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Hans-ulrich Schmincke - One of the best experts on this subject based on the ideXlab platform.
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Large-volume basaltic Hyaloclastite eruption along a propagating land/lake lithosphere fracture at Lake Van (Eastern Anatolia): impact of volcanism on the evolution of Lake Van V
Bulletin of Volcanology, 2018Co-Authors: Hans-ulrich Schmincke, Mari Sumita, Deniz CukurAbstract:The Incekaya Hyaloclastite cone (eastern Anatolia, Turkey), the focal point along a major eruptive fissure, was the main source of an unusually large explosive basaltic eruption. The ca. 80 ka-old eruption began onshore with scoria cones from a 5 km N-S fracture propagating toward Lake Van (surface area of 3755 km^2). At the intersection with the fault-bounded lake basin, a ca. 400-m-high subaerial Hyaloclastite edifice formed, which can be crudely subdivided into a main lower massive bulk of hydrothermally altered lithic-rich Hyaloclastites (CL) topped unconformably by a > 30-m-thick, well-bedded fallout tephra (CU). The CU tephras are correlated with (1) widespread onshore Hyaloclastite fallout deposits mostly west-southwest of the cone and (2) a ca. 2-m-thick, ca 80-ka-old bedded Hyaloclastite (V-60), part of a 220 m ICDP (International Continental Scientific Drilling Program) core, drilled in Lake Van, 27 km N of Incekaya. The Hyaloclastite unit was seismically identified as being the most widespread and well-defined reflector throughout much of western Lake Van. A minimum volume of > 9 km^3 fallout Hyaloclastite tephra is estimated when the area of the seismic reflector is extrapolated to the coast and 2 km inland. Seismic reflectors also suggest at least two (Hyaloclastite?) intralake cones rising up to 388 m above the lake sediment surface 1.5 km NW off Incekaya cone and were possibly erupted along the same fracture. The total volume of Hyaloclastites includes (a) subaerial Incekaya cone, (b) the inferred subaqueous continuation of the cone(s), (c) the bedded intralake and onshore deposits, and, tentatively, (d) a widespread (seismically defined) mass flow deposits directly beneath Incekaya reflector of roughly 20 km^3 and may represent the deposits of explosively erupted basaltic magma. Sideromelane shards, the main clast type, are dominantly angular, and most show ≪ 50 vol.% vesicles. Less common tachylite clasts are poorly vesicular (
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large volume basaltic Hyaloclastite eruption along a propagating land lake lithosphere fracture at lake van eastern anatolia impact of volcanism on the evolution of lake van v
Bulletin of Volcanology, 2018Co-Authors: Hans-ulrich Schmincke, Mari Sumita, Deniz CukurAbstract:The Incekaya Hyaloclastite cone (eastern Anatolia, Turkey), the focal point along a major eruptive fissure, was the main source of an unusually large explosive basaltic eruption. The ca. 80 ka-old eruption began onshore with scoria cones from a 5 km N-S fracture propagating toward Lake Van (surface area of 3755 km2). At the intersection with the fault-bounded lake basin, a ca. 400-m-high subaerial Hyaloclastite edifice formed, which can be crudely subdivided into a main lower massive bulk of hydrothermally altered lithic-rich Hyaloclastites (CL) topped unconformably by a > 30-m-thick, well-bedded fallout tephra (CU). The CU tephras are correlated with (1) widespread onshore Hyaloclastite fallout deposits mostly west-southwest of the cone and (2) a ca. 2-m-thick, ca 80-ka-old bedded Hyaloclastite (V-60), part of a 220 m ICDP (International Continental Scientific Drilling Program) core, drilled in Lake Van, 27 km N of Incekaya. The Hyaloclastite unit was seismically identified as being the most widespread and well-defined reflector throughout much of western Lake Van. A minimum volume of > 9 km3 fallout Hyaloclastite tephra is estimated when the area of