Cameroon line

Coordinates: 3°30′0″N 8°42′0″E / 3.50000°N 8.70000°E / 3.50000; 8.70000
Source: Wikipedia, the free encyclopedia.
Map of the Gulf of Guinea, showing the chain of islands formed by the Cameroon volcanic line

The Cameroon line (French: Ligne du Cameroun, Portuguese: Linha dos Camarões, Spanish: cordillera de Camerún) is a 1,600 km (1,000 mi) long chain of volcanoes that includes islands in the Gulf of Guinea and mountains on the African mainland, from Mount Cameroon on the coast towards Lake Chad on the northeast.[1] They form a natural border between eastern Nigeria and the West Region of Cameroon. The islands, which span the equator, have tropical climates and are home to many unique plant and bird species. The mainland mountain regions are much cooler than the surrounding lowlands, and also contain unique and ecologically important environments.

The Cameroon volcanic line is geologically unusual in extending through both the ocean and the continental crust. Various hypotheses have been advanced by different geologists to explain the line.[1][2]

Geography

Mount Cameroon craters left after the eruptions in 2000

In the Gulf of Guinea, the Cameroon line consists of six offshore volcanic swells that have formed islands or seamounts. From the southwest to the northeast the island groups are Annobón (or Pagalu), São Tomé, Príncipe and Bioko. Two large seamounts lie between São Tomé and Príncipe, and between Príncipe and Bioko.

On the mainland, the line starts with

Oku Massif.[1] East of Oku there are further volcanic mountains in the Ngaoundere Plateau, some of which appear to have similar origins.[3]

Island chain

Annobón

The southernmost island in the chain is Annobón, also known as Pagalu, with an area of about 17.5 km2 (6.8 sq mi). It is an extinct volcano that rises from deep water to 598 m (1,962 ft) above sea level. The island belongs to Equatorial Guinea.

The average temperature is 26.1 °C (79.0 °F), with little seasonal variation. Most rain falls from November to May, with annual precipitation averaging 1,196 mm (47.1 in) - less than on the mainland.[4] Annobón has lush valleys and steep mountains, covered with rich woods and luxuriant vegetation.[5]

The small population lives in one community, practicing some agriculture but mainly living by fishing.[4]

São Tomé

Rainforest trekking is one of São Tomé's attractions
Beach scenery on São Tomé.

São Tomé island is 854 km2 (330 sq mi) in area, lying almost on the equator. The entire island is a massive shield volcano which rises from the floor of the Atlantic Ocean, over 3,000 m (10,000 ft) below sea level, and reaches 2,024 m (6,640 ft) above sea level in the Pico de São Tomé.[6] The oldest rock on São Tomé is 13 million years old.[7] Most of the lava that has erupted on São Tomé over the last million years has been basalt. The youngest dated rock on the island is about 100,000 years old, but numerous more recent cinder cones are found on the southeast side of the island.[8]

Due to the prevailing southwesterly winds, there is great variability in rainfall. In the rain shadow to the northeast of São Tomé the vegetation is dry savannah, with only 60 cm (24 in) of rain each year. By contrast, the lush south and west of the island receive about 6 m (20 ft) of rain, mostly falling in March and April.

critically endangered (the São Tomé ibis, São Tomé fiscal and São Tomé grosbeak).[10] Schistometopum thomense, a bright yellow species of caecilian, is endemic to São Tomé.[11]

As of 2010,

palm products. There are large reserves of oil in the ocean between Nigeria and São Tomé which have not yet been exploited.[12]

Príncipe

Príncipe is the smaller of the two major islands of

Pico de Príncipe, in a thickly forested area within the Obo National Park. The north and centre of the island were formerly plantations but have largely reverted to forest. As with São Tomé, the island has always been isolated from the mainland and therefore has many unique species of plants and animals, including six endemic birds.[10]

Príncipe has a population of around 5,000 people. Other than

Forro speakers.[13]

Bioko

Coastline of Bioko

Bioko is just 32 km (20 mi) off the coast of Cameroon, on the continental shelf. The island used to be the end of a peninsula attached to the mainland, but was cut off when sea levels rose 10,000 years ago at the end of the last ice age.[14] With an area of 2,017 km2 (779 sq mi) it is the largest island in the Cameroon line.[15]

Bioko has three basaltic shield volcanoes, joining at the lower levels.

