Iceland: Tectonics, Volcanics, and Glacial Features: 247 [1. ed.] 1119427096, 9781119427094

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Geophysical Monograph 247

Iceland

Tectonics, Volcanics, and Glacial Features Tamie J. Jovanelly

This Work is a co‐publication of the American Geophysical Union and John Wiley and Sons, Inc.

This Work is a co‐publication between the American Geophysical Union and John Wiley & Sons, Inc. This edition first published 2020 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009 © 2020 the American Geophysical Union All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions

Published under the aegis of the AGU Publications Committee Brooks Hanson, Executive Vice President, Science Carol Frost, Chair, Publications Committee For details about the American Geophysical Union visit us at www.agu.org. Wiley Global Headquarters 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Limit of Liability/Disclaimer of Warranty While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials, or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Library of Congress Cataloging‐in‐Publication data is available. Hardback: 9781119427094 Cover Design: Wiley Cover Image: © Tamie Jovanelly Set in 10/12pt Times New Roman by SPi Global, Pondicherry, India Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

This book would have never been imagined if it were not for my wonderful husband, Joe(y) Cook, whose dreams and love are as far reaching as an Askja ash cloud. And believe me, that is really far.

CONTENTS Preface...................................................................................................................................................ix Introduction1 1. The Geologic Framework of Iceland������������������������������������������������������������������������������������������������������ 3

Part I  Tectonics

5

2. Overview of Tectonics in Iceland............................................................................................................ 7 3. Tectonics of the Reykjanes Peninsula and Southwestern Region........................................................... 15 4. Tectonics of the South and Southeastern Regions................................................................................. 27 5. Tectonics of the Northeastern Region.................................................................................................. 31 6. Tectonics of the Western Region.......................................................................................................... 39

Part II  Volcanics

43

7. Overview of Volcanics in Iceland......................................................................................................... 45 8. Volcanics of the Reykjanes Peninsula and Southwestern Region.......................................................... 61 9. Volcanics of the South and Southeastern Regions................................................................................ 65 10. Volcanics of the North and Northeastern Regions................................................................................ 87 11. Volcanics of the Western Region........................................................................................................ 107

Part III  Glacial Features

115

12. Overview of Glacial Features in Iceland............................................................................................. 117 13. Glacial Features of the Reykjanes Peninsula and Southwestern Region.............................................. 137 14. Glacial Features of the South and Southeastern Regions.................................................................... 145 15. Glacial Features of the Northern and Western Regions...................................................................... 159 Glossary���������������������������������������������������������������������������������������������������������������������������������������������������� 173 References������������������������������������������������������������������������������������������������������������������������������������������������ 177 Index��������������������������������������������������������������������������������������������������������������������������������������������������������� 199

vii

PREFACE glossary, and GPS coordinates for most locations for easy reference. The support I had in writing this book was truly endless. Encouragement came in the form of multiple bouquets of flowers and sugar‐free Red Bulls delivered to my office by husband (Joe Cook), positive reviewer feedback (Dr. Kent Murray and Dr. Sheila Seaman), and advice from colleagues (Dr. Ed Harvey and wife Carol Rogers, and Dr. Mary Anne Holmes). Substantial project contributions came from Nathan Mennen who prepared all the figures for the book, Emily Larrimore who wrote some of the interesting displayed boxes you will find within the text, and Amanda Tomlinson who formatted references and glossary terms. I also need to thank the many Berry College Geology “Home Team” students who traveled with me to Iceland: Mallory Paulk, Maggie Midkiff‐Maddrey, Russell Maddrey, Matthew Bentley, Emma Cook, Emily Larrimore, Carley Carder, Amanda Tomlinson, JT Keiffer, Timothy Wooley, and Andrew Elgin. Undoubtedly, they were the guinea pigs for this book and provided insight into the content it should contain. I cannot believe how much fun I had writing this book and I want to thank John Wiley & Sons, Inc. Publishers and the American Geophysical Union for providing me with this opportunity. I enjoyed the whole process: manuscript collecting, reading, learning, and the solitude of the writing process. For me, every day was a chance to study more about the country and  science that I love so dearly. With that stated, I  realize that I am standing on the shoulders of the foundational Icelandic geologists that came before me: Helgi Björnsson, Páll Einarsson, Agust Gudmundsson, Guðrún Larsen, Kristjan Sæmundsson, Oddur Sigurðsson, and Thor Thordarsen. Each of these lifelong explorers have written books and documents that I encourage the reader to look at first hand for more complete understanding. During the 15 months or so that it took to write the manuscript I often reflected back on my Advisor, Dr. Sheri Fritz, at the University of Nebraska‐Lincoln who worked tirelessly on manuscript writing out of a labor of love, or so it seemed. Her commitment to science has always inspired me to work harder.

