Eastern Iceland Research Articles

Development of permeable networks by viscous-brittle deformation in a shallow rhyolite intrusion. Part 1: Field evidence

Efficient outgassing of shallow magma bodies reduces the risk of explosive eruption. Silica-rich magmas are too viscous for exsolved gas bubbles to escape the system through buoyant forces alone, and so volatile overpressure is often released through deformation-related processes. Here we present a case study on magma emplacement-related deformation in a shallow (∼500 m depth) rhyolite intrusion (the Sandfell laccolith, Eastern Iceland) to investigate the establishment of degassing (volatile exsolution) and outgassing (gas escape) networks in silicic sub-volcanic intrusions. We observe viscous and brittle deformation features: from vesiculated flow bands that organized into ‘pore channels’ in the ductile regime, to uniform bands of tensile fractures (‘fracture bands’) that grade into breccia and gouge in the brittle regime. Through field mapping, structural analysis, and anisotropy of magnetic susceptibility (AMS) measurements, we show that areas with higher degrees of brittle deformation are proximal to abruptly changing AMS fabrics, and flow band orientations and point to laccolith-wide strain partitioning in the magma. We associate the changes in flow fabrics and the intensity of brittle deformation to the transition from dominantly horizontally flowing magma during initial sill-stacking to up to the NE magma flow linked to the propagation of a trap-door fault from the N to the SE. The establishment of intrusion-scale brittle permeable networks linked to changes in strain partitioning that facilitated magma flow during different stages of laccolith growth would have profoundly assisted the outgassing of the entire laccolith. Magmatic fracturing captures viscous and brittle processes working in tandem as an efficient outgassing mechanism, and should be considered in sub-volcanic intrusions elsewhere.

Call type repertoire of killer whales (Orcinus orca) in Iceland and its variation across regions

AbstractKiller whales (Orcinus orca) have group‐specific call repertoires that can be used to track groups and populations using passive acoustic monitoring. To provide a detailed description of the Icelandic killer whale repertoire and its variation, we analyzed acoustic data collected in five locations between 1985 and 2016. Calls were classified manually, and CART and random forest analyses were employed to validate the manual classification. A total of 91 call categories (including call types and subtypes) were defined. Most call categories were recorded in more than one location, with the highest proportion shared between herring grounds in Vestmannaeyjar (South) and Breiðafjörður (West). However, both locations included call categories that were not recorded elsewhere in Iceland. Recordings from past herring wintering grounds in eastern Iceland included few call categories that matched other locations. Sample sizes from Reykjanes (Southwest) and Skjálfandi (North) were small and did not include unique call categories. The relative occurrence of call categories in Vestmannaeyjar changed little over a 14‐year period (2002–2016), although shorter‐term changes between years were observed that appeared to correlate to changes in individuals identified. This comparison of acoustic repertoires provides valuable information on the social structure and movement patterns of herring‐eating killer whales around Iceland.

Using multivariate autoregressive state‐space models to examine stock structure of Greenland halibut in the North Atlantic

AbstractAccurate information on population structure is essential for effective fisheries management. Greenland halibut (Reinhardtius hippoglossoides) in the North Atlantic is managed as four separate offshore stocks. We use Multivariate Autoregressive State‐Space (MARSS) models to assess population structure by means of abundance and biomass trends in four regions (Norwegian Sea, Iceland, Southeast Greenland, and Northwest Atlantic) where three offshore stocks are recognized: (1) Baffin Bay–Davis Strait (Northwest Atlantic stock), (2) Southeast Greenland and Iceland (West Nordic stock (WNS)), and (3) the Barents and Norwegian Seas (Northeast Arctic stock). We formulated model alternatives, using bottom trawl survey data from each region for 1996–2019, to evaluate support for different population structures. Abundance and biomass observations from each region were linked to growth rate parameters in MARSS models and the impact of climate (North Atlantic Oscillation Index) and fishing (commercial catches) on stock dynamics was investigated. Top models identified the Northwest Atlantic as an independent population. Best‐fit models treated Greenland halibut in the WNS as two independent populations (east and west), with potential connections between eastern Iceland and the western Barents Sea. These results suggest a mismatch between current stock perception and management boundaries in the Northeast Atlantic.

