Andean orogeny

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https://en.wikipedia.org/wiki/Andean_orogeny

 

Simplified sketch of the present-situation along most of the Andes

 

The Andean orogeny (Spanish: Orogenia andina) is an ongoing process of orogeny that began in the Early Jurassic and is responsible for the rise of the Andes mountains. The orogeny is driven by a reactivation of a long-lived subduction system along the western margin of South America. On a continental scale the Cretaceous (90 Ma) and Oligocene (30 Ma) were periods of re-arrangements in the orogeny. Locally the details of the nature of the orogeny varies depending on the segment and the geological period considered.

Overview

Subduction orogeny has been occurring in what is now western South America since the break-up of the supercontinent Rodinia in the Neoproterozoic. The Paleozoic Pampean, Famatinian and Gondwanan orogenies are the immediate precursors to the later Andean orogeny. The first phases of Andean orogeny in the Jurassic and Early Cretaceous were characterized by extensional tectonics, rifting, the development of back-arc basins and the emplacement of large batholiths. This development is presumed to have been linked to the subduction of cold oceanic lithosphere. During the mid to Late Cretaceous (ca. 90 million years ago) the Andean orogeny changed significantly in character. Warmer and younger oceanic lithosphere is believed to have started to be subducted beneath South America around this time. Such kind of subduction is held responsible not only for the intense contractional deformation that different lithologies were subject to, but also the uplift and erosion known to have occurred from the Late Cretaceous onward. Plate tectonic reorganization since the mid-Cretaceous might also have been linked to the opening of the South Atlantic Ocean. Another change related to mid-Cretaceous plate tectonic changes was the change of subduction direction of the oceanic lithosphere that went from having south-east motion to having a north-east motion at about 90 million years ago. While subduction direction changed it remained oblique (and not perpendicular) to the coast of South America, and the direction change affected several subduction zone-parallel faults including Atacama, Domeyko and Liquiñe-Ofqui.

Paleogeography of the Late Cretaceous South America. Areas subject to the Andean orogeny are shown in light grey while the stable cratons are shown as grey squares. The sedimentary formations of Los Alamitos and La Colonia that formed in the Late Cretaceous are indicated.

 

Low angle subduction or flat-slab subduction has been common during the Andean orogeny leading to crustal shortening and deformation and the suppression of arc volcanism. Flat-slab subduction has occurred at different times in various part of the Andes, with northern Colombia (6–10° N), Ecuador (0–2° S), northern Peru (3–13° S) and north-central Chile and Argentina (24–30° S) experiencing these conditions at present.

The tectonic growth of the Andes and the regional climate have evolved simultaneously and have influenced each other. The topographic barrier formed by the Andes stopped the income of humid air into the present Atacama desert. This aridity, in turn, changed the normal superficial redistribution of mass via erosion and river transport, modifying the later tectonic deformation.

In the Oligocene the Farallon Plate broke up, forming the modern Cocos and Nazca plates ushering a series of changes in the Andean orogeny. The new Nazca Plate was then directed into an orthogonal subduction with South America causing ever-since uplift in the Andes, but causing most impact in the Miocene. While the various segments of the Andes have their own uplift histories, as a whole the Andes have risen significantly in last 30 million years (Oligocene–present).

Orogeny by segment

Colombia, Ecuador and Venezuela (12° N–3° S)

Map of a north-south sea-parallel pattern of rock ages in western Colombia. This pattern is a result of the Andean orogeny.

 

Tectonic blocks of continental crust that had separated from northwestern South America in the Jurassic re-joined the continent in the Late Cretaceous by colliding obliquely with it. This episode of accretion occurred in a complex sequence. The accretion of the island arcs against northwestern South America in the Early Cretaceous led to the development of a magmatic arc caused by subduction. The Romeral Fault in Colombia forms the suture between the accreted terranes the rest of South America. Around the Cretaceous–Paleogene boundary (ca. 65 million years ago) the oceanic plateau of the Caribbean large igneous province collided with South America. The subduction of the lithosphere as the oceanic plateau approached South America led to the formation of a magmatic arc now preserved in the Cordillera Real of Ecuador and the Cordillera Central of Colombia. In the Miocene an island arc and terrane (Chocó terrane) collided against northwestern South America. This terrane forms parts of what is now Chocó Department and Western Panamá.

The Caribbean Plate collided with South America in the Early Cenozoic but shifted then its movement eastward. Dextral fault movement between the South American and Caribbean plate started 17–15 million years ago. This movement was canalized along a series of strike-slip faults, but these faults alone do not account for all deformation. The northern part of the Dolores-Guayaquil Megashear forms part of the dextral fault systems while in the south the megashear runs along the suture between the accreted tectonic blocks and the rest of South America.

Northern Peru (3–13° S)

The seaward tilting of the sedimentary strata of Salto del Fraile Formation in Peru was caused by the Andean orogeny.

Long before the Andean orogeny the northern half of Peru was subject of the accretion of terranes in the Neoproterozoic and Paleozoic. Andean orogenic deformation in northern Peru can be traced to the Albian (Early Cretaceous). This first phase of deformation —the Mochica Phase— is evidenced in the folding of Casma Group sediments near the coast.

Sedimentary basins in western Peru changed from marine to continental conditions in the Late Cretaceous as a consequence of a generalized vertical uplift. The uplift in northern Peru is thought to be associated with the contemporary accretion of the Piñón terrane in Ecuador. This stage of orogeny is called the Peruvian Phase. Besides coastal Peru the Peruvian Phase affected or caused crustal shortening along the Cordillera Oriental and the tectonic inversion of Santiago Basin in the Sub-Andean zone. The bulk of the Sub-Andean zone was however unaffected by the Peruvian Phase.

After a period without much tectonic activity in the Early Eocene the Incaic Phase of orogeny occurred in the Mid and Late Eocene. No other tectonic event in the western Peruvian Andes compare with the Incaic Phase in magnitude. Horizontal shortening during the Incaic Phase resulted in the formation of the Marañón fold and thrust belt. An unconformity cutting across the Marañón fold and thrust belt show the Incaic Phase ended no later than 33 million years ago in the earliest Oligocene.

Topographic map of the Andes by NASA. The southern and northern ends of the Andes are not shown. The Bolivian Orocline is visible as a bend in the coastline and the Andes lower half of the map.

