From Iceland — Icelandic Volcanism: Where, Why & How?

Icelandic Volcanism: Where, Why & How?

Published June 6, 2011

Icelandic Volcanism: Where, Why & How?

Iceland sure has been in the global news a lot this past year or so and a lot of that has been to do with volcanoes. So why does this little country, stranded in the middle of the North-Atlantic, have so many volcanoes and why are they so damned troublesome for the rest of the world? I was lucky enough to cover the Eyjafjallajökull eruption for Grapevine last year and following the recent eruption at Grímsvötn I’ve been drafted to try to answer the many volcano-related questions that may be whizzing through your head. So hold onto your hats, ladies and gentlemen, for this whistle-stop tour through the complex science that is Icelandic volcanology…


Iceland looks like a very lonely country, stuck most out of place amongst a sea of, well, sea. So why has it sprung up right there, of all places? To answer this question we must strip back the watery layer and take a good look at the ocean floor.

When we do so, we see that the island of Iceland is located on the crossing place of two major linear features. The first of these is a huge rift, running roughly north to south, splitting the Atlantic in two. This is the Mid-Atlantic Ridge, a zone where two of the plates comprising the Earth are spreading apart, creating new land as they do so. In this case we have the North American plate on the west side and the Eurasian plate to the east. The ridge itself is slightly raised from the surrounding ocean floor, but nowhere near the water surface, so this alone cannot explain Iceland’s prominence.

For this we must turn to the other feature—a raised strip of ocean floor running between the Faeroe Islands and Scotland to the southeast and Greenland to the northwest. What caused this? Well, the current belief is that it is due to a so-called ‘hot spot’. The exact reasons for hot-spot formation is still very much under debate in the scientific community, but the basic fact is that there appears to be an area of anomalous heating under one spot on the crust, in this case under one part of the Mid-Atlantic Ridge. This causes more volcanism—and in turn, more land production—along this section of the ridge. It is believed that as the plates spread apart along the ridge, the greater production at this point caused a raised ridge to form as the plates moved away from the hot-spot.

But what about Iceland itself? Honestly I don’t think anyone is quite sure why Iceland sits so high above that ridge. For some reason, more land is being produced faster now than in the past and this has allowed an island to form above the surface of the ocean. What it does mean, however, is that that hot-spot still resides beneath Iceland and this can account for much of the country’s volcanism.


Not necessarily. On a basic level most people would consider a volcano as a hole in the ground that erupts liquid rock, or magma—called lava once it reaches the surface. But there is actually a vast range of different volcano types, all with different eruption styles and hazards that come with them. Most volcanically active areas of the world are typified by one or two types, but Iceland is rather unique in that it possesses almost the full range of types.

The type of eruption a volcano produces—and by extension therefore the type of volcanic edifice formed—depends largely on the type of lava produced. And this in itself depends mostly on where the lava comes from. Without going into too much detail, the explosivity of an eruption is generally related to how viscous (thick) the lava is.

Think of the volcanoes in Hawaii, for example. A good example of typical hot-spot volcanism, the activity here most often comes in the form of spectacular fountains of glowing orange lava erupted from a crater of elongated fissure (collectively known as the vent). The lavas here have very low viscosity—in other words they flow very easily—and are generally named ‘basalts’ due to their chemical composition. Small bubbles of gas within the lava can escape easily and they essentially propel the lava high into the air. Over the years, the lava spreads a long way from the vent, resulting in large, flattish volcanoes that we typically refer to as ‘shield volcanoes’.
Taking a step up from Hawaii we can look at a volcano like Sakurajima in southern Japan, which has been erupting virtually every single day for decades. This volcano produces more viscous ‘andesite’ lavas, which trap gas bubbles within them. Very simply, these bubbles grow in size while trapped in the thick magma, eventually bursting at the surface, often resulting in an explosion of glowing fragments of lava. Volcanoes such as this tend to produce more ash and can cause more problems for air traffic (something fresh in our minds after last year). They usually form steeper-sided peaks, similar to the typical conical volcano image you may have in your head.

