Minor earthquake in Höfsjökull volcano. Continued earthquake activity in Tjörnes Fracture Zone

It has been quiet in Iceland for the past few weeks. Mostly due to bad weather. But that drops the SIL network ability to detect minor earthquake activity in Iceland.

Hofsjökull volcano

Hofsjökull volcano is among Iceland most quiet volcanoes. With no eruption documented in historical times. Hofsjökull volcano is also a interesting volcano. As it more like two volcanoes in one. Why that is I am not sure. Hofsjökull volcano is covered with a glacier that is up to 650 meters thick in the central caldera.

The earthquake in Hofsjökull volcano. Copyright of this image belongs to Icelandic Meteorological Office.

This earthquake was small. Just magnitude 1.3. But with the depth of 22,4 km. So it took place in the magma chamber, or close to it. Rather then any crustal earthquake activity. But it is hard to know for sure at the moment. During a normal year. Hofsjökull volcano can have up to 10 earthquakes during the year. Some years there are no earthquakes at all in Hofsjökull volcano.

Tjörnes Fracture Zone

Earthquake swarm that started on 20th of October in TFZ continues to this day. But currently the earthquake activity is just minor earthquakes at the moment. But the swarm is ongoing now for one month. There are also no signs of this earthquake swarm stopping. Even if the earthquakes are just minor ones for now.

Current earthquake activity in TFZ today and past few days. Copyright of this image belongs to Icelandic Meteorological Office.

I fully expect the earthquake activity to continue in TFZ. When it might stop is impossible to know for now. So keeping an eye of this area of TFZ is a good idea. The risk of large earthquake taking place in this area remains unchanged last I knew.

The dormant volcanoes in Iceland

While all the active volcanoes in Iceland get all the press and coverage. There are volcanoes in Iceland that are not so active and have remain dormant for thousands of years now and currently do not show any signs of activity. Some have some hot spring activity. But that is about it for them. The following volcanoes in Iceland have not erupted in historical times. Some have not even erupted in past 12.000 years best to my knowledge. But as history has proved in other parts of the world. An dormant volcano can start to erupt when it feels like it and without any warning at all in some cases.

This is based on Global Volcanism Program data. So it might be subject to errors and updates.


Hrómundartindur – Last eruption is unknown. This volcano is an stratovolcano.

Grensdalur – Last eruption was in Pleistocene and its current status is Pleistocene-Fumarolic. This volcano is an stratovolcano.

Grímsnes – Last eruption is believed to have taken place around 3500 BC. But this volcano is marked as tephrochronology. This volcano makes crater rows when it erupts.
Geysir – This volcano last erupted in Pleistocene and is marked as Pleistocene-Geysers. This is an stratovolcano.

Hveravellir – This volcano last erupted around the year 950 AD. It is currently marked as radiocarbon. This is an subglacial volcano.

Hofsjökull – This volcano has erupted sometimes in the last 12.000 years. It has the marking holocene. This is an subglacial volcano. The second volcano in this system is not marked in GVP database. But that volcano is named Kerlingarfjöll. I do not know when it last erupted. There is also an embeeded second volcano in Hofsjökull volcano that is Independent from the main Hofsjökull volcano (yes, this is the case it seems). I do not have any idea when that did last erupt.

Esjufjöll – This is an sub-glacier volcano in Vatnajökull glacier. Its last eruption is uncertain. But is believed to taken place in the year 1927. From the year 2000 this volcano has been showing signs of increased activity. This volcano is an stratovolcano.

Fremrinamur – This volcano is last believed to have erupted in the year 1200 BC. This volcano is an stratovolcano.

Þeystareykjabunga – This volcano last erupted around the year 900 BC. This volcano is an shield volcano.

Snæfellsjökull – This volcano last erupted around 200 AD. It is currently marked as radiocarbon. This is an stratovolcano.

Helgrindur (also known as Lysuhóll) – This volcano has no record of eruption. But it is thought that it erupted sometimes in the past 12.000 years. But exact time is not known. This volcano makes pyroclastic cones. This volcano is the smallest volcano system in Iceland.

Tindafjallajökull – It is unknown if this volcano has erupted during holocene. But the GVP information say that dozen of small eruptions took place in early holocene. This is an stratovolcano.

Snæfell – This volcano is located east of Vatnajökull glacier and south of Kverkfjöll volcano. It was once believed that this volcano was extinct. But new research into it has changed that opinion. This is most likely an stratovolcano. But I do not have it confirmed. This volcano has at least not erupted for several thousands years or more. Currently there are no information about this volcano in the Global Volcanism Program database.

Skrokkalda – This volcano is dormant. But I do not know if it has erupted in historical times or not, but I do no think that is the case. I am also not sure what type of volcano it is.

There are few more volcanoes missing due the fact that they are not in the Global Volcanism Program database. But I have them on a map that I own. So they are going to be added later when I update this blog post.

Overview of Hjofsjökull volcano activity

Hofsfjökull volcano is not among the most activate volcanoes in Iceland. It has never erupted since Iceland got inhabited. The last eruption that took place in Hofsjökull volcano is undated, but took place sometimes in the last 12.000 years or so.

Here is a short overview of the activity since the year 1999 in Hofsjökull volcano. All pictures are from the web site of Icelandic Met Office and copyright of them belongs to them.

Last earthquake swarm took place in Week 47, in the year 1999.

In Week 21, in the year 2008 a harmonic tremor pulse was detected from Hofsjökull volcano. This started at 13:30 UTC on the 24. May 2008.