 

In the period after the Eocene the Northern Peruvian Andes were subject to the Quechua Phase of orogeny. The Quechua Phase is divided into the sub-phases Quechua 1, Quechua 2 and Quechua 3. The Quechua 1 Phase lasted from 17 to 15 million years ago and included a reactivation of Inca Phase structures in the Cordillera Occidental. 9–8 million years ago, in the Quechua 2 Phase, the older parts of the Andes in northern Peru were thrusted to the northeast. Most of the Sub-Andean zone of northern Peru deformed 7–5 million years ago (Late Miocene) during the Quechua 3 Phase. The Sub-Andean stacked in a thrust belt.

The Miocene rise of the Andes in Peru and Ecuador led to increased orographic precipitation along its eastern parts and to the birth of the modern Amazon River. One hypothesis links these two changes by assuming that increased precipitation led to increased erosion and this erosion led to filling the Andean foreland basins beyond their capacity and that it would have been the basin over-sedimentation rather than the rise of the Andes that made drainage basins flow to the east. Previously the interior of northern South America drained to the Pacific. 

Bolivian Orocline (13–26° S)

Early Andean subduction in the Jurassic formed a volcanic arc in northern Chile known as La Negra Arc. The remnants of this arc are now exposed in the Chilean Coast Range. Several plutons were emplaced in the Chilean Coast Range in the Jurassic and Early Cretaceous including the Vicuña Mackenna Batholith. Further east at similar latitudes, in Argentina and Bolivia, the Salta rift system developed during the Late Jurassic and the Early Cretaceous.

Pisco Basin, around latitude 14° S, was subject to a marine transgression in the Oligocene and Early Miocene epochs (25–16 Ma). In contrast Moquégua Basin to the southeast and the coast to south of Pisco Basin saw no transgression during this time but a steadily rise of the land.

From the Late Miocene onward the region that would become the Altiplano rose from low elevations to more than 3,000 m.a.s.l.. It is estimated that the region rose 2000 to 3000 meters in the last ten million years. Together with this uplift several valleys incised in the western flank of the Altiplano. In the Miocene the Atacama Fault moved, uplifting the Chilean Coast Range and creating sedimentary basins east of it. At the same time the Andes around the Altiplano region broadened to exceed any other Andean segment in width. Possibly about 1000 km of lithosphere has been lost due to lithospheric shortening. During subduction the western end of the forearc region flexured downward forming a giant monocline. By contrast the region east of the Altiplano is characterized by deformation and tectonics along a complex fold and thrust belt. Over-all the region surrounding the Altiplano and Puna plateaux has been horizontally shortened since the Eocene.

The Altiplano and its largest lake as seen from Ancohuma. The uplift of the Altiplano plateau is one of the most striking features of the Andean orogeny.

In southern Bolivia lithospheric shortening has made the Andean foreland basin to move eastward relative to the continent at an average rate of ca. 12–20 mm per year during most of the Cenozoic. Along the Argentine Northwest the Andean uplift has caused Andean foreland basins to separate into several minor isolated intermontane sedimentary basins. Towards the east the piling up of crust in Bolivia and the Argentine Norwest caused a north-south forebulge known as Asunción arch to develop in Paraguay.

The uplift of the Altiplano is thought to be indebted to a combination of horizontal shortening of the crust and to increased temperatures in the mantle (thermal thinning). The bend in the Andes and the west coast of South America known as the Bolivian Orocline was enhanced by Cenozoic horizontal shortening but existed already independently of it.

Besides direct causes the particular characteristics of the Bolivian Orocline–Altiplano region are attributed to a variety of deeper causes. These causes include a local steepening of the subduction angle of Nazca Plate, increased crustal shortening and plate convergence between the Nazca and South American plates, an acceleration in the westward drift of the South American Plate, and a rise in the shear stress between the Nazca and South American plates. This increase in shear stress could in turn be related to the scarcity of sediments in the Atacama trench which is caused by the arid conditions along Atacama Desert. Capitanio et al. attributes the rise of Altiplano and the bending of the Bolivian Orocline to the varying ages of the subducted Nazca Plate with the older parts of the plate subducting at the centre of the orocline. As Andrés Tassara puts it the rigidity of the Bolivian Orocline crust is derivative of the thermal conditions. The crust of the western region (forearc) of the orocline has been cold and rigid, resisting and damming up the westward flow of warmer and weaker ductile crustal material from beneath the Altiplano. The Cenozoic orogeny at the Bolivian orocline has produced a significative anatexis of crustal rocks including metasediments and gneisses resulting in the formation of peraluminous magmas. These characteristics imply that the Cenozoic tectonics and magmatism in parts of Bolivian Andes is similar to that seen in collisional orogens. The peralumineous magmatism in Cordillera Oriental is the cause of the world-class mineralizations of the Bolivian tin belt.

Tilted strata of the Yacoraite Formation at Serranía de Hornocal in northernmost Argentina. The Andean orogeny caused the tilting of these originally horizontal strata.

The rise of the Altiplano is thought by scientist Adrian Hartley to have enhanced an already prevailing aridity or semi-aridity in Atacama Desert by casting a rain shadow over the region.

Central Chile and Argentina (26–39° S)

At the latitudes between 17 and 39° S the Late Cretaceous and Cenozoic development of the Andean orogeny is characterized by an eastward migration of the magmatic belt and the development of several foreland basins. The eastward migration of the arc is thought to be caused by subduction erosion.

At the latitudes of 32–36° S —that is Central Chile and most of Mendoza Province— the Andean orogeny proper began in the Late Cretaceous when backarc basins were inverted. Immediately east of the early Andes foreland basins developed and their flexural subsidence caused the ingression of waters from the Atlantic all the way to the front of the orogen in the Maastrichtian. The Andes at the latitudes of 32–36° S experienced a sequence of uplift in the Cenozoic that started in the west and spread to the east. Beginning about 20 million years ago in the Miocene the Principal Cordillera (east of Santiago) began an uplift that lasted until about 8 million years ago. From the Eocene to the early Miocene, sediments accumulated in the Abanico Extensional Basin, a north-south elongated basin in Chile that spanned from 29° to 38° S. Tectonic inversion from 21 to 16 million years ago made the basin to collapse and the sediments to be incorporated to the Andean cordillera. Lavas and volcanic material that are now part of Farellones Formation accumulated while the basin was being inverted and uplifted. The Miocene continental divide was about 20 km to the west of the modern water divide that makes up the Argentina–Chile border. Subsequent river incision shifted the divide to the east leaving old flattish surfaces hanging. Compression and uplift in this part of the Andes has continued into the present. The Principal Cordillera had risen to heights that allowed for the development of valley glaciers about 1 million years ago.

