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{{Short description|Chain of volcanoes in
[[File:Gulf of Guinea (English).jpg|thumb|upright=1.5|Map of the Gulf of Guinea, showing the chain of islands formed by the Cameroon volcanic line
The '''Cameroon line''' ({{Lang-fr|Ligne du Cameroun}}, {{lang-pt|Linha dos Camarões}}, {{Lang-es|cordillera de Camerún}}) is a {{convert|1,600|km|mi|sigfig=1|abbr=on}} 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.{{sfn|Burke|2001}} They form a natural border between eastern [[Nigeria]] and the [[West Region (Cameroon)|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.{{sfn|Burke|2001}}{{sfn|Foulger|2010|pp=1ff}}
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==Geography==
[[File:Mount Cameroon craters.jpg|thumb|[[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 [[seamount]]s. From the southwest to the northeast the island groups are [[Annobón]] (or Pagalu), [[São Tomé Island|São Tomé]], [[Príncipe]] and [[Bioko]]. Two large seamounts lie between São Tomé and Príncipe, and between Príncipe and Bioko.
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====Annobón====
{{Main|Annobón}}
The southernmost island in the chain is Annobón, also known as Pagalu, with an area of about {{convert|17.5|km2|sqmi|abbr=on}}. It is an extinct volcano that rises from deep water to {{convert|598|m|ft|abbr=on}} above sea level. The island belongs to [[Equatorial Guinea]].
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====São Tomé====
{{Main|São Tomé Island}}▼
[[File:sao tome forest.jpg|thumb|right|Rainforest trekking is one of São Tomé's attractions]]
[[File:São Tomé - Resort Pestana Equador.jpg|thumb|Beach scenery on São Tomé.]]
▲{{Main|São Tomé Island}}
São Tomé
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 {{convert|60|cm|in|abbr=on}} of rain each year. By contrast, the lush south and west of the island receive about {{convert|6|m|ft|abbr=on}} of rain, mostly falling in March and April.{{sfn|Becker|2008|pp=4}} The climate is hot and humid with the rainy season from October to May. The higher slopes of the island are forested and form part of the Obo National Park.{{sfn|African Bird Club}} São Tomé has never been connected to Africa, and therefore has many unique plants and birds.{{sfn|Becker|2008|pp=3}} Of the bird species, 16 are [[endemism|endemic]] and six are near endemic, of which four are only shared with Príncipe. Six species are considered [[Vulnerable species|vulnerable]], and three are [[critically endangered]] (the [[São Tomé ibis]], [[São Tomé fiscal]] and [[São Tomé grosbeak]]).{{sfn|African Bird Club}} ''[[Schistometopum thomense]]'', a bright yellow species of [[caecilian]], is endemic to São Tomé.<ref>AmphibiaWeb (2011). ''[http://amphibiaweb.org/cgi-bin/amphib_query?query_src=aw_lists_alpha_&where-genus=Schistometopum&where-species=thomense Schistometopum thomense.]'' Accessed May 1, 2011.</ref>
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====Príncipe====
{{Main|Príncipe}}
Príncipe is the smaller of the two major islands of [[São Tomé and Príncipe]], with an area of {{convert|136|km2|sqmi|abbr=on}}. Volcanic activity stopped around 15.7 million years ago, and the island has been deeply eroded apart from spectacular towers of [[phonolite]]. The island is surrounded by smaller islands including [[Ilheu Bom Bom]], [[Ilhéu Caroço]], [[Tinhosa Grande]] and [[Tinhosa Pequena]], and lies in ocean {{convert|3,000|m|ft|abbr=on}} deep. It rises in the south to {{convert|946|m|ft|abbr=on}} at [[Pico de Príncipe]], in a thickly [[forest]]ed area within the Obo National Park. The north and centre of the island were formerly [[plantation]]s 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.{{sfn|African Bird Club}}
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====Bioko====
[[File:biokoisland.jpg|thumb|Coastline of Bioko]]▼
{{Main|Bioko}}
▲[[File:biokoisland.jpg|thumb|Coastline of Bioko]]
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]].{{sfn|McNeil Jr.|2010}} With an area of {{convert|2017|km2|sqmi|0|abbr=on}} it is the largest island in the Cameroon line.{{sfn|McColl|2005|pp=298}}▼
▲Bioko is just {{convert|32|km|mi|abbr=on}} 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 [[
Bioko has three basaltic shield volcanoes, joining at the lower levels. [[San Carlos (volcano)|San Carlos]] is {{convert|2,260|m|ft|abbr=on}} 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.{{sfn|San Carlos - Smithsonian}} [[Pico Basilé|Santa Isabel]] is the largest volcano at {{convert|3,007|m|ft|abbr=on}} 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.{{sfn|Santa Isabel - Smithsonian}} [[San Joaquin (Equatorial Guinea)|San Joaquin]], also known as Pico Biao or Pico do Moka, is {{convert|2,009|m|ft|abbr=on}} high, on the southeast of the island. The summit is cut by a small lake-filled [[caldera]], and there is a [[Volcanic crater lake|crater lake]] on the NE flank. San Joaquin has been active during the last 2,000 years.{{sfn|San Joaquin - Smithsonian}}