My first venture to Iceland was in 2006 when it was still off the radar of most tourists. With the keys to a Toyota Yaris and a series of paper road maps, my older brother Jim and I circled the island in 11 days. Although we were perpetually lost, we had found a pristine landscape with amazing views, incredible geology, and no road signs. I was hooked. Over the course of the next decade I would visit nearly every summer, bringing with me fortunate undergraduate students who could keep up with the hiking, as well as my desire to explore everything about the island—including the sampling of the dreadful rotten shark cuisine. My preparation for teaching Physical Geology and Advanced Geological Field Studies courses in Iceland became the inspiration to write this book. As a geologist, I hope to not only capture the island’s natural beauty, but also to enhance it through detailed descriptions that link the relationships between ­structure, process, and time to the island’s evolution. I reviewed innumerable peer‐reviewed scientific papers in order to deliver the reader with the most up‐to‐ date research on interesting, and sometimes debated, geological theories regarding an island being split in half due to plate tectonic motion. This text is not just intended for academics, but also for novice geologists who want to understand the magnificent scenery at a deeper level. To encourage this, I provide background introductions and figures that offer information on foundational geological concepts. Additionally, the book has been intentionally organized for travelers to use, by highlighting Iceland’s most popular destinations and putting each region into a contextual perspective. More specifically, the book is organized into three main sections: tectonics (Chapters 2–6), volcanics (Chapters 7–11), and glacial features (Chapters 12–15). The book can be read from cover to cover, or it can be utilized as a travel guide by traversing the island from the capital city Reykjavik counter clockwise. For the latter use, the reader can refer to the introductions to each Part (Chapters 2, 7, 12) followed by chapters organized into four cardinal quadrants describing the southwest (Chapters 3, 8, 13), southeast (Chapters 4, 9, 14), northeast (Chapters 5 and 10), and northwest (Chapters 6, 11, 15). The book provides an index,

ix

Introduction

1 The Geologic Framework of Iceland

and perpendicular transform faults with a (to the east) parallel offset ridge (Figure 1.1) [Einarsson, 2008]. Prior to the current tectonic setting, where the Kolbeinsey and Reykjanes ridges form the spreading centers, the extinct Aegir Ridge to the east, which ran parallel to these systems [Kristjánsson, 1979; Weir et  al., 2001; Tronnes, 2002], was important in the initiation of the North Atlantic Ocean during the Eocene (about 56 to 34  million years ago) as rifting and seafloor spreading began separation of Greenland from Norway. At 24 million years ago (Late Oligocene to Early Miocene), as the overall size and temperature of the mantle plume continued to dissipate, the Reykjanes–Kolbeinsey plate boundary was centered over the hot spot (Figure  1.1) [Fitton et  al., 1997; Kodaira et al., 1998; Holbrook et al., 2001]. Since then, the main Reykjanes and Kolbeinsey ridges have moved 240  km to the northwest so that the plume is now located under Iceland’s largest ice cap, Vatnajökull. Consequently, this is also where the crust is thickest (~40 km; Sigmundsson, 2006]. The position of the hot spot has been established through a combination of earthquake data [Oskarsson et  al., 1985; Einarsson, 1991; Weisenberger, 2010], seismic crustal structure and tomography data [Flòvenz and Gunnarson, 1991; Foulger et al., 2006], and seismic reflection and refraction data [Holbrook et al., 2001]. The plume‐origin hypothesis suggests that volcanism was initiated by ascending mantle‐derived magma from beneath thick continental lithosphere and subsequently from beneath oceanic lithosphere as rifting continued and the ocean basin grew [Sigvaldason, 1974a].