the seismic reflector is extrapolated to the coast and 2 km inland. Seismic reflectors also suggest at least two (Hyaloclastite?) intralake cones rising up to 388 m above the lake sediment surface 1.5 km NW off Incekaya cone and were possibly erupted along the same fracture. The total volume of Hyaloclastites includes (a) subaerial Incekaya cone, (b) the inferred subaqueous continuation of the cone(s), (c) the bedded intralake and onshore deposits, and, tentatively, (d) a widespread (seismically defined) mass flow deposits directly beneath Incekaya reflector of roughly 20 km3 and may represent the deposits of explosively erupted basaltic magma. Sideromelane shards, the main clast type, are dominantly angular, and most show ≪ 50 vol.% vesicles. Less common tachylite clasts are poorly vesicular (< 50 vol.%). Structural transitions and interlayering between tachylite and sideromelane are ubiquitous. Fluidal and pumiceous lapilli are present in the basal massive facies. Bulk rock and glass compositions indicate constant composition of the slightly evolved Al-rich basalt magma. Olivine (Fo78–82) and plagioclase (An70–80) microphenocrysts, many skeletal with growth features, and microlites make up < 1 vol.% and suggest rapid magma ascent. The high explosive energy of the eruption is interpreted to be due to (1) high magma discharge rates and shearing in the eruptive jet and (2) magma-water interaction conditions. Approximate temporal coincidences with the Incekaya eruption include the following: (a) an abrupt cessation in the supply of evolved tephra from the adjacent Suphan Volcano to the lake sediments, which ended abruptly for ca. 60 ky, (b) an extreme fall in lake level by ca. 150 m, and (c) a drastic increase in pore water salinity (Na+ and Cl− (g/L)) and pH.
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Englacial vs lacustrine origin of volcanic table mountains : evidence from iceland
Bulletin of Volcanology, 1999Co-Authors: Reinhard Werner, Hans-ulrich SchminckeAbstract:Detailed facies analysis of Hyaloclastites and associated lavas from eight table mountains and similar "Hyaloclastite volcanoes" in the Icelandic rift zone contradict a rapid and continuous, "monogenetic", entirely subglacial evolution of most volcanoes studied. The majority of the exposed Hyaloclastite deposits formed in large, stable lakes as indicated by widespread, up to 300-m-thick, continuous sections of deep water, shallow water and emergent facies. Salient features include extensively layered or bedded successions comprising mainly debris flow deposits, turbidites, base surge and fallout deposits consisting of texturally and compositionally variable, slightly altered Hyaloclastites, as well as sheet and pillow lavas. In contrast, chaotic assemblages of coarser-grained, more poorly sorted and more strongly palagonitized Hyaloclastite tuffs and breccias, as well as scoria and lava are interpreted to have formed under sub- or englacial conditions in small, chimney-like ice cavities or ice-bound lakes. Irregularly shaped and erratically arranged Hyaloclastite bodies produced at variable water levels appear to have resulted mainly from rapid changes of the eruptive environment due to repeated build-up and drainage of ice-bound lakes as well as the restricted space between the ice walls. We distinguish a "deep water" facies formed during high water levels of the lake, a hydroclastic shallow water and emergent facies (leakage of the lake or growth of the volcano above the water surface). Our model implies the temporary existence of large, stable lakes in Iceland probably formed by climatically induced ice melting. The highly complex edifices of many table mountains and similar volcanoes were constructed during several eruptive periods in changing environments characterized by contrasting volcanic and sedimentary processes.
Deniz Cukur - One of the best experts on this subject based on the ideXlab platform.
