Keilir (; 378 m asl) is a

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

subglacial mound or perhaps a conical tuya 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 in

Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...

.''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 ''For the volcanic landforms around Krýsuvík, see also: Krýsuvík (volcanic system)'' Krýsuvík (also Krísuvík, both pronounced in Icelandic) is an area in Southwest Iceland at about 35 km from Reykjavík. Geography and access It i ...

and Reykjanesfólkvangur. It is about 17 miles southwest of the capital city Reykjavík, In March 2021 the mountain began to emit earthquakes followed by a fissure lava eruption further southwest at Fagradalsfjall.

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 (geology), rock layers (Stratum, strata) and layering (stratification). It is primarily used in the study of sedimentary rock, sedimentary and layered volcanic rocks. Stratigrap ...

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 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, they rem ...

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 moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

(area of lava cap 0,020 km2) which could mean that the volcanic mountain is a tuya (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 volcanic object created when magma hardens within a vent on an active volcano. When present, a plug can cause an extreme build-up of high gas pressure if rising volatile-charged mag ...

(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 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, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

and could determine the thickness of ice which covered the vent during eruption. The Antarctic cone did lie under a polar ice sheet and not a

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

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 plutonic complexes and seems to be a

parasitic cone A parasitic cone (also adventive cone or satellite 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 volcano. These fractures occur ...

of a

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

, 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 warrior's 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 v ...

Þráinskjöldur, though the last one is younger than the subglacial formation. Interestingly, similar tectonics, rift zones, are behind both expressions of volcanism: the West Antarctic Rift System 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 Lapilli 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'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli range f ...

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

(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 Reykjanes Peninsula 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. i ...

is to be expected from the top in good weather.

References

{{reflist Reykjanes Subglacial volcanoes of Iceland Mountains of Iceland Pleistocene volcanoes