The island of Iceland, with its northern tip just 61 km south of the Arctic Circle, has a long constructive history that started 130 million years ago during the last Pangea cycle. Spreading of new ocean floor at mid‐ocean ridges to separate continents following this breakup and the onset of a large mantle plume (radius about 300 km) beneath Greenland are thought to have led to excessive mantle upwelling [Wolfe et  al., 1997; Holbrook et al., 2001; Rickers et al., 2013]. These events coincide with dates of continental flood basalts and mid‐Cretaceous volcanism along the Arctic Mendelev Ridge, Alpha Ridge, and Ellesmere Island [Lawver and Müller, 1994; Johnston and Thorkelsen, 2000; Sigmundsson, 2006]. Uplift accompanying igneous intrusions in northwest Europe, Greenland, and Canada between 64 and 52 million years ago immediately initiated passive volcanic margins, thereby setting the stage for magmatic upwelling and island creation [Saunders et al., 2007]. During the past 60 million years, the overall northwest migration of the North American plate carrying Greenland and the southeast migration of the Eurasia Plate have determined the position of the Iceland hot spot. The process of rifting has separated the two major plates, Eurasia and North America (Figure 1.1), with the Mid‐Atlantic Ridge forming a divergent plate boundary between them. Seafloor spreading is occurring at approximately 2 cm per year, or 20 km per million years [Sella et  al., 2002; Geirsson et  al., 2006]. Two Icelandic microplates (or blocks, i.e., Hreppar in the south and Tjörnes in the north) have formed at the intersection of (to the west) the Reykjanes and Kolbeinsey ridges

Iceland: Tectonics,Volcanics, and Glacial Features, Geophysical Monograph 247, First Edition. Tamie J. Jovanelly. © 2020 American Geophysical Union. Published 2020 by John Wiley & Sons, Inc.

3

4  The Geologic Framework of Iceland

Figure 1.1  Tectonic context of Iceland. At present Iceland is divided by the Kolbeinsey Ridge in the north and the Rekjanes Ridge in the south. The yellow circles show the position of the mantle plume from 50 million years ago to present. [Modified from Fitton et al. [1997]; design credit Nathan Mennen.]

Alternative hypotheses that consider mechanisms for large magma generation include excess magmatism from melting of mantle and/or recycled ocean‐crust material [Foulger, 2006] and a rift model whereby the development of a North Atlantic spreading center is solely reliant on plate‐tectonic mechanisms and not hot‐ spot development [Ellis and Stoker, 2014].

Most of the 350,000 km2 basaltic plateau making up Iceland lies below sea level, with about 30% of the island being above sea level, up to a maximum relief of 2110 m above the ocean surface [Gudmundsson, 2000]. The submarine shelf surrounding the island ranges 50–200 km wide and gently slopes to depths of 400 m [Thordarson and Larsen, 2007].

Part I: Tectonics

2 Overview of Tectonics in Iceland

2.1. PRESENT TECTONIC SETTING

by identifying distinct sedimentary horizons containing plant remains between lava formations. Of these zones, that in the east has been the most active during the past 2–3 million years (Myr). The Eastern Volcanic Zone, and numerous other past rift‐jump structures, were mapped using subaerial lava‐flow bodies (e.g., dipping of Tertiary basalt strata) that could be identified on surface geologic maps through various aged folded basalts (Figure  2.1; Böðvarsson and Walker, 1964; Jóhannesson and Sæmundsson, 2009; Hjartarson et  al., 2017]. The South Iceland Seismic Zone, an area characterized by high earthquake activity, accommodates the offset between the East and West Volcanic Zones [Stefánsson et  al., 2006; Einarsson, 1991]. The East Volcanic Zone intersects the North Volcanic Zone at the triple junction beneath Vatnajökull, formed by the MIB (Figure 2.1). The North Volcanic Zone connects to the (KR) via the Tjörnes Fracture Zone. Here, the term “fracture zone” describes the zone that connects the parallel off‐set ridge axis to the KR segment of the MAR. Transform plate movement along the Tjörnes Fracture Zone has resulted in another major center of seismicity and deformation. The Snæfellsnes Volcanic Belt reactivated at 2 Ma and is moving to the southeast, whereas the southern part of the East Volcanic Zone is currently propagating to the southwest. The Reykjanes Volcanic Belt in southwest Iceland is the subaerial expression of the RR and connects to the West Volcanic Zone.

