The Cameroon line (, , ) is a 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. 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.
Geography
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 Mount Cameroon and extends northeast in a range known as the Western High Plateau, home to the Cameroonian Highlands forests. Volcanic swells further inland are Manengouba, Bamboutu and the Oku Massif. East of Oku there are further volcanic mountains in the Ngaoundere Plateau, some of which appear to have similar origins.
Island chain
Annobón
The southernmost island in the chain is Annobón, also known as Pagalu, with an area of about . It is an extinct volcano that rises from deep water to above sea level. The island belongs to Equatorial Guinea.
The average temperature is , with little seasonal variation. Most rain falls from November to May, with annual precipitation averaging - less than on the mainland. Annobón has lush valleys and steep mountains, covered with rich woods and luxuriant vegetation.
The small population lives in one community, practicing some agriculture but mainly living by fishing.
São Tomé
São Tomé island is 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 below sea level, and reaches above sea level in the Pico de São Tomé. The oldest rock on São Tomé is 13 million years old. 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.
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 of rain each year. By contrast, the lush south and west of the island receive about of rain, mostly falling in March and April. 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. São Tomé has never been connected to Africa, and therefore has many unique plants and birds. Of the bird species, 16 are endemic and six are near endemic, of which four are only shared with Príncipe. Six species are considered vulnerable, and three are critically endangered (the São Tomé ibis, São Tomé fiscal and São Tomé grosbeak). Schistometopum thomense, a bright yellow species of caecilian, is endemic to São Tomé.
As of 2010, São Tomé and Príncipe, an independent nation, had an estimated population of 167,000, most of whom lived on São Tomé island. The main language is Portuguese, but there are many speakers of Forro and Angolar (Ngola), two Portuguese-based creole languages. The economy is mainly based on tourism. Agriculture is important near the north and east coasts, with the chief exports being cocoa, coffee, copra, and palm products. There are large reserves of oil in the ocean between Nigeria and São Tomé which have not yet been exploited.