2004 Denver Annual Meeting (November 7–10, 2004)
Paper No. 66-7
Presentation Time: 9:30 AM-9:45 AM


COX, Rónadh1, RAKOTONDRAZAFY, A.F. Michel2, and RAKOTONDRAMAZAVA, Hery Tiana2, (1) Geosciences, Williams College, Williamstown, MA 01267, rcox@williams.edu, (2) Département des Sciences, Université d'Antananarivo, Antananarivo, 101

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.

2004 Denver Annual Meeting (November 7–10, 2004)
General Information for this Meeting
Session No. 66
Geomorphology II
Colorado Convention Center: 107/109
8:00 AM-12:00 PM, Monday, 8 November 2004

Geological Society of America Abstracts with Programs, Vol. 36, No. 5, p. 171

© Copyright 2004 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.