The southwestern side of Bioko is rainy for most of the year, with annual rainfall in some locations of {{convert|10,000|mm|in|0|abbr=on}}. The climate is tropical at lower altitudes, becoming about {{convert|1|C-change|F-change|abbr=on}} cooler for each {{convert|150|m|ft|0|abbr=on}} of elevation. There is [[Canopy (biology)|open canopy]] montane forest above {{convert|1,500|m|ft|abbr=on}} on Pico Basilé, Gran Caldera de Luba and Pico Biao, with [[subalpine]] grassland above {{convert|2,500|m|ft|abbr=on}}. Bioko has exceptional numbers of [[endemic species]] of flora and fauna, partly due to the great range of altitudes, particularly birdlife. The [[montane forest]] is protected by the {{convert|330|km2|sqmi|abbr=on}} [[Basilé National Park]] and the {{convert|510|km2|sqmi|abbr=on}} [[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, ''[[Mandrillus leucophaeus poensis]]''.{{sfn|Mount Cameroon - WWF|ignore-err=yes}}
Bioko is a part of [[Equatorial Guinea]]. The island has a population of 334,463 inhabitants (2015 Census),<ref>{{cite encyclopedia|url=http://www.britannica.com/EBchecked/topic/65951/Bioko|title=Bioko|encyclopedia=Encyclopædia Britannica|access-date=28 April 2015}}</ref> most of whom are [[Bubi people|Bubi]]. The rest of the population are ''[[Fernandinos]]'', [[Spaniard]]s and immigrants from [[Río Muni]], [[Nigeria]] and [[Cameroon]].{{sfn|McColl|2005|pp=298}} 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.{{sfn|Equatorial Guinea - CIA}}
===Western High Plateau===
{{Main|Western High Plateau}}
The Western High Plateau, also called the Western Highlands or the Bamenda Grassfields, continues the Cameroon line into the mainland of
====Volcanism====
[[File:Lake Oku Cameroon.jpg|thumb|right|[[Lake Oku]] is a [[Volcanic crater lake|crater lake]] on the plateau.]]
The Western High Plateau features several dormant [[volcano]]es, including the [[Bamboutos Mountains]], [[Mount Oku]], and [[Mount Kupe]].{{sfn|Gwanfogbe|Meligui|Moukam|Nguoghia|1983|pp=8}}
[[Volcanic crater lake|Crater lake]]s dot the plateau, the result of dead volcanoes filling with water.{{sfn|Neba|1999|pp=17}} This includes [[Lake Barombi Mbo]] and [[Lake Bermin]], which have the highest number of [[Endemism|endemic]] fish species per area recorded anywhere in the world.<ref name=FEW>Freshwater Ecoregions of the World (2008). ''[http://www.feow.org/ecoregion_details.php?eco=519 Western Equatorial Crater Lakes.] {{webarchive|url=https://web.archive.org/web/20111005204252/http://www.feow.org/ecoregion_details.php?eco=519 |date=2011-10-05 }}''</ref>
The {{convert|4,095|m|ft|abbr=on}} Mount Cameroon on the coastline, which may have been observed by the [[Carthage|Carthaginian]] [[Hanno the Navigator]] in the 5th century BC, erupted in 2000.{{sfn|Mount Cameroon - Smithsonian
====Climate====
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====Environment====
Volcanism has created fertile black and brown soils.{{sfn|Gwanfogbe|Meligui|Moukam|Nguoghia|1983|pp=19}} 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.{{sfn|Gwanfogbe|Meligui|Moukam|Nguoghia|1983|pp=18}} 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 palm]]s grow in the valleys and depressions.{{sfn|Neba|1999|pp=34}}
==Geology==
Geologists disagree over which volcanic regions should be included in the Cameroon
There are varying theories for the similarities between the oceanic and continental volcanoes.{{sfn|Rankenburg|Lassiter|Brey|2004}}
===Surrounding plate===
[[File:Cameroon line.svg|thumb|upright=1.3|Major geographical features near Cameroon line]]
The Cameroon line bisects the angle where the coast of
The coastline roughly corresponds to the coast of the Borborema geological province of
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.{{sfn|Dorbath|Dorbath|Fairhead|Stuart|1986}}
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A major earthquake in 1986 could indicate that the shear zone is reactivating.{{sfn|New Scientist|1987}}
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.{{sfn|Petters|1978}} During the [[Santonian]] age, around 84 million years ago, the Benue Trough underwent intense compression and folding.{{sfn|Obaje|Wehner|Abubakar|Isah|2004}} Since then it has been tectonically quiet.{{sfn|Burke|2001}}▼
▲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.{{sfn|Petters|1978}}
===Hypotheses===
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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.{{sfn|Burke|2001}}
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.{{sfn|Foulger|2010|pp=1ff}}
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.{{sfn|Tatchum|Tabod|Manguelle-Dicoum|2006}}
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}}
*{{cite journal
|title = A teleseismic delay time study across the Central African Shear Zone in the Adamawa region of Cameroon, West Africa
|first1 = C.
