How Iceland’s Volcanoes Work

Iceland is visited for its beautiful landscapes and wonderful hot mineral springs. Both are due primarily to the volcanoes. Although they are a constant threat, without them there was no this beautiful island. It is the volcanoes (in cooperation with glaciers) that make up Iceland’s magnificent landscape.

See what volcanoes actually are, how they form and operate, and learn more about some of them. Your trip to Iceland will be much more interesting because of it.

If
you want to find out where Iceland’s volcanoes came from in the first place and how the island itself was formed, read the previous article in this series: How Iceland Was Formed – Island Geology in a Nutshell

V
olcanoes heat water far beyond 100 ºC / 212 ºF, can inundate up to several thousand km² with lava, cause floods destroying everything in their path and earthquakes really more frequent than one might think. You can read about what their effects are in Iceland in the next article in this series: Stunning effects of Iceland’s volcanism

However, let’s start with what a volcano even is….

What is a volcano

Colloquially, we refer to a volcano as that part of it that is clearly visible on the surface of the earth. However, in scientific terms, this is only a part of the volcano, called a volcanic mountain. This is because the entire volcano also consists of its internal and underground structure, which is often larger than the mountain itself on the surface.

A single volcano can produce several or even dozens of volcanic mountains (for Icelandic examples of such volcanoes, see below in the text). Also, many individual volcanoes can be part of a larger volcanic system (in Iceland this is the absolute norm). These differences can lead to misunderstandings, because for a scientist a volcano can have many mountains, but it can also exist even without a clearly visible volcanic mountain – for example, when the volcano has not yet formed, it has already collapsed or has largely “gone up in the air.” On the other hand, for a tourist, one mountain is one volcano, and if the mountain is not there, we think the volcano is not there either. Depending on the methodology adopted, dozens to hundreds of volcanoes can be identified in Iceland….

So let’s take a closer look at the structure and classification of volcanoes.

Volcano structure

The construction of a single volcano is quite simple. The main engine is the so-called focus (volcanic chamber), which is located several to even tens of kilometers below the crater. This is the place in the Earth’s crust or mantle where molten rocks form magma.

Volcano cross section

The magma chamber is connected to the Earth’s surface by a long, straight conduit or otherwise channel. The conduit ends in a crater, which is the final place from which the magma comes out. It is usually shaped like a funnel or bowl and is usually located right in the middle of the volcano.

The magma, after coming to the surface, changes its properties slightly and is now called lava. The crater may be surrounded by a caldera, which is larger than it and is formed by the collapse of rocks over a partially emptied volcanic chamber. The classic view of a volcano also distinguishes parasitic craters, which are formed by the enormous pressure of magma seeking the easiest outlet.

An example of a classic volcano is, for example, Bardarbunga (Bárðarbunga) in the Vatnajokull (Vatnajökull) massif. This is a huge volcano – its collapsed caldera alone is 8×11 km / 5×7 miles in size and 700 m / 765 yards (sic!) deep. This caldera, by the way, is entirely filled with Vatnajokull glacier ice. In contrast, the latest eruption of the Bardarbunga volcano, the one that formed one of Iceland’s youngest lava fields – Holuhraun (2015) – occurred through a parasitic crater, fortunately located below the main part of the glacier. As a result, at least the flooding associated with the eruption was of limited force.

Nature, of course, completely disregards scientific classifications and descriptions and creates a huge number of types, kinds and shapes of volcanoes. Perhaps the fascinating beauty of Iceland comes precisely from the fact that on a relatively small island we can see a huge variety of these “volcanic variations.” It’s worth learning more about them, because although many of them look beautiful at first glance, they become even more fascinating when we learn what they really are and how they were formed.

Types of volcanoes in Iceland

Map of Iceland’s volcanic systems^[10]

Currently, volcanic activity in Iceland is concentrated in the NE-SW zone. It consists of a total of as many as about 200 volcanoes clustered in 32 groups (volcanic systems) ^[10] About 30 Icelandic volcanoes have manifested their activity in the last 1100 years.

It is of course impossible to describe all of Iceland’s volcanoes, but we will introduce the profiles of a few of them below to illustrate their diversity and to show how different types of volcanoes function in practice.

