Elsevier

Journal of Geodynamics

Volume 43, Issue 1, January 2007, Pages 118-152
Journal of Geodynamics

Volcanism in Iceland in historical time: Volcano types, eruption styles and eruptive history

https://doi.org/10.1016/j.jog.2006.09.005Get rights and content

Abstract

The large-scale volcanic lineaments in Iceland are an axial zone, which is delineated by the Reykjanes, West and North Volcanic Zones (RVZ, WVZ, NVZ) and the East Volcanic Zone (EVZ), which is growing in length by propagation to the southwest through pre-existing crust. These zones are connected across central Iceland by the Mid-Iceland Belt (MIB). Other volcanically active areas are the two intraplate belts of Öræfajökull (ÖVB) and Snæfellsnes (SVB). The principal structure of the volcanic zones are the 30 volcanic systems, where 12 are comprised of a fissure swarm and a central volcano, 7 of a central volcano, 9 of a fissure swarm and a central domain, and 2 are typified by a central domain alone.

Volcanism in Iceland is unusually diverse for an oceanic island because of special geological and climatological circumstances. It features nearly all volcano types and eruption styles known on Earth. The first order grouping of volcanoes is in accordance with recurrence of eruptions on the same vent system and is divided into central volcanoes (polygenetic) and basalt volcanoes (monogenetic). The basalt volcanoes are categorized further in accordance with vent geometry (circular or linear), type of vent accumulation, characteristic style of eruption and volcanic environment (i.e. subaerial, subglacial, submarine).

Eruptions are broadly grouped into effusive eruptions where >95% of the erupted magma is lava, explosive eruptions if >95% of the erupted magma is tephra (volume calculated as dense rock equivalent, DRE), and mixed eruptions if the ratio of lava to tephra occupy the range in between these two end-members. Although basaltic volcanism dominates, the activity in historical time (i.e. last 11 centuries) features expulsion of basalt, andesite, dacite and rhyolite magmas that have produced effusive eruptions of Hawaiian and flood lava magnitudes, mixed eruptions featuring phases of Strombolian to Plinian intensities, and explosive phreatomagmatic and magmatic eruptions spanning almost the entire intensity scale; from Surtseyan to Phreatoplinian in case of “wet” eruptions and Strombolian to Plinian in terms of “dry” eruptions. In historical time the magma volume extruded by individual eruptions ranges from ∼1 m3 to ∼20 km3 DRE, reflecting variable magma compositions, effusion rates and eruption durations.

All together 205 eruptive events have been identified in historical time by detailed mapping and dating of events along with extensive research on documentation of eruptions in historical chronicles. Of these 205 events, 192 represent individual eruptions and 13 are classified as “Fires”, which include two or more eruptions defining an episode of volcanic activity that lasts for months to years. Of the 159 eruptions verified by identification of their products 124 are explosive, effusive eruptions are 14 and mixed eruptions are 21. Eruptions listed as reported-only are 33. Eight of the Fires are predominantly effusive and the remaining five include explosive activity that produced extensive tephra layers. The record indicates an average of 20–25 eruptions per century in Iceland, but eruption frequency has varied on time scale of decades. An apparent stepwise increase in eruption frequency is observed over the last 1100 years that reflects improved documentation of eruptive events with time. About 80% of the verified eruptions took place on the EVZ where the four most active volcanic systems (Grímsvötn, Bárdarbunga–Veidivötn, Hekla and Katla) are located and 9%, 5%, 1% and 0.5% on the RVZ–WVZ, NVZ, ÖVB, and SVB, respectively. Source volcano for ∼4.5% of the eruptions is not known.

Magma productivity over 1100 years equals about 87 km3 DRE with basaltic magma accounting for about 79% and intermediate and acid magma accounting for 16% and 5%, respectively. Productivity is by far highest on the EVZ where 71 km3 (∼82%) were erupted, with three flood lava eruptions accounting for more than one half of that volume. RVZ–WVZ accounts for 13% of the magma and the NWZ and the intraplate belts for 2.5% each. Collectively the axial zone (RVZ, WVZ, NVZ) has only erupted 15–16% of total magma volume in the last 1130 years.

Introduction

Volcanism in Iceland is diverse for an oceanic island and has featured nearly all volcano types and eruption styles known on Earth (Thorarinsson and Sæmundsson, 1979, Thorarinsson, 1981). The volcano types range from archetypal mafic lava shields to classical conical-shape stratovolcanoes, whereas the styles of volcanic activity span the spectrum of effusive as well as phreatomagmatic and magmatic explosive eruptions in the settings of subaerial to subglacial and submarine environments. Although volcanism is typified by effusive basalt eruptions, it is also characterized by a range of explosive styles that includes felsic eruptions of subplinian and Plinian intensities as well as the renowned mafic subglacial phreatomagmatic eruptions. The effects of many Icelandic eruptions have been felt on a hemispheric scale and the very largest ones may have had global impact.

Just over 25 years ago, Thorarinsson and Sæmundsson (1979), published a decisive paper titled “Volcanic activity in historical times”, outlining the contemporary understanding of Icelandic volcanoes. Since then significant advances have been made and it is time to present a new review of the volcanic activity in Iceland in historical times, which is the principal objective of this paper.

Section snippets

Geologic setting

The Iceland basalt plateau is situated at the junction of two large submarine physiographic structures, the Mid-Atlantic Ridge and the Greenland–Iceland–Faeroe Ridge (Fig. 1). It rises more than 3000 m above the surrounding sea floor, has crustal thickness of 10–40 km and covers about 350,000 km2 (e.g. Gudmundsson, 2000 and references therein). About 30% of this area (∼103,000 km2) is above sea level, the remainder forms the 50–200 km wide shelf around the island. The construction of the Iceland

Volcano types and forms

The term volcano encompasses a wide range of structures that are constructed around the vent(s) by the volcanic activity, whether they are formed by single (monogenetic) or multiple (polygenetic) eruptions (Macdonald, 1972, Cas and Wright, 1987, Francis and Oppenheimer, 2004). The overall architecture of a volcano is primarily determined by the type of magma erupted, the habitual eruption behaviour, the shape of the vent system and the environmental setting (i.e. subaerial, subglacial,

Distribution and frequency of historical eruptions

Volcanic activity in Iceland in historical time (i.e. over the last 1100 years) is a direct continuation of the prehistoric Holocene activity and generally confined to the same volcanic regions. A total of 16 volcanic systems have been volcanically active during this period. Of 9 volcanic systems on the RVZ–WVZ sector of the axial zone (Fig. 2), 4 have erupted in the last 1100 years. Those are the, Reykjanes–Svartsengi, Krýsuvík, Brennisteinsfjöll, and Langjökull systems on the WVZ–RVZ (Fig. 3;

Summary

  • 1.

    Holocene volcanism in Iceland is confined to the neovolcanic zones, which are represented by discrete 15–50 km wide volcanic zones and belts that collectively cover about one-third of Iceland. Volcanic systems are the principal structure of the volcanic zones and in total there are 30 active systems in Iceland. The volcanic systems are comprised of a fissure swarm or a central volcano or both. A fissure swarm and a central volcano coexist on 12 systems, whereas 7 are only comprised of a central

Acknowledgements

We are grateful to Ágúst Gudmundsson and Magnús T. Gudmundsson for constructive reviews and suggestions, which resulted in significant improvements on the initial draft of this manuscript.

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