GEOLOGICAL VERSUS HUMAN CONTROLS ON LAVAKA FORMATION AND EXTREME EROSION IN MADAGASCAR

Extreme erosion characterizes Madagascar's central highlands. The country has a "world-record erosion rate" according to the World Bank, and the widespread hillslope gullies called lavaka have been described as “cat claw-marks” on the landscape and are featured in tourist guidebooks as examples of catastrophic denudation of the country’s landscape. Lavakas are caused mainly by groundwater sapping. They initiate suddenly and grow rapidly, shedding sediment volumes on the order of 8000 m3 in a few months. They cause substantial infrastructural problems because they undermine roads and bridges, and because the debris flows that issue from them swamp agricultural land. They are widely cited as being largely or entirely anthropogenic in origin, and the major source of high sediment loads in Madagascar's rivers. The connection between human influence and lavaka formation is not simple, however. Lavakas form exclusively on lateritised Precambrian gneisses, and initial results of field and GIS analysis of lavakas in the Miarinarivo area of central Madagascar suggest that their formation is partially controlled by the relationship between the structural grain of the saprolitised bedrock and the orientation and geometry of the hillslope. Lavakas do not appear to form in dipslopes, and form most readily when the gneissic foliation dips at a moderate angle into a convex-up hillside. There is clear anthropogenic influence on lavaka formation: they often initiate downslope of cart and cattle tracks, and upslope of roadcuts. But in spite of published assertions that they are caused entirely by humans, there is strong evidence that they were a feature of the Malagasy landscape before the arrival of people. The deeply lateritised highlands of Madagascar are very steep, with convex hillslopes commonly greater than 50°. The laterites and saprolites are unconsolidated and mechanically incompetent. We interpret lavakas as a primary response of the unstable landscape to recent tectonic over-steepening, with initiation accelerated by anthropogenic factors.