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Large-volume basaltic Hyaloclastite eruption along a propagating land/lake lithosphere fracture at Lake Van (Eastern Anatolia): impact of volcanism on the evolution of Lake Van V
Bulletin of Volcanology, 2018Co-Authors: Hans-ulrich Schmincke, Mari Sumita, Deniz CukurAbstract:The Incekaya Hyaloclastite cone (eastern Anatolia, Turkey), the focal point along a major eruptive fissure, was the main source of an unusually large explosive basaltic eruption. The ca. 80 ka-old eruption began onshore with scoria cones from a 5 km N-S fracture propagating toward Lake Van (surface area of 3755 km^2). At the intersection with the fault-bounded lake basin, a ca. 400-m-high subaerial Hyaloclastite edifice formed, which can be crudely subdivided into a main lower massive bulk of hydrothermally altered lithic-rich Hyaloclastites (CL) topped unconformably by a > 30-m-thick, well-bedded fallout tephra (CU). The CU tephras are correlated with (1) widespread onshore Hyaloclastite fallout deposits mostly west-southwest of the cone and (2) a ca. 2-m-thick, ca 80-ka-old bedded Hyaloclastite (V-60), part of a 220 m ICDP (International Continental Scientific Drilling Program) core, drilled in Lake Van, 27 km N of Incekaya. The Hyaloclastite unit was seismically identified as being the most widespread and well-defined reflector throughout much of western Lake Van. A minimum volume of > 9 km^3 fallout Hyaloclastite tephra is estimated when the area of the seismic reflector is extrapolated to the coast and 2 km inland. Seismic reflectors also suggest at least two (Hyaloclastite?) intralake cones rising up to 388 m above the lake sediment surface 1.5 km NW off Incekaya cone and were possibly erupted along the same fracture. The total volume of Hyaloclastites includes (a) subaerial Incekaya cone, (b) the inferred subaqueous continuation of the cone(s), (c) the bedded intralake and onshore deposits, and, tentatively, (d) a widespread (seismically defined) mass flow deposits directly beneath Incekaya reflector of roughly 20 km^3 and may represent the deposits of explosively erupted basaltic magma. Sideromelane shards, the main clast type, are dominantly angular, and most show ≪ 50 vol.% vesicles. Less common tachylite clasts are poorly vesicular (
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large volume basaltic Hyaloclastite eruption along a propagating land lake lithosphere fracture at lake van eastern anatolia impact of volcanism on the evolution of lake van v
Bulletin of Volcanology, 2018Co-Authors: Hans-ulrich Schmincke, Mari Sumita, Deniz CukurAbstract:The Incekaya Hyaloclastite cone (eastern Anatolia, Turkey), the focal point along a major eruptive fissure, was the main source of an unusually large explosive basaltic eruption. The ca. 80 ka-old eruption began onshore with scoria cones from a 5 km N-S fracture propagating toward Lake Van (surface area of 3755 km2). At the intersection with the fault-bounded lake basin, a ca. 400-m-high subaerial Hyaloclastite edifice formed, which can be crudely subdivided into a main lower massive bulk of hydrothermally altered lithic-rich Hyaloclastites (CL) topped unconformably by a > 30-m-thick, well-bedded fallout tephra (CU). The CU tephras are correlated with (1) widespread onshore Hyaloclastite fallout deposits mostly west-southwest of the cone and (2) a ca. 2-m-thick, ca 80-ka-old bedded Hyaloclastite (V-60), part of a 220 m ICDP (International Continental Scientific Drilling Program) core, drilled in Lake Van, 27 km N of Incekaya. The Hyaloclastite unit was seismically identified as being the most widespread and well-defined reflector throughout much of western Lake Van. A minimum volume of > 9 km3 fallout Hyaloclastite tephra is estimated when the area of the seismic reflector is extrapolated to the coast and 2 km inland. Seismic reflectors also suggest at least two (Hyaloclastite?) intralake cones rising up to 388 m above the lake sediment surface 1.5 km NW off Incekaya cone and were possibly erupted along the same fracture. The total volume of Hyaloclastites includes (a) subaerial Incekaya cone, (b) the inferred subaqueous continuation of the cone(s), (c) the bedded intralake and onshore deposits, and, tentatively, (d) a widespread (seismically defined) mass flow deposits directly beneath Incekaya reflector of roughly 20 km3 and may represent the deposits of explosively erupted basaltic magma. Sideromelane shards, the main clast type, are dominantly angular, and most show ≪ 50 vol.% vesicles. Less common tachylite clasts are poorly vesicular (< 50 vol.%). Structural transitions and interlayering between tachylite and sideromelane are ubiquitous. Fluidal and pumiceous lapilli are present in the basal massive facies. Bulk rock and glass compositions indicate constant composition of the slightly evolved Al-rich basalt magma. Olivine (Fo78–82) and plagioclase (An70–80) microphenocrysts, many skeletal with growth features, and microlites make up < 1 vol.% and suggest rapid magma ascent. The high explosive energy of the eruption is interpreted to be due to (1) high magma discharge rates and shearing in the eruptive jet and (2) magma-water interaction conditions. Approximate temporal coincidences with the Incekaya eruption include the following: (a) an abrupt cessation in the supply of evolved tephra from the adjacent Suphan Volcano to the lake sediments, which ended abruptly for ca. 60 ky, (b) an extreme fall in lake level by ca. 150 m, and (c) a drastic increase in pore water salinity (Na+ and Cl− (g/L)) and pH.
