Geology of Tanzania

The oldest Precambrian rocks in Tanzania form the stable continental crust of the Tanzania Craton and date to the Archean more than 2.5 billion years ago. The craton includes the vestiges of two Archean orogenic belts, the Dodoman Belt in central Tanzania, and the Nyanzian-Kavirondian in the north. The remains of these two belts produce lenses of sedimentary and volcanic rocks within granites and migmatite. The Dodoman Belt stretches for 480 kilometers, broadening westward and is composed of banded quartzites, aplite, sericitic schist, pegmatite and ironstone.

The Nyanzian Belt, by contrast, is mainly acid and basic basalt, dolerite, trachyte, rhyolite and tuff in separated zones south and east of Lake Victoria. The Kavirondian System is closely related to the Nyanzian, but separated by an unconformity and a zone of intense deformation. The Kavirondian is believed to be the remains of a molasse that formed by the Nyanzian as evidenced by sedimentary rocks such as mudstone, conglomerate as well as volcanics and grit.^[1]

The Ubendian Belt is a highly metamorphosed and folded metamorphic zone with intrusive granites southwest of the Tanzania Craton, paralleling Lake Tanganyika and Lake Rukwa. It likely formed in the Paleoproterozoic. Most rocks in the belt are pelite or various types of volcanic rock. Gneiss, hornblende, biotite, garnet and kyanite are common in the belt. ^[2]

Northwestern Tanzania has the Mesoproterozoic Karagwe-Ankole Belt, with argillite, moderately metamorphosed phyllite, quartzites and sericitic schists. For the most part, the belt is only moderately folded, although deformation increases near granite intrusions. The belt is associated with Bukoban Series conglomerates, bedded sandstones and basalt flows which extend from the Ugandan border and terminate at the Rukwa Rift.^[3]

The Mozambique Belt is a structurally and metamorphically complex terrane that abuts the edge of the Tanzania Craton and formed during the Neoproterozoic. The formation of the belt is related to the widespread Pan-African orogeny and most rocks are highly metamorphosed pyroxene-gneiss, charnockite, biotite, hornblende. It also contains smaller occurrences of graphitic schist, quartzites and crystalline limestone. ^[3]

During the Paleozoic, after the Pan-African orogeny and the formation of the supercontinent Gondwana the Mozambique Belt experienced millions of years of erosion. This erosive period continued as Gondwana joined with Euramerica to become the supercontinent Pangaea.^[3]

In the Mesozoic, a major rift valley spanning what is today southern Africa and southern South America began to fill with sediments, depositing the first rock units of the Karoo Supergroup—the most extensive stratigraphic unit in southern Africa. In the Tanzania region, situated in the interior of Pangaea, the Karoo Supergroup deposited between the Late Carboniferous and the Early Jurassic, with sequences of terrestrial sediments. These sediments formed diverse rock units including sandstones, conglomerates, tillite, shale, red and gray mudstones and periodically, limestone deposits. Many of the Tanzanian Karoo rocks from this period contain plant and animal fossils.

With the fragmentation of Pangaea, the modern coastal region of Tanzania experienced a long lasting marine transgression, which deposited limestone, marl, sandstone and shale throughout the Late Jurassic, Cretaceous and almost all of the past 66 million years of the Cenozoic, up until the last 2.5 million years of the Quaternary. In the interior, the beginning East Africa Rift created large rift troughs, which filled with terrestrial sediments. Carbonatite volcanoes began erupting in the Cretaceous, a process that continued into the Cenozoic.^[3]

Alkaline volcanic rocks, including olivine basalt, alkaline basalt, nephelinite, phonolite, trachyte and pyroclastic flows from the Neogene are common north of Lake Nyasa and Lake Natron.^[4]

The East African Rift system plays a key role in Tanzania's current day structural geology. Graben rift valleys often have volcanism associated with them. The Western Rift is filled with Lake Tanganyika and Lake Nyasa, while the section from Lake Natron to Lake Nyasa is part of the Eastern Rift (also known as the Gregory Rift). Lake Rukwa and the Selous Basin occupy small, subsidiary rift basins. Structural geologists infer that Lake Victoria is situated in a downwarp in the crust between the two major rifts.

Ol Doinyo Lengai is the world's only active volcano with carbonatite lava and last had a large-scale eruption in 1966.^[5]