Spatial analysis and controlling factors of landslides in East Icelandic fjords

The fjords of eastern Iceland display many landslide landforms. These phenomena, visible by remote observation of satellite images and Digital Elevation Models (DEM), were inventoried then measured and analysed. A total of 290 landslides were recorded in a spreadsheet database and in a Geographic Information System (GIS). For each landslide, location, morphometry (length, width, surface area, thickness, estimated volume, etc.) data, as well as potential control variables, particularly geological (lithology, dip) or explanatory (orientation, age of deglaciation of the slope affected by the landslide) were recorded. These variables and their distribution were studied by spatial and statistical analysis. This study highlights a higher density of landslides in the northern part of the study area, which could be explained by past ice sheet current directions, which would have reinforced the pressure exerted on the slopes leading to the postglacial decompression phenomenon. The over-representation of west- and south-facing landslides leads to the hypothesis of climatic control: the sunnier slopes underwent more rapid deglaciation, which favoured the instability of the slopes. The data collected also suggest that the lithology (Tertiary basalts) is a control variable and the period during which the landslide area is deglaciated an explanatory variable for landslide initiation. The landslides observed are thus part of a paraglacial dynamic of slope instability after their deglaciation.

Geology of a Neogene caldera cluster in the Borgarfjörður eystri–Loðmundarfjörður area, Eastern Iceland

The Borgarfjörður eystri–Loðmundarfjörður area in Eastern Iceland hosts several volcanic centres in a relatively small area. Large volumes of silicic, as well as an unusual amount of intermediate rocks, occur in this area, alongside other volcanic and sub-volcanic features that formed in the Miocene between 14 and 12:2 Ma. We compiled the first comprehensive geological map of the area and summarise the geology based on more than 40 years of fieldwork. We identify regionally extensive marker horizons that comprise intermediate (icelandite) and mafic (olivine basalt and porphyritic basalt) lavas. These marker horizons, along with new paleomagnetic data and some previous radiogenic dating, allow us to bracket a phase of magmatic activity that is one of the oldest in Eastern Iceland. We describe the prominent features of the volcanic centres, including remnants of collapse calderas in Njarðvík, Dyrfjöll, Breðuvík and other ignimbrite-producing vent structures in Kækjuskörð and Herfell. Notably, the area also contains extremely well exposed examples of volcanic vents, cone sheets, and unique caldera-lake sediments. We conclude with open questions to inspire future research on this understudied area.

The explosive basaltic Katla eruptionin 1918, south Iceland, I: Course of events, tephra fall and flood routes

The 23-day long eruption of the ice-covered Katla volcano in 1918 began on October 12 and was over by November 4. Seismicity preceding and accompanying the onset had already started by 11:30 on October 12, while the eruption broke through the glacier around 3 PM. The plume rose to 14–15 km on the first day. The eruption caused widespread tephra fall, accompanied by lightning and thunder. Tephra fall from the intense first phase (October 12–14) was reported from Höfn, 200 km east of Katla, Reykjavík, 150 km to the west and Akureyri, 240 km to the north. The initial phase was followed by more sporadic activity for a week, and a second intense phase (October 22–24), with heavy tephra fall in populated areas east and south of the volcano. Skaftártunga (25–35 km east of Katla), was the worst hit farming district, with reported tephra thickness of 6.5–10 cm in total, collecting into drifts tens of cm thick. The Vík village suffered almost continuous tephra fall for 13 hours on October 24 and 25, leaving a 2 to 4 cm thick tephra layer on the ground. Tephra reached Reykjavík four times but minor tephra fallout («1 mm) occurred. Tephra also reached northern, western and eastern Iceland. In addition to producing the 0.9–1.0 km3 tephra layer, which may as freshly fallen have been 1.1–1.2 km³, the eruption was accompanied by a jökulhlaup that flooded the Mýrdalssandur plain and neighbouring areas. The jökulhlaup on October 12 had two separate phases. The first phase is considered to have flowed supraglacially down the lower parts of the Kötlujökull outlet glacier into the Leirá, Hólmsá and Skálm rivers (northern fork), and the Sandvatn and Múlakvísl rivers (southern fork). It was much more widespread than the second phase which emerged from below the glacier snout and was confined to the western part of Mýrdalssandur. That phase carried huge icebergs and massive sediment load onto the sandur plain.