Mari Sumita - One of the best experts on this subject based on the ideXlab platform.
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Large-volume basaltic Hyaloclastite eruption along a propagating land/lake lithosphere fracture at Lake Van (Eastern Anatolia): impact of volcanism on the evolution of Lake Van V
Bulletin of Volcanology, 2018Co-Authors: Hans-ulrich Schmincke, Mari Sumita, Deniz CukurAbstract:The Incekaya Hyaloclastite cone (eastern Anatolia, Turkey), the focal point along a major eruptive fissure, was the main source of an unusually large explosive basaltic eruption. The ca. 80 ka-old eruption began onshore with scoria cones from a 5 km N-S fracture propagating toward Lake Van (surface area of 3755 km^2). At the intersection with the fault-bounded lake basin, a ca. 400-m-high subaerial Hyaloclastite edifice formed, which can be crudely subdivided into a main lower massive bulk of hydrothermally altered lithic-rich Hyaloclastites (CL) topped unconformably by a > 30-m-thick, well-bedded fallout tephra (CU). The CU tephras are correlated with (1) widespread onshore Hyaloclastite fallout deposits mostly west-southwest of the cone and (2) a ca. 2-m-thick, ca 80-ka-old bedded Hyaloclastite (V-60), part of a 220 m ICDP (International Continental Scientific Drilling Program) core, drilled in Lake Van, 27 km N of Incekaya. The Hyaloclastite unit was seismically identified as being the most widespread and well-defined reflector throughout much of western Lake Van. A minimum volume of > 9 km^3 fallout Hyaloclastite tephra is estimated when the area of the seismic reflector is extrapolated to the coast and 2 km inland. Seismic reflectors also suggest at least two (Hyaloclastite?) intralake cones rising up to 388 m above the lake sediment surface 1.5 km NW off Incekaya cone and were possibly erupted along the same fracture. The total volume of Hyaloclastites includes (a) subaerial Incekaya cone, (b) the inferred subaqueous continuation of the cone(s), (c) the bedded intralake and onshore deposits, and, tentatively, (d) a widespread (seismically defined) mass flow deposits directly beneath Incekaya reflector of roughly 20 km^3 and may represent the deposits of explosively erupted basaltic magma. Sideromelane shards, the main clast type, are dominantly angular, and most show ≪ 50 vol.% vesicles. Less common tachylite clasts are poorly vesicular (
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large volume basaltic Hyaloclastite eruption along a propagating land lake lithosphere fracture at lake van eastern anatolia impact of volcanism on the evolution of lake van v
Bulletin of Volcanology, 2018Co-Authors: Hans-ulrich Schmincke, Mari Sumita, Deniz CukurAbstract:The Incekaya Hyaloclastite cone (eastern Anatolia, Turkey), the focal point along a major eruptive fissure, was the main source of an unusually large explosive basaltic eruption. The ca. 80 ka-old eruption began onshore with scoria cones from a 5 km N-S fracture propagating toward Lake Van (surface area of 3755 km2). At the intersection with the fault-bounded lake basin, a ca. 400-m-high subaerial Hyaloclastite edifice formed, which can be crudely subdivided into a main lower massive bulk of hydrothermally altered lithic-rich Hyaloclastites (CL) topped unconformably by a > 30-m-thick, well-bedded fallout tephra (CU). The CU tephras are correlated with (1) widespread onshore Hyaloclastite fallout deposits mostly west-southwest of the cone and (2) a ca. 2-m-thick, ca 80-ka-old bedded Hyaloclastite (V-60), part of a 220 m ICDP (International Continental Scientific Drilling Program) core, drilled