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Eastern Arc Mountains Forests of Tanzania
2. Description
2.a Description of Property
EASTERN ARC MOUNTAINS
The name
The term ‘Eastern Arc’ was introduced in 1985 to describe the arc of forest-capped ancient crystalline mountains
of eastern Tanzania and south-east Kenya, which are under the influence of the Indian Ocean climatic regime
and, therefore, contain predictable local climates (Lovett 1985).
Geographical distribution
Thirteen separate mountain blocks comprise the Eastern Arc, covering a total area of approximately 23,000 km 2 of
which some 3,500 km 2 is forested. The mountains are separated from each other but, together, they form a broad
arc shape of some 600 km in length. Twelve of the 13 mountain blocks are found in eastern Tanzania within 15
Districts and 5 Regions. The other mountain block (Taita Hills) is found in Kenya (Map 1, Executive Summary).
Geology and Geomorphology
The Eastern Arc Mountains were formed by uplifting associated with the Rifting of the African plate which caused
the development of the ‘Great Rift Valley’. The mountains are at least 30 million years old and are made up of
ancient rocks dating from the Precambrian epoch.
The Eastern Arc Mountains rise to a maximum altitude of 2,635 m at Kimhandu peak in the Ulugurus, although
more typically the maximum altitudes of different blocks are between 2,200 m and 2,500 m.
The mountain blocks typically rise dramatically from the surrounding plains, often with very steep drops down to
the plains, especially on the eastern and southern margins. Some of the larger blocks have flat tops, which are
slightly undulating. There are many rocky outcrops throughout the mountains and fast flowing rivers drain to the
lowlands.
The rocks of the Eastern Arc are highly deformed after hundreds of millions of years of uplift, erosion and further
uplift. They comprise a mixture of migmatites and granites, with numerous quartz veins. The rocks are all base
poor and weather to form a nutrient poor and generally sandy soil that is suitable for agriculture, but is not very
productive. Only in areas that have been more recently covered by forest, and hence have higher humous
content, are the soils good for agricultural productivity. These areas are typically located at higher altitudes where
the rainfall is also more conducive for agriculture.
Climate regime
The Eastern Arc Mountains are under the direct climatic influence of the Indian Ocean, and the climatic regime is
believed to have been stable over millions of years, as indicated by biogeographical affinities to the forests of
West Africa, Madagascar, and Asia. The Arc forests survived the driest and coldest periods of the last ice ages,
as the Indian Ocean did not cool appreciably and rainfall patterns may not have been greatly disrupted. Today,
the climate in the mountains remains wetter, and less seasonal than the surrounding lowlands.
The Uluguru Mountains receive up to 3,000 mm rain a year on the eastern slopes, and annual rainfall on most
other mountains exceeds 1,500 mm in wetter parts. There is, however, some evidence that the climate has
become drier and more seasonal in recent decades, with a lower likelihood of the forests being enveloped in mist.
Ministry of Natural Resources and Tourism 19