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 c. 300 million years before present (myr bp), encompassing the Karroo Rifting, the breakup of Gondwanaland, the relative ages of the block mountains, volcanic mountains, plains and plateau, the East African Rift Valley system, and the vertical movements affecting the continental margin of East Africa. Reference is made to the general geology and topography of West Africa for comparison and the debate about the pre-drift position of Madagascar is briefly reviewed. Variations in soil types in relation to parent rock, age, climate, organic material and relief are considered.

Introduction

A wide variety of rock types are found in East Africa representing all three of the major groups: igneous, metamorphic, and sedimentary. They range in age from over 2000 myr BP to unconsolidated sediments accumulating at present, but represent two geological time spans: the Precambrian (> 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 entirely different geological conditions, resulting in distinct properties.