Volcanoes are described because of a number of selected characteristics, and we can fit each volcano into a number of such groups of varying rank. Therefore, at the outset, it is worth sorting out these concepts.

We can divide volcanoes in five basic ways:

by place of occurrence
- Surface volcanoes
- Submarine volcanoes
- Subglacial volcanoes
by type of eruption
- Central volcanoes (eruption occurs from a single location)
- Linear volcanoes (eruption occurs along a longer fissure)
- Mixed volcanoes
by level of activity ^[11]
- Active volcanoes – those that have had at least one eruption in the last 10,000 years (in Holocene);
  - Actively erupting – volcanoes where magma is currently reaching the surface
  - Dormant (or ‘potentially active’) – are active volcanoes that are not currently manifesting any form of activity
- Extinct – those that have not had an eruption in the last 10,000 years and so are considered without potential to erupt again in future;
by material extracted
- Efluvial (lava)
- Explosive (ash)
- Mixed (usually these are stratovolcanoes, or stratified volcanoes)
by shape of volcanic mountain
- Disc-shaped (a vast mountain with a low slope)
- Tapered (mountain with steep slopes)
- Table (a cone with an extensive, flat, truncated summit)

The criteria for these divisions are strongly interrelated. For example, the shape of a volcanic mountain is influenced by virtually all features of a volcano. Thus, a complete description of a volcano can include many names, although each name still describes the same volcano, simply giving different features.

For the purposes of this article, we will group Icelandic volcanoes according to the products they produce. In addition to lava in the typical sense, volcanoes also yield gases and so-called pyroclastic materials. The name of the latter refers to loose fragments of lava shot into the air during an eruption. They solidify already in flight or shortly after falling to the ground. Pyroclastic materials also include larger portions of rock not melted or entrained by magma as it travels through the channel and crater.

Lava volcanoes

Effusive (lava) volcanoes are defined as those from which only or almost only lava is ejected. As a result, eruptions are usually very calm and mild. Lava in such volcanoes can reach temperatures of 1300 ºC / 2400 ºF. Very often it is so-called visceral lava – helluhraun – melting, shiny and smooth. Basalt is formed under such conditions.

Skjaldbreiour

Skjaldbreiour is a volcano you could say the flagship of the Euphrates group of volcanoes. It is located in southern Iceland, about 65 km northeast of Reykjavik, and is 1016 m high and 15 km in diameter. Right next door, only about 5 km east of the volcano’s foot, is the Hloduvellir mountain hut or lodge, from which the volcano is beautifully visible. It is estimated that the last activity of Skjaldbreiour took place about 9000 years ago.

Skjaldbreidur volcano

Due to its shape, Skjaldbreiour belongs to shield volcanoes. They occur in the form of flat hills with slopes sloping a maximum of a few degrees. They are often very extensive, their shield is slightly domed or conical, and they often also have a caldera. They are symmetrical with a crater and a caldera (if they have one) in the center, which allows them to be classified also as central volcanoes. This type of volcanoes are classified as long-lasting ones – their activity can last for decades!

Laki

Laki is a linear eruptive volcano located in the southern part of Iceland, about 30 km north of Kirkjubaejarklaustur. It belongs to the Grímsvötn volcano complex (also called Grímsvötn-Laki) ^[10]
During the eruption of lineament volcanoes, lava spews quietly from the fissures, without major explosions. In Iceland, such fissures are called gjá (e.g. Almannagjá, Flosagjá and Peningagjá in Thingvellir National Park, Grjotagjá and Stóragjá near Lake Myvatn, or Eldgjá quite near Laki).

Linear volcano: Laki

In 1783, the Laki eruption took place, which was one of the largest eruptions in history. Lava erupted from a 27-kilometer-long (17 miles) fissure for eight months, flooding some 565 km² (220 sq mi). During the eruption 120 Mt SO₂ was ejected into the atmosphere and about 14.7 km³ of lava flowed out. Many craters formed in the fissure. Today there are about 100 of them there.

Eldborg

Eldborg is a efluvial volcano located in the western part of the island, about 75 km north of Reykjavik, and is classified as a central volcano. The distinctive volcanic cone – such as Eldborg has – is called a spatter cone/ring (literally, a splashed cone/ring). It is a specific type of cone volcano whose crater has very steep edges. Such a peculiar form is formed by lava, which solidifies almost immediately depositing on the crater walls. Because of this, the crater walls are made up of irregular and sharp edges.