The present tectonic setting of Iceland is driven by the continued spreading of the Mid‐Atlantic Ridge (MAR); specifically, the Kolberinsey Ridge (KR) in the north and the Reykjanes Ridge (RR) in the south (Figure 1.1). These subaerial expressions of the MAR in Iceland are characterized by various seismically and volcanically active centers often referred to as neovolcanic zones [Einarsson, 1991]. Three major neovolcanic zones are recognized where the main processes are normal faulting and volcanic fissuring: North Volcanic Zone, West Volcanic Zone, and East Volcanic Zone (Figure 2.1). These neovolcanic zones are bounded by perpendicular transform faults that connect the RR and KR with a parallel offset ridge axis, with the Mid‐Iceland Belt (MIB) forming a triple junction beneath Vatnajökull [Sigmundsson, 2006]. The island has undergone dynamic change through a series of rift jumps that first began 24 million years ago (Ma) in northern Iceland, which initiated the first rift zone [Harðarson et  al., 1997; Hjartarson et  al., 2017]. Magmatic upwelling through rift jumping is a prominent process in the evolution of Iceland [Hjartarson et  al., 2017]. As described by Mittelstaedt et  al. [2008) rift jumps are induced by magmatic heating from an off‐axis hot spot (at present, under Vatnajökull), which results in a change in the location of the ridge axis. The magma produced by the hot spot thins the lithospheric crust thereby initiating new rifting to form a new ridge axis. In Iceland, this process is combined with east and west divergence of two continental plates, resulting in the rift axes becoming less active as they move away (e.g., relocate) from the hot spot intensity. Denk et al. [2011] recognizes unconformities that accompany rift jumps in Iceland

2.2. BACKGROUND GEOLOGY Effusive volcanism during the Tertiary from seafloor spreading in the North Atlantic region began to build up a massive basalt plateau (estimated 350,000  km2)

Iceland: Tectonics,Volcanics, and Glacial Features, Geophysical Monograph 247, First Edition. Tamie J. Jovanelly. © 2020 American Geophysical Union. Published 2020 by John Wiley & Sons, Inc.

7

8 TECTONICS

Figure 2.1  Iceland’s volcanic zones, associated plate boundaries, and general geologic age of bedrock. KR, Kolbeinsey Ridge; RR, Reykjanes Ridge; EVZ, East Volcanic Zone; WVZ, West Volcanic Zone; NVZ, North Volcanic Zone; SISZ, South Iceland Seismic Zone; MIB, Mid‐Iceland Volcanic Belt; TFZ, Tjörnes Fracture Zone; ÖVB, Öræfi Volcanic Flank; RVB, Reykjanes Volcanic Belt; SVB, Snæfellsnes Volcanic Belt. [Adapted from Sæmundsson [1979]; design credit Nathan Mennen.]

[Sæmundsson, 1979]. Lavas of similar composition have been found in northwest Britain, Faroe Islands, and Greenland, which help confirm plate movement and provide documentation of the scale of this depositional event [Roberts and Hunter, 1979]. The overall age of the island exposed above sea level is geologically young, with the oldest rocks found to the east and west (14–16  Ma; Moorbath et  al., 1968; McDougall et  al., 1984), whereas rocks in the northern region may be only 12  Ma [Sæmundsson, 1986]. Walker [1960] completed the first published lithological account of the Tertiary units on Iceland. Iceland is divided geologically into three main groups: Tertiary Basalt Formation (Upper Tertiary), Grey Basalt Formation (Upper Pliocene to Lower Pleistocene), Mòberg Formation (Upper Pleistocene), and the Upper Pleistocene and Holocene unconsolidated

or poorly lithified beds such as till, glaciofluvial deposits, marine and fluvial sediments, as well as soils (Table 2.1; Gardner, 1885; Walker, 1960; Sæmundsson, 1979]. Due to its abundance (covering about half the total area of Iceland) and its extensive exposure in the east and west, there is even an Icelandic term for the dark basalt, blágrýtismyndun [Sæmundsson, 1979]. The Tertiary Basalt Formation is composed mainly of basaltic lava flows (>83%) comprising tholeiite petrology (typical of continental plateau basalts and mid‐ocean ridges), olivine (typical of ocean basins), and porphyritic basalts (representative of intrusive and extrusive processes) (Thórarinsson et  al., 1959; Klein and Langmuir, 1987; Shorttle and Maclennan, 2011]. Although dominated by basalt, rhyolitic lavas (8%), andesitic lavas (3%), and interbasaltic beds

OVERVIEW OF TECTONICS IN ICELAND  9 Table 2.1  Geologic timescale and associated major climate events in Iceland Stage

0–2.5 ka

Late Bog Period (sub‐Atlantic)

2.5–5 ka

Late Birch Period (sub‐Boreal)