in Lake Van, 27 km N of Incekaya. The Hyaloclastite unit was seismically identified as being the most widespread and well-defined reflector throughout much of western Lake Van. A minimum volume of > 9 km3 fallout Hyaloclastite tephra is estimated when the area of the seismic reflector is extrapolated to the coast and 2 km inland. Seismic reflectors also suggest at least two (Hyaloclastite?) intralake cones rising up to 388 m above the lake sediment surface 1.5 km NW off Incekaya cone and were possibly erupted along the same fracture. The total volume of Hyaloclastites includes (a) subaerial Incekaya cone, (b) the inferred subaqueous continuation of the cone(s), (c) the bedded intralake and onshore deposits, and, tentatively, (d) a widespread (seismically defined) mass flow deposits directly beneath Incekaya reflector of roughly 20 km3 and may represent the deposits of explosively erupted basaltic magma. Sideromelane shards, the main clast type, are dominantly angular, and most show ≪ 50 vol.% vesicles. Less common tachylite clasts are poorly vesicular (< 50 vol.%). Structural transitions and interlayering between tachylite and sideromelane are ubiquitous. Fluidal and pumiceous lapilli are present in the basal massive facies. Bulk rock and glass compositions indicate constant composition of the slightly evolved Al-rich basalt magma. Olivine (Fo78–82) and plagioclase (An70–80) microphenocrysts, many skeletal with growth features, and microlites make up < 1 vol.% and suggest rapid magma ascent. The high explosive energy of the eruption is interpreted to be due to (1) high magma discharge rates and shearing in the eruptive jet and (2) magma-water interaction conditions. Approximate temporal coincidences with the Incekaya eruption include the following: (a) an abrupt cessation in the supply of evolved tephra from the adjacent Suphan Volcano to the lake sediments, which ended abruptly for ca. 60 ky, (b) an extreme fall in lake level by ca. 150 m, and (c) a drastic increase in pore water salinity (Na+ and Cl− (g/L)) and pH.
Thure E. Cerling - One of the best experts on this subject based on the ideXlab platform.
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Peak discharge of a Pleistocene lava-dam outburst flood in Grand Canyon, Arizona, USA
Quaternary Research, 2006Co-Authors: Cassandra R. Fenton, Robert H. Webb, Thure E. CerlingAbstract:Abstract The failure of a lava dam 165,000 yr ago produced the largest known flood on the Colorado River in Grand Canyon. The Hyaloclastite Dam was up to 366 m high, and geochemical evidence linked this structure to outburst-flood deposits that occurred for 32 km downstream. Using the Hyaloclastite outburst-flood deposits as paleostage indicators, we used dam-failure and unsteady flow modeling to estimate a peak discharge and flow hydrograph. Failure of the Hyaloclastite Dam released a maximum 11 × 109 m3 of water in 31 h. Peak discharges, estimated from uncertainty in channel geometry, dam height, and hydraulic characteristics, ranged from 2.3 to 5.3 × 105 m3 s−1 for the Hyaloclastite outburst flood. This discharge is an order of magnitude greater than the largest known discharge on the Colorado River (1.4 × 104 m3 s−1) and the largest peak discharge resulting from failure of a constructed dam in the USA (6.5 × 104 m3 s−1). Moreover, the Hyaloclastite outburst flood is the oldest documented Quaternary flood and one of the largest to have occurred in the continental USA. The peak discharge for this flood ranks in the top 30 floods (>105 m3 s−1) known worldwide and in the top ten largest floods in North America.
