The geology of Madagascar comprises a variety of rocks of Precambrian age which make up the larger part of the east and centre of the island. They are intruded by basalts and rhyolites of Mesozoic to Cenozoic age. In contrast, the western part of the island is formed from sedimentary rocks of Carboniferous to Quaternary age. Archean rocks occur from the northeast portion of the island down to the south in the Ranotsara shear zone. Rocks in the northern portion of Madagascar are greenstone belts, from the Archean or Paleoproterozoic age.
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Tectonics
A NW-SE trending sinistrel shear belt separates northern and southern crustal terranes. The Ampanihy ductile shear zone was created by flattening events associated with granulite metamorphism, isoclinal folding, flattened sheaths, steep to vertical foliations and sheath-like geometry of massif-type anorthosite bodies.
Economic geology
Madagascar is the world's leading producer many colored gemstones, including: sapphires, rubies, multi-coloured tourmalines, emerald, amethysts, cordierites, aquamarines and garnets. Madagascar is also a major source of graphite, making it the second-largest producer in Africa. Additionally, Madagascar is the Africa's third leading producer of chromite. Titanium oxide is found in significant quantities in beach sands located near Toalagnaro, however, 75% of the coastal forest zone in that area would have to be destroyed in order to exploit it, which has not been done.
Huge oil fields lie to the west of Madagascar, however they would require steam injection to extract, which is both technologically difficult and expensive. The oil fields of Tsimiroro and Bemolanga are estimated to be a combined 11.5 billion barrels.