Euphrates spatter cone volcano: Eldborg

Volcanoes of this type are small and short-lived (their eruptions last days or months at most), and their lava fields are relatively small.

Ash volcanoes

Explosive volcanoes are also called ash volcanoes. Nowadays there are very few of them. The main product of such volcanoes is pyroclastic materials and volatile products. Eruptions of this type are very violent. They are associated with such phenomena as ash flows, avalanches and pyroclastic precipitation, and so-called burning clouds. One of the most dangerous phenomena is ash flows, or lahars. Volcanic ash along with fragments of clotted lava when saturated with water behaves like mud. The water can come from a crater lake, from a glacier on the volcano, or from precipitation. Lahar flows down the slopes of the volcano at speeds of tens of kilometers per hour, and its flow can reach up to a distance of several kilometers!

Graenavatn

The Graenavatn volcano lies about 25 kilometers south of Reykjavik and is an example of a rare central, explosive, maar-type volcano. Maars are characteristic craters associated with the eruption of gases that are released from magma and groundwater. They do not have a cone, but a tuff dike surrounding a funnel-shaped depression.

Maar type explosive volcano: Graenavatn

Volcanic tuffs are volcanic ash and sand cemented by water. The diatreme, or volcanic chimney typical of explosive eruptions, is here filled with pyroclastic material cemented back by volcanic ash, the so-called volcanic breccia. Lakes are very often formed in the depressions created by explosions.

Hverfjall

Hverfjall is located in the northern part of Iceland in the south of the complex volcanic system Krafla, on the western shore of Lake Myvatn. It is a central volcano with a truncated cone made of tuff that measures 150 meters. The walls of the crater, which is about a kilometer in diameter, are quite steep. Hverfjall was formed by a so-called phreatomagmatic eruption. In this type of volcano, the products of the eruption are very small in volume (less than 0.5 km³), but the nature of the eruption is very special.

Inside the tuff cone: Hverfjall

Phreatomagmatic eruptions are eruptions caused by direct contact between water and magma. This usually occurs in a focus or volcanic conduit. Water that comes into contact with hot magma momentarily turns to steam. If there are channels in the cone through which the steam can escape then it escapes freely with them, but if all outlets are blocked it collects inside. When there is a large amount of it or it is produced in a sudden way – an explosion occurs. In such cases, the main product is steam, but lava also happens. Also ejected is rock material that was blasted during the eruption. The Hverfjall volcano formed during just such an eruption about 2,700 years ago.

Threngslaborgir

Threngslaborgir (Þrengslaborgir) is a volcano located southeast of Lake Myvatn belonging to the Heidarspordar (Heiðarsporðar)^[10] volcanic system. This is a very active system, which also includes the Ludent volcano, from whose eruptions one of the eras in the region’s history is named (see Geological History of the Myvatn Region).

Thanks to the eruption of Threngslaborgir around Lake Myvatn, completely unusual effects have been created due to the contact between water and lava: the Skutustadagigar pseudocraters and the Dimmuborgir lava field

Heidarspordar lavas [10]

Surface water (in this case the lake), upon contact with lava, naturally heats up and evaporates. However, if the water is trapped under the incoming lava (e.g., groundwater flowing from a spring covered by fresh lava) more and more vapor is formed, until its pressure is high enough to burst the lava flowing through. This results in the formation of so-called pseudocraters (trail craters). They often form entire linear clusters. Unlike true craters, pseudo-craters have no connection to the focus, so they do not belong to the volcano. We can observe the effects of the Threngslaborgir eruption of about 2,300 years ago south and east of Lake Myvatn. These are the linear craters of Threngslaborgir itself (Þrengslaborgir) and the pseudo-craters created after its eruption: Skutustadagigar (Skútustaðagígar) and Dimmuborgir (usually depicted as a lava field, as it is difficult to see craters there today).

Heiðarsporðar is the volcanic system that gave rise to Þrengslaborgir, Dimmuborgir and Skútustaðagígar. Þrengslaborgir is part of a row of craters formed during an eruption called Laxárhraun Yngra, which occurred about 2,300 years ago. Dimmuborgir and Skútustaðagígar are groups of pseudocraters that were formed by the same eruption.