Virtual Geosites as Innovative Tools for Geoheritage Popularization: A Case Study from Eastern Iceland

In this paper, we have adopted a modern, cutting-edge methodology to make geoheritage sites (geosites) available and explorable worldwide, through both immersive and non-immersive virtual reality, particularly suitable also in COVID-19 times. In doing this, we have focused our attention on five different outcroppings, shallow magma bodies in Iceland: such geological objects, although being often underestimated, are, on the contrary, very suitable for geoheritage popularization purposes. These outstanding outcrops have been transformed in virtual outcrops (VOs) through UAV-based photogrammetry 3D modelling, and have been uploaded on a brand-new, dedicated online resource (GeoVires Virtual Reality Lab for Earth Sciences) which is accessible worldwide for Earth Science teaching and communication. As already stressed above, the choice of these Icelandic shallow magma bodies has been suggested by the fact that such geological objects, although extraordinarily challenging both in terms of geotourism and teaching, are seldom the object of attention from the international scientific community. The five VOs are defined here as virtual geosites (VGs) because they are, indeed, geosites that are fully accessible with a smartphone, a tablet, or a PC; moreover, each is provided with a detailed description and notes available during 3D exploration. Our work could represent a model for future, similar efforts aimed at popularizing Earth Sciences and making geoheritage available to a broad public through VGs.

Magmatic Processes at Snæfell Volcano, Iceland, Constrained by Zircon Ages, Isotopes, and Trace Elements

AbstractWe present the first zircon‐based U‐Pb geochronology, trace element concentrations, and O and Hf isotope compositions for Snæfell, an off‐rift volcano in eastern Iceland. These data provide constraints on the longevity and petrogenetic conditions of the Snæfell magmatic system. U‐Pb zircon ages range from 545 ± 59 to 266 ± 16 ka (2SE), but differences between grain core and mantle ages within each sample reveal zircon residence times of 100–200 kyr—far longer than observed at other Neovolcanic systems in Iceland. Zircon δ18O is restricted to ∼3.5–4‰, and zircon εHf ranges ∼+13 to ∼+17, which is substantially more radiogenic than Snæfell basalts. This combined O and Hf isotopic perspective suggests rhyolite petrogenesis at Snæfell can be attributed to fractional crystallization of mantle‐derived basaltic magmas with limited influence of pre‐existing crustal material. Trace element evidence further characterizes the magmatic source material: Sc/Yb <∼0.01 suggests an amphibole‐free petrogenetic environment, and Ti concentrations in zircon <5 ppm suggest a cool, near‐solidus, crystallization environment for the majority of the zircon's pre‐eruptive history, with elevated Ti in zircon surfaces suggesting a late thermal perturbation, perhaps a mafic input that remobilized (to the point of eruption triggering) near‐solidus magmas. These zircon‐based conclusions are broadly consistent with previous interpretations of rhyolite petrogenesis conditions at Snæfell but provide a multi‐faceted perspective with more detailed resolution of source materials, magma generating processes, system longevity, and pre‐eruptive conditions.

Interpreting Inverse Magnetic Fabric in Miocene Dikes From Eastern Iceland

AbstractAnisotropy of Magnetic Susceptibility (AMS) is a valid tool to investigate magma flow direction within dikes. However, geometrically inverse magnetic fabric characterized by maximum magnetic susceptibility axis (kmax) perpendicular to the dike wall may complicate the interpretation of flow trajectories. To better understand the nature of this fabric, we present a multiscale study on 19 dikes (383 samples) in the Miocene Alftafjordur volcanic system (Iceland), where 80% of the samples show a geometrically inverse magnetic fabric. We carried out (1) AMS measurements at different magnetic fields and temperatures, along with Anisotropy of Anhysteretic Remanent Magnetization (AARM) analysis; (2) hysteresis loops and FORC diagrams; (3) thin section analysis; (4) structural fieldwork. A variable Ti‐content (0.1 < x < 0.6, Fe3‐xTixO4) titanomagnetite is the main magnetic carrier, and the contribution of the paramagnetic elongated crystals to the magnetic fabric is negligible. Single domain is not the prevailing domain state of the magnetic particles, suggesting that its occurrence cannot be the main cause for the inverse fabric. AMS analysis at different fields and temperatures along with AARM allow us to exclude any mineral phase change of the titanomagnetite across the dike. Nevertheless, kmax is parallel to a diffuse horizontal column‐like fracture pattern perpendicularly oriented with respect to the dike strike. This suggests that the Ti‐magnetite mineral orientation during dike cooling was affected by the fracture network progressively developing columnar basalts. This study demonstrates that the interpretation of AMS data on old and deep volcanic bodies is not straightforward and observations at different scales are required.