Before the Miocene uplift of the Principal Cordillera was over, the Frontal Cordillera to the east started a period of uplift that lasted from 12 to 5 million years ago. Further east the Precordillera was uplifted in the last 10 million years and the Sierras Pampeanas has experienced a similar uplift in the last 5 million years. The more eastern part of the Andes at these latitudes had their geometry controlled by ancient faults dating to the San Rafael orogeny of the Paleozoic. The Sierras de Córdoba (part of the Sierras Pampeanas) where the effects of the ancient Pampean orogeny can be observed, owes it modern uplift and relief to the Andean orogeny in the late Cenozoic. Similarly the San Rafael Block east of the Andes and south of Sierras Pampeanas was raised in the Miocene during the Andean orogeny. In broad terms the most active phase of orogeny in area of southern Mendoza Province and northern Neuquén Province (34–38° S) happened in the Late Miocene while arc volcanism occurred east of the Andes.

At more southern latitudes (36–39° S) various Jurassic and Cretaceous marine transgressions from the Pacific are recorded in the sediments of Neuquén Basin. In the Late Cretaceous conditions changed. A marine regression occurred and the fold and thrust belts of Malargüe (36°00 S), Chos Malal (37° S) and Agrio (38° S) started to develop in the Andes and did so in until Eocene times. This meant an advance of the orogenic deformation since the Late Cretaceous that caused the western part of Neuquén Basin to stack in the Malargüe and Agrio fold and thrust belts. In the Oligocene the western part of the fold and thrust belt was subject to a short period of extensional tectonics whose structures were inverted in the Miocene. After a period of quiescence the Agrio fold and thrust belt resumed limited activity in the Eocene and then again in the Late Miocene.

In the south of Mendoza Province the Guañacos fold and thrust belt (36.5° S) appeared and grew in the Pliocene and Pleistocene consuming the western fringes of the Neuquén Basin.

Southern Patagonian Andes (48–55° S)

Syncline next to Nordenskjöld Lake in Torres del Paine National Park. The syncline formed during the Andean orogeny.

The early development of the Andean orogeny in southernmost South America affected also the Antarctic Peninsula. In southern Patagonia at the onset of the Andean orogeny in the Jurassic, extensional tectonics created the Rocas Verdes Basin, a back-arc basin whose southeastern extension survives as the Weddell Sea in Antarctica. In the Late Cretaceous the tectonic regime of Rocas Verdes Basin changed leading to its transformation into a compressional foreland basin –the Magallanes Basin– in the Cenozoic. This change was associated with an eastward move of the basin depocenter and the obduction of ophiolites. The closure of Rocas Verdes Basin in the Cretaceous is linked to the high-grade metamorphism of Cordillera Darwin Metamorphic Complex in southern Tierra del Fuego.

As the Andean orogeny went on, South America drifted away from Antarctica during the Cenozoic leading first to the formation of an isthmus and then to the opening of the Drake Passage 45 million years ago. The separation from Antarctica changed the tectonics of the Fuegian Andes into a transpressive regime with transform faults.

About 15 million years ago in the Miocene the Chile Ridge begun to subduct beneath the southern tip of Patagonia (55° S). The point of subduction, the triple junction has gradually moved to the north and lies at present at 47° S. The subduction of the ridge has created a northward moving "window" or gap in the asthenosphere beneath South America.

Geology of the Appalachians

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https://en.wikipedia.org/wiki/Geology_of_the_Appalachians

 

The "Pennsylvania Salient" in the Appalachians, appears to have been formed by a large, dense block of mafic volcanic rocks that became a barrier and forced the mountains to push up around it. 2012 image from NASA's Aqua satellite.

 

Generalized east-to-west cross section through the central Hudson Valley region. USGS image.

 

The geology of the Appalachians dates back to more than 480 million years ago. A look at rocks exposed in today's Appalachian Mountains reveals elongate belts of folded and thrust faulted marine sedimentary rocks, volcanic rocks and slivers of ancient ocean floor – strong evidence that these rocks were deformed during plate collision. The birth of the Appalachian ranges marks the first of several mountain building plate collisions that culminated in the construction of the supercontinent Pangaea with the Appalachians and neighboring Little Atlas (now in Morocco) near the center. These mountain ranges likely once reached elevations similar to those of the Alps and the Rocky Mountains before they were eroded.

Geological history

Paleozoic Era

Paleogeographic reconstruction showing the Appalachian Basin area during the Middle Devonian period.

 

During the earliest Paleozoic Era, the continent that would later become North America straddled the equator. The Appalachian region was a passive plate margin, not unlike today's Atlantic Coastal Plain Province. During this interval, the region was periodically submerged beneath shallow seas. Thick layers of sediment and carbonate rock were deposited on the shallow sea bottom when the region was submerged. When seas receded, terrestrial sedimentary deposits and erosion dominated.

During the Middle Ordovician Period (about 458-470 million years ago), a change in plate motions set the stage for the first Paleozoic mountain building event (Taconic orogeny) in North America. The once quiet Appalachian passive margin changed to a very active plate boundary when a neighboring oceanic plate, the Iapetus, collided with and began sinking beneath the North American craton. With the creation of this new subduction zone, the early Appalachians were born.

Along the continental margin, volcanoes grew, coincident with the initiation of subduction. Thrust faulting uplifted and warped older sedimentary rock laid down on the passive margin. As mountains rose, erosion began to wear them down. Streams carried rock debris downslope to be deposited in nearby lowlands.

This was just the first of a series of mountain building plate collisions that contributed to the formation of the Appalachians. Mountain building continued periodically throughout the next 250 million years (Caledonian, Acadian, Ouachita, Hercynian, and Alleghenian orogenies). Continent after continent was thrust and sutured onto the North American craton as the Pangean supercontinent began to take shape. Microplates, smaller bits of crust, too small to be called continents, were swept in, one by one, to be welded to the growing mass.