5–7.2 ka

Early Bog Period (Atlantic)

Sub‐stage

7.2–9.3 ka Early Birch Period (Boreal) 9.3–10 ka

Pre‐Boreal

Late Pleistocene

Younger Dryas Weichselian

12–20 ka

Allerød Older Dryas

20–110 ka

Major events

Ice Age glaciers melt Cooling in northern hemisphere; glaciers grow Warmer climate Icelandic ice sheets quickly retreat Eurasian ice sheet at maximum; last glacial stage

115–130 ka

Eemian

Last interglacial stage

130–300 ka

Saale

Glacial stage

300–700 ka 0.7–2.5 Ma

2.5–3.3 Ma

Start of full‐scale glaciations

Pacific Ocean fauna arrive in Iceland. Bering Strait opens Climate begins to cool

3.3–7 Ma

Warm, temperate climate 7–12 Ma

Middle Miocene

Late Miocene

11–12 ka

12–18 Ma

Early Miocene

Teriatery

Pliocene

CENOZOIC

Early Middle Pleistocene Pleistocene

Quaternary

10–11 ka

Formation

Upper Pleistocene Formation

Age

Plio‐Pleistocene Formation

Epoch

18–25 Ma

Teriatery Basalt Formation

Period

Holocene

Era

Origination of Iceland

Note. ka, thousand years ago; Ma, million years ago. Modified from Thordarson and Höskuldsson [2014]; design credit Nathan Mennen.

10 TECTONICS

c­omposed of tephra and sediment (6%) also can be found [Einarsson, 1994]. Over time, vesicles and fractures present in rock can become infilled post‐depositionally with minerals such as quartz, jasper, chalcedony, calcite, and zeolites. Large calcite crystals sometimes found are referred to as “Iceland spar” [Einarsson, 1960]. Other than interbasaltic red‐bed clays, sedimentary rocks ( 5) have been linked to strike‐slip faulting [Árnadóttir et  al., 2004] and crustal stretching [Keiding et  al., 2009]. Currently, it is common for the earthquake swarms to begin in the west, which then triggers aftershock sequences eastward along the fault [Keiding et  al., 2009] that are likely tied to bookshelf faulting (Figure 3.2) [Einarsson, 2008]. Bookshelf faulting occurs when stress is released in transform fault zones during strike‐slip earthquakes. In turn, the release in pressure causes the blocks between the fault zones to rotate. The fissure swarms (Photo 2) on the western part of the Reykjanes Peninsula, which reflect extensional and normal faulting on northeast‐striking planes [Einarsson, 1991], were formed beneath Pleistocene glaciers (i.e., hyaloclastite ridges) or are postglacial eruptive features. In contrast, the fissures on the eastern side are predominantly right‐lateral strike‐slip faults that crosscut the plate boundary in north–south patterns [Einarsson, 1991]. As described by Jakobsson et al. [1978] and Sæmundsson [1979], these eruptive fissures are spaced on average approximately 5 km apart.

A major rift jump from the Snæfellsnes Peninsula (Figure 2.1) to the Reykjanes Peninsula occurred approximately 6–7 Ma to initiative spreading there [Sæmundsson, 1979; Jóhannesson, 1980]. It is on the Reykjanes Peninsula where three systems merge: the West Volcanic Zone (WVZ), the Reykjanes Volcanic Belt (RVB), and the oblique‐trending RR, i.e. the MAR [Keiding et al., 2006]. The unique steepness of the RR as it meets the Reykjanes Peninsula makes it one of the only places in the world where a mid‐ocean ridge is visible on land. This change in dip has been attributed to faster spreading rates, as confirmed by topographic and seismic studies [Einarsson, 2001]. Although there has been a gradual decline in activity of the RVB and WVZ, since between 2 and 3 Ma when the rift started migrating east [Keiding et  al., 2008; Sæmundsson, 1974], small‐scale earthquake activity (M 5  km3 per century [Thordarson and Larsen, 2007; Thordarson and Höskuldsson, 2008]. This total includes the Earth’s largest eruption of the past 200 years, as measured by volume, at Bárðarbunga in 2014–2015. This eruption lasted six months and produced 85 km2 of lava, which equates to the size of Manhattan. The largest tholeiitic eruption since 1100  CE occurred over a 50‐day duration at Laki (1783–1784 CE) producing 370 km2 of lava. 7.2. VOLCANIC MORPHOLOGY The morphology of Iceland’s volcanoes varies widely in size and shape, and there is variation in overall life spans ranging from 0.5 to 1.5  Myr [Jakobsson et  al., 1978; Sæmundsson, 1978, 1979; Jakobsson, 1979a] (Figure 7.3). Undoubtedly, Iceland is unique in the diversity of volcanic features represented. Moreover, the complexities of extensional rift tectonics and the continued presence of a hot spot leads to interesting discussion of volcano formation and structure. Although there are currently 33 active volcanoes on the island, there are also more than 50 extinct central volcanic features representing 0.78–0.15 Ma (Figure 7.2). 7.2.1. Stratovolcanoes and Associated Features

