The geological evolution of East Africa - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 19

Magadi-Natron and Manyara basins (Pickering,
1968; Crossley, 1979).
The central highlands ofAfrica, comprising the
mountains of easten Zare, westen Uganda,
Rwanda and Burundi, lie along the line of the
westen arm of the Rift Valley and were formed by
a combinaion of block-fauling and volcanism.
Uplift of the giant block of Precambrian rocks of
the Rwenzori Range on the Zaire-Uganda border
occurred mainly in the Pliocene and Qu atenary.
Nyamlagira and Nyirogongo are sill acive
volcanoes.
Evidence has already been cited to indicate that
the basin of Lake Nyasa is a very old structure. Le
Founier et al. (1985) note that the basin of Lake
Tanganyika has evolved over a period of 20 myr.
However, ages of the lakes appear to be much
younger with Lake Tanganyika estimated to be
mid-Pliocene (c. 7 myr BP ) and Lake Nyasa of
early Pleistocene (2.0 myr BP ) age. Lake Kivu
formed during the late Pleistocene through the
damming of river valleys by lava flows.
In comparison with the Rift Valley lakes, the
Lake Victoria basin lies in a depression formed by
crustal sagging between the two arms of the Rift
Valley. The basin started developing con-
temporaneously with the rifing to the west and
east and there is evidence of an earlier lake or
system of lakes than the modem Lake Victoria,
which is believed to be a mere 30 000 years old
(Beadle, 1974).

centres. By the beginning of the Upper Pleisto-
cene ( c. 120 000 yrs BP ) , volcanic activiy became
increasingly sporadic with smaller volumes erup-
ted in each episode. Shira and Mawenzi are now
exinct while the Kibo cenre, which has been
dormant during the Holocene or last 10 000
years, shows only residual fumarolic aciviy
(Downie & Wilkinson, 1972). fhese authors con-
clude that there has been no erupive acivity
within the last 200 years.
The lahars or volcanic mudflows which
resulted from the collapse of the original craters
and their lakes that occurred at late stages in the
development of both Mount Meru and Mawenzi
are likely to have been far more devasing to the
forests on their slopes than the younger lava flows
because of their much greater volume and extent.
The Mount Meru lahar spread eastwards
across the Sanya Plains almost as far as Moshi
and, according to Guest & Leeedal (1953),
occurred in prehistoric imes, while they esimate
that the last erupion from Meru occurred around
1877. Since then acivity has been conined to
fumaroles. Earthquakes sill occur from ime to
time, thus Meru is still classified as being an
active volcano whereas Kilimanjaro is dormant.
The Mawenzi lahar occurred in the Lower
Pleistocene ( c. 500 000 BP ) (Downie & Wilkinson,
1972) and spread northeastwards over an area of
1166 m2 between Laitokitok and Chyulu Range.
Kibo has a much more restricted lahar associ-
ated with it, mainly found between 670 and 1330
m west of Moshi. Downie & Wilkinson (1972)
believe it resulted from a series oflandslides prior
to the last Inner Crater erupions during the
Upper Pleistocene (11 000 BP ) .
In southern Tanzania, the isolated province of
the Rungwe Volcanics comprising the Poroto
Mountains, Tukuyu, Kiejo and Rungwe Moun-
tains, lies at the Y-shaped juncion of the Rukwa,
Ruaha and Nyasa roughs. These lavas are similar
in composition to the alkali basalts of the north.
The earliest lavas erupted in the late Pliocene (5-
2.5 myr BP ) (Harkin, 1960), after the main rift
movement. Like Kilimanjaro, the major acivity
occurred in the Pleistocene, the Rungwe volcano
being the youngest, and the extrusives of the
Poroto Mountains the oldest. The last erupion

Volcanics of Teiay to Recent age
In the early Pleistocene the rift system extended
eastwards by a system ofoblique faults and down-
warped basins from the Natron-Manyara trough
to the Kilimanjaro region. Here it encountered
the north-south aults delimiing the Pangani
Rift. The formaion of this east-west rough, the
Kilimanjaro Depression, was the trigger for the
onset of the Kilimanjaro volcanism, the oldest
lavas of which are believed to be 1.0 myr old
(Downie & Wilkinson, 1972).
Iniially volcanic acivity was widespread over
the whole area now occupied by the mountain but,
with ime, erupions were concentrated irst to the
Shira, then Mawenzi and finally to the Kibo