Viedma () is a

subglacial volcano A subglacial volcano, also known as a glaciovolcano, is a volcanic form produced by subglacial eruptions or eruptions beneath the surface of a glacier or ice sheet which is then melted into a lake by the rising lava. Today they are most ...

whose existence is questionable. It is supposedly located below the ice of the

Southern Patagonian Ice Field The Southern Patagonian Ice Field ( es, Hielo Continental or '), located at the Southern Patagonic Andes between Chile and Argentina, is the world's second largest contiguous extrapolar ice field. It is the bigger of two remnant parts of the Pat ...

, an area disputed between

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

and

Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...

. The 1988 eruption deposited ash and pumice on the

ice field An ice field (also spelled icefield) is a mass of interconnected valley glaciers (also called mountain glaciers or alpine glaciers) on a mountain mass with protruding rock ridges or summits. They are often found in the colder climates and highe ...

and produced a mudflow that reached

Viedma Lake Viedma Lake ( es, Lago Viedma, ) is a Patagonian lake in the province of Santa Cruz, Argentina, situated near its border with Chile. Measuring approximately 50 miles (80 kilometers) in length, it is a major elongated trough lake formed from melt ...

. The exact position of the edifice is unclear, both owing to the ice cover and because the candidate position, the "Viedma

Nunatak A nunatak (from Inuit ''nunataq'') 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 are also called glacial islands. Examples are natural pyramidal peaks. ...

", does not clearly appear to be of volcanic nature.

Geography and geomorphology

Viedma is located in the southern

Patagonian Andes Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and gl ...

, southwest of

Mount FitzRoy Monte Fitz Roy (also known as Cerro Chaltén, Cerro Fitz Roy, or simply Mount Fitz Roy) is a mountain in Patagonia, on the border between Argentina and Chile.lake of the same name lies southwest of the volcano. The area is poorly accessible and the volcanic history poorly known. Viedma is part of the

Austral Volcanic Zone The Andean Volcanic Belt is a major volcanic belt along the Andes, Andean cordillera in Argentina, Bolivia, Chile, Colombia, Ecuador, and Peru. It is formed as a result of subduction of the Nazca Plate and Antarctic Plate underneath the South Am ...

. This volcanic zone consists of six volcanoes, from north to south

Lautaro Lautaro (Anglicized as 'Levtaru') ( arn, Lef-Traru " swift hawk") (; 1534? – April 29, 1557) was a young Mapuche toqui known for leading the indigenous resistance against Spanish conquest in Chile and developing the tactics that would conti ...

, Aguilera, Viedma, Reclus,

Monte Burney Monte Burney is a volcano in southern Chile, part of its Austral Volcanic Zone which consists of six volcanoes with activity during the Quaternary. This volcanism is linked to the subduction of the Antarctic Plate beneath the South America Plate ...

and

Fueguino Fueguino is a volcanic field in Chile. The southernmost volcano in the Andes, it lies on Tierra del Fuego's Cook Island and also extends over nearby Londonderry Island. The field is formed by lava domes, pyroclastic cones, and a crater lake. ...

. These volcanoes form a long chain of volcanoes, the most southern of which is a

volcanic complex A complex volcano, also called a compound volcano or a volcanic complex, is a mixed landform consisting of related volcanic centers and their associated lava flows and pyroclastic rock. They may form due to changes in eruptive habit or in t ...

lava dome In volcanology, a lava dome is a circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6% of eruptions on ...

s and

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

s on Cook Island. Few things are known with certainty about the volcanic edifice of Viedma as it is mostly buried beneath glacial ice and the "Viedma Nunatak" was later revealed to not be a volcano. In 1956 Louis Lliboutry proposed that volcanic activity may occur in

fissure vent 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 eruption, explosive activity. The vent is often a few metres wide an ...

s buried beneath glaciers; between eruptions they would be concealed below the ice; Lliboutry considered dark bands on the ice to be

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 ...

deposits, a view supported by a 1958-1959 expedition that found

pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular vol ...

on the

Viedma Glacier Viedma Glacier is a large glacier that is part of the huge Southern Patagonian Ice Field, located at the southern end of mainland South America. Viedma Glacier is a valley glacier and its moraine-rich terminus flows into the western end of Lake ...

. Other reports of volcanic phenomena in the region exist, and the existence of a large

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

beneath the

has been suggested as an explanation for volcanism there.

