The Seal Nunataks are a group of 16

island An island (or isle) is an isolated piece of habitat that is surrounded by a dramatically different habitat, such as water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island ...

s called nunataks emerging from the

Larsen Ice Shelf The Larsen Ice Shelf is a long ice shelf in the northwest part of the Weddell Sea, extending along the east coast of the Antarctic Peninsula from Cape Longing to Smith Peninsula. It is named after Captain Carl Anton Larsen, the master of the No ...

east of

Graham Land Graham Land is the portion of the Antarctic Peninsula that lies north of a line joining Cape Jeremy and Cape Agassiz. This description of Graham Land is consistent with the 1964 agreement between the British Antarctic Place-names Committee an ...

, Antarctic Peninsula. The Seal Nunataks have been described as separate

volcanic vent A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates are ...

s of ages ranging from

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological 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 finally confirmed in ...

. There are unconfirmed reports of

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...

volcanic activity.

Geography and geomorphology

The Seal Nunataks are part of

and were embedded within the

until its northern margin collapsed between 1986 and 1996, rising from about below sea level. They were discovered by the Norwegian

Carl Anton Larsen Carl Anton Larsen (7 August 1860 – 8 December 1924) was a Norwegian-born whaler and Antarctic explorer who made important contributions to the exploration of Antarctica, the most significant being the first discovery of fossils for which ...

in December 1893, who also identified them as volcanoes and named them ''Seal Islands''. The Argentina Base Aérea Teniente Benjamín Matienzo is located in the Seal Nunataks. The Seal Nunataks are part of a volcanic province in West Antarctica which extends over the Antarctic Peninsula, Marie Byrd Land and Ross Island. In the Antarctic Peninsula,

Alexander Island Alexander Island, which is also known as Alexander I Island, Alexander I Land, Alexander Land, Alexander I Archipelago, and Zemlja Alexandra I, is the largest island of Antarctica. It lies in the Bellingshausen Sea west of Palmer Land, Antarc ...

James Ross Island James Ross Island is a large island off the southeast side and near the northeastern extremity of the Antarctic Peninsula, from which it is separated by Prince Gustav Channel. Rising to , it is irregularly shaped and extends in a north–south ...

and the Seal Nunataks form this volcanic province. Alexander Island was active between 48 and 18 million years ago. James Ross Island lies about northeast of the Seal Nunataks. The Seal Nunataks volcanic group contains at least sixteen volcanic cones, the largest of which is high Murdoch Nunatak. From north to south they are Lindenberg island, Larsen Nunatak,

Evensen Nunatak Evensen Nunatak () is a nunatak northwest of Dallmann Nunatak in the Seal Nunataks group, off the east coast of the Antarctic Peninsula. It was first charted by the Falkland Islands Dependencies Survey in August 1947, and named by them for Capt ...

, Dallmann and Murdoch Nunatak, Akerlundh,

Bruce The English language name Bruce arrived in Scotland with the Normans, from the place name Brix, Manche in Normandy, France, meaning "the willowlands". Initially promulgated via the descendants of king Robert the Bruce (1274−1329), it has been ...

and Bull Nunatak, Donald, Pollux and Christensen Nunatak, Arctowski and Gray Nunatak, Oceana Nunatak, Hertha Nunatak and Castor Nunatak. The nunataks consist of ridges with lengths of less than flanked with

scree Scree is a collection of broken rock fragments at the base of a cliff or other steep rocky mass that has accumulated through periodic rockfall. Landforms associated with these materials are often called talus deposits. Talus deposits typically ...

deposits and occasionally by primary volcanic features. According to

Otto Nordenskjöld Nils Otto Gustaf Nordenskjöld (6 December 1869 – 2 June 1928) was a Finnish and Swedish geologist, geographer, and polar explorer. Early life Nordenskjöld was born in Hässleby in Småland in eastern Sweden, in a Finland Swedish family th ...

in 1901-1903, Christensen Nunatak features a crater. Castor, Christensen and Hertha are the only vents which show evidence of subaerial activity in form of lava flows; elsewhere the nunataks are constructed by

hyaloclastite Hyaloclastite is a volcanoclastic accumulation or breccia consisting of glass (from the Greek ''hyalus'') fragments (clasts) formed by quench fragmentation of lava flow surfaces during submarine or subglacial extrusion. It occurs as thin margin ...

and

pillow lava Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of discont ...

s which form beneath ice. The volcanoes appear to align along northwest-striking lines.