By about 300 million years ago (Pennsylvanian Period) Africa was approaching the North American craton. The collisional belt spread into the Ozark-Ouachita region and through the Marathon Mountains area of Texas. Continent vs. continent collision raised the Appalachian-Ouachita chain to a lofty mountain range on the scale of the present-day Himalaya. The massive bulk of Pangea was completed near the end of the Paleozoic Era (Permian Period) when Africa (Gondwana) plowed into the continental agglomeration, with the Appalachian-Ouachita mountains near the core.

Mesozoic Era and later

Pangea began to break up about 220 million years ago, in the Early Mesozoic Era (Late Triassic Period). As Pangea rifted apart a new passive tectonic margin was born and the forces that created the Appalachian, Ouachita, and Marathon Mountains were stilled. Weathering and erosion prevailed, and the mountains began to wear away.

By the end of the Mesozoic Era, the Appalachian Mountains had been eroded to an almost flat plain. It was not until the region was uplifted during the Cenozoic Era that the distinctive topography of the present formed. Uplift rejuvenated the streams, which rapidly responded by cutting downward into the ancient bedrock. Some streams flowed along weak layers that define the folds and faults created many millions of years earlier. Other streams downcut so rapidly that they cut right across the resistant folded rocks of the mountain core, carving canyons across rock layers and geologic structures. The ridges of the Appalachian Mountain core represent erosion-resistant rock that remained after the rock above and beside it was eroded away.

Geologic provinces

Map of Appalachian geological provinces

The Appalachian Mountains span across five geologic provinces (as defined by the USGS): the Appalachian Basin, the Blue Ridge Mountains, the Piedmont Province, the Adirondack Province, and the New England Province.

The Appalachian Basin

The Appalachian Basin is a foreland basin containing Paleozoic sedimentary rocks of Early Cambrian through Early Permian age. From north to south, the Appalachian Basin Province crosses New York, Pennsylvania, eastern Ohio, West Virginia, western Maryland, eastern Kentucky, western Virginia, eastern Tennessee, northwestern Georgia, and northeastern Alabama. The northern end of the Appalachian Basin extends offshore into Lakes Erie and Ontario as far as the United States–Canada border. The Appalachian Basin province covers an area of about 185,500 square miles (480,000 km²). The province is 1,075 miles (1,730 km) long from northeast to southwest and between 20 to 310 miles (30 to 500 km) wide from northwest to southeast.

The northwestern flank of the basin is a broad homocline that dips gently southeastward off the Cincinnati Arch. A complexly thrust faulted and folded terrane (Appalachian Fold and Thrust Belt or Eastern Overthrust Belt), formed at the end of the Paleozoic by the Alleghenian orogeny, characterizes the eastern flank of the basin. Metamorphic and igneous rocks of the Blue Ridge Thrust Belt that bounds the eastern part of the Appalachian Basin Province were thrust westward more than 150 miles (240 km) over lower Paleozoic sedimentary rocks.

Coal, oil, and gas production

The Appalachian Basin is one of the most important coal producing regions in the US and one of the biggest in the world. Appalachian Basin bituminous coal has been mined throughout the last three centuries. Currently, the coal primarily is used within the eastern U.S. or exported for electrical power generation, but some of it is suitable for metallurgical uses. Economically important coal beds were deposited primarily during Pennsylvanian time in a southeastward-thickening foreland basin. Coal and associated rocks form a clastic wedge that thickens from north to south, from Pennsylvania into southeast West Virginia and southwestern Virginia.

The Appalachian Basin has had a long history of oil and gas production. Discovery of oil in 1859 in the Drake Well, Venango County, Pennsylvania, marked the beginning of the oil and gas industry in the Appalachian Basin. The discovery well opened a prolific trend of oil and gas fields, producing from Upper Devonian, Mississippian, and Pennsylvanian sandstone reservoirs, that extends from southern New York, across western Pennsylvania, central West Virginia, and eastern Ohio, to eastern Kentucky.

A second major trend of oil and gas production in the Appalachian Basin began with the discovery in 1885 of oil and gas in Lower Silurian Clinton sandstone reservoirs in Knox County, Ohio. By the late 1880s and early 1900s, the trend extended both north and south across east-central Ohio and included several counties in western New York where gas was discovered in Lower Silurian Medina Group sandstones. About 1900, large oil reserves were discovered in Silurian and Devonian carbonate reservoirs in east-central Kentucky. Important gas discoveries from the Lower Devonian Oriskany Sandstone in Cambridge County, Ohio, in 1924, Schuyler County, New York, in 1930, and Kanawha County, West Virginia, in 1936 opened a major gas-producing trend across parts of New York, Pennsylvania, Maryland, Ohio, West Virginia, Kentucky, and Virginia.

Another drilling boom in the Appalachian Basin occurred in the 1960s in Morrow County, Ohio, where oil was discovered in the Upper Cambrian part of the Knox Dolomite.

Crystalline Appalachians

Geological map of the southern Crystalline Appalachians

The Blue Ridge, Piedmont, Adirondack, and New England Provinces are collectively known as the Crystalline Appalachians, because they consist of Precambrian and Cambrian igneous and metamorphic rocks.

The Blue Ridge Thrust Belt Province underlies parts of eight states from central Alabama to southern Pennsylvania. Along its western margin, the Blue Ridge is thrust over the folded and faulted margin of the Appalachian basin, so that a broad segment of Paleozoic strata extends eastward for tens of miles, buried beneath these subhorizontal crystalline thrust sheets. At the surface, the Blue Ridge consists of a mountainous to hilly region, the main component of which are the Blue Ridge Mountains that extend from Georgia to Pennsylvania. Surface rocks consist mainly of a core of moderate-to high-rank crystalline metamorphic or igneous rocks, which, because of their superior resistance to weathering and erosion, commonly rise above the adjacent areas of low-grade metamorphic and sedimentary rock. The province is bounded on the north and west by the Paleozoic strata of the Appalachian Basin Province and on the south by Cretaceous and younger sedimentary rocks of the Gulf Coastal Plain. It is bounded on the east by metamorphic and sedimentary rocks of the Piedmont Province.

