Þorbjörn () is a 243 m high volcanic mountain next to the town of

Grindavík Grindavík () is a fishing town in the Southern Peninsula district of Iceland, not far from Þorbjörn, a tuya (a type of flat-topped, steep-sided volcano). It is one of the few towns with a harbour on this coast. Most of the inhabitants work ...

( Gullbringusýsla) on

Reykjanes peninsula Southern Peninsula (, ) is an administrative unit and part of Reykjanesskagi (pronounced ), or Reykjanes Peninsula, a region in southwest Iceland. It was named after Reykjanes, the southwestern tip of Reykjanesskagi. The region has a populatio ...

Iceland Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...

.Íslandshandbókin. Náttúra, saga of sérkenni. Reykjavík 1989, p. 65 Blue Lagoon can be easily seen from the summit.

Name

The name of Þorbjörn or ''Þorbjarnarfell'' , the “mountain of Þorbjörn” is a popular man's name in Iceland connected with the son of a farmer in the region. When a group of bandits were tyrannizing the farmers in the area and disappeared into hiding after their raids, the young man pretended to be part of the group and discovered their hiding place in a cave within the mountain. The bandits were captured and hanged. Since then, the small canyon within the mountain is called “Thieves’ Canyon” (''Þjófagjá'' ).

History

The

U.S. Army The United States Army (USA) is the primary land service branch of the United States Department of Defense. It is designated as the Army of the United States in the United States Constitution.Article II, section 2, clause 1 of the United Stat ...

built a jeep track up the mountain during

World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

, which remains. Transmitters from the telephone company Sími and Icelandic National Television

RÚV Ríkisútvarpið (, ; abbr. RÚV ) is Iceland's national public broadcasting, public-service broadcasting organization. Founded in 1930, it operates from studios in the country's capital, Reykjavík, as well as regional centres around the count ...

occupy the site where the

U.S. Navy The United States Navy (USN) is the maritime service branch of the United States Department of Defense. It is the world's most powerful navy with the largest displacement, at 4.5 million tons in 2021. It has the world's largest aircraft ...

had installed radar stations. These were given to the Icelandic Government when the US Navy left the country in 2006.

Geography

The mountain is near the tip of

Reykjanes Reykjanes () is a small headland on the south-western end of the Reykjanes Peninsula in Iceland, giving the main peninsula its name. Volcanic action is responsible for forming the entire peninsula. The nearest town is Keflavik. The name, , trans ...

peninsula between Svartsengi Power Station with the Blue Lagoon and the town of Grindavík at road 43 to the east. Another road, route 426, runs around its western side from Grindavík to Svartsengi.

Geology

Þorbjörn grew from subglacial eruptions during a cold spell in the

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

. The subglacial volcano is located within the

Reykjanes volcanic system Reykjanes () is a small headland on the south-western end of the Reykjanes Peninsula in Iceland, giving the main peninsula its name. Volcanic action is responsible for forming the entire peninsula. The nearest town is Keflavik. The name, , tran ...

or Svartsengi volcanic system, depending on author, and enclosed by

Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

lava fields. A visible tectonic

graben In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...

runs over the top of the mountain forming a small canyon, up to 80 m deep. The mountain is a symbol of Reykjanes' geology.Reynir Ingibjartsson: 25 Gönguleiðir á Reykjanesskaga. Náttúrann við Bæjarveggin. Reykjavík , pp. 70-75

Pleistocene volcanoes on Reykjanes peninsula

volcanism on the Reykjanes peninsula is represented by the larger

shield volcano A shield volcano is a type of volcano named for its low profile, resembling a shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more viscous lava ...

es and subglacially-formed ridge volcanoes or

tuya A tuya is a flat-topped, steep-sided volcano formed when lava erupts through a thick glacier or ice sheet. They are rare worldwide, being confined to regions which were covered by glaciers and had active volcanism during the same period. As lava ...

s. Analysis of aerial photographs of shield volcanoes and glaciovolcanic edifices makes clear that the latter have much steeper slopes. Reykjanes has several kinds of subglacial volcanoes: ''tindars'', also called

subglacial mound A subglacial mound is a type of subglacial volcano formed when lava erupts beneath a thick glacier or ice sheet. The magma forming these volcanoes was not hot enough to melt a vertical pipe right through the overlying glacial ice, instead forming ...