Many of Iceland’s volcanoes contain (or once contained) a central vent underlying the summit crater, which would classify them as stratovolcanoes or composite cones. Here,

geysers. Geotourism is advantageous for Icelandic society as it can promote conservation and sustainability, educate the general public on potential risks and hazards, and can benefit local economies and encourage rural development [Ólafsdóttir and Dowling, 2014]. Volcanic eruptions obviously cause negative effects on society that can result in financial loss, environmental degradation, and even fatalities, but the unique volcanic nature of Iceland has also provided many economic, energy, and tourist opportunities. Further research and development of models to predict the negative impacts and challenges caused by future eruptions are of vital importance so that the benefits of Iceland’s geothermal activity can outweigh its costs.

the volcano’s cone‐shape structure, or edifice, is built by the symmetrical accumulation of lava or pyroclastic material around this central vent system through repeated eruptions (i.e., polygenetic eruption type), which may or may not be bimodal in magmatic composition. They commonly have a stratified appearance with alternating lava flows, airfall tephra, pyroclastic flows, volcanic lahars, and/or debris flows (Figure 7.4). The three largest stratovolcanoes, Öræfajökull (64.5400, −23.8071; 2100 m), Eyjafjallajökull (63.631, −19.6083; 1667 m), and Snæfellsjökull (64.8400, −23.9017; 1446 m), are currently in volcanic belts where little or no crustal spreading occurs (Figure 7.2, Photo 15). Although there are numerous calderas existing today in Iceland, the most recent formation of a caldera occurred after the Bárðarbunga (64.6411, −17.5281) eruption in 2015. A caldera forms by the collapse of a volcano into itself to form a large caldron‐like depression in its place. The depression forms after the rapid evacuation of a magma chamber that has been left without structural support for the crust above the magma chamber. 7.2.2. Shield Volcanoes and Associated Features

Skjaldbreiður (64.4092, −20.7525) (Photo 16) is a large‐volume (>1 km3) basalt lava shield volcano with a summit crater [Rossi, 1996]. Shield volcanoes in Iceland can be distinguished from stratovolcanoes by their heights being typically about one‐twentieth of their widths. Additionally, the lower slopes are often gentle (~3°), but the middle slopes become steeper (~10°) and then flatten at the summit. This gives shield volcanoes a flank morphology that is convex in an

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Figure 7.3  Volcano morphology and time line, with relative sizes, shapes, and life spans of various types of volcanoes. The volcano images are vertically exaggerated two times and the craters and calderas are exaggerated four times. The approximate erosional expectances of the volcanic structures are listed in years. [Modified from Decker and Decker [2005]; design credit Nathan Mennen.]

Figure 7.4  Basic volcano morphology. [Design credit Nathan Mennen.]

upward direction. Their overall broad shape results from the extrusion of low‐viscosity mafic lava with low gaseous content that spreads outward from the summit area. The resulting surface structures will be determined by magma discharge rate and the steepness of the slope over which the lava flows. “Aa” lava flows develop when there is a high discharge rate and steep slopes (Photo 17). The partially solidified front of the flow steepens due to the mass of flowing lava behind it

until it separates from the flow. At this time, the general mass behind it moves forward. The top of the flow cools quickly while the molten magma underneath cools slowly as it is sheltered from contact with the atmosphere; a jagged appearance results. In contrast, gentle slopes and lower discharge events form smooth undulating or ropy masses called pahoehoe that are typically 0.2 >0.75 ? ? ?

? 4 4 ? ? ?