The Adirondack and New England Provinces include sedimentary, metasedimentary, and plutonic igneous rocks, mainly of Cambrian and Ordovician age, similar lithologically to rocks in the Blue Ridge and Piedmont Provinces to the south. The uplifted, nearly circular Adirondack Mountains consist of a core of ancient Precambrian rocks that are surrounded by upturned Cambrian and Ordovician sedimentary rocks.

Farthest South

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Amundsen's Norwegian party stand at the South Pole, 17 December 1911. They had reached 90°S two days earlier.

Farthest South refers the most southerly latitude reached by explorers before the conquest of the South Pole in 1911. Significant steps on the road to the pole were the discovery of lands south of Cape Horn in 1619, Captain James Cook's crossing of the Antarctic Circle in 1773, and the earliest confirmed sightings of the Antarctic mainland in 1820. From the late 19th century onward, the quest for Farthest South latitudes became in effect a race to reach the pole, which culminated in Roald Amundsen's success in December 1911.

In the years before reaching the pole was a realistic objective, other motives drew adventurers southward. Initially, the driving force was the discovery of new trade routes between Europe and the Far East. After such routes had been established and the main geographical features of the earth had been broadly mapped, the lure for mercantile adventurers was the great fertile continent of "Terra Australis" which, according to myth, lay hidden in the south. Belief in the existence of this supposed land of plenty persisted well into the 18th century; explorers were reluctant to accept the truth that slowly emerged, of a cold, harsh environment in the lands of the Southern Ocean. 

James Cook's voyages of 1771–74 demonstrated conclusively the likely hostile nature of any hidden lands. This caused a shift of emphasis in the first half of the 19th century, away from trade and towards sealing and whaling, and then exploration and discovery. After the first overwintering on continental Antarctica in 1898–99 (Adrien de Gerlache), the prospect of reaching the South Pole appeared realistic, and the race for the pole began. The British were pre-eminent in this endeavour, which was characterised by the rivalry between Robert Falcon Scott and Ernest Shackleton during the Heroic Age of Antarctic Exploration. Shackleton's efforts fell short; Scott reached the pole in January 1912 only to find that he had been beaten by the Norwegian Amundsen.

Early voyagers

Antarctica and surrounding islands, showing Tierra del Fuego and the Auckland Islands

 

In 1494, the principal maritime powers, Portugal and Spain, signed a treaty which drew a line down the middle of the Atlantic Ocean and allocated all trade routes to the east of the line to Portugal. That gave Portugal dominance of the only known route to the east—via the Cape of Good Hope and Indian Ocean, which left Spain, and later other countries, to seek a western route to the Pacific. The exploration of the south began as part of the search for such a route.

Unlike the Arctic, there is no evidence of human visitation or habitation in Antarctica or the islands around it prior to European exploration. However, the most southerly parts of South America were already inhabited by tribes such as the Selk'nam/Ona, the Yagán/Yámana, the Alacaluf and the Haush. The Haush in particular made regular trips to Isla de los Estados, which was 29 kilometres (18 mi) from the main island of Tierra del Fuego, suggesting that some of them may have been capable of reaching the islands near Cape Horn. Fuegian Indian artefacts and canoe remnants have also been discovered on the Falkland Islands, suggesting the capacity for even longer sea journeys.

While the natives of Tierra del Fuego were not capable of true oceanic travel, there is some evidence of Polynesian visits to some of the sub antarctic islands to the south of New Zealand, although these are further from Antarctica than South America. There are also remains of a Polynesian settlement dating back to the 13th century on Enderby Island in the Auckland Islands. According to ancient legends, around the year 650 the Polynesian traveller Ui-te-Rangiora led a fleet of Waka Tīwai south until they reached "a place of bitter cold where rock-like structures rose from a solid sea". It is unclear from the legends how far south Ui-te-Rangiora penetrated, but it appears that he observed ice in large quantities. A shard of undated, unidentified pottery, reported as found in 1886 in the Antipodes Islands, has been associated with this expedition.

Ferdinand Magellan

Ferdinand Magellan

Although Portuguese by birth, Ferdinand Magellan transferred his allegiance to King Charles I of Spain, on whose behalf he left Seville on 10 August 1519, with a squadron of five ships, in search of a western route to the Spice Islands in the East Indies. Success depended on finding a strait or passage through the South American land masses, or finding the southern tip of the continent and sailing around it. The South American coast was sighted on 6 December 1519, and Magellan moved cautiously southward, following the coast to reach latitude 49°S on 31 March 1520. Little if anything was known of the coast south of this point, so Magellan decided to wait out the southern winter here, and established the settlement of Puerto San Julian.

In September 1520, the voyage continued down the uncharted coast, and on 21 October reached 52°S. Here Magellan found a deep inlet which proved to be the strait he was seeking, later to be known by his name. Early in November 1520, as the squadron navigated through the strait, they reached its most southerly point at approximate latitude 54°S. This was a record Farthest South for a European navigator, though not the farthest southern penetration by man; the position was north of the Tierra del Fuego archipelago, where there is evidence of human settlement dating back thousands of years.

Francisco de Hoces

The first sighting of an ocean passage to the Pacific south of Tierra del Fuego is sometimes attributed to Francisco de Hoces of the Loaisa Expedition. In January 1526 his ship San Lesmes was blown south from the Atlantic entrance of the Magellan Strait to a point where the crew thought they saw a headland, and water beyond it, which indicated the southern extremity of the continent. It is speculation as to which headland they saw; conceivably it was Cape Horn. In parts of the Spanish-speaking world it is believed that de Hoces may have discovered the strait later known as the Drake Passage more than 50 years before Sir Francis Drake, the British privateer.

Sir Francis Drake

Sir Francis Drake

Sir Francis Drake sailed from Plymouth on 15 November 1577, in command of a fleet of five ships under his flagship Pelican, later renamed the Golden Hinde. His principal objective was plunder, not exploration; his initial targets were the unfortified Spanish towns on the Pacific coasts of Chile and Peru. Following Magellan's route, Drake reached Puerto San Julian on 20 June. After nearly two months in harbour, Drake left the port with a reduced fleet of three ships and a small pinnace. His ships entered the Magellan Strait on 23 August and emerged in the Pacific Ocean on 6 September.

Drake set a course to the north-west, but on the following day a gale scattered the ships. The Marigold was sunk by a giant wave; the Elizabeth managed to return into the Magellan Strait, later sailing eastwards back to England; the pinnace was lost later. The gales persisted for more than seven weeks. The Golden Hinde was driven far to the west and south, before clawing its way back towards land. On 22 October, the ship anchored off an island which Drake named "Elizabeth Island", where wood for the galley fires was collected and seals and penguins captured for food.