Bergrún Arna Óladóttir ^[10]

Mixed volcanoes

Mixed volcanoes eject lava and ash alternately. In doing so, they form a distinct volcanic cone which structure consists of successive alternating layers. They are also known as stratovolcanoes, layered volcanoes or composite volcanoes.

selected types of volcanoes and their creation process

How some types of volcanoes are created

Eldfell

Eldfell volcano is located on the Icelandic island of Heimaey, in the Vestmannaeyjar, 8 km south of the main island of Iceland. It is precisely an efluvial-explosive volcano. At the same time, it is a central volcano and has a conical volcanic mountain of the scoria type. The lava coming out of the volcano is typical block lava, called apalhraun in Iceland. Because of its properties, its front gets overbuilt, making it hit with greater force. You can read more about the different types of lava in the following guide – Stunning effects of volcanism in Iceland.

Eldfell volcano

Even today, the Eldfell lava field is quite dangerous. The edges of the solidified lava fragments are very sharp and precarious. The 1973 eruption had a huge impact on the lives of Icelanders and caused great damage. Alkaline lava gushed out of the volcano for more than half a year. Great damage was also caused by the pyroclastic materials spewed during the eruption, which covered a nearby town (named after the archipelago and volcanic system on which it lies: Vestmannaeyjar).

A volcano with similar characteristics to Eldfell – i.e., also an eruptive, explosive volcano with a scoria-type cone – is also, for example, Grabrok. This volcano is much easier to access – it lies just off Road 1, about 5 km southwest of the junction with Road 60 and about 30 km northwest of Borgarnes, at the height of Cape Snaefellsnes. A parking lot can be found near the crater, and comfortable landings lead to the top of the volcano. The volcanic mountain is much lower than Eldfell, so we will climb it in a few minutes. There is an interesting view from the top – very clearly visible lava waves from the last eruption, and between them “huddled” modern farm buildings…. The Grábrók eruption is dated to about 3,600 years ago ^[10].

Hekla

Hekla is a famous, powerful and typical mixed volcano. It lies in southern Iceland, about 65 km east of the town of Selfoss and about 70 km northwest of Vik. Its eruptions are euphoric and explosive, and it ejects acid lava in addition to pyroclastic material. It is considered a stratovolcano, but it is located in a 5.5 kilometer long fissure, making it not shaped like a typical cone. Hekla’s shape is more like an upturned boat hull – it appears conical from the front, but has outstretched side ridges.

Hekla Volcano

In 1947-1948, there was as much as an 11-month eruption of Hekla, during which the column of ejected pyroclastic materials and ash in its initial phase was as high as 30 kilometers, and the ash reached as far as Finland. The earliest historical eruption of Hekla is dated to 1104, and it was even stronger. It is estimated that as much as 2.5 km³ of dust was ejected into the atmosphere then. Since then, 23 more eruptions have been recorded (including several in the late 20th century), making Hekla one of Iceland’s most active volcanoes. In total, it is estimated that for 1,000 years Hekla has produced 5 km³ of ash and 13-14 km³ of lava. We write more about Hekla in a separate article: Hekla – Queen of Icelandic Volcanoes.

Eyjafjallajokull

Eyjafjallajökull is one of the volcanoes enclosing the Thorsmork region to the south.

Eyjafjallajokull volcano as seen from Thorsmork

It lies in southern Iceland, about 35 km northwest of Vik and just above Skogar, northwest of the Skogafoss. This volcano became widely known in 2010, when it erupted after 187 years of dormancy, blocking air traffic in almost the entire northern hemisphere. The fountain of lava was 150 meters high and the ash cloud was as high as 7 kilometers / 4.5 miles, so it is an eruptive volcano. The lava spewed from a 100-meter fissure with twelve craters and had temperatures as high as 1,200 ºC / 2,200 ºF. Its slope has an angle of about 6º, and its caldera is 7 to 15 km / 4 to 9 miles wide in diameter. So it doesn’t fit very well into the basic classification, because it is a stratovolcano, but in appearance it looks more like a shield volcano. What is very interesting about it, however, is that it is a subglacial volcano, which is different from all previously described surface volcanoes.