12

1766

72

1.3

5

13

1845

77

0.63

4

14 15 16

1947 1970 1980

102 16 10

0.8 0.2 0.12

4 4 4

17 18

1991 2000

0.15 0.11

4 4

9.5 9

Notes A highly destructive eruption that ejected its tephra over the entire northern half of Iceland Formed the Efrahvolshraun lava flow Second largest tephra erution; formed the Selsundshraun lava flow Flourine from the ejected tephra poisoned livestock Formed the Nordurhraun lava flow

Produced upwards of 60,000 m3 s−1 of tephra which was eventually carried as far as Norway Hekla’s largest volcanic eruption recorded; 3–5 cm of tephra deposited; flooding caused from melted ice and snow at Hekla’s peaks This 7 month long eruption caused widespread death among local livestock and wildlife This eruption spread along Hekla’s 6.9 km fissure, depositing upwards of 20 cm of tephra Hekla’s most recent volcanic eruption produced an ash plume 10 km high, which traveled over 300 km from the eruption site

Note. Modified from Larsen et al. [1999], Oladóttir et al. [2011], and Thórarinsson [1970]; design credit Nathan Mennen. VEI, volcanic explosivity index.

1990] revealing a composite caldera that Gudmundsson and Milsom [1997] divided into three regions. A combined magnetic and seismic reflection survey of the main caldera indicated that the caldera floor is made of volcanic clastic sediments, lava flows and sills [Gudmundsson, 1992]. The Grímsvötn caldera lake (10–12 km wide, 200–300 m deep) is largely confined to the main caldera and is overlain by a floating ice‐shelf 240–260 m thick. Direct surface expressions of hydrothermal activity are visible at its highest peak, Grímsfjall (1722  m), on the southern caldera rim, in the form of fumaroles, steam outlets, and ice caves. Most of the phreatomagmatic eruptions persist for days to weeks. Volcanic eruptions at Grímvötn often  coincide with jökulhlaups when the water level in  the caldera lake rises to a critical height of 1425– 1450 m  a.s.l. Geothermal activity continuously melts

the overlying ice, and meltwater accumulates in a subglacial lake within the caldera until the surrounding ice is breached. When this happens, water escapes to cause a jökulhlaup in the River Skeiðará, after having traveled ~50 km beneath the Skeiðarárjökull outlet glacier. Jökulhlaups occur there every 1–10 years and last from days to weeks, each time releasing 0.4–4 km3 of water [Björnsson, 2003]. A devastating jökulhlaup occurred in November 1996 following an eruption of Grímsvötn, however, no lives were lost due to detailed monitoring (e.g., hydrological, seismic, geodetic) carried out by the IMO. The flooding event itself did not occur until 13 days after a fissure eruption occurred between the Bárðarbunga volcano and Lake Grímsvötn, but indicators from caldera subsidence and subsurface monitoring of water flow allowed for proper evacuation and the closing of

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Figure 9.4  The aviation zones shut down (red) or restricted (orange) by the 2010 eruption of Eyjafjallajökull. [Deltafalcon/Wikimedia Commons; Public Domain.]

the Ring Road (at Hringvegur) prior to the arrival of the flood wave. The velocity of the glacial River Skeiðará peaked at 55,000  m3  s−1, or more than five times that of a “normal” megaflood. Ultimately, a section of road across the Skeiðará sandur was washed away by the intensity of the flood event (Photo 24). Smaller jökulhlaups have occurred since the 1996 event (e.g., 2004), when appropriate warnings have also been issued by the IMO. To provide a historic perspective, from 1600 to 1934 about one jökulhlaup occurred every decade with a discharge of about 5–7  km3 of water; post‐1938 two or three jökulhlaups would occur every decade with correspondingly smaller discharges between 0.5 and 3.5  km3 [Björnsson, 1992; Björnsson and Gudmundsson, 1993].

All historic eruptions at Grímsvötn took place from the ice‐covered central volcano, with exception of the 1783–1784 Laki fissure eruption [Thórarinsson, 1974; Gudmundsson and Björnsson, 1991]. The most recent eruption occurred on 21 May 2011 when a 20–25  km ash cloud was accompanied by multiple earthquakes [Petersen et  al., 2012]. The ash cloud from this eruption was classified as 10 times larger than the 2004 eruption and produced an order of magnitude larger volume of magma (0.2–0.3  km3 DRE) [Sigmarsson et al., 2013; Jude‐Eton et al., 2012]. Although flight travel was interrupted in northern Europe, it was much less widespread than the 2010 disruption after the Eyjafjallajökull eruption. Another notable event occurred on 28 December

Volcanics of the South and Southeastern Regions  71

Photo 24  The remnants of a bridge wiped out along the Ring Road (Route 1) by a jökulhlaup flooding event in 1996 linked to the eruption of Grímsvötn. This is known as the Skeiđará Bridge Monument. [Courtesy of Russell Maddrey.]