San Carlos is 2,260 m (7,410 ft) high with a broad summit caldera, lying at the extreme SW of the island. The volcano dates from the Holocene age and has been active within the last 2000 years.[16] Santa Isabel is the largest volcano at 3,007 m (9,865 ft) in height, and contains many satellite cinder cones. Three eruptions have been reported from vents on the southeast flank during the late-19th and early-20th centuries.[17] San Joaquin, also known as Pico Biao or Pico do Moka, is 2,009 m (6,591 ft) high, on the southeast of the island. The summit is cut by a small lake-filled caldera, and there is a crater lake on the NE flank. San Joaquin has been active during the last 2,000 years.[18]

The southwestern side of Bioko is rainy for most of the year, with annual rainfall in some locations of 10,000 mm (394 in). The climate is tropical at lower altitudes, becoming about 1 °C (1.8 °F) cooler for each 150 m (492 ft) of elevation. There is

montane forest is protected by the 330 km2 (130 sq mi) Basilé National Park and the 510 km2 (200 sq mi) Luba Crater Scientific Reserve
. There has been little habitat loss, and the southern slopes have remained almost completely undisturbed. Although hunting pressure is rising, the fauna in the inaccessible southern part of the island is mostly intact. This includes an endemic subspecies of drill,

Bioko is a part of

Spaniards and immigrants from Río Muni, Nigeria and Cameroon.[15] Cocoa production was once the main export, but has declined in recent years. Farming, fishing and logging remain important. Natural gas is produced in offshore wells, processed on the island and exported via tanker.[21]

Western High Plateau

The Western High Plateau, also called the Western Highlands or the Bamenda Grassfields, continues the Cameroon line into the mainland of Cameroon. The plateau rises in steps from the west. To the east, it terminates in mountains that range in height from 1,000 m (3,300 ft) to 2,500 metres (8,200 ft).[22] The plateau gives way to the Adamawa Plateau to the northeast, a larger but less rugged region.[23]

Volcanism

Lake Oku is a crater lake on the plateau.

The Western High Plateau features several dormant

Bamboutos Mountains, Mount Oku, and Mount Kupe.[22]
Crater lakes dot the plateau, the result of dead volcanoes filling with water.[23] This includes Lake Barombi Mbo and Lake Bermin, which have the highest number of endemic fish species per area recorded anywhere in the world.[24]

The 4,095 m (13,435 ft) Mount Cameroon on the coastline, which may have been observed by the

Oku volcanic plain, released a cloud of carbon dioxide gas that killed at least 1,200 people.[27]

Climate

The region has cool temperatures, heavy rainfall, and

Sudan climate becomes predominant.[30]

Northwest Province

The Western High Plateau's relief and high rainfall make it a major watershed for Cameroon.[31] Important rivers in the region include the Manyu, which rises in the Bamboutos Mountains and becomes the Cross River on its lower course, and the Nkam, which is known as the Wouri River on its lower course.[29] The region gives rise to important tributaries to the Sanaga River.[32] These rivers have a long high-water period during the wet season and a short low-water period during the dry season.[33]

Environment

Volcanism has created fertile black and brown soils.[34] The Western High Plateau was once heavily forested. However, repeated cutting and burning by humans has forced the forest back to areas along the waterways and has allowed grasslands to expand into the area.[35] Sudan savanna forms the dominant vegetation. This consists of grassfields—leading to the name Bamenda grassfields around the city of Bamenda—and short shrubs and trees that shed their foliage during the dry season as a defence against brush fires and dry weather. Raffia palms grow in the valleys and depressions.[36]

Geology

Geologists disagree over which volcanic regions should be included in the Cameroon volcanic line. All definitions include the islands and the continental stretch up to Oku. Based on similarities in age and composition, some also include the

Biu plateau of Nigeria to the north of the Yola arm of the Benue Trough, and the Jos Plateau
to the west of the Benue Trough.

There are varying theories for the similarities between the oceanic and continental volcanoes.[37]

Surrounding plate

Major geographical features near Cameroon line

The Cameroon line bisects the angle where the coast of Africa makes a 90° bend from the southern coast along the west of the

West African craton
. The coastline roughly corresponds to the coast of the Borborema geological province of northeastern Brazil, which began to separate from this part Africa around 115 million years ago.

The Central African Shear Zone (CASZ), a lineament that extends from the Sudan to coastal Cameroon, runs under the continental section of the Cameroon line. It is visible in the Foumban Shear Zone, which was active before and during the opening of the South Atlantic in the Cretaceous period.[38] The western end of the

Pernambuco fault in Brazil.[39]
A major earthquake in 1986 could indicate that the shear zone is reactivating.[40]

The Benue Trough lies to the west of the Cameroon line. The Benue Trough was formed by rifting of the central West African basement, beginning at the start of the Cretaceous era. A common explanation of the trough's formation is that it is an aulacogen, an abandoned arm of a three-armed radial rift system. The other two arms continued to spread during the break-up of Gondwana, as South America separated from Africa.[41] During the Santonian age, around 84 million years ago, the Benue Trough underwent intense compression and folding.[42] Since then it has been tectonically quiet.[1]