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|year = 1986
|pages = 751–766
|doi = 10.1111/j.1365-246X.1986.tb00658.x▼
▲|doi = 10.1111/j.1365-246X.1986.tb00658.x
|bibcode = 1986GeoJ...86..751D
|doi-access= free
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|publisher=Macmillan Education
|isbn=0-333-36690-5}}
*{{cite
|publisher=Geological Society of London
|year=1995
|title=40Ar/39Ar chronology, petrology and geodynamic setting of Mesozoic to early Cenozoic magmatism from the Benue Trough, Nigeria
|first1=H. |last1=Maluski |first2=C. |last2=Coulon |first3=M. |last3=Popoff |first4=P. |last4=Baudin
|volume=152
|issue=2
|pages=311–326
|doi=10.1144/gsjgs.152.2.0311
|bibcode=1995JGSoc.152..311M
|s2cid=128685924
}}
*{{cite journal |title=The Cameroon Volcanic Line Revisited: Petrogenesis of Continental Basaltic Magmas from Lithospheric and Asthenospheric Mantle Sources
|first1=A. |last1=Marzoli |first2=E. M. |last2=Piccirillo |first3=P. R. |last3=Renne |first4=G. |last4=Bellieni
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*{{cite news |url=https://www.nytimes.com/2010/09/17/health/17aids.html
|title=Precursor to H.I.V. Was in Monkeys for Millenniums
|first= Donald G. Jr.|last=McNeil
|date=September 16, 2010
|work=[[The New York Times]]
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| name=Cameroon
| vn=224010
|access-date=2011-02-01
|ref={{sfnref|Mount Cameroon - Smithsonian}}}}
*{{WWF ecoregion | ref={{harvid|Mount Cameroon - WWF}}
|id=at0121
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|volume=27| issue = 2
|pages=191–206
|date=April 2004|doi=10.1111/j.1747-5457.2004.tb00053.x|bibcode=2004JPetG..27..191O|s2cid=129544171 }}
*{{cite journal |journal=Geologische Rundschau
|volume=73| issue = 3
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|name=San Carlos
|vn=224002
|access-date=2011-02-04
|ref={{sfnref|San Carlos - Smithsonian}}}}
*{{cite gvp
|name=Santa Isabel
|vn=224004
|access-date=2011-02-04
|ref={{sfnref|Santa Isabel - Smithsonian}}}}
*{{cite gvp
|name=San Joaquin
|vn=224003
|access-date=2011-02-04
|ref={{sfnref|San Joaquin - Smithsonian}}}}
*{{cite web
|url=https://www.cia.gov/the-world-factbook/countries/sao-tome-and-principe/
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|title=São Tomé and Principe Geography
|publisher=CIA
|work=World Factbook
}}
*{{Cite gvp
|name=Sao Tome
|vn=224001
|access-date=2011-02-04
|ref={{sfnref|Sao Tome - Smithsonian}}}}
*{{cite web |ref={{harvid|São Tomé and Príncipe (US State Dept)|2010}}
|url=https://2009-2017.state.gov/r/pa/ei/bgn/5434.htm
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|volume= 3| issue = 1
|year=2006
|doi=10.1088/1742-2132/3/1/009 |pages=82–89|bibcode = 2006JGE.....3...82N |doi-access=free}}
{{Refend}}
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