1998 when a week‐long eruption took place but did not trigger a glacial outburst. Bárðarbunga is the second most active volcano in historic time (with Hekla a close third) and second tallest peak in Iceland (2009 m). Bárðarbunga is part of a volcanic system that is approximately 200 km long and 25  km wide. The caldera is about 80  km2, up to 10 km wide, and 700 m deep. The crater is hidden by 850 m of glacial ice. Although the eruption frequency is less than that of Grímsvötn, activity on the two systems often behaved concurrently [Larsen et al., 1998; Sigmarsson et  al., 2000]. Also similar to Grímsvötn, the Bárðarbunga central volcano has been capped with ice historically, but approximately 70% of the overall system is ice free [Sæmundsson, 1978]. Despite this, there have only been 3 of 23 confirmed eruptions along the ice‐free fissure swarm [Larsen, 1984; Thordarson and Larsen, 2007].

9.2. KATLA AND EYJAFJALLAJÖKULL Katla (63.633, −19.083), which translates to “kettle” is a gentle sloping (80,000  km2), along with the two other equally intense events (VEI ≥ 4.0) of H4 at 2250 BCE and H3 at 1050  BCE [Larsen and Thórarinsson, 1977]. The

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finding of two additional tephra markers in the marine record further constrain the age of the H5 and H4 tephras. Specifically, Gudmundsdóttir et  al. [2011] established stratigraphical correlation of two tephra layers within the TFZ (Figure 2.1) on the north Icelandic marine shelf. These marine records are referred to as Hekla  DH (4632  BCE) and Hekla  Ö (4042 BCE). Here it should be noted that although all 18 recorded historic eruptions (Table 9.1, Figure 9.3), as well as the H3–H5 tephras, are found preserved in the tephra record, sometimes complications can arise in distinguishing between Hekla and nearby Katla deposits [Jóhannsdóttir, 2007]. Additionally, it has been noted by Jagan [2010] that there are difficulties in discerning eruptions from the same volcano if the analysis is based solely on major element composition. For example, Jagan [2010] and Larsen et al. [1999] described similarities in the mineralogical composition of the tephra deposits H1510 and H1947 (8 and 14 in Table 9.1), and only an understanding of their position in the stratigraphic column can resolve any confusion. A comprehensive database designated to the study of tephrochronology can be found on the internet (www. tephrabase.org) to aid in the mapping and further understanding of tephra in Iceland and Europe. Hekla’s unique increase in eruption pattern throughout its history delineates it from other Icelandic volcanoes. The initial phase of each Hekla eruption is always a highly explosive subplinan to Plinian type eruption [Janebo, 2016; Gudnason et al., 2017] that has produced silica rich tephra in volumes ranging from 0.02 km3 to 2.2 km3 (namely the H3 and 1104 CE events; Figure 9.3, Table  9.1) [Larsen et  al., 1999]. The second eruption

phase is typically basaltic andesite deposits and then mafic fissure eruptions usually follow this. Since the first eruption after human settlement in 874 CE, Hekla has erupted 18 times, with the last event occurring in 2000. Up to the 1970s the recurrence interval of Hekla eruptions was around two per century, but since 1970 the volcano has erupted about every 10 years: 1970, 1980, 1991, and again in 2000 (Table  9.1) [Thórarinsson and Sigvaldason, 1972; Grönvold et  al., 1983; Gudmundsson et  al., 1992; Höskuldsson and Olafsdottir, 2002]. The duration and magnitude of the explosive phase is directly correlated with the amount of time spent in dormancy prior to the eruption (Figure  9.7). Additionally, the increasing frequency of Hekla eruptions has been linked to the magmatic chemical composition of its eruption products [Sigvaldason, 1974; Höskuldsson et  al., 2007]. After dormancy lasting 100 years or more, high‐silica magma (rhyodacite) was ejected from the volcano (i.e. H3, H4, H5, and 1104 CE). After the 1104 CE event there is a shift from the ryhodacite composition to the less silica‐rich basaltic andesite, which also corresponds to an increase in frequency of eruption (