascending mist signals its presence well before arrival, and its persistent, photogenic rainbow in sunny weather has helped make 'The Beast' the main attraction at Jökulsárgljúfur.

The 'Collapsing Waterfall' is the second cascade in the progression and drops 44 meters through the height of three lava flows and is 100 meters wide. As discussed, the canyon floor coincides with the top of a lava flow. Without evidence of any vertical incision into the flows, other than at the knickpoints themselves, abrasion is limited and confirms that toppling of basalt columns is the dominant role of erosion in the canyon.

With a drop of 45 meters and 100 meters wide, the waterfall's angulation in the channel and downstream strike are determined by faulting in the crust, atypical finding. And like Selfoss, it has excavated through three basalt flows and a number of erosion-susceptible sedimentary interbeds.

Mighty Dettifoss from the West Rim
The dark sediment and debris-laden meltwater is in stark contrast to the white spray that billows up from the plungepool. The grassy platform is an abandoned paleo-terrace. Contrasts exist between the opposing flooded and vegetated and unvegetated terraces and the thick blanket of milky gray water moving against the sharply delineated rocky banks. Go there: 65°48'51.86"N, 16°23'0.97"W  

The audible roar and low-level rumble under one's feet is hypnotic. A massive curtain of grayish meltwater continues to endlessly pour over the brink. It's staggering how much water is bound up within the glacial reservoir that's hundreds of kilometers upstream. Imagine the falls at extreme flood stage? With an average flow of 193 cu m/sec and 200 to 500 contingent on Vatnajökull summer melt, a jökulhlaup could be more than twice that! 

Looking downstream at the roughly uniform canyon width, canyon-forming erosion must occur at the knickpoint rather than the canyon's walls related to extreme flood discharge. As knickpoints propagate upstream, rock is typically removed over the thickness of a lava flow(s). It exposes the surfaces of pristine rock to cosmic rays that bombard the Earth. 

Accumulating cosmogenic nuclides (form in surface rocks due to cosmic bombardment) are datable and have allowed the model of canyon formation at Jökulsárgljúfur to be deciphered in association with its distinctive geomorphology. As mentioned, the river follows a path downslope dictated on a grand scale by the NS-trend of an EW structural fabric that is the result of extension both regionally and across the island. Every exposed flow depicts the classic columnar jointing characteristic of cooled and solidified basalt. The talus slope doesn't appear to have been reworked, suggesting that recent flood events have not (yet) occurred.

Jökulsárgljúfur directly below Dettifoss
It's truly difficult to direct your gaze way from this spectacular landscape. Well-worn footpaths crisscross a sloping, grassy paleo-terrace that leads to the edge of the perpetually mist-shrouded canyon. Both trans-canyon terraces are concordant with lava flows that coincide with the top of the falls. The line of vehicles in the distance are on Route 864

Three concordant (cross-canyon paired) strath terraces (cut into bedrock rather than worn into alluvium) are found at different levels in the canyon that indicate the position of dry paleo-riverbeds and are associated with upstream knickpoint migration. Their presence and orientation suggest that they were abandoned by waterfall retreat. In addition, the gradual decrease 
in ages of the terraces towards the waterfalls likely precludes formation in a single event such as catastrophic flooding.

Strath Terraces at Selfoss and Dettifoss
Modified from Baynes 2015.

The micro-scale features in bedrock - the tool marks, various flutes and scours, etc. - indicate that fluvial abrasion does play an important role in generating relief via vertical incision. And again, strong structural controls are incurred by the lithology and stratigraphy of the lava flows since the paleo-terraces, including the terrace of the present-day riverbed, correspond to the tops of lava flows.

Digital Elevation of Jökulsárgljúfur and its Three Largest Waterfalls
Each of three waterfalls possesses a paired strath terrace (colored) that is directly associated with the upstream migration of a knickpoint within the canyon. Oblique south view with 2x vertical exaggeration. Modified from Sticky de Quay et al, 2018.

Downstream of Hafragilsfoss, the third cascade, the canyon cuts through a volcanic fissure and associated flows dated at ~8,500 years ago without evidence of entering the canyon, hence predating it, helping to constrain its origin and bear no relationship to knickpoint or waterfall generation, which elsewhere is often the case.

Dettifoss (and Prometheus) from the East Bank
Like so many other iconic sites in Iceland, Dettifoss is no stranger to the big screen. Here it is in the opening scene of Prometheus. The movie was impressive, but the waterfall stole the show.

As mentioned, the canyon is some 30 km long beginning with Selfoss where it's more canyon-like with high vertical walls in the first 9 km. Further north in the canyon's middle third (photo), canyon Jökulsárgljúfur is a 9 km-long, wide open, scenic valley and then returns to canyon-like in the final 11.5 km-long lower third. 

Below waterfall Réttarfoss in the valley section, we're in the region of Forvöð east of the river and Hólmatungur on the west. Fed by numerous springs, lavish low-level vegetation blankets a soil-coated, sloping terrace that leads to the canyon (right to left mid-photo). Persistent runoff has exposed a number of densely cobbled streambeds that display a multi-millennial, Holocene history of flooding. In the distance, the volcanic summits of the lower, largely abandoned Melrakkaslétta peninsula rise to the occasion. Few visitors seem to venture this far downcanyon, once they've seen Dettifoss, the big attraction.

Open Valley Section of Jökulsárgljúfur facing East

Temperatures in Iceland in the last 20 or 30 years have increased 3-4x more than the average rise in the Northern Hemisphere. It suggests that Iceland's climate-vulnerable glaciers - 269 of them - will gradually retreat to the highest summits in 150 to 200 years. The most recent to succumb and lose glacial status, since it no longer flows under its own weight, is Okjökull having been repenned Ok in 2014.

Thinning of Iceland's ice cap's - at 40 sq km-loss annually - has significant consequences. Decreased ice pressure will result in isostatic rebound that will exceed the concomitant rise in sea level. It will not only starve Iceland's usable harbors, but as deglaciation progresses, meltwater flow will increase the volume of glacial rivers and impounded glacial lakes. Magma production will increase with subsequent surface volcanism that will generate jökulhlaups. Thus, 
the future at Jökulsárgljúfur for additional canyon-forming and continued upstream knickpoint migration appears favorable.

Unfortunately, our tour of Jökulsárgljúfur ended rather abruptly. Construction delays on 862 prevented us from exploring the lower canyon, scabland and sandur-delta region in a timely manner. It was time to return to the Ring Road and continue our geo-journey to the west into the Mývatn Lake District and the active Krafla Volcanic System of the North Volcanic Zone. 

Please visit what we discovered in our upcoming post Part V. Here's a small example of what we saw.

East View of Lake Mývatn and the Katla Volcanic System

"Sjáumst bráðlega"...Julia and I will see you soon in Part V!