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GEOLOGYANDHYDROGRAPHY

The Zanzibar archipelago, consisting of the islands of

Unguja and Pemba and several surrounding islets, arepart of the ancient Miocene Rufiji/Ruvu delta. The core

of these islands consists of rocks ranging in age fromMiocene to recent clacareous 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 the 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 un-

inhabited 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 the mainland

Tanzania.

Pemba is a simple fault block and rose earlier than

Unguja; it is thuscomposed of lower stratigraphic rocks,

and geologically much older than Unguja.

Topographically, it is a single ridge andwatershedwith 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 wasconnected 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).

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