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1.
Tephra layers with Icelandic provenance have been identified across the North Atlantic region in terrestrial, lacustrine, marine and glacial environments. These tephra layers are used as marker horizons in tephrochronology including climate studies, archaeology and environmental change. The major element chemistries of 19 proximally deposited Holocene Icelandic silicic tephra layers confirm that individual volcanic systems have unique geochemical signatures and that eruptions from the same system can often be distinguished. In addition, glass trace element chemistry highlights subtle geochemical variations between tephra layers which appear to have identical major element chemistry and thus allows for the identification of some, if not all, tephra layers previously considered identical in composition. This paper catalogues the compositional variation between the widespread Holocene Icelandic silicic tephra deposits. 相似文献
2.
Kirti Sharma Stephen Self Stephen Blake Thorvaldur Thordarson Gudrun Larsen 《Journal of Volcanology and Geothermal Research》2008
The explosive rhyolitic eruption of Öræfajökull volcano, Iceland, in AD 1362 is described and interpreted based on the sequence of pyroclastic fall and flow deposits at 10 proximal locations around the south side of the volcano. Öræfajökull is an ice-clad stratovolcano in south central Iceland which has an ice-filled caldera (4–5 km diameter) of uncertain origin. The main phase of the eruption took place over a few days in June and proceeded in three main phases that produced widely dispersed fallout deposits and a pyroclastic flow deposit. An initial phase of phreatomagmatic eruptive activity produced a volumetrically minor, coarse ash fall deposit (unit A) with a bi-lobate dispersal. This was followed by a second phreatomagmatic, possibly phreatoplinian, phase that deposited more fine ash beds (unit B), dispersed to the SSE. Phases A and B were followed by an intense, climactic Plinian phase that lasted ∼ 8–12 h and produced unit C, a coarse-lapilli, pumice-clast-dominated fall deposit in the proximal region. At the end of Plinian activity, pyroclastic flows formed a poorly-sorted deposit, unit D, presently of very limited thickness and exposed distribution. Much of Eastern Iceland is covered with a very fine distal ash layer, dispersed to the NE. This was probably deposited from an umbrella cloud and is the distal representation of the Plinian fallout. A total bulk fall deposit volume of ∼ 2.3 km3 is calculated (∼ 1.2 km3 DRE). Pyroclastic flow deposit volumes have been crudely estimated to be < 0.1 km3. Maximum clast size data interpreted by 1-D models suggests an eruption column ∼ 30 km high and mass discharge rates of ∼ 108 kg s− 1. Ash fall may have taken place from heights around 15 km, above the local tropopause (∼ 10 km), with coarser clasts dispersed below that under a different wind regime. Analyses of glass inclusions and matrix glasses suggest that the syn-eruptive SO2 release was only ∼ 1 Mt. This result is supported by published Greenland ice-core acidity peak data that also suggest very minor sulphate deposition and thus SO2 release. The small sulphur release reflects the low sulphur solubility in the 1362 rhyolitic melt. The low tropopause over Iceland and the 30-km-high eruption column certainly led to stratospheric injection of gas and ash but little sulphate aerosol was generated. Moreover, pre-eruptive and degassed halogen concentrations (Cl, F) indicate that these volatiles were not efficiently released during the eruption. Besides the local pyroclastic flow (and related lahar) hazard, the impact of the Öræfajökull 1362 eruption was perhaps restricted to widespread ash fall across Eastern Iceland and parts of northern Europe. 相似文献
3.
Katla volcano,Iceland: magma composition,dynamics and eruption frequency as recorded by Holocene tephra layers 总被引：1，自引：0，他引：1
Bergrún Arna Óladóttir Olgeir Sigmarsson Gudrun Larsen Thor Thordarson 《Bulletin of Volcanology》2008,70(4):475-493
The Katla volcano in Iceland is characterized by subglacial explosive eruptions of Fe–Ti basalt composition. Although the
nature and products of historical Katla eruptions (i.e. over the last 1,100 years) at the volcano is well-documented, the
long term evolution of Katla’s volcanic activity and magma production is less well known. A study of the tephra stratigraphy
from a composite soil section to the east of the volcano has been undertaken with emphasis on the prehistoric deposits. The
section records ∼8,400 years of explosive activity at Katla volcano and includes 208 tephra layers of which 126 samples were
analysed for major-element composition. The age of individual Katla layers was calculated using soil accumulation rates (SAR)
derived from soil thicknesses between 14C-dated marker tephra layers. Temporal variations in major-element compositions of the basaltic tephra divide the ∼8,400-year
record into eight intervals with durations of 510–1,750 years. Concentrations of incompatible elements (e.g. K2O) in individual intervals reveal changes that are characterized as constant, irregular, and increasing. These variations
in incompatible elements correlate with changes in other major-element concentrations and suggest that the magmatic evolution
of the basalts beneath Katla is primarily controlled by fractional crystallisation. In addition, binary mixing between a basaltic
component and a silicic melt is inferred for several tephra layers of intermediate composition. Small to moderate eruptions
of silicic tephra (SILK) occur throughout the Holocene. However, these events do not appear to exhibit strong influence on
the magmatic evolution of the basalts. Nevertheless, peaks in the frequency of basaltic and silicic eruptions are contemporaneous.