Hypotheses

The

basaltic rocks in the oceanic and continental sectors of the Cameroon line are similar in composition, although the more evolved rocks are quite distinct. The similarity in basaltic rocks may indicate they have the same source. Since the lithosphere mantle below Africa must be different in chemical and isotopic composition from the younger lithosphere below the Atlantic, one explanation is that the source is in the asthenosphere rather than in metasomatized lithosphere.[43]
A different view is that the similarities are caused by shallow contamination of the oceanic section, which could be caused by sediments from the continent or by rafted crustal blocks that were trapped in the oceanic lithosphere during the separation between South America and Africa.[37]

According to some geologists, there is evidence that a mantle plume has existed in the region for about 140 million years, first remaining in roughly the same position while the African plate rotated above it, and then remaining stationary under the Oku area since around 66 million years ago.[1] In this theory, the abnormal heat rising in a mantle plume would lead to melting of the upper mantle, which raises, thins and weakens the crust and facilitates rifting. This may have been repeated several times in the Benue Trough between 140 Ma and 49 Ma.[44][45] One plume-related hypothesis for the later development of the Cameroon Line around 30 Ma is that it coincides with development of a shallow mantle convection system centered on the mantle plume, and is related to thinning and extension of the crust along the Cameroon line as pressures relaxed in the now stationary plate.[1]

The traditional mantle plume hypothesis is disputed by scientists who point out that features of the region are quite different from what is predicted by that hypothesis, and that a source in a lithospheric fracture is more likely to be the explanation.[2] One explanation for the origin of the volcanic line is likely leakage of magma from reactivated Precambrian faults,[46] while another scenario is the rising of mantle material from African Large low-shear-velocity provinces travels under Congo Craton and through existing fractures ultimately feed the volcanic activities.[47] The puzzling feature, that the composition of the magmas is the same both in the land volcanoes and the oceanic ones is likely explained by recent studies that show the underlying lithosphere is the same. A gravity study of the southern part of the

Adamawa plateau has shown a belt of dense rocks at an average depth of 8 km running parallel to the Foumban shear zone. The material appears to be an igneous intrusion that may have accompanied reactivation of the shear zone, and may be associated with the Cameroon line.[48]

References

  1. ^ a b c d e f Burke 2001.
  2. ^ a b Foulger 2010, pp. 1ff.
  3. ^ Marzoli et al. 1999.
  4. ^ a b Fa 1991, p. 168.
  5. ^ Appleton 1857.
  6. ^ Sao Tome - CIA 2011.
  7. ^ a b Becker 2008, pp. 3.
  8. ^ Sao Tome - Smithsonian.
  9. ^ Becker 2008, pp. 4.
  10. ^ a b c African Bird Club.
  11. ^ AmphibiaWeb (2011). Schistometopum thomense. Accessed May 1, 2011.
  12. ^ São Tomé and Príncipe (US State Dept) 2010.
  13. ^ Becker 2008, pp. 3ff.
  14. ^ McNeil 2010.
  15. ^ a b McColl 2005, pp. 298.
  16. ^ San Carlos - Smithsonian.
  17. ^ Santa Isabel - Smithsonian.
  18. ^ San Joaquin - Smithsonian.
  19. ^ Mount Cameroon - WWF.
  20. ^ "Bioko". Encyclopædia Britannica. Retrieved 28 April 2015.
  21. ^ Equatorial Guinea - CIA.
  22. ^ a b Gwanfogbe et al. 1983, pp. 8.
  23. ^ a b Neba 1999, pp. 17.
  24. ^ Freshwater Ecoregions of the World (2008). Western Equatorial Crater Lakes. Archived 2011-10-05 at the Wayback Machine
  25. ^ Mount Cameroon - Smithsonian.
  26. ^ Conserving Bamenda - Birdlife.
  27. ^ BBC 1986.
  28. ^ Gwanfogbe et al. 1983, pp. 16–17.
  29. ^ a b Gwanfogbe et al. 1983, pp. 17.
  30. ^ Neba 1999, pp. 19.
  31. ^ Gwanfogbe et al. 1983, pp. 24.
  32. ^ Neba 1999, pp. 40.
  33. ^ Gwanfogbe et al. 1983, pp. 25.
  34. ^ Gwanfogbe et al. 1983, pp. 19.
  35. ^ Gwanfogbe et al. 1983, pp. 18.
  36. ^ Neba 1999, pp. 34.
  37. ^ a b Rankenburg, Lassiter & Brey 2004.
  38. ^ Dorbath et al. 1986.
  39. ^ Stuart et al. 1985.
  40. ^ New Scientist 1987.
  41. ^ Petters 1978.
  42. ^ Obaje et al. 2004.
  43. ^ Fitton 1987.
  44. ^ Ofoegbu 1984.
  45. ^ Maluski et al. 1995.
  46. ^ Njonfang et al. 2008.
  47. S2CID 255023093
    .
  48. ^ Tatchum, Tabod & Manguelle-Dicoum 2006.

Sources

3°30′0″N 8°42′0″E / 3.50000°N 8.70000°E / 3.50000; 8.70000