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No description available.

The mountainous Reydarfjordur area of Eastern Iceland is composed of at least 25,000 ft, of gently dipping Tertiary lavas. A part of this westerly dipping succession, averaging 2,000 ft, in thickness, and containing a considerable portion of acid. rocks, has been studied. in detail and mapped for 25 miles North/South along the strike, It has been shown that volcanicity during the eruption of this 2,000 ft. unit The Reydarfjordur Acid Volcanic Succession — was rhythmic and six separate phases of volcanicity have been recognised. Each phase reaches its maximum development in or near the Reydarfjordur Acid Centre where there were usually eruptions of acid lavas often preceded by large scale pyroclastic eruptions initiating each phase. Contemporaneously, basic and intermediate lavas were extruded from north—south fissures to form the Flank Succession. In the Reydarfjordur/Faskrudsfjordur area acid—intermediate, rhyolitic andesite lavas form a considerable proportion of the succession, but in the peripheral regions tholeiite and, basaltic andesite lavas predominate, and in regions distant from the centre the lavas erupted at this time were normal flood basalts. The significance of this is discussed and comparisons are drawn between the Reydarfjordur area and. other areas of acid volcanicity in Eastern Iceland. A detailed description is given of four rhyolite basalt composite lavas (the previously undescribed extrusive products of composite dykes) while the Appendix contains petrographic and some chemical data on the commonly occurring lava types and some notes on flow structure in acid lavas.

Subglacial rhyolite eruptions at Torfajokull, Iceland have produced a variety of volcanic edifices during the last glacial period (115-11 ka). These range from small-volume (< 0.1 km3) volcanoes, such as Bhlhnukur and Dalakvislfell, to larger volume (-1 km3) flat-topped tuyas such as South-east Rauoufossafjoll. Lithofacies associations at each volcano record distinct phases of volcano-ice interaction beneath temperate glaciers at least 350 m thick. All eruptions began with the construction of a pile of glassy fragmental material within a subglacial cavity. Fragmentation at Bhlhnukur was primarily caused by quenching, when rising magma encountered meltwater. Fragmentation at Southeast Rauoufossafjoll was apparently more energetic, and generated phreatomagmatic ash over 300 m thick. Dalakvisl is intermediate between the other two localities. Most fragmental deposits are massive, suggesting that a sustained meltwater lake did not develop during eruptions, in contrast with evidence from many basaltic volcanoes. Instead, meltwater drained away in a number of discrete channels, some of which have been identified. The eruption at Blahnukur apparently terminated before the glacier surface had been pierced, whereas the eruption at South-east Rau6ufossafjoll produced a cap of flat-lying subaerial lava flows about 1.5 km in length. Numerical models are presented, in which simple patterns of ice melting and deformation are used to simulate the evolving size of subglacial cavities during eruptions. The radius of the cavity is compared to the radius of the growing subglacial volcano. The models predict that, at low magma discharge rates and beneath thick ice, cavities will become completely filled with volcanic debris and the eruption will be dominantly intrusive, forming the types of lithologies observed at Blahnukur. Cavities never become filled at higher magma discharge rates, and an explosive phreatomagmatic eruption is predicted, which would form the types of lithologies observed at South-east Rauoufossafjol1.