The observed pattern of change in tephra composition within individual time intervals suggests different conditions in the
plumbing system beneath Katla volcano. At present, the cause of change of the magma plumbing system is not clear, but might
be related to eruptions of eight known Holocene lavas around the volcano. Two cycles are observed throughout the Holocene,
each involving three stages of plumbing system evolution. A cycle begins with an interval characterized by simple plumbing
system, as indicated by uniform major element compositions. This is followed by an interval of sill and dyke system, as depicted
by irregular temporal variations in major element compositions. This stage eventually leads to a formation of a magma chamber,
represented by an interval with increasing concentrations of incompatible elements with time. The eruption frequency within
the cycle increases from the stage of a simple plumbing system to the sill and dyke complex stage and then drops again during
magma chamber stage. In accordance with this model, Katla volcano is at present in the first interval (i.e. simple plumbing
system) of the third cycle because the activity in historical time has been characterized by uniform magma composition and
relatively low eruption frequency. 相似文献
4.
Darren J. Larsen Gifford H. Miller Áslaug Geirsdóttir Thorvaldur Thordarson 《Quaternary Science Reviews》2011,30(19-20):2715-2731
A suite of environmental proxies in annually laminated sediments from Hvítárvatn, a proglacial lake in the central highlands of Iceland, are used to reconstruct regional climate variability and glacial activity for the past 3000 years. Sedimentological analysis is supported by tephrostratigraphy to confirm the continuous, annual nature of the laminae, and a master varve chronology places proxies from multiple lake cores in a secure geochronology. Varve thickness is controlled by the rate of glacial erosion and efficiency of subglacial discharge from the adjacent Langjökull ice cap. The continuous presence of glacially derived clastic varves in the sediment fill confirms that the ice cap has occupied the lake catchment for the duration of the record. Varve thickness, varve thickness variance, ice-rafted debris, total organic carbon (mass flux and bulk concentration), and C:N of sedimentary organic matter, reveal a dynamic late Holocene climate with abrupt and large-scale changes in ice-cap size and landscape stability. A first-order trend toward cooler summers and ice-cap expansion is punctuated by notable periods of rapid ice cap growth and/or landscape instability at ca 1000 BC, 600 BC, 550 AD and 1250 AD. The largest perturbation began ca 1250 AD, signaling the onset of the Little Ice Age and the termination of three centuries of relative warmth during Medieval times. Consistent deposition of ice-rafted debris in Hvítárvatn is restricted to the last 250 years, demonstrating that Langjökull only advanced into Hvítárvatn during the coldest centuries of the Little Ice Age, beginning in the mid eighteenth century. This advance represents the glacial maximum for at least the last 3 ka, and likely since regional deglaciation 10 ka. The multi-centennial response of biological proxies to the Hekla 3 tephra deposition illustrates the significant impact of large explosive eruptions on local environments, and catchment sensitivity to perturbations. 相似文献
5.
6.
T. C. Jude-Eton T. Thordarson M. T. Gudmundsson B. Oddsson 《Bulletin of Volcanology》2012,74(5):1057-1082
The basaltic, phreatomagmatic eruption of Grímsv?tn volcano, Iceland, in November 2004 (G2004) lasted for 5?days, during which time two separate vents were active. Significant deposition of tephra occurred in the first 45?h only. We have subdivided the deposit into seven units (A–G) on the basis of differences in texture, grain size and componentry, and the presence of sharp contacts between the layers. The distribution of tephra lobes was used to infer the vent of origin for each unit. The G2004 deposit is poorly sorted overall and consists of non-vesicular to highly vesicular juvenile components. Units A and B comprise almost exclusively non- to poorly vesicular glass fragments, whereas units C–G contain at least 30?vol.% highly vesicular pumice. The proportion of non-juvenile fragments increases significantly in the final unit (unit F) of the main phase; non-juvenile fragments are restricted to the coarse (>0 Φ) fraction of the deposit. Main phase units C and E account for 80% of the total deposit volume, including the entire distal portion, and are interpreted to represent a mixture of (1) a widely dispersed component that fell from the upper margins of a strongly inclined (～45°), 6–10?km high plume and (2) a locally dispersed (<3?km from source) component originating from pyroclastic density currents and minor tephra jets. 相似文献
7.
8.
