Miombo Ecoregion Vision Report - Biodiversity Foundation for Africa

2. PHYSICAL FEATURES AND PROCESSES
2.1 Extent and Physical Determinants
Miombo Ecoregion Report, page 9
The Miombo Ecoregion covers over 3.8 million km 2 in central and southern Africa, extending
from the west coast in Angola to the east coast in Mozambique and Tanzania. It includes all or
part of 11 countries – Angola, Namibia, Botswana, South Africa, Zimbabwe, Zambia,
Democratic Republic of Congo (DRC), Mozambique, Malawi, Tanzania and Burundi. Much of
the ecoregion is on the ancient African plateau with an altitude of 800 to 1250 m above sea level,
but in the east the ecoregion transcends the escarpment and elements of the ecoregion can be
found in the east African coastal zone, at 200 to 300 m altitude (Figure 2). In spite of the
favourable elevation, the biological elements of the ecoregion give way to other biomes in the
northeast, south and southwest. Elevation therefore does not fully determine the Miombo
Ecoregion boundary.
Overall, the Miombo Ecoregion boundary appears to be determined by the interaction of
topography, precipitation and temperature. Climate is probably the most important determinant.
The ecoregion occurs in the unimodal rainfall zone, except in the northeast in central Tanzania
where rainfall tends to be bimodal. Mean annual rainfall is in the range of 600 to 1400 mm
(Figure 3) and occurs from November to April. To the northwest, the boundary roughly follows
the 1400 mm isohyet while to the northeast and south, the boundary follows the 600 mm isohyet.
It is obvious that the biological elements of the ecoregion are ill-adapted to humid and arid
conditions. In central and southern Africa both humid and arid conditions are associated with
mean maximum temperatures higher than 30 o C. Much of the region therefore lies in the warm
subhumid zone with a mean maximum temperature of 24–27 o C.
2.2 Geology
The geological foundation of much of south and central Africa consists of large, stable, Archaean
crustal blocks called cratons, of relatively low metamorphic grade, separated by broad zones of
more highly metamorphosed rocks known as mobile belts. The cratonic nuclei include the
Congo, Tanzania, Kaapvaal and Zimbabwe cratons; dating indicates a long and complex
geological history and gives ages ranging from around 3500 to 2600 million years (Ma). Cratonic
stabilisation did not occur everywhere at the same time. In the Zimbabwe craton, which is the
best exposed, some early stabilisation had been effected by 3300 Ma with final stabilisation at
about 2600 Ma, accompanying the emplacement of vast volumes of potassium-rich granites.
The cratons now consist of the deformed remains of volcano-sedimentary piles (greenstone belts)
intruded by various, and numerous, granites. Metamorphic grade in the greenstone belts is such
that primary textures and structures are often well preserved and the primary nature of the rocks
is clear. For example, basaltic lavas dominate the volcanic pile and numerous examples of
pillows indicate that eruption occurred under water. Also pockets of limestone, although now
recrystallised, show fossil stromatolites in places indicating that, here at least, water depths were
shallow. Typically, the basaltic greenstones weather to give fertile, reddish soils. Prominent
among the associated chemical sediments are banded iron formations and ferruginous cherts,
which often outcrop as prominent well-wooded ridges.