Introduction

In his classic paper on the vegetation of what is now Tanzania, the geographer Clement Gillman (1949) recognised the importance of the disjunct arc of mountains in the east of the country as condensers of moisture brought inland from the Indian Ocean. At this they are remarkably efficient: on the eastern side of the mountains rainfall can be well over 2000 mm per year; yet in the rain shadow only a few tens of kilometres to the west, it can be below 500 mm per year. In his list of Indian Ocean condensers, Gillman included the northern volcanic mountains of Meru and Kilimanjaro, and then ran the arc southwards along the disjunct crystalline block-faulted mountains to the Kipengere range above Lake Nyasa. Here he pointed out that high rainfall in the great amphitheatre of volcanic and crystalline mountains surrounding the northern end of Lake Nyasa was largely attributable to convection from the lake surface.

The volcanoes of Meru and Kilimanjaro are geologically recent, having been formed within the last million years. However, the crystalline block-faulted mountains of the Eastern Arc are very old, with initiation of faulting dating from 290–180 Myr BP (million years before present) and reactivation of the faults creating the modern mountains during the last 7 Myr BP (Griffiths, Chapter 2). Indications are that the Indian Ocean climate was comparatively stable during Pleistocene climatic fluctuations (Lovett, Chapter 3).