Christopher W. Hamilton Thorvaldur Thordarson Sarah A. Fagents 《Bulletin of Volcanology》2010,72(4):449-467
To determine the relationships between rootless cone emplacement mechanisms, morphology, and spatial distribution, we mapped
the Hnúta and Hrossatungur groups of the 1783–1784 Laki lava flow in Iceland. We based our facies maps on Differential Global
Positioning System (DGPS) measurements, photogeological interpretations, and supporting field observations. The study area
covers 2.77 km2 and includes 2216 explosion sites. To establish the timing of rootless cone formation we incorporated tephrochronological
constraints from eighty-eight stratigraphic sections and determined that the Hnúta and Hrossatungur groups are composite structures
formed by the emplacement of six geographically and chronologically discrete domains. Rootless eruptions initiated in domain
1 on the first day of the Laki eruption (June 8, 1783) and lasted 1–2 days. The second episode of rootless activity began
in domain 2 on June 11 and lasted 1–3 days. The four domains of the Hrossatungur group dominantly formed after June 14 and
exhibit a complex emplacement sequence that reflects interactions between the Laki lava, contemporaneously emplaced rootless
cones, and an existing topographic ridge. In the study area, we identify three distinct rootless cone archetypes (i.e., recurring
morphological forms) that are related to tube-, channel-, and broad sheet lobe-fed eruptions. We assert that emplacement of
lava above compressible substrates (e.g., unconsolidated sediments) may trigger rootless eruptions by causing subsidence-induced flexure and failure of the basal crust,
thereby allowing molten lava (fuel) to come into direct contact with groundwater (coolant) and initiating analogs to explosive
molten fuel–coolant interactions (MFCIs). 相似文献
9.
Siwan M. Davies Gudrun Larsen Stefan Wastegård Chris S. M. Turney Valerie A. Hall Lisa Coyle Thor Thordarson 《第四纪科学杂志》2010,25(5):605-611
The Eyjafjöll AD 2010 eruption is an extraordinary event in that it led to widespread and unprecedented disruption to air travel over Europe – a region generally considered to be free from the hazards associated with volcanic eruptions. Following the onset of the eruption, satellite imagery demonstrated the rapid transportation of ash by westerly winds over mainland Europe, eventually expanding to large swathes of the North Atlantic Ocean and the eastern seaboard of Canada. This small‐to‐intermediate size eruption and the dispersal pattern observed are not particularly unusual for Icelandic eruptions within a longer‐term perspective. Indeed, the Eyjafjöll eruption is a relatively modest eruption in comparison to some of the 20 most voluminous eruptions that have deposited cryptotephra in sedimentary archives in mainland Europe, such as the mid Younger Dryas Vedde Ash and the mid Holocene Hekla 4 tephra. The 2010 eruption, however, highlights the critical role that weather patterns play in the distribution of a relatively small amount of ash and also highlights the spatially complex dispersal trajectories of tephra in the atmosphere. Whether or not the preservation of the Eyjafjöll 2010 tephra in European proxy archives will correspond to the extensive distributions mapped in the atmosphere remains to be seen. The Eyjafjöll 2010 event highlights our increased vulnerability to natural hazards rather than the unparalleled explosivity of the event. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
10.
M. Nakagawa K. Wada T. Thordarson C. P. Wood J. A. Gamble 《Bulletin of Volcanology》1999,61(1-2):15-31
Ruapehu volcano erupted intermittently between September and November 1995, and June and July 1996, producing juvenile andesitic
scoria and bombs. The volcanic activity was characterized by small, sequential phreatomagmatic and strombolian eruptions.
The petrography and geochemistry of dated samples from 1995 (initial magmatic eruption of 18 September 1995, and two larger
events on 23 September and 11 October), and from 1996 (initial and larger eruptions on 17–18 June) suggest that episodes of
magma mixing occurred in separate magma pockets within the upper part of the magma plumbing system, producing juvenile andesitic
magma by mixing between relatively high (1000–1200 °C)- and low (∼1000 °C)- temperature (T) end members. Oscillatory zoning
in pyroxene phenocrysts suggests that repeated mixing events occurred prior to and during the 1995 and 1996 eruptions. Although
the 1995 and 1996 andesitic magmas are products of similar mixing processes, they display chronological variations in phenocryst
clinopyroxene, matrix glass, and whole-rock compositions. A comparison of the chemistry of magnesian clinopyroxene in the
four tephras indicates that, from 18 September through June 1996, the tephras were derived from at least two discrete high-temperature
(high-T) batches of magma. Crystals of magnesian clinopyroxene in the 23 September and 11 October tephras appear to be derived
from different high-T magma batches. Whole-rock and matrix-glass compositions of all tephras are consistent with their derivation
from distinct mixed melts. We propose that, prior to 1995 there was a shallow low-temperature (low-T) magma storage system
comprising crystal-rich mush and remnant magma from preceding eruptive episodes. Crystal clots and gabbroic inclusions in
the tephras attest to the existence of relict crystal mush. At least two discrete high-T magmas were then repeatedly injected
into the mush zone, forming discrete and mixed magma pockets within the shallow system. The intermittent 1995 and 1996 eruptions
sequentially tapped these magma pockets.
Received: 1 April 1998 / Accepted: 22 December 1998 相似文献