Geology and Hydrography

The Zanzibar archipelago, consisting of the islands of Unguja and Pemba and several surrounding islets, is part of the ancient Miocene Rufiji/Ruvu delta. The core of these islands consists of rocks ranging in age from Miocene to recent calcareous sediment with limestone of marine origin that was subsequently more or less covered by sands, silt and clays brought by rivers from the East African mainland. The abundance of coral limestones on both islands is thought to indicate the possibility that the sea might have covered much of the islands at some point in time. However, due to periods of isostatic movement and block faulting over coastal Tanzania and offshore zone these islands remained above sea level as blocks of the original delta.

The islands are about 40 km from the coast of East Africa separated by Pemba Channel on the north and Zanzibar Channel on the south. Unguja Island is about 40 km south-east of Pemba and about 56 km north of Latham Island or Fungu Kizimkazi, which is a small uninhabited island also part of the Zanzibar archipelago. Pemba Channel is much deeper and can reach up to 800 m in depth off the continental shelf of mainland Tanzania.

Pemba is a simple fault block and rose earlier than Unguja; it is thus composed of lower stratigraphic rocks, and geologically much older than Unguja. Topographically, it is a single ridge and watershed with a raised east coast. It is dissected by hills and ridges and has a marked indented western coastline with low-lying shore and numerous marine inlets and dense mangrove forests. The highest point on Pemba Island is at Siniongoni about 90m above sea level.

Unguja Island is elongate and indented only sparsely with a stand of mangrove forests much smaller compared to that of Pemba. Believed to have emerged from the sea later compared to Pemba, it was connected to the mainland of Tanzania as late as during the beginning of the Pleistocene age. The topography of Unguja Island is generally flat but with a central ridge running from north to south whose highest point is at Masingini about 120m above sea level.
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The ocean surrounding Unguja, Pemba and the related islands consists of a confluence of ocean currents that culminate into the East Africa Coastal Current (EACC), which mostly flows northwards but pockets of residual currents flowing southwards have been recorded. It is believed that the northward flows are common during the Southwest Monsoon (April to October) and southwards flows in the Northeast Monsoon (November to March).