Iceland, being the subaerial expression of the Mid Atlantic Ridge, is an ideal place to study the tectonic, magmatic and volcanic processes which have contributed to the evolution of the Earth. Such studies greatly benefit from knowledge of the timing of events and timescales of geological processes. Unfortunately there are few absolute ages (e.g. K-Ar, Ar-Ar ages) published for Iceland, because of the relatively young and K-poor nature of the volcanic rocks. Rhyolites make up ~10% of the outcropping volcanic rocks in Iceland (Walker, 1966) and can contain up to -4% K2O, making them potentially useful for Ar-Ar dating studies. Icelandic volcanics can contain a large proportion of Ar with the same isotopic composition as air, suggesting the presence of some kind of atmospheric contaminant. This causes problems for Ar-Ar studies and further work is needed to investigate the source of this contamination. Two volcanoes have been studied: Kerlingarfjoll, a rift zone volcano from central Iceland, and Ljosufjoll, a flank zone volcano from the Snaefellsnes Peninsula. Kerlingarfjoll has erupted at least twenty-one times since 336 ka, with a general eruptive periodicity of ~40 kyrs, coinciding with times of maximum obliquity of the Earth's tilt. At least two different groups of rhyolite have been erupted, which are separated in time and space. The geochemical differences within and between these two groups may be attributed to chemical diffusion processes within the magma. The spatial distribution of eruptions has decreased over time, along with the volcanic production rate, leading to the conclusion that Kerlingarfoll silicic magma chamber is either in decline or extinct. Ljosufjoll has been producing alkaline silicic magma since at least 1.6 Ma, but erosion and burial by later deposits means that few eruptions older than ~200 ka are exposed. As well as rhyolites, Ljosufjoll has erupted a comparable amount of trachytes, most notably a 4.5km long subaerial lava flow which is the most recent silicic eruption at the volcano (erupted <100 ka). The rhyolite compositions are consistent with geochemical models of extreme levels of fractional crystallisation of trachyte parent magma. This, combined with a large proportion of inherited 40Ar in some feldspars has led to the conclusion that silicic volcanism at Ljosufjoll is due to the presence of a trachytic mush chamber, of temperature 600-650°C and at a depth of 6.5-7 km. The rhyolites form by expulsion of the interstitial melt, probably in response to crustal loading during glaciation, while the trachytes are formed by re-melting of the crystalline mush. Subglacial eruptions have occurred at both Kerlingarfjoll and Ljosufjoll during interglacial periods. This has implications for climate studies as it places time constraints on environmental parameters, such as the presence or absence of ice and ice thickness.

The November 2004 explosive eruption of Grímsvötn volcano, Iceland (G2004) commenced as a subglacial event within the interior of the Vatnajökull ice cap before breaking through the ice cover to generate a 6-10 km high eruption column. This produced a tephra apron extending >50 km across the ice surface northwards from the eruption site, incorporating 0.047 km3 (DRE) of plagioclase-bearing, sparsely porphyritic, basaltic tephra. This study focuses on quantifying the key eruption parameters and evaluating the stratigraphy, grain size and geochemical characteristics of the proximal facies of the subaerial deposit with a level of detail and precision that has never previously been possible for a modern glacio-volcanic event. The G2004 deposit consists of a finely layered sequence which is subdivided 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. It is poorly sorted and finegrained with a median clast diameter of 1.5 Φ. The grain size characteristics and clast morphologies are indicative of intense phreatomagmatic fragmentation, despite a significant component of highly vesicular juvenile glass. A wide range in thicknesses and geometries of depositional units reflects variations in intensity and style of activity. 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 (i) a widely dispersed component that fell from the upper margins of a strongly inclined (~45°) 6-10 km high plume and (ii) a locally dispersed (<3 km from source) component originating from (1) pyroclastic density currents generated by shallow explosions and tephra jets and (2) sedimentation from the jet region and lower convective column margins. The other units are only locally dispersed around the vent. A significant proportion of fine material was deposited in the near-vent region due to particle aggregation processes. The bulk of the G2004 deposit is therefore identified as the product of continuous incremental deposition during the passage of a single quasi-steady current supplied by a sustained explosive phreatomagmatic eruption, with a variable contribution of material from concurrent tephra fallout. Major oxide, trace element and volatile composition of the G2004 tephra were analysed and compared with that of the G1998 and Laki events. Results indicate that the G2004 magma originated within a shallow-level, compositionally stratified chamber and was discharged through an independent plumbing system. The parent magmas for each of these three Grímsvötn events were produced by different degrees of partial melting within a similar lower-crust or mantle source, but are not related by fractional crystallization or magma mixing. Despite episodic intense vesiculation, the G2004 magma was fragmented at very shallow levels by almost exclusively phreatomagmatic mechanisms – the effect of which was to arrest the degassing process such that only 75% of the potential magmatic sulphur budget escaped to the atmosphere.