The Seal Nunataks are a group of 16

island An island or isle is a piece of land, distinct from a continent, completely surrounded by water. There are continental islands, which were formed by being split from a continent by plate tectonics, and oceanic islands, which have never been ...

s called

nunatak A nunatak (from Inuit language, Inuit ) is the summit or ridge of a mountain that protrudes from an ice field or glacier that otherwise covers most of the mountain or ridge. They often form natural pyramidal peaks. Isolated nunataks are also cal ...

s 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 ...

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 ...

Antarctic Peninsula The Antarctic Peninsula, known as O'Higgins Land in Chile and Tierra de San Martin in Argentina, and originally as Graham Land in the United Kingdom and the Palmer Peninsula in the United States, is the northernmost part of mainland Antarctica. ...

. The Seal Nunataks have been described as separate

volcanic vent A volcano is commonly defined as a vent or fissure 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 fo ...

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 ( ; 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 ...

. There are unconfirmed reports of

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 ...

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 h ...

in December 1893, who also identified them as volcanoes and named them ''Seal Islands''. The Argentina

Base Aérea Teniente Benjamín Matienzo Matienzo Base (, or more often ''Base Matienzo'' or ''Estación Matienzo'') is an Argentine Antarctic base and scientific research station named after Lieutenant Benjamín Matienzo, an Argentine aviation pioneer. It is located in Larsen Nunatak ...

is located in the Seal Nunataks. The Seal Nunataks are part of a volcanic province in

West Antarctica West Antarctica, or Lesser Antarctica, one of the two major regions of Antarctica, is the part of that continent that lies within the Western Hemisphere, and includes the Antarctic Peninsula. It is separated from East Antarctica by the Transan ...

which extends over the

Marie Byrd Land Marie Byrd Land (MBL) is an unclaimed region of Antarctica. With an area of , it is the largest unclaimed territory on Earth. It was named after the wife of American naval officer Richard E. Byrd, who explored the region in the early 20th centu ...

and

Ross Island Ross Island is an island in Antarctica lying on the east side of McMurdo Sound and extending from Cape Bird in the north to Cape Armitage in the south, and a similar distance from Cape Royds in the west to Cape Crozier in the east. The isl ...

. 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, Antarcti ...

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–so ...

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 The Falkland Islands and Dependencies Aeri ...

, 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 Bull Nunatak () is a nunatak which lies west of Bruce Nunatak in the Seal Nunataks group, off the east coast of the Antarctic Peninsula. It was first charted in 1902 by the Swedish Antarctic Expedition under Otto Nordenskiöld, and named by him f ...

, Donald, Pollux and Christensen Nunatak, Arctowski and Gray Nunatak, Oceana Nunatak, Hertha Nunatak and Castor Nunatak. The nunataks are for the most part

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 consisting 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. The term ''scree'' is ap ...

deposits and occasionally by primary volcanic features. According to

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

in 1901-1903, Christensen Nunatak features a

crater A crater is a landform consisting of a hole or depression (geology), depression on a planetary surface, usually caused either by an object hitting the surface, or by geological activity on the planet. A crater has classically been described ...

. Castor, Christensen and Hertha are the only vents which show evidence of subaerial activity in form of

lava flow 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 ...

s; 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 marg ...

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 discontinu ...

s and dykes. The volcanoes appear to align along northwest-striking lines.

Geology

The

was a site of

subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

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 d ...

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 ...

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 that ...

. 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. Small volumes of magma ascending would generate the dispersed volcanic fields, like Seal Nunataks. There is only little evidence of the

basement A basement is any Storey, floor of a building that is not above the grade plane. Especially in residential buildings, it often is used as a utility space for a building, where such items as the Furnace (house heating), furnace, water heating, ...

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 from ...

, where sediments of

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

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-graben ...

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

Lineament ''See also Line (geometry)'' A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault. Typically a lineament will appear as a fault-aligned valley, a series of fault or fold-align ...

s associated with the former

of oceanic fracture zones west of the Antarctic Peninsula may have played a role in establishing the alignments of the Seal Nunataks volcanoes.

Composition

The Seal Nunataks consist of

basalt Basalt (; ) is an aphanite, 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 planetary surface, surface of a terrestrial ...

, basalt which contains

clinopyroxene The pyroxenes (commonly abbreviated 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 ions of calcium (Ca), sodium (Na), iron (Fe ...

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

and

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

Andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...

and

basanite Basanite () is an igneous, volcanic ( extrusive) rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoids, pyroxenes, olivine, and calcic plagioclase and forms from magma low in silica and enriched in alkali metal ox ...

have been reported as well. The rocks define numerous suites, both alkali basalt, subalkaline basalts and basaltic andesites are represented.

Xenolith A xenolith ("foreign rock") is a rock (geology), rock fragment (Country rock (geology), country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusi ...

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 alumi ...

and

spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , a diminutive form of ''spine,'' in reference to its pointed crystals. Prop ...

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 (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

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

fractional crystallization Fractional crystallization may refer to: * Fractional crystallization (chemistry), a process to separate different solutes from a solution * Fractional crystallization (geology) Fractional crystallization, or crystal fractionation, is one of the ...

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 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 ...

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 Lindenberg Island is a circular volcanic island in diameter, lying north of Robertson Island and some east-northeast of Cape Fairweather, off the east coast of the Antarctic Peninsula. It was discovered by a Norwegian whaling expedition unde ...

, 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 another 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

s 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. However, some reported fumarolic activity may be due to solar evaporation of snow, and tephra layers were only found in moraines, implying that it might not have been genuine volcanic eruptions.

References

* * * * * * * {{Authority control Volcanoes of Graham Land Pyroclastic cones Islands of Graham Land Oscar II Coast Nunataks of Graham Land