Temporal and spatial evolution of the Neogene age Breiðdalur central volcano through 39Ar/40Ar and U-Pb age determination

The volcanic sequences in eastern Iceland provide a cross-section of magmatic activity from the Iceland rift system throughout the Miocene. This study focuses on a high precision age determination campaign of a single Miocene volcanic system, the Breiðdalur central volcano and associated dyke swarm, in order to quantify its evolution. The central volcano consists of: a lower flank of mafic lavas erupted onto a basement of plateau basalts; an upper flank of intermediate and mafic lavas; an 8–10 km wide caldera with associated infill formations; and a summit sequence of lavas and tuffs. This study indicates that the central volcano had a lifespan of around 1 My, from 10.2–9.1 Ma, and started with mafic magmatism. Intermediate magmatism began around 10.04 Ma and the caldera formed between 9.8 and 9.5 Ma, evolving via multiple nested-caldera collapses. Effusive silicic magmatism was predominantly in the youngest sequences between 9.5 and 9.1 Ma. After 9.1 Ma the activity at the central volcano effectively ceased. The volcano became buried by plateau basalt sequences predominantly derived from eruptions at the associated Breiðdalur dyke swarm. Activity on the dyke swarm began around 9.9 Ma and continued to until 7.8 Ma, an additional 1.3 My.

Late Miocene to late Pleistocene geomagnetic secular variation at high northern latitudes

SUMMARYWe report a palaeomagnetic study of Icelandic lavas of late Miocene to late Pliocene age to test the geocentric axial dipole hypothesis at high northern latitudes. Cores were sampled from 125 sites in the Fljótsdalur valley in eastern Iceland, and hand samples were taken for 17 new incremental heating 40Ar/39Ar age determinations. 96 per cent of the cores were oriented using both a Brunton compass and a sun compass. Comparison of the magnetic and sun azimuths reveals deviations of ±5°, ±10° and ±20°, respectively, for 42, 16 and 3 per cent of the data points, indicating that core sampling intended for palaeosecular variation (PSV) studies at high northern latitudes should be oriented by sun. A total of 1279 independent specimens were subjected to AF- and thermal-demagnetization for palaeodirectional analysis, and well-grouped site mean directions were obtained for 123 sites of which 113 were found to be independent sites. Applying a selection criteria of k > 50 and N ≥ 5 (Nmean = 9.5), we obtain a combined grand mean direction for 46 normal and 53 reverse (for VGPlat > ±45°) polarity sites of declination = 5.6° and inclination = 77.5° that is not significantly different from that expected from a GAD field. The corresponding palaeomagnetic pole position (VGPlat = 86.3°N, VGPlon = 21.2°E, dp/dm = 4.0°/4.3°) is coincident with the North Pole within the 95 per cent confidence limits. An updated age model is constructed based on the 40Ar/39Ar ages, showing that the majority of the Fljótsdalur lavas fall within 2–7 Ma. We combine the Fljótsdalur data with existing data from the nearby Jökuldalur valley. The 154 palaeodirections are well-dispersed between 1 and 7 Ma and constitute a high-quality data set for PSV analysis. Our results partly support previous conclusions of a generally higher dispersion during reverse polarity intervals. However, when comparing our Matutayma data with Brunhes age data from Jan Mayen, we find no evidence of a higher VGP scatter during the Matuyama as previously suggested. When comparing our VGP scatter to the two commonly used models for VGP dispersion: Model G and TK03, we find a good fit for all 1–7 Ma VGP scatter data SB(1–7) to Model G, whereas SB(1–7) is not fitted by TK03, even when considering the uncertainty of SB(1–7). We also find that all VGP scatter estimates, except that for the Gilbert subset, are consistent with Model G, while the discrepancy with TK03 is generally larger.

