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, 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.

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 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]. 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].

2.1. PRESENT TECTONIC SETTING

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 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.

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) [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 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 (<10% of the Tertiary succession) and fossils are rare (Box 2.1). For example, only 50 genera or species of plants have been documented on the Tjörnes Peninsula [Einarsson, 1994; Thordarson and Höskuldsson, 2014].

Era	Period	Epoch	Age	Stage	Sub‐stage	Formation	Major events
CENOZOIC	Quaternary	Holocene	0–2.5 ka	Late Bog Period (sub‐Atlantic)		Upper Pleistocene Formation
			2.5–5 ka	Late Birch Period (sub‐Boreal)
			5–7.2 ka	Early Bog Period (Atlantic)
			7.2–9.3 ka	Early Birch Period (Boreal)
			9.3–10 ka	Pre‐Boreal			Ice Age glaciers melt
		Late Pleistocene	10–11 ka	Weichselian	Younger Dryas		Cooling in northern hemisphere; glaciers grow
			11–12 ka		Allerød		Warmer climate
			12–20 ka		Older Dryas		Icelandic ice sheets quickly retreat
			20–110 ka				Eurasian ice sheet at maximum; last glacial stage
		Middle Pleistocene	115–130 ka	Eemian			Last interglacial stage
			130–300 ka	Saale			Glacial stage
			300–700 ka
		Early Pleistocene	0.7–2.5 Ma			Plio‐Pleistocene Formation	Start of full‐scale glaciations
	Teriatery	Pliocene	2.5–3.3 Ma				Pacific Ocean fauna arrive in Iceland. Bering Strait opens
			3.3–7 Ma			Teriatery Basalt Formation	Climate begins to cool
		Late Miocene	7–12 Ma				Warm, temperate climate
		Middle Miocene	12–18 Ma
		Early Miocene	18–25 Ma				Origination of Iceland

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

Most of the Tertiary landscape‐altering events resulted from fissure eruptions, although the presence of central or shield volcanoes can be spectulated through the numerous feeder dikes that cross cut the basalt piles. Most feeder dikes have north to south orientation, their mean thickness is about 3 m, and widths range from 1 to 10 m [Sæmundsson, 1986]. Textbook examples of dike swarms found in the eastern fjords can make up between 3 and 15% of the rock outcrop. Thordarson [2012] describes the dike swarms as subsurface components of fissure swarms in active volcanic systems, whereby the swarms are linked to localized deposits of andesite–rhyolite lava and tephra marking the location of extinct central volcanoes; 40 extinct volcanic systems have been identified in the Tertiary succession.

Deposition of the Tertiary basalts ended at 2.5 Ma coinciding with the onset of an extensive glacial period [Eiríksson, 2008]. The crustal response to the weight of ice overburden was translated through normal faulting with throws between 10 and 20 m and main fault strikes generally trending N–E or N–SW. The characteristic dip of the Tertiary Basalt Formation is between 5° and 15°, but the direction of dip is variable depending on region (Figure 2.1; Einarsson, 1960; Sigmundssson, 2006). Measurements of Tertiary Basalt Formation dips identify folds in the basalt that relate to the rift‐jump complexes previously described. Conformable boundaries between the Tertiary and Quaternary basalt deposits in Iceland are not obvious [Einarrson, 1994]. Dating of changes in polarity of Earth’s magnetic field, however, have made it possible to delineate Tertiary and Quaternary basalts, as the Quaternary Period spans the present Brunhes normal polarity epoch and the previous Matuyana predominately reversed polarity epoch [Kristjansson and McDougall, 1982]. The top of the penultimate normal polarity epoch, the Gauss, marks the end of the Tertiary Period at approximately 2.6 Ma.

There are two notable Plio‐Pleistocene depositional sequences that ultimately make up about 25% of exposed rock at the surface: Grey Basalt (3.0–0.7 Ma) and Móberg Formations (0.7 Ma to 10,000 ka). Although superficially similar in appearance to the Tertiary Basalt, the Grey Basalt is characterized as being lighter in color and coarser in texture [Kjartansson, 1960]. The Móberg Formation is described as a palogonite tuff and breccia that is a result of hydration and alteration processes [Kjartansson, 1960].

In the Pliocene Epoch at about 5 Ma climate began to cool, and by Early Pleistocene times (2.5 Ma) a major ice cap had formed, which by Late Pleistocene times (26 ka) covered the entire island of Iceland, lasting until the present Holocene Interglacial (Figure 2.2). In the Weichselian Glacial stage, between the Eemian and Holocene interglacials, five glacial events took take place [Thordarson and Höskuldsson, 2014], culminating in the Last Glacial Maximum, when ice likely extended well beyond the current shoreline. Early reports of a glacial moraine found 130 km offshore [Ólafsdóttir, 1975; Norðahl et al., 2008] were followed by other geomorphological studies that determined the ice extent was only 10–20 km on the coastal shelf [Hjort et al., 1985]. Regardless of the offshore position of the ice margin, Iceland’s landscape was influenced dominantly by the expansion and retreat of Pleistocene ice sheets. The resulting Pliocene–Quaternary geomorphologic evolution includes features such as the mörberg ridges (Figure 2.3) and tabletop mountains (Photo 1), which dominate the landscape in the neovolcanic zones. Moreover, the substantial meltwaters from Weichselian ice sheets 1 km thick produced abundant fluvial deposits and spectacular erosional landscape features such as canyons.