BASALTIC VOLCANISM, VOLCANOES AND VOLCANIC ROCKS OF MARINE AND LACUSTRINE ENVIRONMENTS JOHN GILBERT JONES A thesis submitted to the University of London for the Ph.D. degree Department of Geology Imperial College March, 1968 2 ABSTRACT Intraglacial basaltic volcanoes near Laugarvatn, south- west Iceland, range from simple pillow lava piles to tuyas - volcanoes of tablemountain form in which a superstructure combining a carapace of sheet laTa on a pedestal of breccia envelops a tuff-mantled pillow lava pile. The pillow lava piles are inferred to be the product of effusion of lava into glacial meltwater, and the tuff tappings which most exhibit record a later phase of explosive emergence from the meltwater pond. The sheet lava and flow-foot breccia of the tuyas are believed to record an ultimate phase of lava effusion in air, following earlier phases of aqueous effusion and explosive emergence, in which flows advanced outwards into encircling meltwater on deltas of breccia formed at their fronts. The little that is known of the morphology and constitution of marine basaltic volcanoes suggests that they are basically similar to the Laugarvatn volcanoes and that concepts derived from the study of the latter have general application. Lxamination of the structural characteristics of Icelandic and Welsh pillow lavas suggests a mode of form- ation and propagation akin to the digital advance of pahoehoe: and a vesicularity study indicates the potential importance of pillow lavas as depth indicators. Review of the literature suggests that explosive activity, induced by extraneous water, is characteristic of eruption of basalt lava from a wet vent in shallow water and in air. Observations of the immersion of basalt lava are reviewed, and the significance of structural records of immersion is indicated. A re-examination of the type 'peperites' suggests that they are the product, not cf brecciation of lava intruded into wet sediment as is currently accepted, but of explosive projection of basaltic ejecta into steadily accumulating lime mud. 3 CONTENTS Page Chapter 1. Introduction 5 Chapter 2. Icelandic intraglacial volcanoes Section 1. The Laugarvatn volcanoes - their 10 shape, gross structure, and pattern of development. Section 2. The Laugarvatn volcanoes - 35 detailed constitution and development. Addendum. Composition of the Laugarvatn 66 lavas. Chapter 3. Pillow lava. Section 1. Pillow lava and pahoehoe. 69 Section 2. Pillow lavas as depth 80 indicators. Addendum. Composition of pillow lava. 94 Chapter 4. Explosive basaltic volcanism in aqueous environments. Section 1. Basaltic eruptions from wet vents 97 in shallow water and in air. Section 2. A lacustrine volcano of central 109 France and the nature of peperites. Chapter 5. Immersion of basalt lava. Section 1. Observed immersion of basalt lava. 126 Section 2. Immersion of basalt lava - its 133. structural expression and stratigraphic significance. Chapter 6. Marine basaltic volcanoes. Section 1. Structural model of a marine 140 basaltic volcano. Section 2. Marine basaltic volcanoes 'high 141 and dry'. Section 3. Basaltic volcanoes of the present 147 ocean basins. Chapter 7. Summary and conclusions. 155 4 Acknowledgements. 161 References cited. 162 Appendix. Clastic rocks of Espiritu Santo Island, New Hebrides. Also bound in thesis; "Intraglacial volcanoes of south- west Iceland and their significance in the interpretation of the form of the marine basaltic volcanoes"; 'Nature', 212, 5062, p.586-588. 5 CHAPTER 1 INTRODUCTION 6 Basalt is the most voluminous of the Earth's volcanic products and is particularly widespread in the ocean basins. Yet the subject of basaltic volcanism in aqueous environments — its processes and products — has long remained an area of almost total ignorance. The reasons for this are clear enough, for the processes of basaltic volcanism in aqueous environments are largely inaccessible to observation as, until recently, have been most of the younger products. The incipient influx of data on the morphology, constituents and geophysical characteristics of totally and largely submerged oceanic volcanoes is creating an urgent need for a sound basis for interpretation. And an understanding of the processes and products of basaltic volcanism in water, based on studies of present or at least Cenozoic aqueous environments, is likewise essential for any substantial progress in the study of older basaltic rocks of aqueous origin in the stratigraphic column. This thesis attempts to present a coherent and comprehensive outline of the principal processes and products of basaltic volcanism in aqueous environments. It incorporates in comparable amount data from my own studies, principally of Icelandic intraglacial volcanoes, and data from the literature. With the objective of the thesis in mind, my fieldwork and literature survey have been directed principally to the elucidation of what I have judged to be the more important aspects of the topic. I have thus made no attempt to provide a complete geological account of the areas of investigation of the kind that might be expected in a local or regional study. Nor