According to Drake's Portuguese pilot, Nuno da Silva, their position at the anchorage was 57°S. However, there is no island at that latitude. The as yet undiscovered Diego Ramírez Islands, at 56°30'S, are treeless and cannot have been the islands where Drake's crew collected wood. This indicates that the navigational calculation was faulty, and that Drake landed at or near the then unnamed Cape Horn, possibly on Horn Island itself. His final southern latitude can only be speculated as that of Cape Horn, at 55°59'S. In his report, Drake wrote: "The Uttermost Cape or headland of all these islands stands near 56 degrees, without which there is no main island to be seen to the southwards but that the Atlantic Ocean and the South Sea meet." This open sea south of Cape Horn became known as the Drake Passage even though Drake himself did not traverse it.

Willem Schouten

On 14 June 1615, Willem Schouten, with two ships Eendracht and Hoorn, set sail from Texel in the Netherlands in search of a western route to the Pacific. Hoorn was lost in a fire, but Eendracht continued southward. On 29 January 1616, Schouten reached what he discerned to be the southernmost cape of the South American continent; he named this point Kaap Hoorn (Cape Horn) after his hometown and his lost ship. Schouten's navigational readings are inaccurate—he placed Cape Horn at 57°48' south, when its actual position is 55°58'. His claim to have reached 58° south is unverified, although he sailed on westward to become the first European navigator to reach the Pacific via the Drake Passage.

Garcia de Nodal expedition

The next recorded navigation of the Drake Passage was achieved in February 1619, by the brothers Bartolome and Gonzalo Garcia de Nodal. The Garcia de Nodal expedition discovered a small group of islands about 60 nautical miles (100 km; 70 mi) south-west of Cape Horn, at latitude 56°30'S. They named these the Diego Ramirez Islands after the expedition's pilot. The islands remained the most southerly known land on earth until Captain James Cook's discovery of the South Sandwich Islands in 1775.

Other discoveries

Other voyages brought further discoveries in the southern oceans; in August 1592, the English seaman John Davis had taken shelter "among certain Isles never before discovered"—presumed to be the Falkland Islands. In 1675, the English merchant voyager Anthony de la Roché visited South Georgia (the first Antarctic land discovered); in 1739 the Frenchman Jean-Baptiste Bouvet de Lozier discovered the remote Bouvet Island, and in 1772 his compatriot, Yves-Joseph de Kerguelen de Trémarec, found the Kerguelen Islands.

Early Antarctic explorers

Captain James Cook

Captain James Cook

 

The second of James Cook's historic voyages, 1772–1775, was primarily a search for the elusive Terra Australis Incognita that was still believed to lie somewhere in the unexplored latitudes below 40°S. Cook left England in September 1772 with two ships, HMS Resolution and HMS Adventure. After pausing at Cape Town, on 22 November the two ships sailed due south, but were driven to the east by heavy gales. They managed to edge further south, encountering their first pack ice on 10 December. This soon became a solid barrier, which tested Cook's seamanship as he manoeuvered for a passage through. Eventually, he found open water, and was able to continue south; on 17 January 1773, the expedition reached the Antarctic Circle at 66°20'S, the first ships to do so. Further progress was barred by ice, and the ships turned north-eastwards and headed for New Zealand, which they reached on 26 March.

During the ensuing months, the expedition explored the southern Pacific Ocean before Cook took Resolution south again—Adventure had retired back to South Africa after a fracas with the New Zealand native population. This time Cook was able to penetrate deep beyond the Antarctic Circle, and on 30 January 1774 reached 71°10'S, his Farthest South, but the state of the ice made further southward travel impossible. This southern record would hold for 49 years.

In the course of his voyages in Antarctic waters, Cook had encircled the world at latitudes generally above 60°S, and saw nothing but bleak inhospitable islands, without a hint of the fertile continent which some still hoped lay in the south. Cook wrote that if any such continent existed it would be "a country doomed by nature", and that "no man will venture further than I have done, and the land to the South will never be explored". He concluded: "Should the impossible be achieved and the land attained, it would be wholly useless and of no benefit to the discoverer or his nation".

Searching for land

Despite Cook's prediction, the early 19th century saw numerous attempts to penetrate southward, and to discover new lands. In 1819, William Smith, in command of the brigantine Williams, discovered the South Shetland Islands, and in the following year Edward Bransfield, in the same ship, sighted the Trinity Peninsula at the northern extremity of Graham Land. A few days before Bransfield's discovery, on 27 January 1820, the Russian captain Fabian von Bellingshausen, in another Antarctic sector, had come within sight of the coast of what is now known as Queen Maud Land. He is thus credited as the first person to see the continent's mainland, although he did not make this claim himself. Bellingshausen made two circumnavigations mainly in latitudes between 60 and 67°S, and in January 1821 reached his most southerly point at 70°S, in a longitude close to that in which Cook had made his record 47 years earlier. In 1821 the American sealing captain John Davis led a party which landed on an uncharted stretch of land beyond the South Shetlands. "I think this Southern Land to be a Continent", he wrote in his ship's log. If his landing was not on an island, his party were the first to set foot on the Antarctic continent.

James Weddell

James Weddell was an Anglo-Scottish seaman who saw service in both the Royal Navy and the merchant marine before undertaking his first voyages to Antarctic waters. In 1819, in command of the 160-ton brigantine Jane which had been adapted for whaling, he set sail for the newly discovered whaling grounds of the South Sandwich Islands. His chief interest on this voyage was in finding the "Aurora Islands", which had been reported at 53°S, 48°W by the Spanish ship Aurora in 1762. He failed to discover this non-existent land, but his sealing activities showed a handsome profit.