F
or many tourists Eyjafjallajökull is an example of Icelandic impossible-to-pronounce proper names (side note: the double L – “ll” – is pronounced as “tl” in Icelandic). Meanwhile, according to many reports, Chuck Norris can pronounce this backwards 😉

Blafjall

The Bláfjall volcano lies about 15 km southwest of the Myvatn in northern Iceland, already mentioned here. It is one of Iceland’s many examples of a subglacial volcano. Because of the island’s climate, until very recently (on a geological time scale) virtually all of Iceland and all of its volcanoes were covered by glaciers. Today, the glaciers have largely retreated and only the highest ones are still covered by ice year-round. Most volcanic mountains, like Bláfjall, have not seen a glacier for a long time. However, we classify volcanoes as subglacial because of the conditions of their formation, not whether they are under a glacier still today. Therefore, Blafjall also belongs to this interesting group.

How the ‘table-top’ volcanoes come to be (Fraedrich, 2018)

Eruptions of subglacial volcanoes are completely normal, but the shape of the volcano itself is sometimes different if the lava outpourings were blocked by the surrounding glacier. In such a situation, in the first stage of an eruption, due to the contact between lava and ice, so-called pillow lava is formed. It then heats up the ice, which melts away. This creates a space into which the lava can now penetrate freely. Eventually a so-called table mountain is created – a solitary mountain with very steep slopes and a flattened or truncated peak.

Bláfjall volcano over Lake Mývatn

Another consequence of a volcano erupting under the ice can be catastrophic flooding. Icelanders even have a special word to describe it – it’s jökulhlaup. The remains of a destroyed bridge over the Skeiðará River near the waterfall of Svartifoss is the result of just such a flood from 1996.

You can read more about floods and other effects of volcanoes in the next guide in this series: Stunning Effects of Iceland’s Volcanism.

Effects of volcano activity in Iceland

So as you can see, a volcano is not only a powerful phenomenon, but also a very diverse one. Therefore, the effects of the existence and action of volcanoes are also very diverse. Undoubtedly, Iceland’s attractiveness is largely based precisely on the fact that there are so many different volcanic formations here, and that they are so fresh – sometimes not yet completely obliterated by the action of erosion.

Unusual effects of Iceland’s volcanism

The primary visible effect of volcanoes in Iceland is, of course, the volcanic mountains described in this article. But there are many more effects of their activity, and water heated to 180 ºC / 360 ºF is not the most surprising of them.

Do you know how many earthquakes are recorded daily (sic!) in Iceland? Or how the dragonglass (in The Game of Thrones) or Obsidian (The Lord of the Rings) was created? And how to move the coastline 800 meters in the fastest way? You will read about this and many other fascinating, though sometimes frightening, effects of Icelandic volcanism in the following article: Stunning effects of Iceland’s volcanism.

In the following article: Iceland’s biggest volcanic eruptions, you will read which volcanic eruption was the biggest, which was the strongest, which was the deadliest, and which was the most costly. We present 8 selected volcanoes in Iceland along with their most famous eruptions.

Bibliography

Czechowski, L. 1994. Plate tectonics and convection in the Earth’s mantle. Wydawnictwo Naukowe PWN, Warsaw
Fraedrich, W. 2018. Iceland from the West to the South. Springer International Publishing (27-92)
Fridriksson, T. 2006. CO₂ emissions and heat flow through soil, fumaroles, and steam heated mud pools at the Reykjanes geothermal area, SW Iceland. Elsevier
Jaroszewki, W., Marks, L., Radomski, A. 1985. Dictionary of Dynamic Geology. Geological Publishers, Warsaw.
Książkiewicz, M. 1979 Dynamic geology. Wydawnictwa Geologiczne, Warsaw
Lamur, A et al. 2018. Disclosing the temperature of columnar jointing in lavas. Nature
Pożaryski, W. 1977. Island in the light of plate tectonics. Geological Review
Stanley, S. M. 2002. Earth History. Publishing House of Science PWN, Warsaw
Thordarson, T., Larsen, D. 2006. Volcanism in Iceland in historical time: Volcano types, eruption styles and eruptive history. Elsevier
Directory of Icelandic Volcanoes – icelandicvolcanoes.is
USGS: What is the difference between an active volcano, a dormant volcano, and an extinct volcano?

How Iceland’s Volcanoes Work

What is a volcano

Volcano structure

Types of volcanoes in Iceland