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Peak discharge of a Pleistocene lava-dam outburst flood in Grand Canyon,
2006Co-Authors: Cassandra R. Fenton, Robert H. Webb, Thure E. CerlingAbstract:The failure of a lava dam 165,000 yr ago produced the largest known flood on the Colorado River in Grand Canyon. The Hyaloclastite Dam was up to 366 m high, and geochemical evidence linked this structure to outburst-flood deposits that occurred for 32 km downstream. Using the Hyaloclastite outburst-flood deposits as paleostage indicators, we used dam-failure and unsteady flow modeling to estimate a peak discharge and flow hydrograph. Failure of the Hyaloclastite Dam released a maximum 11 × 10 9 m 3 of water in 31 h. Peak discharges, estimated from uncertainty in channel geometry, dam height, and hydraulic characteristics, ranged from 2.3 to 5.3 × 10 5 m 3 s �1 for the Hyaloclastite outburst flood. This discharge is an order of magnitude greater than the largest known discharge on the Colorado River (1.4 × 10 4 m 3 s �1 ) and the largest peak discharge resulting from failure of a constructed dam in the USA (6.5 × 10 4 m 3 s �1 ). Moreover, the Hyaloclastite outburst flood is the oldest documented Quaternary flood and one of the largest to have occurred in the continental USA. The peak discharge for this flood ranks in the top 30 floods (N10 5 m 3 s �1 ) known worldwide and in the top ten largest floods in North America.
Hugh Tuffen - One of the best experts on this subject based on the ideXlab platform.
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the formation of helgafell southwest iceland a monogenetic subglacial Hyaloclastite ridge sedimentology hydrology and volcano ice interaction
Journal of Volcanology and Geothermal Research, 2006Co-Authors: Herdís H. Schopka, Magnús T. Gudmundsson, Hugh TuffenAbstract:Abstract Helgafell, a little-eroded basaltic Hyaloclastite ridge in southwest Iceland, formed in a single eruption under a Pleistocene ice sheet. The ice thickness at the eruption site was at least 500 m and it was probably located some 15 km from the glacier's snout. The eruption created a 2 km long, 0.8 km wide and initially 300 m high ridge. Gravity modelling indicates that Helgafell has a bulk density of 1800 kg m − 3 , and that Holocene lavas around it are 40–80 m thick. This confirms that Helgafell is predominantly made of Hyaloclastite, with pillow lavas and intrusions only making up a few percent of the total volume. The southeast side is made of unsorted eruption-fed Hyaloclastites considered to have been piled up against an ice wall, while moderately to well-sorted water-transported material is found on the northwest side. Glacier flow and meltwater drainage was towards the northwest. The absence of basal pillow lavas suggests that magma fragmentation occurred from the onset of the eruption until its end. The lithofacies preserved indicate a fully subglacial eruption, although a final subaerial eruptive phase may have taken place through an ice chimney. The volatile contents (H 2 O: 0.26–0.37 wt.%) of several glass samples from the southeast side of the mountain indicate water pressures of ∼ 1 MPa throughout the eruption. Efficient syn-eruptive drainage of meltwater coupled with rapid ice subsidence probably led to partial dynamic support of the ice, causing water pressure in the vault to be much lower than the static load of the overlying ice. Observed lack of correlation between elevation and volatile content may be a consequence of gradual reduction in dynamic support as the eruption rate declined and the edifice grew higher. Helgafell demonstrates that explosive activity may occur under ∼ 500 m thick ice, and it may be an analogue to the ridge formed in the Gjalp eruption in 1996.