Weddell's ships, Jane and Beaufoy, under full sail

In 1822 Weddell, again in command of Jane and this time accompanied by a smaller ship, the cutter Beaufoy, set sail for the south with instructions from his employers that, should the sealing prove barren, he was to "investigate beyond the track of former navigators". This suited Weddell's exploring instincts, and he equipped his vessel with chronometers, thermometers, compasses, barometers and charts. In January 1823 he probed the waters between the South Sandwich Islands and the South Orkney Islands, looking for new land. Finding none, he turned southward down the 40°W meridian, deep into the sea that now bears his name. The season was unusually calm, and Weddell reported that "not a particle of ice of any description was to be seen". On 20 February 1823, he reached a new Farthest South of 74°15'S, three degrees beyond Cook's former record. Unaware that he was close to land, Weddell decided to return northward from this point, convinced that the sea continued as far as the South Pole. Another two days' sailing would likely have brought him within sight of Coats Land, which was not discovered until 1904, by William Speirs Bruce during the Scottish National Antarctic Expedition, 1902–04. On his return to England, Weddell's claim to have exceeded Cook's record by such a margin "caused some raised eyebrows", but was soon accepted.

Benjamin Morrell

In November 1823, the American sealing captain Benjamin Morrell reached the South Sandwich Islands in the schooner Wasp. According to his own later account he then sailed south, unconsciously following the track taken by James Weddell a month previously. Morrell claimed to have reached 70°14'S, at which point he turned north because the ship's stoves were running short of fuel—otherwise, he says, he could have "reached 85° without the least doubt". After turning, he claimed to have encountered land which he described in some detail, and which he named New South Greenland. This land proved not to exist. Morrell's reputation as a liar and a fraud means that most of his geographical claims have been dismissed by scholars, although attempts have been made to rationalise his assertions.

James Clark Ross

James Clark Ross's 1839–43 Antarctic expedition in HMS Erebus and HMS Terror was a full-scale Royal Naval enterprise, the principal function of which was to test current theories on magnetism, and to try to locate the South Magnetic Pole. The expedition had first been proposed by leading astronomer Sir John Herschel, and was supported by the Royal Society and the British Association for the Advancement of Science. Ross had considerable past experience in magnetic observation and Arctic exploration; in May 1831 he had been a member of a party that had reached the location of the North Magnetic Pole, and he was an obvious choice as commander.

Captain Sir James Clark Ross

The expedition left England on 30 September 1839, and after a voyage that was slowed by the many stops required to carry out work on magnetism, it reached Tasmania in August 1840. Following a three-month break imposed by the southern winter, they sailed south-east on 12 November 1840, and crossed the Antarctic Circle on 1 January 1841. On 11 January a long mountainous coastline that stretched to the south was sighted. Ross named the land Victoria Land, and the mountains the Admiralty Range. He followed the coast southwards and passed Weddell's Farthest South point of 74°15'S on 23 January. A few days later, as they moved further eastward to avoid shore ice, they were met by the sight of twin volcanoes (one of them active), which were named Mount Erebus and Mount Terror, in honour of the expedition's ships.

The Great Ice Barrier (later to be called the "Ross Ice Shelf") stretched away east of these mountains, forming an impassable obstacle to further southward progress. In his search for a strait or inlet, Ross explored 300 nautical miles (560 km; 350 mi) along the edge of the barrier, and reached an approximate latitude of 78°S on or about 8 February 1841. He failed to find a suitable anchorage that would have allowed the ships to over-winter, so he returned to Tasmania, arriving there in April 1841.

The following season Ross returned and located an inlet in the Barrier face that enabled him, on 23 January 1842, to extend his Farthest South to 78°09'30"S, a record which would remain unchallenged for 58 years. Although Ross had not been able to land on the Antarctic continent, nor approach the location of the South Magnetic Pole, on his return to England in 1843 he was knighted for his achievements in geographical and scientific exploration.

Explorers of the Heroic Age

The oceanographic research voyage known as the Challenger Expedition, 1872–76, explored Antarctic waters for several weeks, but did not approach the land itself; its research, however, proved the existence of an Antarctic continent beyond reasonable doubt.

The impetus for what would become known as the Heroic Age of Antarctic Exploration came in 1895, when in an address to the Sixth International Geographical Congress in London, Professor Sir John Murray called for a resumption of Antarctic exploration: "a steady, continuous, laborious and systematic exploration of the whole southern region". He followed this call with an appeal to British patriotism: "Is the last great piece of maritime exploration on the surface of our Earth to be undertaken by Britons, or is it to be left to those who may be destined to succeed or supplant us on the Ocean?" During the following quarter-century, fifteen expeditions from eight different nations rose to this challenge. In the patriotic spirit engendered by Murray's call, and under the influence of RGS president Sir Clements Markham, British endeavours in the following years gave particular weight to the achievement of new Farthest South records, and began to develop the character of a race for the South Pole.

Carsten Borchgrevink

Carsten Borchgrevink, who led the Southern Cross Expedition, 1898–1900

The Norwegian-born Carsten Egeberg Borchgrevink had emigrated to Australia in 1888, where he worked on survey teams in Queensland and New South Wales before accepting a schoolteaching post. In 1894 he joined a sealing and whaling expedition to the Antarctic, led by Henryk Bull. In January 1895 Borchgrevink was one of a group from that expedition that claimed the first confirmed landing on the Antarctic continent, at Cape Adare. Borchgrevink determined to return with his own expedition, which would overwinter and explore inland, with the location of the South Magnetic Pole as an objective.

Borchgrevink went to England, where he was able to persuade the publishing magnate Sir George Newnes to finance him to the extent of £40,000, equivalent to £4.2 million in 2016, with the sole stipulation that, despite the shortage of British participants, the venture be styled the "British Antarctic Expedition". This was by no means the grand British expedition envisaged by Markham and the geographical establishment, who were hostile and dismissive of Borchgrevink. On 23 August 1898 the expedition ship Southern Cross left London for the Ross Sea, reaching Cape Adare on 17 February 1899. Here a shore party was landed and was the first to over-winter on the Antarctic mainland, in a prefabricated hut.

In January 1900, Southern Cross returned, picked up the shore party and, following the route which Ross had taken 60 years previously, sailed southward to the Great Ice Barrier, which they discovered had retreated some 30 miles (48 km) south since the days of Ross. A party consisting of Borchgrevink, William Colbeck and a Sami named Per Savio landed with sledges and dogs. This party ascended the Barrier and made the first sledge journey on the barrier surface; on 16 February 1900 they extended the Farthest South record to 78°50'S. On its return to England later in 1900, Borchgrevink's expedition was received without enthusiasm, despite its new southern record. Historian David Crane commented that if Borchgrevink had been a British naval officer, his contribution to Antarctic knowledge might have been better received, but "a Norwegian seaman/schoolmaster was never going to be taken seriously".