Icelandic permafrost dynamics since the Last Glacial Maximum – model results and geomorphological implications

Iceland’s periglacial realm is one of the most dynamic on the planet, with active geomorphological processes and high weathering rates of young bedrock resulting in high sediment yields and ongoing mass movement. Permafrost is discontinuous in Iceland’s highlands and mountains over c. 800 m a.s.l, and sporadic in palsa mires in the central highlands. During the late Pleistocene and Holocene, Iceland’s periglacial environment varied considerably in time and space, dominated by glacial fluctuations and periglacial processes. To evaluate the dynamics of permafrost in Iceland since the last deglaciation, we use the output of a coupled climate/ice sheet model to force a transient permafrost model (CryoGRID 2) from the Last Glacial Maximum (LGM) through to the present. We find that permafrost was widespread across the deglaciated areas of western, northern and eastern Iceland after the LGM, and that up to 20% of Iceland’s terrestrial area was underlain by permafrost throughout the late Pleistocene. This influenced geomorphological processes and landform generation: the early collapse of the marine-based ice sheet together with the aggradation of permafrost in these zones initiated the formation of abundant and now relict rock glaciers across coastal margins. Permafrost degraded rapidly after the Younger Dryas, with a marked impact on slope stability. Permafrost that formed during the Little Ice Age is again thawing rapidly, and an escalation in slope failure and mass-movement might be currently underway. Our study demonstrates that large regions of Iceland have been underlain by permafrost for millennia, facilitating landform development and influencing the stability of steep slopes.

Corrigendum: Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland—Implications for Rhyolite Intrusion Growth and Volcanic Hazards

Corrigendum: Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland—Implications for Rhyolite Intrusion Growth and Volcanic Hazards

Volcanoes, medicine, and monasticism: Investigating mercury exposure in medieval Iceland

AbstractThis study aimed to evaluate the possible use of mercury as a medical treatment at the medieval monastic hospital Skriðuklaustur (ad 1494–1554) in eastern Iceland. The individuals excavated from Skriðuklaustur exhibited a wide range of pathological conditions, including the only skeletal evidence of venereal syphilis in Iceland. Skeletal remains from the Skeljastaðir cemetery (ca. ad 1000–1104) in southern Iceland were also analysed in light of the site's proximity to the mercury emitting volcano Hekla. The eruption produced a severe toxic fallout resulting in the mass mortality of livestock and is believed to have caused the abandonment of Skeljastaðir and the other farms in the region. The skeletal analyses and sampling were conducted according to standard anthropological methods. Mercury concentrations were determined in human (n = 50), faunal (n = 23), and soil (n = 22) samples using inductively coupled plasma mass spectrometry. Individuals from both sites exhibited elevated mercury concentrations. At Skriðuklaustur, some individuals showed normal concentrations, but those with pathological conditions (e.g., hydatidosis, syphilis, tuberculosis, and other non‐specific infections) had the highest concentrations overall. On the other hand, all the individuals analysed from Skeljastaðir exhibited elevated mercury concentrations, some of which were remarkably high. A few of the individuals buried at Skeljastaðir post‐date the eruption, possibly indicating that some of them experienced heavy exposure to volcanic emissions. The less extreme concentrations at Skriðuklaustur may be a result of attention to dosage and the temporary nature of mercurial treatments. None of the faunal and soil samples presented with concentrations exceeding the normal limit, implying that diagenesis was not a concern in this research. The conclusion is that a variety of factors from medical treatment to scholarly work lead to mercury exposure at Skriðuklaustur, whereas at Skeljastaðir, volcanogenic emissions are implicated.

The subsurface circulation of the Iceland Sea observed with RAFOS floats

The pathways of dense waters located above the sill depth of Denmark Strait were investigated in the Iceland Sea using 52 acoustically tracked RAFOS floats. These floats were deployed in summer 2013 and 2014, with a target depth of 500 m, resulting in a total of 40.9 float-years of track data covering the Iceland Sea basin. In the interior Iceland Sea basin, the float tracks showed a double gyre circulation, out of which floats eventually escaped towards the Norwegian Sea in the East Icelandic Current, with some appearing to be en route to the Faroe Bank Channel. Four floats exited through Denmark Strait and surfaced in the Labrador and Irminger Seas. Four other floats deployed west of the Kolbeinsey Ridge at 70°N show the connection between the East Greenland Current and the East Icelandic Current. Floats deployed east of the Kolbeinsey Ridge along the Icelandic slope were captured in a region with no clear main flow. Eddy motions, mainly small scale (radii of 0.5–3 km), are seen throughout the Iceland Sea. Several floats were grounded on the Icelandic slope both east and west of the Kolbeinsey Ridge due to upslope currents, which created a rim of cold water along the slope. While this water was indicative of the presence of the North Icelandic Jet, no connection between the eastern Iceland Sea and Denmark Strait sill was found. Our investigation of wind stress curl fields from atmospheric reanalysis data suggests that high wind stress curl conditions may have been unfavorable for a westward connection by the North Icelandic Jet at the time of the float observations.

Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland—Implications for Rhyolite Intrusion Growth and Volcanic Hazards

Felsic magma commonly pools within shallow mushroom-shaped magmatic intrusions, so-called laccoliths or cryptodomes, which can cause both explosive eruptions and collapse of the volcanic edifice. D ...

Provenance Variation in Phenology and Frost Tolerance in Subalpine Fir (Abies lasiocarpa) Planted in Denmark and Iceland

In Iceland and Denmark, there is an interest in planting Abies lasiocarpa for use as Christmas trees. To search for usable genetic material for both countries, 26 provenances of subalpine fir, covering most of its natural range, were planted in eastern Iceland and Jutland, Denmark. Flushing, bud set and survival rates were assessed. Artificial freezing of twigs, from field trials in eastern Iceland and Denmark, was done to rank the provenances for frost tolerance in the spring and autumn. The northernmost provenances showed earliest bud set, highest autumn frost tolerance and a latitudinal cline was delineated. Differences between provenances in flushing and spring frost tolerance were less than that found for bud set and autumn frost tolerance. The southernmost provenances showed earliest flushing and the most spring frost damage on buds. Mortality of single provenances in the field tests could not be attributed to low freezing tolerances in the autumn or spring. The southernmost provenances of Abies lasiocarpa from New Mexico and Arizona showed the highest survival rate in the field trial in East Iceland, while the eastern provenances showed a low survival rate except for two provenances from Utah and Wyoming. The western provenances from Washington state showed the best survival in Denmark, followed by the southernmost provenances.

The mode of emplacement of Neogene flood basalts in Eastern Iceland: The plagioclase ultraphyric basalts in the Grænavatn group

Plagioclase ultraphyric basalt lava with high fraction of solids have a mode of emplacement that is poorly understood. In this study we conduct detailed mapping of a PUB group in eastern Iceland, namely the Grænavatn group, and assess the group architecture, flow morphology and internal structure with additional constraints from petrography, petrology and crystal size distribution, to derive information on emplacement dynamics of plagioclase ultraphyric basalts. We also derive information on the plumbing system of the group with reference to the source of the macrocysts. The group is exposed in steep glacially carved fjords and can be traced for more than 70km along strike. The flows have mixed architecture of simple and compound flows. Individual flow lobes have thicknesses in the range of 1–24m and many reach widths and lengths exceeding 1000m. The flows vary from rubbly to slabby pahoehoe, but are predominantly of pahoehoe type. The aspect ratio of the group and the nature of the flows indicate fissure-fed eruptions. The plagioclase macrocrysts (5–30mm) are An-rich, exhibit bimodal size distribution and the modal proportions within the group varies from 15–40%. Clinopyroxene macrocrysts are also present ranging from 1–6%. The lowermost flow is thickest and carries the greatest crystal cargo load. The morphology of the lava flows suggests low viscous behavior, at odds with the high crystal content. The very calcic plagioclase macrocrysts (An80–85) are in disequilibrium with the groundmass and plagioclase microlaths therein (An50–70), meaning that the crystal-laden magmas quickly ascended from deeper crustal levels to the surface. The flows with highest crystal content may have maintained high temperatures by heat exchange with the primitive macrocrysts in the flows and developed non-Newtonian behavior such as shear thinning. Such conditions would have enabled the flows to advance rapidly during episodes with high effusion rates forming the simple flows, and subsequently maintained by insulated lobe-by-lobe emplacement. Occasionally surges disrupted the crust to form rubbly and slabby pahoehoe. A Herschel-Bulkley model is applied to the flows yielding effusion rates in the order of 103–104m 3/s for the largest eruptions. Variation in crystal proportions within the group and individual flows is considered to be the result of magmas penetrating a crystal mush, followed by disaggregation and mobilization, sorting by flotation, mixing of magma batches and transportation of heterogeneous crystal-laden magmas to the surface with opportunities for staging en route.

