Keilir (; 378 m asl) is a

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 ...

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 ...

or perhaps a conical

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 ...

G.B.M.Pedersen, P. Grosse: ''Morphometry of subaerial shield volcanoes and glaciovolcanoes from Reykjanes Peninsula, Iceland: Effects of eruption environment. Journal of Volcanology and Geothermal Research'' 282, (2014), 115-133.
See also for maps therein. 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 og sérkenni''. Reykjavík 1989, p. 53 Basal area is 0.773 km2, summit area 0.004 km2, basal width 0.99 km, summit width 0.07 km, volume 0.0362 km3. It is located within the area of Krýsuvík volcanic system and Reykjanesfólkvangur. It is about 17 miles southwest of the capital city

Reykjavík Reykjavík is the Capital city, capital and largest city in Iceland. It is located in southwestern Iceland on the southern shore of Faxaflói, the Faxaflói Bay. With a latitude of 64°08′ N, the city is List of northernmost items, the worl ...

, In March 2021 the mountain began to emit earthquakes followed by a fissure lava eruption further southwest at

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 ...

Formation

Keilir was formed during a subglacial

fissure eruption A fissure vent, also known as a volcanic fissure, eruption fissure or simply a fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is often a few metres wide and may be many kilom ...

which concentrated in the end at one vent.Snæbjörn Guðmundsson: ''Vegavísir um jarðfræði Íslands.'' Reykjavík 2015, p. 22-23 Except the cone, also some small subglacially formed hills to the north are results of this eruption.Ari Trausti Guðmundsson, Pétur Þorsteinsson: ''Íslensk fjöll. Gönguleiðir á 152 tind.'' Reykjavík 2004, p. 156-157

Eruptions under the Weichselian glaciers on Reykjanes Peninsula

When

stratigraphy Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...

is considered in detail, it tells about the different parts of this eruption: The eruption thawed the glacier ice and formed a subglacial lake in which the volcano continued to develop. The water very soon touched the magma within the vent and caused explosive activity.

Tephra Tephra is fragmental material produced by a Volcano, volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, ...

set down in layers into the subglacial lake. With time, the tephra built up a hill and small elongated mountain over the vent(s). When such an eruption is continued over a longer time span, the water in the end does not reach the vent(s) any more and lava begins to flow. In the top region of Keilir, there is a small cap of

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

(area of lava cap 0,020 km2) which could mean that the volcanic mountain is a

(the lava being from subaerial eruptions at the end of the eruption series) or perhaps just represents a

volcanic plug A volcanic plug, also called a volcanic neck or lava neck, is a volcano, volcanic object created when magma hardens within a Volcanic vent, vent on an active volcano. When present, a plug can cause an extreme build-up of high gas pressure if risi ...

(the lava cooled and plugged up the vent). Ice thickness and more exact time of eruption in the case of Keilir are not known, just that it took place during the

( Weichselian).

Comparison to an Antarctic subglacial tuff cone

''Smellie'' and other scientists newly discovered a similar monogenetic subglacial

tuff cone Volcanic cones are among the simplest volcanic landforms. They are built by ejecta from a volcanic vent, piling up around the vent in the shape of a cone with a central crater. Volcanic cones are of different types, depending upon the nature and s ...

within the ice of

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...

and could determine the thickness of ice which covered the

vent Vent or vents may refer to: Science and technology Biology *Vent, the cloaca region of an animal *Vent DNA polymerase, a thermostable DNA polymerase Geology *Hydrothermal vent, a fissure in a planet's surface from which geothermally heated water ...

during eruption. The Antarctic cone did lie under a polar

ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacier, glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are the Antarctic ice sheet and the Greenland ice sheet. Ice s ...

and not a

temperate glacier A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires d ...

as was the case with Keilir. Additionally, the Antarctic cone is much older (around 640 000 years), whereas Keilir is up to 100 000 years old. And the tuff cone from Victoria Land, Antarctica, is placed next to known

granitic A granitoid is a broad term referring to a diverse group of coarse-grained igneous rocks that are widely distributed across the globe, covering a significant portion of the Earth's exposed surface and constituting a large part of the continental ...

plutonic complexes and seems to be a

parasitic cone A parasitic cone (also adventive cone, satellite cone, satellitic cone or lateral cone) is the cone-shaped accumulation of volcanic material not part of the central vent of a volcano. It forms from eruptions from fractures on the flank of the ...

of a

stratovolcano A stratovolcano, also known as a composite volcano, is a typically conical volcano built up by many alternating layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with ...

, whereas Keilir is located adjacent to presently active volcanic systems and looks as if it were placed on top of the

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 ...

Þráinskjöldur, though the last one is younger than the subglacial formation. Interestingly, similar tectonics,

rift zone A rift zone is a feature of some volcanoes, especially shield volcanoes, in which a set of linear cracks (or rifts) develops in a volcanic edifice, typically forming into two or three well-defined regions along the flanks of the vent. Believed ...

s, are behind both expressions of volcanism: the

West Antarctic Rift System The West Antarctic Rift System is a series of rift valleys between East and West Antarctica. It encompasses the Ross Embayment, the Ross Sea, the area under the Ross Ice Shelf and a part of Marie Byrd Land in West Antarctica, reaching to th ...

on the one hand,J. L. Smellie, etal.: ''A tuff cone erupted under frozen-bed ice (northern Victoria Land, Antarctica): linking glaciovolcanic and cosmogenic nuclide data for ice sheet reconstructions.'' (2017).
Retrieved 25 August 2020. and in the case of Keilir the Reykjanes Rift as part of the rift zones crossing Iceland. Also similar are their rocks –

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

lapilli tuff Lapilli (: lapillus) is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' is Latin for "little stones". By definition lapilli range from in diam ...

(in Icelandic: móberg). Helgi Páll Jónsson: ''Eldfjallagarður og jarðminjar á Reykjanesskaga.'' MS ritgerð. Leiðbeinendur Ólafur Ingólfsson, prófessor. Jarðvísindadeild Verkfræði- og náttúruvísindasvið Háskóli Íslands. (2011)
Retrieved 25 August 2020.

Landmark

As the mountain is easily recognized from afar, eg. from Reykjavík, but also from sea, it was used as a landmark by fishermen and seamen for many centuries.

Hiking

There is a hiking trail leading up on the mountain from ENE. At the top, a guestbook can be found within a box. An excellent view over a big part of

and

Faxaflói Faxaflói (), sometimes Faxa Bay, Faxe Bay or Faxi Bay,Thorstein Thorsteinsson. 1930. ''Iceland, 1930: A Handbook Published on the Fortieth Anniversary of Landsbanki Íslands (National Bank of Iceland)''. Reykjavík: Ríkisprentsmidjan Gutenberg. ...

is to be expected from the top in good weather.

References

{{reflist Southern Peninsula (Iceland) Subglacial volcanoes of Iceland Mountains of Iceland Pleistocene volcanoes