Geology

The Antarctic Peninsula was a site of subduction from about 200 million years ago until 4 million years ago, when a number of

spreading ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diver ...

s collided with the subduction zone starting from 50 million years ago at

and caused it to cease. The

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...

became inactive; however volcanic activity continued in different form over the entire peninsula. In the case of the Seal Nunataks, volcanism commenced immediately after subduction had ceased and was probably facilitated by the presence of

fracture zone A fracture zone is a linear feature on the ocean floor—often hundreds, even thousands of kilometers long—resulting from the action of offset mid-ocean ridge axis segments. They are a consequence of plate tectonics. Lithospheric plates on eit ...

s in the already subducted

slab Slab or SLAB may refer to: Physical materials * Concrete slab, a flat concrete plate used in construction * Stone slab, a flat stone used in construction * Slab (casting), a length of metal * Slab (geology), that portion of a tectonic plate tha ...

. Alternatively, the development of a

slab window In geology, a slab window is a gap that forms in a subducted oceanic plate when a mid-ocean ridge meets with a subduction zone and plate divergence at the ridge and convergence at the subduction zone continue, causing the ridge to be subducted ...

may have aided in the onset of volcanism. There is only little evidence of the basement at

Robertson Island Robertson Island is an ice-covered island, long in a northwest-southeast direction and wide, lying at the east end of the Seal Nunataks off the east coast of the Antarctic Peninsula. Captain Carl Anton Larsen discovered Robertson Island fr ...

, where sediments of

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

age crop out. The tectonic structure in the area appears to be a

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...

bordered by Robertson Island in the south, the Larsen Rift.

Composition

The Seal Nunataks consist of

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

, basalt which contains

clinopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe I ...

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

and

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...

Xenolith A xenolith ("foreign rock") is a rock fragment ( country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in ig ...

s consisting of

lherzolite Lherzolite is a type of ultramafic igneous rock. It is a coarse-grained rock consisting of 40 to 90% olivine along with significant orthopyroxene and lesser amounts of calcic chromium-rich clinopyroxene. Minor minerals include chromium and alu ...

and spinel have been found at Seal Nunataks. Their geochemistry implies that the

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

s are primitive mantle-derived melts which underwent only a little fractional crystallization.

Eruptive history

Potassium-argon dating has yielded ages between 4 million years ago and "recent" for the rocks in the volcanic field, but the reliability of the dates decreases the younger they are. Pillow lavas are usually about 1.5 million years old while subaerial activity occurred 700,000 years ago. The youngest dates were obtained on Donald and Gray, the dates being less than 200,000 years ago.

Glacial A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betwe ...

erratics on some of the nunataks imply that they were covered with glaciers in the past. The discoverer Captain Larsen observed volcanic activity on Christensen Nunatak and Lindenberg Island, but the reliability of these reports is considered to be unclear.

Fumarole A fumarole (or fumerole) is a vent in the surface of the Earth or other rocky planet from which hot volcanic gases and vapors are emitted, without any accompanying liquids or solids. Fumaroles are characteristic of the late stages of volcani ...

s have been observed on Christensen, Dallman and Murdoch, and eruptions were reported on the last two in 1982. This activity consisted of the possible formation of lava flows and a

pyroclastic 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 ...

, respectively.

References

* * * * * {{refend Volcanoes of Graham Land Pyroclastic cones Landforms of Graham Land Islands of Graham Land Oscar II Coast

Geography and geomorphology

Geology

Composition

Eruptive history

See also

References