
The Rio Grande Rise, also called the Rio Grande Elevation or Bromley Plateau, is an
aseismic ocean ridge in the southern
Atlantic Ocean
The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...
off the coast of
Brazil
Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...
. Together with the
Walvis Ridge off Africa, the Rio Grande Rise forms a V-shaped structure of mirrored hotspot tracks or seamount chains across the northern
South Atlantic
The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for ...
.
In 2013, Brazilian scientists announced that they found granite boulders on the Rio Grande Rise and speculated that it could be the remains of a submerged continent, which they called the "Brazilian Atlantis". Other researchers, however, noted that such boulders can end-up on the ocean floor by less speculative means.
Brazil proposes the area as part of its
continental shelf
A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an islan ...
since December 2018, but it has yet to be approved by the Commission on the Limits of the Continental Shelf. It is far beyond the country's
exclusive economic zone
An exclusive economic zone (EEZ), as prescribed by the 1982 United Nations Convention on the Law of the Sea, is an area of the sea in which a sovereign state has exclusive rights regarding the exploration and use of marine natural resource, reso ...
and would thus be part of its
extended continental shelf The extended continental shelf, scientific continental shelf, or outer continental shelf, refers to a type of maritime area, established as a geo-legal paradigm by the United Nations Convention on the Law of the Sea (UNCLOS). Through the process kno ...
.
Geology
The Rio Grande Rise separates the
Santos and
Campos Basin
The Campos Basin is one of 12 coastal sedimentary basins of Brazil. It spans both onshore and offshore parts of the South Atlantic with the onshore part located near Rio de Janeiro. The basin originated in Neocomian stage of the Cretaceous peri ...
s and is composed of western and eastern areas, which have different geological backgrounds. The western area has numerous
guyot
In marine geology, a guyot (), also called a tablemount, is an isolated underwater volcanic mountain (seamount) with a flat top more than below the surface of the sea. The diameters of these flat summits can exceed . Guyots are most commonly fo ...
s and
seamount
A seamount is a large submarine landform that rises from the ocean floor without reaching the water surface (sea level), and thus is not an island, islet, or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abruptly a ...
s and a basement dated to . The eastern area is covered by
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 and may represent an abandoned spreading centre. In the western area, volcanic
breccia
Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix.
The word has its origins in the Italian language ...
and layers of ash indicate widespread volcanism during the
Eocene
The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
, which coincides with the formation of volcanic rocks onshore. During this period, parts of the western plateau were uplifted over sea level and short-lived volcanic islands formed.
When West
Gondwana
Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...
(i.e. South America) broke away from Africa during the
Early Cretaceous
The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...
(), the
South Atlantic
The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for ...
opened up from its southern to its northern end. In this process, the voluminous
Paraná and Etendeka continental flood basalt
A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive rock, intrusive (sill (geology), sills, dike (geology), dikes) and extrusive rock, extrusive (lava flows, tephra deposits), arising when magm ...
s formed in what is now Brazil and
Namibia
Namibia, officially the Republic of Namibia, is a country on the west coast of Southern Africa. Its borders include the Atlantic Ocean to the west, Angola and Zambia to the north, Botswana to the east and South Africa to the south; in the no ...
. This event is linked to the
Tristan-Gough hotspot, now located near the
Mid-Atlantic Ridge
The Mid-Atlantic Ridge is a mid-ocean ridge (a Divergent boundary, divergent or constructive Plate tectonics, plate boundary) located along the floor of the Atlantic Ocean, and part of the List of longest mountain chains on Earth, longest mountai ...
, close to
Tristan da Cunha
Tristan da Cunha (), colloquially Tristan, is a remote group of volcano, volcanic islands in the South Atlantic Ocean. It is one of three constituent parts of the British Overseas Territories, British Overseas Territory of Saint Helena, Ascensi ...
and the
Gough Island
Gough Island ( ), also known historically as Gonçalo Álvares, is a rugged volcanic island in the South Atlantic Ocean. It is a dependency of Tristan da Cunha and part of the British overseas territory of Saint Helena, Ascension and Tristan d ...
s. During the
Maastrichtian
The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...
(), the orientation of spreading changed, which is still visible on the African side, and volcanism ended on the American side. This process resulted in the Tristan-Gough seamount chains on either side of the Tristan-Gough hotspot.
