Introduction
The realm of volcanism is widespread in Iceland. It comprises a variety of phenomena that sometimes differ vastly from the usual image we have of a volcano, forming a conical mountain that has smoke rising from its top.
When we talk about the volcanic activity in Iceland two questions come to mind:
- How many eruptions have occurred during the history of Iceland?
- How many volcanoes are there on Iceland?
I will quickly address these two questions before we dive deeper into the architecture of Icelandic volcanism.
How often do eruptions occur?
The answer to the question how many eruptions have occurred in Iceland through its geological history is difficult to answer. The different magmatic sequences exposed on the island are not always easily distinguishable into single eruption cycles. We have, though, the observations humankind made about Iceland’s recent activity. Those observations show that eruptions take place about every three to five years. If we use those numbers to calculate the amount of eruptions back in time, it means that since the first humans settled to icy shores 1000 years ago, at least 200 eruptions have taken place. This number overlaps well with the amounts of eruptions that have been mentioned in the manuscripts that the Icelandic settlers have put on to paper since the 12th century. If we use this number to extrapolate even further, it suggests that the amount of eruptions since Iceland’s creation, 24 million years ago, might be in the order of 5 million.
How many volcanoes are there?
The amount of volcanoes present on Iceland depends on the definition one would like to apply. We, of course, only want to look at the active volcanoes. But this does not really narrow it down completely. If we look back on historical observations, we can count 22 central volcanoes that have erupted multiple times. But there are volcanoes that have only erupted once. By pure definition, these volcano would be considered extinct. The area those central volcanos are located in, on the other hand, can still be active.
To avoid this definition problem, geologists have introduced the concept of a volcanic system. If we add those volcanic systems into our list of active volcanoes, we end up with 30 systems.
Tectonic map of Iceland with active volcanic systems and extinct volcanoes.
Simplified after Jóhannesson and Sæmundsson (1998). Based on data from The Icelandic Institute of Natural History (license). Names of volcanic zones from en.wikipedia.org.
What are volcanic systems?
A volcanic system outlines the essential geological structures in Iceland. They include a fissure swarm and a central volcano and can occur by themselves or in a combined system with both features exposed.
The two surface expressions of Icelandic volcanism differ in their respective plumbing system. Fissure swarms are fed by a deep-seated magma reservoir, whereas a central volcano is fed by a secondary, shallower, magma chamber. Both volcanic systems have their own distinct tectonic environment and a different magma chemistry. Each volcanic system has an average life expectancy between 0.5 to 1.5 million years.
Sketch of one half of a volcanic system in Iceland. An elongated magma chamber is located at the base of the crust. The reservoir feeds fissures that form parallel swarms at the surface. It also feeds a secondary reservoir that underlies a central volcano.
Drawn by the author. Inspired by Gudmundsson et al. (2009) and Thordarson and Höskuldsson (2014).
The fissure swarms are related to the rifting episodes. They form long and narrow bands (5-20 km wide and 50-100 km long). The bands are formed by cracks and faults as well as volcanic fissures. In general, they are believed to represent the surface expressions of equally shaped magma chambers located at the base of the crust (deeper than 10 km). Magma chamber and fissure swarm are orientated parallel to the rift zone, underlining that the fundamental process responsible for their creation is the plate spreading.
The most common surface expressions are wide cracks and graben. Volcanic activity is usually expressed as a row of small volcanic cones or elongated ridges, where they interacted with overlying glaciers.
Bardarbunga Volcanic System.
Photograph by Feo Pitciarn. flickr.com
Where volcanic eruptions occur repeatedly at the same location, they form edifices that are termed central volcanoes. The cause for these focal points of volcanic activity are secondary magma chambers that are located only 2-6 km beneath the surface. Where present, the central volcanoes usually represent largest formation and often have a collapsed crater on their top, called caldera. Central volcanoes within the volcanic rifts in Iceland are, as a rule, transected by fissure swarms.
The Hekla central volcano.
Photograph by Sverrir Thorolfsson. flickr.com
Activity of volcanic systems is not continuous. Rather, the system is activated during certain episodes. These episodes can last a few years up to several decades. These episodes are characterized by increased earthquake activity and volcanic eruptions.
Previous posts
Geology of Iceland - Part 1: Introduction
Geology of Iceland - Part 2: The Iceland hotspot
Geology of Iceland – Part 3: an island being ripping apart
References
- Gudmundsson, A., Friese, N. Andrew, R., Philipp, S.L., ertl, G., Letourneur, L. (2009). Effects of dyke emplacement and plate pull on mechanical interaction between volcanic systems and central volcanoes in Iceland. Special Publications of IAVCEI. link
- Jóhannesson, H. and Saemundsson, K. (1998). Geological Map of Iceland. 1:500 000. Tectonics. Icelandic Insititue of Natural History, Reykjavik (1st edition). link
- Sigmundsson, F. (2006). Iceland Geodynamics: Crustal Deformation and Divergent Plate Tectonics. Springer, 231 p. link
- Thordarson, T. and Höskuldsson, Á. (2014). Classic Geology in Europe 3: Iceland. Second Edition. Dunedin Academic Press Ltd, 256 pp. link