has it been my intention to provide a compendium of all published data, interpretation and 7 opinion related in any way to the topic. Concerning the products of basaltic volcanism in aqueous environments, this thesis confines itself almost entirely to description and discussion of macroscopic forms, structures and textures. This does not imply any denial of the relevance and importance of the microscopic, chemical and other aspects of the subject. However, I judged a comprehensive, integrated framework of basic macroscopic observation to be of more value to the subject in its present state than a more intensive examination of any limited aspect. A large number of thin sections has been examined in the course of the work and care has been taken to see that interpretations based on macroscopic observations do not conflict with microscopic characteristics. The fieldwork on which this thesis is based includes six months in Iceland, principally in the Laugarvatn area; three weeks in Auvergne, central France; and three weeks in the Strumble Head area of Pembrokeshire, Wales. In addition pillow lava localities on the Italian mainland and in Elba and Sicily were briefly visited. The field— work in France proved much less productive than I had anticipated, exposure of the peperites being very poor and their nature quite contrary to what current literature had let me to expect. With hindsight the time would have been devoted to a field of study more productive within the context of the thesis. The thesis has been written as a series of short papers, several of which are published or in press. Chapter 2, Section 1 and Chapter 3, Section 1 contain the substance of papers to be published in the Quarterly Journal of the Geological Society of London and in the Journal of Geology, respectively. And a considerable part 8 of Chapter 6, Section 1 has been published in 'Nature' under the title "Intraglacial volcanoes of southwest Iceland and their significance in the interpretation of the form of the marine basaltic volcanoes" (bound in the thesis). I have included in the thesis only such material as has been accepted for publication or that I consider to be in a condition suitable for publication. Appended is a paper entitled 'Clastic rocks of Espiritu Santo Island, New Hebrides", published in the Bulletin of the Geological Society of America. It is an outcome of fieldwork in the New Hebrides undertaken prior to my registration as a candidate for the Ph.D. degree in the University of London. 9 CHAPTER 2 ICELANDIC INTRAGLACIAL VOLCANOES 10 SECTION 1 THE LAUGARVATN VOLCANOES - THEIR SHAPE, GROSS STRUCTURE, AND PATTERN OF DEVELOPMENT 1. Introduction The intention of this section is to describe and interpret the shape, gross structure and interrelationship of a group of basaltic volcanoes in south-west Iceland. These volcanoes occur in the Ticinity of the village of Laugarvatn, about 30km E.N.E. of Reykjavik. They stand in a cluster immediately north of the road which runs from Thingvellir, through Laugarvatn to Geysir, and about 20km south-west of the Langjokull icecap. They are part of a Quaternary volcanic belt with a NE-S7; trend which runs from the Langjokull in the north-east to Cape Reykjanes in the south-west (Map 1, inset). All are part of the "Moberg Formation" of Icelandic geologists (Kjartansson, 1959). According to Kjartansson, the rocks of the Moberg Formation show uniform magnetic polarity of the present cycle and are generally considered to be of late Pleistocene age. The constituent rock types are briefly described and discussed at the beginning of the section to facilitate description of the volcanoes themselves. More detailed lithologic descriptions are given in Section 2. 2. Rock types and their significance All the rocks of which the volcanoes of the Laugarvatn region are built consist of a basalt which, when fully crystalline, contains essential olivine, clinopyroxene, plagioclase and opaques. 11 2.1. Lavas Sheet lava and pillow lava are important components of the Laugarvatn volcanoes. The term sheet lava denotes extruded lava which in section is seen to consist of tabular units with lateral extent many times their thickness. It includes both fragmental (aa) and non- fragmental (pahoehoe) varieties. The term pillow lava is restricted to lavas consisting of abundant units which in section display numerous spherical and elliptical outlines and which contain little or no fragmental material. The contrasting forms of sheet and pillow lava are believed to be a consequence of extrusion in air and water respectively Pillow lavas are generally accepted as a criterion of the aqueous environment and such an interpretation of pillow structure in the Laugarvatn area is in full accord with other facts and inferences. 2.2. Fragmental rocks In the Laugarvatn volcanoes fragmental basaltic rocks - palagonitic glassy breccias and vitric tuffs - are as abundant as the basalt lavas. The breccias are predominantly unsorted, glass-rich fragmental rocks with clasts ranging from coarse fragments of glass-encrusted lava, often of highly irregular outline, to glass fragments of sand grade.