At the top of this simplified scale come the ‘rhyolite’ volcanoes, which have very thick lavas that are very resistant to flowing. A typical rhyolite volcano (if there is such a thing!) could perhaps be something like Chaiten in Chile. They tend to erupt lavas in the form of domes (which also occur at andesite volcanoes), rather than spectacular explosions. However, these domes are often unstable and may then collapse, producing the fearsome ‘pyroclastic flow’—an avalanche of rock, debris and hot gases. Rhyolite volcanoes often feature enormous craters, or ‘calderas’. Explosions may result in some cases from rhyolitic activity, although they are not common. However, some of the largest eruptions in history have been of this type—think of the infamous Yellowstone, USA…


As I hinted at earlier, Iceland has a bit of almost everything crammed all into one place! Let’s look at last year’s Eyjafjallajökull eruption as an example, shall we?

The first stage of the eruption, which breached the narrow strip of exposed land between Eyjafjallajökull and Mýrdalsjökull glaciers, was a small (but rather photogenic!) fissure eruption. It was somewhat similar to the sort of eruption you might see in Hawaii—a long crack in the ground, producing towering fountains of glowing basalt lava and sending rivers of molten rock pouring down the mountainside. These lava flows even had lava falls, showing how easily they could flow. A spectacular ‘tourist eruption’, the first stage didn’t really provide much of a threat to anyone.

About a month later, however, things changed. No sooner had the fissures calmed down, but a new one opened—this time on the summit of the volcano, directly beneath the glacier ice. This time the lava was much different (andesite—explaining why this happened requires an article of its own) and this produced a much more explosive eruption. This high explosivity, aided by the lava coming in contact with cold melting ice from the glacier, produced the now-infamous ash cloud that shut down air space across mainland Europe. While ash this fine and troublesome is possibly quite unusual for an Icelandic eruption, explosive activity is really rather common.

In fact, glaciers play a major role in Icelandic volcanism. In past times, when the entire country was buried under ice, the weight of the ice was enough to constrain many eruptions and prevent them from breaking through it. Most of the long ridges and flat-topped hills (‘tuya’) you may see as you drive around the country are the result of volcanism constrained by ice. Additionally, as mentioned previously, the water produced as it melts can cause explosions—something responsible for even lavas that are traditionally less explosive producing quite violent eruptions. Take the latest activity at Grímsvötn—a basaltic eruption, if it had taken place under plain air it would probably have been quite benign. But because of fragmentation due to all that ice and water… well, you’ve probably already seen the result!

“But what about the rhyolite volcanoes”, I hear one or two of you ask? Well, if you visit one of the huge calderas like Askja you are standing inside what is known as a ‘caldera’, which are often formed at least partly as a result of very large rhyolite explosions. And if you are lucky enough to visit the Landmannalaugar area, those fantastic yellow and orange colours are caused by exposed rhyolite rocks. In fact there are even exposed rhyolitic lava flows and domes in this area if you look for them.

And you know what the most confusing thing is? Most volcanoes in Iceland are in fact ‘volcanic systems’, with a pronounced ‘central volcano’ with a long ‘swarm’ of fissures branching off from it. This is the result of different types of volcanism within the same system—often more explosive at the central volcano and more gently effusive along the fissures. Despite this, the source of the lavas for each eruption in each system is more or less the same. Such diverse volcanism even within a few square kilometres poses something of a dilemma for scientists eager to understand what is going on and makes Icelandic eruptions arguably even harder to forecast than most!


It is clear that Icelandic volcanism is tremendously varied. It’s impossible to tackle in any detail in such a short space (believe me, I could go on for days, but I suspect there wouldn’t be anyone left reading by the end), but I hope I have at least managed to touch on some things that may pique your interest.

In terms of what exactly Iceland can expect in the future, no-one really knows. Volcanoes cannot be well predicted, only roughly forecasted. To this end, however, we can say that in the long term it will most likely be ‘more of the same’. Iceland is a growing country and it will continue to be volcanically active for a long time to come. There will be some large eruptions and plenty of smaller ones too. Some will produce beautiful, glowing fountains and some… well, Europe would be well advised to have contingencies in place in case of another Eyjafjallajökull!

More on the volcano:
Killer Volcanoes: A Comparative History
Volcanology? That’s from Star Trek, right?

See more Eruption Iceland stories.

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