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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:647351 |
| Date | January 2005 |
| Creators | Flude, Stephanie |
| Contributors | Burgess, Ray |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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