Physical Environment of MADAGASCAR
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Figure 3. Madagascar: Topography and Drainage
Madagascar can be divided into five geographical regions: the east coast, the Tsaratanana Massif, the central highlands, the west coast, and the southwest. The highest elevations parallel the east coast, whereas the land slopes more gradually to the west coast (see fig. 3).
The east coast consists of a narrow band of lowlands, about fifty kilometers wide, formed from the sedimentation of alluvial soils, and an intermediate zone, composed of steep bluffs alternating with ravines bordering an escarpment of about 500 meters in elevation, which gives access to the central highlands. The coastal region extends roughly from north of Baie d'Antongil, the most prominent feature on the east coast of the island formed by the Masoala Peninsula, to the far south of the island. The coastline is straight, with the exception of the bay, offering less in the way of natural harbors than the west coast. The Canal des Pangalanes (Lakandranon' Ampalangalana), an 800-kilometerlong lagoon formed naturally by the washing of sand up on the island by the Indian Ocean currents and by the silting of rivers, is a feature of the coast; it has been used both as a means of transportation up and down the coast and as a fishing area. The beach slopes steeply into deep water. The east coast is considered dangerous for swimmers and sailors because of the large number of sharks that frequent the shoreline.
The Tsaratanana Massif region at the north end of the island contains, at 2,880 meters, the highest point on the island and, north of this, the Montagne d'Ambre (Ambohitra), which is of volcanic origin. The coastline is deeply indented; two prominent features are the excellent natural harbor at Antsiranana (Diégo Suarez), just south of the Cap d'Ambre (Tanjon' i Bobaomby), and the large island of Nosy-Be to the west. The mountainous topography to the south, however, limits the potential of the port at Antsiranana by impeding the flow of traffic from other parts of the island.
The central highlands, which range from 800 to 1,800 meters in altitude, contain a wide variety of topographies: rounded and eroded hills, massive granite outcrops, extinct volcanoes, eroded peneplains, and alluvial plains and marshes, which have been converted into irrigated rice fields. The central highlands extend from the Tsaratanana Massif in the north to the Ivakoany Massif in the south. They are defined rather clearly by the escarpments along the east coast, and they slope gently to the west coast. The central highlands include the Anjafy High Plateaux; the volcanic formations of Itasy (Lake Itasy itself is found in a volcanic crater) and the Ankaratra Massif, reaching a height of 2,666 meters; and the Ivakoany Massif in the south. The Isalo Roiniforme Massif lies between the central highlands and the west coast. Antananarivo, the national capital, is located in the northern portion of the central highlands at 1,468 meters above sea level. A prominent feature of the central highlands is a rift valley running north to south, located east of Antananarivo and including Lac Alaotra, the largest body of water on the island, having a length of forty kilometers. The lake is located 761 meters above sea level and is bordered by two cliffs, rising 701 meters to the west and 488 meters to the east, which form the walls of a valley resembling the rift valleys of East Africa. This region has experienced geological subsidence, and earth tremors are frequent here.
The west coast, composed of sedimentary formations deposited in several layers over time, is more indented than the east coast, especially in the northwest, thus offering a number of fine harbors sheltered from cyclones, such as the harbor at Mahajanga. Deep bays and well-protected harbors have attracted explorers, traders, and pirates from Europe, Africa, and the Middle East since ancient times; thus, the area has served as an important bridge between Madagascar and the outside world. Yet the broad alluvial plains found on the coast between Mahajanga and Toliara, which are believed to have great agricultural potential, are thinly inhabited and remain largely unexploited.
The southwest is bordered on the east by the Ivakoany Massif and on the north by the Isala Roiniforme Massif. It includes two regions along the south coast, the Mahafaly Plateau and the desert region occupied by the Antandroy people.
The Mananara and Mangoro rivers flow from the central highlands to the east coast, as does the Maningory, which flows from Lake Alaotra. Other rivers flowing east into the Indian Ocean include the Bemarivo, the Ivondro, and the Mananjary. These rivers tend to be short because the watershed is located close to the east coast. Owing to the steep elevations, they flow rapidly, often over spectacular waterfalls. The rivers flowing to the west coast and emptying into the Mozambique Channel tend to be longer and slower, because of the more gradual slope of the land. The major rivers on the west coast are the Sambirano, the Mahajamba, the Betsiboka (the port of Mahajanga is located at the mouth), the Mania, the North and South Mahavavy, the Mangoky, and the Onilahy. The Ikopa, which flows past Antananarivo, is a tributary of the Betsiboka. The Mangoky River has a basin area of some 50,000 square kilometers; the Ikopa River and the Betsiboka River have basin areas of 18,550 and 11,800 square kilometers, respectively. The principal river in the south, the Mandrare, has a basin area of some 12,435 square kilometers, but it runs dry during certain months in this desert region. Important lakes, aside from Alaotra, include Lake Kinkony in the northwest and Lake Ihotry in the southwest.
Madagascar has been called the "Great Red Island" because of the supposed preponderance of red lateritic soils. The red soils predominate in the central highlands, although there are much richer soils in the regions of former volcanic activity--Itasy and Ankaratra, and Tsaratamana to the north. A narrow band of alluvial soils is found all along the east coast and at the mouths of the major rivers on the west coast; clay, sand, and limestone mixtures are found in the west; and shallow or skeletal laterite and limestone are located in the south.
Data as of August 1994
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