Viedma Nunatak

The Viedma

is commonly interpreted to be the site of the volcano, after observations made in 1944-1945 and 1950 by survey flights including controversial sightings of

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 which also led to the nunatak becoming known as the "Volcan Viedma"; but the lack of clear evidence and the difficulty of field sampling has rendered its identification with the Viedma vent contentious. The nunatak is about high above sea level. Most of the mountain is buried beneath the

of the

and only parts of it crop out. The outcropping nunatak is elongated in north-south direction. A number of structures interpreted as

crater Crater may refer to: Landforms *Impact crater, a depression caused by two celestial bodies impacting each other, such as a meteorite hitting a planet *Explosion crater, a hole formed in the ground produced by an explosion near or below the surfac ...

s and concave depressions are found especially on the southern part of the volcano, some of which are lined up in north-south direction. The northern sector of the volcano was apparently more heavily affected by glacial erosion; conversely, several craters in the southern sector appear to be young. The nunatak shows clear evidence of glacial action, including

glacial striation Glacial striations or striae are scratches or gouges cut into bedrock by glacial abrasion. These scratches and gouges were first recognized as the result of a moving glacier in the late 18th century when Swiss alpinists first associated them w ...

s and coverage by

glacial drift In geology, drift is a name for all sediment (clay, silt, sand, gravel, boulders) transported by a glacier and deposited directly by or from the ice, or by glacial meltwater. Drift is often subdivided into (unsorted and) unstratified drift (glaci ...

s; the Viedma glacier may once have crossed the nunatak in its central part. Rock samples taken from the nunatak are

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...

rocks, including

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...

and

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...

s, and no evidence of magmatic rocks was found in a 1958-1959 expedition. Different observations have yielded different sizes and numbers of supposed craters; González-Ferrán ''et al.'' 1995 reported several craters and

s with sizes ranging , while Kobayashi ''et al.'' 2010 observed fewer craters and none of them larger than . These craters were later interpreted as being actually glacial

cirque A (; from the Latin word ') is an amphitheatre-like valley formed by glacial erosion. Alternative names for this landform are corrie (from Scottish Gaelic , meaning a pot or cauldron) and (; ). A cirque may also be a similarly shaped landform ...

s containing tarn lakes.

Geology

Off the southernmost west coast of

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...

, the

Antarctic Plate The Antarctic Plate is a tectonic plate containing the continent of Antarctica, the Kerguelen Plateau, and some remote islands in the Southern Ocean and other surrounding oceans. After breakup from Gondwana (the southern part of the superconti ...

subduct Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

s beneath the

South American Plate The South American Plate is a major tectonic plate which includes the continent of South America as well as a sizable region of the Atlantic Ocean seabed extending eastward to the African Plate, with which it forms the southern part of the Mid-A ...

at a rate of . This subduction process is responsible for volcanism in the

. The Austral Volcanic Zone is one of four volcanic zones in the Andes, the other three are the

Northern Volcanic Zone The Andean Volcanic Belt is a major volcanic belt along the Andean cordillera in Argentina, Bolivia, Chile, Colombia, Ecuador, and Peru. It is formed as a result of subduction of the Nazca Plate and Antarctic Plate underneath the South Ame ...

, the

Central Volcanic Zone The Andean Volcanic Belt is a major volcanic belt along the Andean cordillera in Argentina, Bolivia, Chile, Colombia, Ecuador, and Peru. It is formed as a result of subduction of the Nazca Plate and Antarctic Plate underneath the South Americ ...

and the

Southern Volcanic Zone The Andean Volcanic Belt is a major volcanic belt along the Andean cordillera in Argentina, Bolivia, Chile, Colombia, Ecuador, and Peru. It is formed as a result of subduction of the Nazca Plate and Antarctic Plate underneath the South American ...

, all of which are separated from the Austral Volcanic Zone and each other by gaps where no volcanic activity occurs. Unlike the Austral Volcanic Zone, volcanism in these other zones is controlled by the subduction of the

Nazca Plate The Nazca Plate or Nasca Plate, named after the Nazca region of southern Peru, is an oceanic tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction, along the Peru–Chile Trench, of the Na ...

beneath the South American Plate. The Austral Volcanic Zone was identified as such in 1976, but some volcanoes were identified and localized later. Owing to their similar composition the three northerly volcanoes Lautaro, Viedma and Aguilera are grouped as the "northern Austral Volcanic Zone". Volcanism in the Austral Volcanic Zone is not well known before the

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 togethe ...