s, originating in fissure eruptions under glaciers, ''flat-topped tuyas'' ( Geitahlíð), ''complex tuyas'' and even ''conical tuyas'' (

Keilir Keilir (; 378 m asl) is a Pleistocene subglacial mound or perhaps a conical tuya ''Tindars/subglacial mounds/hyaloclastic ridge''s originate from subglacial fissure "eruptions" which never emerged. The retreat of the Pleistocene glaciers and the drop in sea level allows these mountains to be researched. Whereas the magma of

s has enough pressure, viscosity or volume to build up a subglacial lake of its own meltwater, it later pushes through the water and ice to build up tephra and/or lava layers on top as well as lava deltas at its slopes.

Formation of the Þorbjörn tuya

It also built up a meltwater lake that Pedersen et al. define as “a pillow-dominated flat-topped tuya without a lava cap”. H. Björnsson explains that though the mountain is crossed by many faults and a graben, so that it looks like a complex formation, its origin is in a single eruption. Haukur Björnsson: Myndun Þorbjarnarfells. BS ritgerð. Leiðbeinandi Ármann Höskuldsson. Jarðvísindadeild Háskóli Íslands 2015
(in Icelandic, abstract also in English) The pillows are formed and cooled in two stages: First the lava touches water and cools down rapidly. In the second step after pillow layers have formed, the meltwater lake or water (if the eruption is underwater in the sea or a lake) cools it further. On the other hand, whether the eruption turns explosive and forms hyaloclastite depends only on the magma pressure. In the upper part of Þorbjörn, there is also a rather eroded crater but, probably because of the erosion, no trace of subaerial lavas has been found. The tuya is slightly elongated in the direction of the volcanic fissure systems of Reykjanes, i.e. southwest to northeast.

Activity since 2020

As is often the case on Reykjanes peninsula, swarms of small

earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...

s and associated ground uplift, thought to be linked to

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

intrusions In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of Intrusive rock, intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety o ...

, began in the region in 2020, and again in 2023. In January 2020, the instruments of IMO (the Icelandic Met Office, in Icelandic: Veðurstofa Íslands) and other

volcano monitoring Prediction of volcanic activity, and volcanic eruption forecasting, is an interdisciplinary monitoring and research effort to predict the time and severity of a volcano's eruption. Of particular importance is the prediction of hazardous eruptions ...

stations showed ground uplift around and to the west of Þorbjörn. The uplift halted after some time and then restarted. Together with repeated earthquake swarms in the region, but also at the tip of Reykjanes peninsula ( Gunnuhver, Sudurnes Geothermal Power Station) as well as under

Fagradalsfjall Fagradalsfjall () is an active tuya volcano formed in the Last Glacial Period on the Southern Peninsula (Iceland), Reykjanes Peninsula, around from Reykjavík, Iceland. Fagradalsfjall is also the name for the wider volcanic system covering an ...

which some authors classify as part of the

Krýsuvík Krýsuvík (also Krísuvík, both pronounced in Icelandic language, Icelandic) is an area in Southwest (Icelandic constituency), Southwest Iceland at about 35 km from Reykjavík. Geography and access It is situated on the Reykjanes penins ...

volcanic system, but others see as an independent volcanic system, more uplift and earthquakes were measured where three smaller volcanic eruptions took place in the years 2021, 2022 and 2023. The volcano-tectonic movements which seem to touch a rather large area were still ongoing in November 2023. Observed in early 2020: * end of January 2020: uplift and earthquakes around Þorbjörn; per day, but gas measurements don't indicate magmatic intrusions near the surface. * mid February 2020: uplift at Þorbjörn stops Veðurstofa Íslands:Stór skjálfti við Grindavík (12.3.2020) Retrieved 6 August 2020.
/ref> * 12 March 2020: A rather heavy earthquake took place not far from

