Durham E-Theses The volcanic geology of Vestur Skaftarfellssysla Iceland Robson, G. R. How to cite: Robson, G. R. (1956) The volcanic geology of Vestur Skaftarfellssysla Iceland, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/9238/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk THESIS presented in candidature for the degree of DOCTOR OP PHILOSOPHY of the University of Durham. The Volcanic Geology of Vestiir - Skaftarfellssysla Iceland. G.R. RolDSon. Being an account of the work carried out at the Geology Department, Durham University (Durham Division) During the period 1949 - 52 under the direc• tion of Professor L. R. Wager, F.R.S. (1949 - 50) and Professor K.C. Dunham, F.R.S. (1950 - 52). • SGIENCf CONTENTS. Page No. 1. PREFACE 1 2. INTRODUCTION 5 (a) The Geology of Iceland 5 ("b) The Region Studied 6 (c) The Geography of the Region 6 PART 1. THE VOLCANO ELDGJA 3. HISTORICAL 12 A' EARLIER WORK 16 5. GENERAL FEATURES OF THE BLDGJA ERUPTIVE PRODUCTS 20 AND CRATERS (a) The Ejecta 20 ("b) The Lavas 25 (c) The craters ^ 36 ,(d) The Origin of Craters in Basaltic Volcanoes 38 (e) Crater Evolution in Eldgja 41 6.,DETAILED DESCRIPTION OF THE ELDGJA I1RATER3 44 (a) The Western Fissure 44 (b) The Activity and Evolution of the Western Fissure 51 (c) The Svartahnuksfjoll Fault Grahen 56 (d) The Central Fissure 57 (e) The Activity and Evolution of the Central Fissure 71 (f) The Eastern Fissure 79 (g) The Activity and Evolution of the Eastern Fissure 89 7. DETAILED DESCRIPTION OP THE ELDGJA LAVA FIELDS 97 (a) The Eastern Lavas 97 ("b.) The Western Lavas 106 8. THE PBTROLOOY OP THE ELDQJA ROCKS 118 (a) The Pyroclastlcs 118 (ID) The Lavas 119 (c) Discussion 126 9. THE RELATIONSHIP OP KATLA TO ELDGJA 131 10. MAGMA TYPES IN ICELAND 133 (a) The Eldgja Magma Type 133 (b) The Grirasvotn Magma Type 133 (c) The Skjaldbreith Magma Type 134 (d) The Hekla Magma Type 136 (e) Relationships "between the Magma Types 138 (f) Comparisons with other Volcanic Provinces 153 TABLES I - XXII 161 - 182 PART 11 THE GEOLOGY OP SKAPTARTUNGA 11. SUMIilARY OP PSEVlOyS WORK ON ICELANDIC GEOLOGY 183 12. THE SKAPTARTUNGA ROCKS 189 13. THE PALAGONITE FORMATION 190 (a) The Lower Group 200 ("b) The Middle Group 204 (c) The Upper Group 206 (d) Petrology 207 (e) Petrology and Correlation in the Palagonite 219 Formation 14. THE DOLSRITE FORMATION 220 lo, (a) Petrography 221 15. THE MYRDALSJOKULL RHYOLITES 223 (a) Petrography 225 16. THE SAITOFBLL TRACHYBASALTS 228 (a) Petrography 230 17. MODERN BASALTIC BXTRUSIVE3 MD PYROCLASTICS 233 (a) Petrography 243 18. THE ORIGIN AND AGE OF THE SKAFTARTUNGA ROCKS- 245 TABLES mil - XXVI 254 - 257 BIBLIOGRAPHY 26« & 265 PREFACE. The writer was introduced to Icelandic Geology as a member of the Durham University Iceland Expedition of 1949. The expedition consisted of eight members, and was committed to an extensive programme of topographical surveying and meteorological and glaciological work. The object to "be studied was the Kotlujokull glacier; the largest of the outflow glaciers of Myrdalsjokull, The expeditions programme of work was severely hampered "by very poor weather and the major part of the programme was abandoned as the summer wore on. The Jlxpeditions two graduate geologists were thus allowed more time to study the geology of the area than had at first seemed probable, and it was possible for them to move about the area with relative freedom, albeit in poor weather, by making use of the camps and duii^s of food established by the expedition on and around the ice cap. During this expedition the Katla lavas, the Sandfell trachybasalts and the Myrdalsjokiill rhyolites were discovered and partially mapped. The area appeared to offer scope for geological research and so, with the help of Professor Wager and the University Exploration Society, and expedition was organized in the summer of 1950 and the writer returned to Iceland in the company of Mr. P.B. Robinson and Mr. E.A. Timothy of the Durham Colleges Department of Geology. It was decided to extend the 1957 EOTI«« work northeastwards from the area studied in 1949, into the country on the eastern horder of Myrdalsjokull and into Skaftartunga. A base camp was established on the eastern bank of the Holmsa river near the mountain Einhymingar, In spite of determined efforts it was found to be impossible to ford this river and gain access to thfi country to the weat except at Maelifellsandur in