The geological evolution of East Africa - Biogeography and Ecology of the Rain Forests of Eastern Africa - page 9

2 The geological evolution of
East Africa

C. JANE GRIFFITHS

Abstract
This chapter examines the geological and
geomorphological processes that have shaped the
present-day landscape of the eastern part of East
Africa. The underlying rock types, their age,
environment of formation and interrelationships
are described. The structural evolution of the
landscape is traced from the Karroo

/Faults
]Volcanics
lm Seds 'l ·

Mesozoic- �

· · �

e r a ry

-
Quaternary)
0 Karroo
� Mozambique
� Precambrian

T ti

300 million
years before present (myr BP), encompassing the
Karroo Rifting, the breakup of Gondwanaland,
the relaive ages of the block mountains, volcanic
mountains, plains and plateau, the East African
Rift Valley system, and the verical movements
affecting the continental margin of East Africa.
Reference is made to the general geology and
topography ofWest Africa for comparison and the
debate about the pre-drift position ofMadagascar
is briefly reviewed. Variaions in soil ypes in rela-
tion to parent rock, age, climate, organic material
and relief are considered.

c.

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Introducion
A wide variety of rock types are found in East
Africa represening all three of the major groups:
igneous, metamorphic, and sedimentary. They
range in age from over 2000 myr BP to uncon-
solidated sediments accumulaing at present, but
represent two geological time spans: the Precam-
brian (>2000-570 myr BP) and the Karroo-
Recent (290-0.01 myr BP) (Figure 2.1 and Table
2.1), separated by a major break in the geological
record. These two groups of rocks were formed
under enirely different geological condiions,
resuling in disinct properties.

400

Figure 2.1. Geological map of he easten part of East Africa.

The Precambrian rocks are highly deformed,
high grade metasedimentary, with minor meta-
igneous, crystalline rocks, including granulites,

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