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The formation of Helgafell, SW-Iceland, a monogenetic subglacial Hyaloclastite ridge: Sedimentology, hydrology and ice-volcano interaction.
2006Co-Authors: Herdís H. Schopka, Mt Gudmundsson, Hugh TuffenAbstract:Helgafell, a little-eroded basaltic Hyaloclastite ridge in southwest Iceland, formed in a single eruption under a Pleistocene ice sheet. The ice thickness at the eruption site was at least 500 m and it was probably located some 15 km from the glacierâ��s snout. The eruption created a 2 km long, 0.8 km wide and initially 300 m high ridge. Gravity modelling indicates that Helgafell has a bulk density of 1800 kg m3, and that Holocene lavas around it are 40â��80 m thick. This confirms that Helgafell is predominantly made of Hyaloclastite, with pillow lavas and intrusions only making up a few percent of the total volume. The southeast side is made of unsorted eruption-fed Hyaloclastites considered to have been piled up against an ice wall, while moderately to well-sorted watertransported material is found on the northwest side. Glacier flow and meltwater drainage was towards the northwest. The absence of basal pillow lavas suggests that magma fragmentation occurred from the onset of the eruption until its end. The lithofacies preserved indicate a fully subglacial eruption, although a final subaerial eruptive phase may have taken place through an ice chimney. The volatile contents (H2O: 0.26â��0.37 wt.%) of several glass samples from the southeast side of the mountain indicate water pressures of ~1 MPa throughout the eruption. Efficient syn-eruptive drainage of meltwater coupled with rapid ice subsidence probably led to partial dynamic support of the ice, causing water pressure in the vault to be much lower than the static load of the overlying ice. Observed lack of correlation between elevation and volatile content may be a consequence of gradual reduction in dynamic support as the eruption rate declined and the edifice grew higher. Helgafell demonstrates that explosive activity may occur under ~500 m thick ice, and it may be an analogue to the ridge formed in the Gja´lp eruption in 1996.
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The formation of Helgafell, southwest Iceland, a monogenetic subglacial Hyaloclastite ridge: Sedimentology, hydrology and volcano–ice interaction
Journal of Volcanology and Geothermal Research, 2006Co-Authors: Herdís H. Schopka, Magnús T. Gudmundsson, Hugh TuffenAbstract:Helgafell, a little-eroded basaltic Hyaloclastite ridge in southwest Iceland, formed in a single eruption under a Pleistocene ice sheet. The ice thickness at the eruption site was at least 500 m and it was probably located some 15 km from the glacierâ��s snout. The eruption created a 2 km long, 0.8 km wide and initially 300 m high ridge. Gravity modelling indicates that Helgafell has a bulk density of 1800 kg m3, and that Holocene lavas around it are 40â��80 m thick. This confirms that Helgafell is predominantly made of Hyaloclastite, with pillow lavas and intrusions only making up a few percent of the total volume. The southeast side is made of unsorted eruption-fed Hyaloclastites considered to have been piled up against an ice wall, while moderately to well-sorted watertransported material is found on the northwest side. Glacier flow and meltwater drainage was towards the northwest. The absence of basal pillow lavas suggests that magma fragmentation occurred from the onset of the eruption until its end. The lithofacies preserved indicate a fully subglacial eruption, although a final subaerial eruptive phase may have taken place through an ice chimney. The volatile contents (H2O: 0.26â��0.37 wt.%) of several glass samples from the southeast side of the mountain indicate water pressures of ~1 MPa throughout the eruption. Efficient syn-eruptive drainage of meltwater coupled with rapid ice subsidence probably led to partial dynamic support of the ice, causing water pressure in the vault to be much lower than the static load of the overlying ice. Observed lack of correlation between elevation and volatile content may be a consequence of gradual reduction in dynamic support as the eruption rate declined and the edifice grew higher. Helgafell demonstrates that explosive activity may occur under ~500 m thick ice, and it may be an analogue to the ridge formed in the Gja´lp eruption in 1996