SIL stations in Iceland showing the harmonic tremor pulse. Picture number one.

SIL stations in Iceland showing the harmonic tremor pulse. Picture number two.

Spectrograph from the SIL station Saurbær (south Iceland, located on the SISZ).

Spectrograph from the SIL station Skrokkalda. This station is located closest to Hofsjökull volcano, along with the SIL station at Hveravöllum hydrothermal area.

During this episode of harmonic tremor one earthquake was detected in north Hofsjökull volcano.

The one earthquake detected in Week 21 in the year 2008. On the week overview for Week 21 in the year 2008 they think that this might have been a swarm of small earthquakes that they where unable to locate. But I do not agree with that, given the data that is published on this event.

Other then this there have been few earthquakes every now and then in Hofsjökull volcano. But besides that it remains quiet and is most likely to be so for the time being.

Deep earthquake in Hofsjökull volcano

Among the most quiet volcanoes in Iceland is Hofsjökull volcano. It has not erupted in at least 12.000 years from it’s main crater (far as I know). But minor fissure eruptions have taken place in the last 12.000 years in the area around Hofsjökull volcano. But they are undated far as I know. Because of that it is now known when the last eruption took place in Hofsjökull volcano. In terms of Icelandic volcanoes, Hofsjökull volcano is more complex then most. South of it lies the Kerlingafjöll volcano. A volcano known for it’s heavy hydrothermal activity. But Kerlingafjöll volcano has not erupted in historical times in Iceland. But they might have erupted in the last 12.000 years without me knowing about it. In the case of Hofsjökull volcano looks are deceiving. According to a geological map that I have of Iceland, in the case of Hofsjökull volcano it has a second magma chamber and a central volcano inside the SW part of it. That volcano is unnamed for oblivious reasons. That volcano might well behave differently then Hofsjökull volcano it self. But I would theorise that this intergrated central volcano inside Hofsjökull volcano is going to be active when Hofsjökull volcano is.

The icecap in Hofsjökull volcano is deep. At it’s deepest point it is 650 meters deep at the most. The deepest points of the icecap are in the main caldera of Hofsjökull volcano. If Chaitén volcano in Chile has teaches geologist anything. It is that a long dormant volcanoes don’t need a long time to go from dormant to active eruption. That is why I find it worrying that deep earthquakes have started to appear in Hofsjökull volcano, even if they are extremely rare so far.

Over the past ten years or so there has been a slight increase in earthquake activity in Hofsjökull volcano along with increase in hydrothermal activity. But so far it has just been a minor earthquakes. The strongest ones getting up to ML3.0 at the strongest. No earthquake swarm have ever been recorded in Hofsjökull volcano to my knowledge (since recording begin in Iceland).. Last earthquake swarm was recorded in Hofsjökull volcano in the year 1999, then a earthquake swarm of 22 earthquakes took place, all of those earthquakes where just minor earthquakes. Besides being one of Iceland largest volcanoes, its location also makes it extremely dangerous. As it is located in the middle of Iceland, and a glacier flood from it could go many different ways down to the ocean and over large populated areas in Iceland. The increase in hydrothermal activity started in Hofsjökull volcano in the year 1994 or about that time. It was at least first documented in that year. A sulphur smell was recorded when the homoeothermal vent was checked out that year in Hofsjökull volcano. Increase in hydrothermal activity in Hofsjökull volcano is interesting as it is followed by a slight increase in earthquake activity over a long period of time.

Location of the two earthquakes. The deep earthquake is the one closer to the caldera. This picture is from Icelandic Met Office web site. The copyright of this picture belongs to them.

The earthquake that started me to get worrying about Hofsjökull volcano is the earthquake that took place 10:11 UTC on 5. April 2011. That earthquake had the depth of 27.6 km. It’s size was ML1.4, so it was a small earthquake. I do not believe that this depth is a result of a error in the SIL system (can happen sometimes). A second earthquake took place at 10:13 UTC, its size was ML1.6 with the depth of 1.6 km.

Whatever that is happening in Hofsjökull volcano is worth watching. Even if it not going to erupt just yet. It is also my opinion that Hofsjökull volcano is not part of Western Iceland Rift Zone (pdf). But its is the most southern part of a failed rift zone often called Skagafjörður volcanic zone (pdf). It is a failed volcanic zone that is mostly extinct, only Hofsjökull volcano and Kerlingarfjöll volcano remain active in that volcanic zone at the most southern end of it.

Other information.

Jarðhiti í Hofsjökli (Tímarit.is, Icelandic, 1996, pdf) – Article with picture of where the hydrothermal vent was found in Hofsjökull glacier.
Hofsjökull (Wiki, english)

Text updated at 18:11 UTC on 6. April 2011. Minor fixes in the text.

More intraplate earthquakes north of Langjökull / Hofsjökull volcanoes

Earlier this night (around 19:19 UTC 28th of October) more intraplate earthquake did start happening north of Langjökull / Hofsjökull volcanoes. The earthquakes are in the same place as before. But please see my earlier post for information about the earlier earthquake swarm at the same location.

Currently it seems that the earthquake swarm is growing in size and frequency of earthquakes. But the earthquakes appears clearly on my Hvammstangi station. The web page for my geophones can be viewed here. The distance of this earthquakes from my Hvammstangi station is about 40 to 80 km. But no more then that I think.

So far all the earthquakes have been less then ML3.0 in size. But if the swarm gets stronger that might change with out any warning. This area of Iceland is unpopulated so it is unlikely that anyone is going to feel those earthquakes at current size levels.