Palaeoclimatic role
A Brazilian-Japanese expedition in 2013 recovered ''in situ'' granitic and metamorphic rocks on the Rio Grande Rise. This can possibly indicate that the plateau includes fragments of continental crust — possible remains of
micro-continent
Continental crustal fragments, partly synonymous with microcontinents, are pieces of continents that have broken off from main continental masses to form distinct islands that are often several hundred kilometers from their place of origin.
Caus ...
s similar to those found on and around
Kerguelen
The Kerguelen Islands ( or ; in French commonly ' but officially ', ), also known as the Desolation Islands (' in French), are a group of islands in the sub-Antarctic region. They are among the most isolated places on Earth, with the closest t ...
in the Indian Ocean and
Jan Mayen
Jan Mayen () is a Norway, Norwegian volcanic island in the Arctic Ocean with no permanent population. It is long (southwest-northeast) and in area, partly covered by glaciers (an area of around the Beerenberg volcano). It has two parts: la ...
in the Arctic Ocean. The existence of such microcontinents is speculative, however, since their remains tend to be covered by younger layers of lava and sediments.
Nevertheless, transoceanic dispersals are hinted at by the fossil record of, for example, flightless birds such as ''
Lavocatavis'', indicating that several islands between Africa and South America made
island hopping
Leapfrogging was an amphibious military strategy employed by the Allies in the Pacific War against the Empire of Japan during World War II. The key idea was to bypass heavily fortified enemy islands instead of trying to capture every island i ...
possible across the Atlantic during the
Tertiary
Tertiary (from Latin, meaning 'third' or 'of the third degree/order..') may refer to:
* Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago
* Tertiary (chemistry), a term describing bonding patterns in organic ch ...
().
At the beginning of the Maastrichtian, the characteristics of water masses differed north and south of the Rio Grande Rice-Walvis Ridge complex. The disappearance of these differences during the Maastritchtian indicates a reorganisation of oceanic circulation patterns that lead to a global homogenisation of intermediate and deep waters. This process seems to have been triggered by the breaching of the Rio Grande Rise-Walvis Ridge complex and the disappearance of epicontinental seaways such as the
Tethys Ocean
The Tethys Ocean ( ; ), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean during much of the Mesozoic Era and early-mid Cenozoic Era. It was the predecessor to the modern Indian Ocean, the Mediterranean Sea, and the Eurasia ...
. The process resulted in the deterioration of
rudist
Rudists are a group of extinct box-, tube- or ring-shaped marine heterodont bivalves belonging to the order Hippuritida that arose during the Late Jurassic and became so diverse during the Cretaceous that they were major reef-building organis ...
-dominated tropical habitats and consequently the extinction of benthic
inoceramid
The Inoceramidae are an extinct family (biology), family of bivalves ("clams") in the Class Mollusca. Fossils of inoceramids are found in marine sediments of Permian to latest Cretaceous in age. Inoceramids tended to live in upper bathyal and ner ...
bivalves.
The origin of modern circulation of cold, deep water — known as the "Big Flush" — is associated with Early
Eocene
The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
() geological events; tectonism that resulted in the opening of the north-east Atlantic and fracture zones that developed in the subsiding Rio Grande Rise, which allowed cold water from the Antarctic
Weddell Sea
The Weddell Sea is part of the Southern Ocean and contains the Weddell Gyre. Its land boundaries are defined by the bay formed from the coasts of Coats Land and the Antarctic Peninsula. The easternmost point is Cape Norvegia at Princess Martha C ...
to flow northward into the North Atlantic. , the generation of cold bottom water in the Antarctic resulted in the formation of
psychrospheric fauna, which today live in temperatures below , in the Atlantic and Tethys. This global distribution suggests that the Rio Grande Rise had been breached by this time, allowing cold, dense water to move north-south through a corridor enhancing the transition from a latitudinal
thermospheric circulation to a meridional
thermohaline circulation
Thermohaline circulation (THC) is a part of the large-scale Ocean current, ocean circulation driven by global density gradients formed by surface heat and freshwater fluxes. The name ''thermohaline'' is derived from ''wikt:thermo-, thermo-'', r ...
.
See also
*
Vitória-Trindade Ridge - a line of seamounts and islands also in the South Atlantic of the coast of Brazil at 20
o South, and caused by a volcanic hot spot 1000 km to the north.
References
Notes
Sources
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{{Coord, 31, S, 35, W, type:mountain, display=title
Underwater ridges of the Atlantic Ocean
Seamount chains