, it was first measured at , but corrected down to later on Veðurstofa Íslands: Landris hafið að nýju við Þorbjörn á Reykjanesi. (17.3.2020) Retrieved 6 August 2020.
/ref> * 17 March 2020: Uplift was measured again at Þorbjörn. But it was slower now. The uplift could be caused by rising magma, but such intrusive events can repeat itself for a rather long time, even years, without any eruptions. * 2 April 2020: A new intrusion was discovered on Reykjanes, in this case at Sýrfell under the tip of the peninsula, i.e. some km to the west of Þorbjörn.
Veðurstofa Íslands:Vísbendingar um nýtt kvikuinnskot á Reykjanesi. (2.4.2020) Retrieved 6 August 2020.
/ref> It stopped in the middle of the month. *These were the results of an interdisciplinary conference in Iceland on 8 April 2020: From January to mid April 2020, around 8,000 earthquakes were registered at the

. This was the most important earthquake series in this region since the first earthquakes measured there. At the same time, uplift was about around Þorbjörn caused by a magmatic intrusion, a sill, at a depth of . Another intrusion was found around the tip of the peninsula under the mountain Sýrfell. It lies deeper at about which means at the border between crust and mantle in this region. The third intrusion was found near Þorbjörn and uplift had not terminated by mid-April 2020. Uplift and intrusion had started on 6 March 2020. It is thought to be another sill, this time at a depth of , but uplift is much slower than with the first intrusion there. The earthquakes are thought to result from strain release. On Reykjanes peninsula, earthquake series are not rare. There was much activity, for instance from 1927 to 1955 and from 1967 to 1977, including a earthquake in 1929 and a 6.0 in 1968, both within the

Brennisteinsfjöll Brennisteinsfjöll (, "Sulfur mountains"
Brennisteinsfjöll. Detailed description. In: Catalogue of Icelandic Volcano ...

. * End of May, beginning of June 2020: Over 700 – mostly small earthquakes measured around Þorbjörn. Uplift started again around the mountain. *End of July 2020: Another earthquake series took place in the region. This time, the

hypocentre A hypocenter or hypocentre (), also called ground zero or surface zero, is the point on the Earth's surface directly below a nuclear explosion, meteor air burst, or other mid-air explosion. In seismology, the hypocenter of an earthquake is its p ...

was below the mountain Fagradalsfjall. Movements were measured at a known fault. The region is well known for repeated earthquake series. A small amount of subsidence was simultaneously registered at Svartsengi.Veðurstofa Íslands: Jarðskjálftavirknin við Fagradalsfjall fer dvínandi.(23.7.2020) Retrieved 6 August 2020.
/ref> The newest events are interpreted as part of the overall volcano-tectonic activity registered on Reykjanes since 2019. This involves four volcanic systems:

Eldey Eldey () is a small, uninhabited island about off the coast of the Reykjanes Peninsula in southwest Iceland. Located west-southwest of Reykjavík, the island of Eldey covers an area of about , and rises to a height of . Its sheer cliffs are h ...

(a small island on

Reykjanes Ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a divergent or constructive plate boundary) located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. In the North Atlantic, the ridge separates the North Am ...

, most of the system is

submarine A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or infor ...

), Reykjanes, Svartsengi (seen by some as part of the Reykjanes system) and Krýsuvík. The

is volcano-tectonically very active. It is the part of Iceland where the

Mid-Atlantic Ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a Divergent boundary, divergent or constructive Plate tectonics, plate boundary) located along the floor of the Atlantic Ocean, and part of the List of longest mountain chains on Earth, longest mountai ...

effectively comes ashore. The previous large eruption series in this region took place in the 13th century (the ''

Reykjanes Fires The Reykjanes Fires (Icelandic language, Icelandic: ''Reykjaneseldar'') were a series of volcanic eruptions that took place on the Reykjanes Peninsula in south-west Iceland between approximately 1210 and 1240. They caused widespread physical and e ...

'', 1220–1240). Thor Thordarson, Armann Hoskuldsson: Iceland. Classic geology of Europe 3. Harpenden 2002, pp. 64-65

Hiking

Some hiking trails lead onto the mountain, e.g. from the southwest.

External links

A.S. Fontaine: ''Mt. Thorbjorn, volcano in Reykjanes: What you need to know.'' In:'' Grapevine''. (17 January 2020)Geologist Páll Einarsson explains the geology of Reykjanes as well as the 2020 earthquake series

. 20 October 2020. (in Icelandic)

References

{{DEFAULTSORT:Thorbjorn (mountain) Þorbjörn Þorbjörn Þorbjörn Þorbjörn Þorbjörn Þorbjörn Pleistocene volcanoes