; it is likely that a lull of volcanism occurred before the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Chile Triple Junction The Chile Triple Junction (or Chile Margin Triple Junction) is a geologic triple junction located on the seafloor of the Pacific Ocean off Taitao and Tres Montes Peninsula on the southern coast of Chile. Here three tectonic plates meet: the South ...

was being subducted in the region.

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 from the Austral Volcanic Zone are adakitic owing to the melting of the subducting

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 i ...

and the interaction of these melts with the crust and mantle. The

basement A basement or cellar is one or more floors of a building that are completely or partly below the ground floor. It generally is used as a utility space for a building, where such items as the furnace, water heater, breaker panel or fuse box, ...

consists of

rocks of

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

age. Outcrops of the basement are found around the volcano and could even occur on the edifice that crops out from the glaciers. The regional basement is formed by these Paleozoic and younger metamorphic and sedimentary rocks as well.

Composition

Viedma like other volcanoes of the Austral Volcanic Zone has erupted

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 predomi ...

and

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...

Phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

s include

amphibole Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is A ...

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

hypersthene Hypersthene is a common rock-forming inosilicate mineral belonging to the group of orthorhombic pyroxenes. Its chemical formula is . It is found in igneous and some metamorphic rocks as well as in stony and iron meteorites. Many references have f ...

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 prope ...

;

orthoclase Orthoclase, or orthoclase feldspar (endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles t ...

, plagioclase and

pyroxene 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 II) ...

also occur as

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 igne ...

s. The rocks form a calcalkaline suite, but there is also an adakitic signature. The xenoliths may reflect a crustal contamination of the magma erupted at Viedma.

Eruption history

Tephra attributed to Viedma has been found in the

Laguna Potrok Aike Potrok Aike is a maar in the Patagonian province of Santa Cruz, Argentina. It has a roughly circular shape and an average diameter of 2,700 metres. It is located just 2,200 metres north of the Argentina-Chile border. It is a maar lake and part of ...

, including three tephra layers 26,991 - 29,416, 40,656 - 48,219 and 41,555 - 57,669 years

before present Before Present (BP) years, or "years before present", is a time scale used mainly in archaeology, geology and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Becaus ...

which may originate either on Viedma or on

. Viedma has erupted during the

; a tephra found in Lago Cardiel and dated 3,345 - 3,010 years

may have originated at Viedma, although Aguilera and Lautaro are also candidate source volcanoes. Due to the absence of traces of volcanism at Viedma Nunatak the

Global Volcanism Program The Smithsonian Institution's Global Volcanism Program (GVP) documents Earth's volcanoes and their eruptive history over the past 10,000 years. The mission of the GVP is to document, understand, and disseminate information about global volcanic a ...

removed its entry for Viedma in 2019.

Historical

Many observations referring to volcanic activity on the

exist, including reports of ashfalls, layers of tephra on glaciers and columns of smoke rising from the ice. A

subglacial eruption Subglacial eruptions, those of ice-covered volcanoes, result in the interaction of magma with ice and snow, leading to meltwater formation, jökulhlaups, and lahars. Flooding associated with meltwater is a significant hazard in some volcan ...

occurred in 1988, depositing ash and

on the Viedma Glacier. These materials later gave rise to a

lahar A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley. Lahars are extreme ...

. The eruption had melted part of the ice and formed a network of valleys; it was assumed that it had taken place at some point between September and November of that year.

Hazards

There are no major population centres close to any volcano in the Austral Volcanic Zone, and the volcanoes are largely unmonitored. Among the know eruptions are large

explosive eruptions, while historical eruptions took place in 1908 at Reclus and 1910 at Monte Burney. Future eruptions of volcanoes in the Austral Volcanic Zone may lead to ash fall at large distances from the volcano, including interruptions in

air traffic Air Traffic are an English alternative rock band from Bournemouth signed to EMI Records. Formed in 2003, the band consists of Chris Wall (piano, lead vocals), David Ryan Jordan (Drums), Tom Pritchard (guitar) and Jim Maddock (bass guitar). ...

and direct ash damage.

References

Sources

* * * * * {{Andean volcanoes Subglacial volcanoes of Argentina Volcanoes of Magallanes Region Volcanoes of Santa Cruz Province, Argentina Andean Volcanic Belt Active volcanoes Mountains of Argentina Polygenetic volcanoes Quaternary volcanoes Quaternary South America