Geology of Tindfjallajökull volcano, Iceland

The geology of Tindfjallajökull volcano, southern Iceland, is presented as a 1:50,000 scale map. Field mapping was carried out with a focus on indicators of past environments. A broad stratocone of interbedded fragmental rocks and lavas was constructed during Tindfjallajökull’s early development. This stratocone has been dissected by glacial erosion and overlain by a variety of mafic to silicic volcanic landforms. Eruption of silicic magma, which probably occurred subglacially, constructed a thick pile of breccia and lava lobes in the summit area. Mafic to intermediate flank eruptions continued through to the end of the last glacial period, producing lavas, hyaloclastite-dominated units and tuyas that preserve evidence of volcano-ice interactions. The Thórsmörk Ignimbrite, a regionally important chronostratigraphic marker, is present on the SE flank of the volcano. The geological mapping of Tindfjallajökull gives insights into the evolution of stratovolcanoes in glaciated regions and the influence of ice in their development

Widespread tephra dispersal and ignimbrite emplacement from a subglacial volcano (Torfajökull, Iceland)

The tephra dispersal mechanisms of rhyolitic glaciovolcanic eruptions are little known, but can be investigated through the correlation of eruptive products across multiple depositional settings. Using geochemistry and geochronology, we correlate a regionally important Pleistocene tephra horizon—the rhyolitic component of North Atlantic Ash Zone II (II-RHY-1)—and the Thórsmörk Ignimbrite with rhyolitic tuyas at Torfajökull volcano, Iceland. The eruption breached an ice mass >400 m thick, leading to the widespread dispersal of II-RHY-1 across the North Atlantic and the Greenland ice sheet. Locally, pyroclastic density currents traveled across the ice surface, depositing the variably welded Thórsmörk Ignimbrite beyond the ice margin and ~30 km from source. The widely dispersed products of this eruption represent a valuable isochronous tie line between terrestrial, marine, and ice-core paleoenvironmental records. Using the tephra horizon, estimates of ice thickness and extent derived from the eruption deposits can be directly linked to the regional climate archive, which records the eruption at the onset of Greenland Stadial 15.2

Subaqueous basaltic magmatic explosions trigger phreatomagmatism: a case study from Askja, Iceland

Sequences of basaltic pillow lavas that transition upwards with systematic gradation from pillow fragment breccias to fluidal bomb-bearing breccia to bomb-bearing lapilli tuffs are common at Askja volcano, Iceland. Based on the detailed textural investigation of three of these sequences, we argue that they record temporally continuous transition from effusive to explosive products that were erupted from and deposited at or near a single subaqueous vent. The recognition of such sequences is important as they provide evidence for controls on the onset of explosive activity in subaqueous environments. Such investigations are complicated by the interplay of magmatic gas expansion, phreatomagmatic and mechanical granulation fragmentation mechanisms in the subaqueous eruptive environment. All of the sequences studied at Askja have textural, componentry and sedimentological characteristics suggestive of a close genetic and spatial relationship between the pillow lavas and all of the overlying glassy clastic deposits. The identification of magma fragmentation signatures in pyroclasts was accomplished through detailed textural studies of pyroclasts within the full range of grain sizes of a given deposit i.e. bomb/blocks, lapilli and fine ash. These textural characteristics were compared and evaluated as discriminators of fragmentation in pyroclastic deposits. The presence of angular vitric clasts within the breccia and lapilli tuff displaying fragile glassy projections indicates little or no postdepositional textural modification. A shift in vesicle and clast textures between the pillow lavas and the large concentration of fluidal bombs in the breccia indicate that the phreatomagmatic explosions were initially triggered by magmatic vesiculation. The initial magmatic gas expansion may have been triggered by depressurization caused by the drainage of the ice-confined lake surrounding Askja. The Fuel Coolant Interactions (FCI) of the more efficient phreatomagmatic explosion was enabled by the increase in the surface area to volume ratio of the fluidal bombs in the water, producing a premix of magma and water. The onset and increasing influence of phreatomagmatic fragmentation is preserved in the presence of very fine blocky ash particles and diminished presence of larger particles such as fluidal bombs. The textural, sedimentological and environmental characteristics of these deposits suggest that phreatomagmatic explosions can be triggered by initial magmatic gas expansion, but that it is likely one of many mechanisms for triggering such explosions