The impact of damming on riverine fluxes to the ocean: A case study from Eastern Iceland.

Anthropogenic water management has extensively altered the world's river systems through impoundments and channel diversions to meet the human's need for water, energy and transportation. To illuminate the effect of such activities on the environment, this study describes the impact of the installation of the Kárahnjúkar Dam in Eastern Iceland on the transport of riverine dissolved- and particulate material to the ocean by the Jökulsá á Dal and the Lagarfljót rivers. This dam, completed in 2007, collects water into the 2.2 km3 Hálslón reservoir and diverts water from the glacial Jökulsá á Dal river into the partially glaciated Lagarfljót lagoon via a headrace tunnel. The impact of the damming was evaluated by sampling water from both the Jökulsá á Dal and the Lagarfljót rivers over a 15 year period spanning from 1998 to 2013.The annual flux of most dissolved elements increased substantially due to the damming. The fluxes of dissolved Zn, Al, Co, Ti and Fe increased most by damming; these fluxes increased by 46–391%. These differences can be attributed to changed saturation states of common secondary minerals in the Jökulsá á Dal due to reduced discharge, increased residence time and dissolution of suspended material, and, to a lesser degree, reduced photosynthesis due to less transparency in the Lagarfljót lagoon. The removal of particulate material and thus decreasing adsorption potential in the Jökulsá á Dal is the likely reason for the Fe flux increase. In contrast, approximately 85% of the original riverine transported mass of particulate material is trapped by the dam; that which passes tends to be relatively fine grained, increasing the average specific surface area of that which continues to flow towards the ocean. Consequently, the particulate geometric surface area flux is decreased by only 50% due to the damming.The blooming of silica diatoms during the spring consumes dissolved silica from the coastal waters until it becomes depleted; making the riverine spring dissolved silica flux an important source of this nutrient. Despite extensive riverine flux changes due to the Kárahnjúkar dam construction, the total spring dissolved silica flux increased, and thus so too the potential for a silica diatom spring bloom in the coastal waters. This is likely because the spring flux is dominated by snow melting downstream of the dam.

A new high-resolution Holocene tephra stratigraphy in eastern Iceland: Improving the Icelandic and North Atlantic tephrochronology

A new and improved Holocene tephra stratigraphy and tephrochronological framework for eastern and northern Iceland is presented. Investigations of a sediment sequence from Lake Logurinn have revealed a comprehensive tephra record spanning the last 10.200 years. A total of 157 tephra layers have been identified, whereof 149 tephra layers have been correlated to its source volcanic system using geochemistry, stratigraphy and age. Fifteen layers have chemical composition of two affinities that possibly represent two very closely spaced eruptions. Thus, these 157 tephra layers are believed to represent 172 explosive eruptions. Nineteen tephra marker layers have been identified in the Lake Logurinn record (G1922, A1875, V1477, V1410, H1636, K1625, O1362, G1354, K1262, V874, Hrafnkatla, Sn-1, Grakolla, HY, H3, H4, HO, LL1755 and Reitsvik-8 tephra markers). New potential tephra markers are the silicic Askja L (∼9400 cal BP), the low titanium basalt layers, LL 1774 (∼10.150 cal BP) and LL 1755 (∼9990 cal BP), assigned to Veidivotn-Bardarbunga and the tephra layers, LL 1527.8 (∼7850 cal BP), LL 911.2 (∼2370 cal BP), LL 908.4 (∼2350 cal BP), LL 781.9 (∼1930 cal BP), LL 644.4 (∼1480 cal BP), not yet correlated to a source volcanic system. A silicic tephra marker layer, Reitsvik 8, correlated to the Fosen tephra in Norway has been identified in Lake Logurinn. The Lake Logurinn tephra record has been connected and integrated with the Icelandic terrestrial tephrochronology and stratigraphy through 102 tephra layers, the marine tephra stratigraphy through 39 layers and overseas through 9 tephra layers. This record is the first high-resolution tephra stratigraphical and chronological framework for the Holocene in eastern Iceland as well as the most detailed and continuous record, and has considerable potential to serve as a key section or a stratotype for the Holocene in eastern Iceland and the North Atlantic.