the extreme north of the area. The base camp was moved to Maelifellsandur from where the whole of the area to be studied was accessible. Because of the remote situation of the base camp it was now necessary to establish subsidiaiy camps from time to time in the southern part of the area and this involved the movement on foot, by the three members of the expedition,of a considerable part of the expeditions equipment, food and specimens, of which there was about half a ton. In spite of these difficulties the lavas of Holmsadalur and the dastem part of the Eldgja fissure were mapped, and the Myrdalsjokull rhyolltes on Kotlukollar were visited for the first time. During this expedition the unique features shown by the craters of the Bldgja volcano had engaged the writer's attention, which came to be directed more and more towards the modem volcanics of the area. In order completely to describe the Eldgja fissure and its lavas a second expedition was organized in 1951 with the help now of professor Dunham and the University Exploration - 2 - Society. The same area was visited with Mr, B.W. Pace. In order that the extensive lava flows of Eldgja could be mapped, horses were bought, and, travelling on horse-back - from two bases, one in the north of the area at Alftavotnskrokur and one in the south at Hrifimess, the whole of the Eldgja fissure and its extensive lava flows on Myrdalssandur and Methallandssandiir were mapped. The writer is indebted to many individuals and organisations for their help during the course of this work. In Iceland, to Mr. Jon Bythorsson and Mr. Agust Bothvarsson of Reykjavik^, and to Mr, Ardni Johsson of Hrifuness, and to the Icelandic National Research Council for permission to undertake the work. In England, to Professors Dunham and Wager for their help and encouragement throughout the work; to Dr. E.A. Vincent for instruction in the methods of chemical analyses, to Mr. R. Phillips for kindly undertaking the partial analysis of an Icelandic rock, and to Dr. P.H. Stewart and the laboratory staff of the Durham Colleges Department of Geology for help in the final stages of the work and during the preparation of the manuscript. The work was made possible through the kind help of the Durham University Ex:gbrati6n Society and by various grants of money from Uhe Royal Society, The Shell Petrole\am Company, The Durham Colleges Research Fund and the Durham Colleges - 3 - Exploration Society, to which bodies gratitude is expressed for their assistance. « 4 - INTRODUCTION. The Geology of Iceland. Iceland is formed entirely of volcanic rocks of which the oldest are the Tertiary plateau basalts of the East and West coasts. In the centre of the island basalt lavas, tuffs and breccias, termed collectively the Palagonit6 Formation, overlie the Tertiary basalts and fill a graben sunk in the basalt plateau, which is between 100 and 200 km. in v/idth, and trends north and south. The Palagonite Formation rocks are themselves overlaiid. in the centre of the island by Post-Glacial basaltic extrusives and pyro- elastics. There is some controversy over the age of the two older groups of rocks and of the graben faulting which intervened. Hawkes (19S8) gives the following provisional datefe. 1. First Volcanic Period. Outpouring of the Thuiean plateau basalts. Eocene (Oligocene?) 2. Tectonic disturbance - uplift along a Barthdrdal fault, Miocene (Oligocene?). 3. Deposition of marine sediments at Tjornes within the central graben. Pliocene. - 5 - 4. Second volcanic period - Accumulation of Palagonite Formation and later of Post-Glacial extrusives. Pliocene - present Day. 5. Glaciation. Quaternary - Present Day. The Region Studied. Vestur ~ Skaftarfellssysla is that part of Iceland between the two ice caps Myrdalsjokull and Vatnajokull. It is bordered to the south by the north Atlantic and to the north by the inland desert. It lies within the Central Graben and thus the oldest rocks exposed are those of the Palagonite Formation. Vestur Skaftarfellssysla is roughly bisected by the river Skafta which flows southward from the northern desert to the Atlantic. 'L': It ^.I'a.in the western part, that lying between the Skafta and Myrdalsjokull, with which the following account is mainly concerned. The Geography of the Region. The region is divisible into three physiographic provinces :- The Coastal Lov/lands. - have been formed by the coalescing - 6 - of river deltas.