Rhyolitic volcano–ice interactions in Iceland

Iceland contains an abundance and diversity of rhyolitic edifices produced during volcano–ice interactions (glaciovolcanism) that is unmatched by any other volcanic province, with an estimated 350 km3 erupted during the past 0.8 Ma from at least fifteen volcanic systems. This review summarises research undertaken during the past decade and provides a new summary of the distribution of rhyolitic glaciovolcanic rocks erupted during the past 0.8 Ma. Descriptions of effusion-dominated edifices focus on the two best-studied edifices — the c.0.6 km3 and 570 m high Prestahnúkur edifice and the smaller-volume (< 0.1 km3) drape of Bláhnúkur. Both show little or no evidence of magmatic fragmentation (i.e. driven by volatile exsolution), but both show abundant evidence of quench fragmentation and meltback of ice walls. A particular feature of Prestahnúkur is its sheet-like lava bodies which are interpreted as sills that intruded the ice–edifice interface. Rhyolitic tuyas are products of sustained eruptions into thick ice, and form distinctive steep-sided edifices with flat or broad dome-like tops. They are 300–700 m high with volumes up to 2 km3 (though 0.5–1.0 km3 is more typical). Initial vigorous phreatomagmatic eruptions within well-drained ice vaults build steep-sided piles of tephra confined by ice walls. Gradual upwards increases in inflated clasts point to the decreasing involvement of meltwater and the increasing ability of the magma to vesiculate. As the eruption progresses and terminates, non-explosive degassing produces lava caps up to 300 m thick. Öræfajökull is a stratovolcano illustrating complex volcano–ice interactions generated when rhyolitic lavas erupt onto steeper slopes and encounter ice of variable thickness and properties (e.g. fracturing). Eruptive units record dramatic ice thickness fluctuations of up to 500 m in valley glaciers between eruptions, which emphasises that useful palaeoenvironmental information can be gleaned from detailed studies of volcano–ice interactions. Considerable value is added when palaeoenvironmental information is combined with Ar–Ar dating. For example Ar–Ar dating of two rhyolitic tuyas at Torfajökull reveal that they erupted prior to the last glacial maximum when temperatures were at least 4 °C cooler than the present day. Ar–Ar dating of Prestahnúkur suggests it erupted during the last interglacial–glacial transition into over 700 m of ice, which corroborates studies arguing for rapid accumulation of land-based ice during global cooling

Products of an effusive subglacial rhyolite eruption: Bláhnúkur, Torfajökull, Iceland

We present field observations from Bláhnúkur, a small volume (3) subglacial rhyolite edifice at the Torfajökull central volcano, south-central Iceland. Bláhnúkur was probably emplaced during the last glacial period (ca. 115–11 ka). The characteristics of the deposits suggest that they were formed by an effusive eruption in an exclusively subglacial environment, beneath a glacier >400 m thick. Lithofacies associations attest to complex patterns of volcano-ice interaction. Erosive channels at the base of the subglacial sequence are filled by both eruption-derived material and subglacial till, which show evidence for deposition by flowing meltwater. This suggests that meltwater was able to drain away from the vent area during the eruption. Much of the subglacial volcanic deposits consist of conical-to-irregularly shaped lava lobes typically 5–10 m long, set in poorly sorted breccias with an ash-grade matrix. A gradational lavabreccia contact at the base of lava lobes represents a fossilised fragmentation interface, driven by magma-water interaction as the lava flowed over poorly consolidated, waterlogged debris. Sets of columnar joints on the upper surfaces of lobes are interpreted as ice-contact features. The morphology of the lobes suggests that they chilled within conically shaped subglacial cavities 2–5 m high. Avalanche deposits mantling the flanks of Bláhnúkur appear to have been generated by the collapse of lava lobes and surrounding breccia. A variety of deposit characteristics suggests that this occurred both prior to and after quenching of the lava lobes. Collapse events may have occurred when the supporting ice walls were melted back from around the cooling lava lobes and breccias. Much larger lava flows were emplaced in the latter stages of the eruption. Columnar joint patterns suggest that these flowed and chilled within subglacial cavities 20 m high and 100–200 m in length. There is little evidence for magma-water interaction at lava flow margins which suggests that these larger cavities were drained of meltwater. As rhyolite magma rose to the base of the glacier, the nature of the subglacial cavity system played an important role in governing the style of eruption and the volcanic facies generated. We present evidence that the cavity system evolved during the eruption, reflecting variations in both melting rate and edifice growth that are best explained by a fluctuating eruption rate

Silicic volcanism at Ljósufjöll, Iceland: insights into evolution and eruptive history from Ar–Ar dating

Ljósufjöll volcano is the largest outcrop of silicic volcanic material in the volcanic Snaefellsnes flank (non-rifting) zone of Iceland. The silicic eruptives range from trachytes to alkaline and peralkaline (comenditic) rhyolites and show evidence for eruption in both subaerial and subglacial environments. Thirteen silicic eruptive units have been identified and mapped by a combination of field observations and geochemical correlation. The trachytes probably formed by fractional crystallisation of a basaltic magma to form an alkali feldspar-rich trachytic mush, with continued fractionation of the interstitial melt producing low Ba and Sr rhyolite magma. Ar–Ar dating of feldspars and matrix (glass or holocrystalline groundmass) has been carried out on twelve of the units. Many units have a complex Ar-isotopic system, with many samples showing evidence for inherited 40Ar in the form of feldspar xenocrysts. The deduced eruption ages span an age range from 650ka. The eruptive history of Ljósufjöll probably extends further back than this but the products of previous eruptions have since been removed by erosion or buried. A syenite xenolith was dated at 1.1Ma, indicating that silicic, alkaline magmatism has been occurring at Ljósufjöll for over one million years. Eruptions of silicic material at Ljósufjöll appear to be more common during times of rapid climate change and fluctuating ice volume

Geochemistry and 3D reconstruction dataset for the Reyðarártindur Pluton, Iceland

-Whole rock major and trace element geochemistry -3D reconstruction of pluton -Feldspar EMP profiles -Sample GPS location