Robert Falcon Scott

The Discovery Expedition of 1901–04 was Robert Falcon Scott's first Antarctic command. Although according to Edward Wilson the intention was to "reach the Pole if possible, or find some new land", there is nothing in Scott's writings, nor in the official objectives of the expedition, to indicate that the pole was a definite goal. However, a southern journey towards the pole was within Scott's formal remit to "explore the ice barrier of Sir James Ross ... and to endeavour to solve the very important physical and geographical questions connected with this remarkable ice formation".

The southern journey was undertaken by Scott, Wilson and Ernest Shackleton. The party set out on 1 November 1902 with various teams in support, and one of these, led by Michael Barne, passed Borchgrevink's Farthest South mark on 11 November, an event recorded with great high spirits in Wilson's diary. The march continued, initially in favourable weather conditions, but encountered increasing difficulties caused by the party's lack of ice travelling experience and the loss of all its dogs through a combination of poor diet and overwork. The 80°S mark was passed on 2 December, and four weeks later, on 30 December 1902, Wilson and Scott took a short ski trip from their southern camp to set a new Farthest South at (according to their measurements) 82°17'S. Modern maps, correlated with Shackleton's photograph and Wilson's drawing, put their final camp at 82°6'S, and the point reached by Scott and Wilson at 82°11'S, 200 nautical miles (370 km; 230 mi) beyond Borchgrevink's mark.

Ernest Shackleton

From left to right: Jameson Adams, Frank Wild and Eric Marshall (photographed by Shackleton) plant the Union flag at their southernmost position, 88°23', on 9 January 1909.

 

After his share in the Farthest South achievement of the Discovery Expedition, Ernest Shackleton suffered a physical collapse on the return journey, and was sent home with the expedition's relief vessel on orders from Scott; he bitterly resented it, and the two became rivals. Four years later, Shackleton organised his own polar venture, the Nimrod Expedition, 1907–09. This was the first expedition to set the definite objective of reaching the South Pole, and to have a specific strategy for doing so.

To assist his endeavour, Shackleton adopted a mixed transport strategy, involving the use of Manchurian ponies as pack animals, as well as the more traditional dog-sledges. A specially adapted motor car was also taken. Although the dogs and the car were used during the expedition for a number of purposes, the task of assisting the group that would undertake the march to the pole fell to the ponies. The size of Shackleton's four-man polar party was dictated by the number of surviving ponies; of the ten that were embarked in New Zealand, only four had survived the 1908 winter.

Ernest Shackleton and three companions (Frank Wild, Eric Marshall and Jameson Adams) began their march on 29 October 1908. On 26 November they surpassed the farthest point reached by Scott's 1902 party. "A day to remember", wrote Shackleton in his journal, noting that they had reached this point in far less time than on the previous march with Scott. Shackleton's group continued southward, discovering and ascending the Beardmore Glacier to the polar plateau, and then marching on to reach their Farthest South point at 88°23'S, a mere 97 nautical miles (180 km; 112 mi) from the pole, on 9 January 1909. Here they planted the Union Jack presented to them by Queen Alexandra, and took possession of the plateau in the name of King Edward VII, before shortages of food and supplies forced them to turn back north. This was, at the time, the closest convergence on either pole. The increase of more than six degrees south from Scott's previous record was the greatest extension of Farthest South since Captain Cook's 1773 mark. Shackleton was treated as a hero on his return to England. His record was to stand for less than three years, being passed by Amundsen on 7 December 1911.

Polar conquest

Roald Amundsen, leader of the first expedition to reach the South Pole, 15 December 1911

In the wake of Shackleton's near miss, Robert Scott organised the Terra Nova Expedition, 1910–13, in which securing the South Pole for the British Empire was an explicitly stated prime objective. As he planned his expedition, Scott saw no reason to believe that his effort would be contested. However, the Norwegian explorer Roald Amundsen, who had been developing plans for a North Pole expedition, changed his mind when, in September 1909, the North Pole was claimed in quick succession by the Americans Frederick Cook and Robert Peary. Amundsen resolved to go south instead.

Amundsen concealed his revised intentions until his ship, Fram, was in the Atlantic and beyond communication. Scott was notified by telegram that a rival was in the field, but had little choice other than to continue with his own plans. Meanwhile, Fram arrived at the Ross Ice Shelf on 11 January 1911, and by 14 January had found the inlet, or "Bay of Whales", where Borchgrevink had made his landing eleven years earlier. This became the location of Amundsen's base camp, Framheim.

After nine months' preparation, Amundsen's polar journey began on 20 October 1911. Avoiding the known route to the polar plateau via the Beardmore Glacier, Amundsen led his party of five due south, reaching the Transantarctic Mountains on 16 November. They discovered the Axel Heiberg Glacier, which provided them with a direct route to the polar plateau and on to the pole. Shackleton's Farthest South mark was passed on 7 December, and the South Pole was reached on 14 December 1911. The Norwegian party's greater skills with the techniques of ice travel, using ski and dogs, had proved decisive in their success. Scott's five-man team reached the same point 33 days later, and perished during their return journey. Since Cook's journeys, every expedition that had held the Farthest South record before Amundsen's conquest had been British; however, the final triumph indisputably belonged to the Norwegians.

Later history

The Amundsen–Scott South Pole Station, photographed in 2006

After Scott's retreat from the pole in January 1912, the location remained unvisited for nearly 18 years. On 28 November 1929, US Navy Commander (later Rear-Admiral) Richard E. Byrd and three others completed the first aircraft flight over the South Pole. Twenty-seven years later, Rear-Admiral George J. Dufek became the first person to set foot on the pole since Scott, when on 31 October 1956 he and the crew of R4D-5 Skytrain "Que Sera Sera" landed at the pole. Between November 1956 and February 1957, the first permanent South Pole research station was erected and christened the Amundsen–Scott South Pole Station in honour of the pioneer explorers. Since then the station had been substantially extended, and in 2008 was housing up to 150 scientific staff and support personnel. Dufek gave considerable assistance to the Commonwealth Trans-Antarctic Expedition, 1955–58, led by Vivian Fuchs, which on 19 January 1958 became the first party to reach the pole overland since Scott.