Citations : lavaka

Gautier Émile Félix : Madagascar – Essai de Géographie physique
page 60
Sur le versant des mamelons il se creuse des chancres énormes, profonds de plusieurs dizaines de mètres, aux bords perpendiculaires et à vif. Ces "lavaka" sont des abîmes qui s'ouvrent brusquement sous les pieds et qui ont presque la régularité de trous faits à l'emporte-pièce. Ils ont une forme semi-circulaire ou presque complètement circulaire. En beaucoup de provinces, ils servent de parcs à boeufs, c'est-à-dire que tous les soirs on y enferme le bétail.
Au fond du trou se trouve toujours une source, plus ou moins pérenne, qui évidemment est la cause première de l'éboulement; ce sont les eaux de circulation souterraine qui ont attaqué la couche de latérite à la base.

American Geophysical Union
Madagascar's central highlands are deeply weathered, with 1-2 m of laterite overlying 10s of metres of saprolite. These unstable materials sit at altitudes on the order of 1000m in recently uplifted, steep terrain characterised by convex hills with slopes averaging 25 degrees and local ridge-valley elevation changes of 100- 500 m. In many areas these slopes bear numerous erosional features called lavakas, with mapped densities up to 25/k$^{2}$ in some areas. Lavakas are tadpole-shaped gullies with vertical sides and flat floors. They are wide at the headwall and taper to a narrow, deeply incised outfall, which connects to the valley drainage. They are not fed by overland flow, but develop by groundwater sapping and subterranean erosion in the porous and friable saprolite beneath the baked lateritic duracrust. The controls on lavaka formation are poorly understood. Interplay between climate, topography, and bedrock geology is known to be important, and human activity is also implicated. However, there are many areas with appropriate rainfall, hillslope geometry and geology where lavakas do not form. Likewise, lavakas form readily in some areas with little human activity, and do not form in other areas that are heavily used. Our analysis suggests that seismic activity may be a significant regional factor driving lavaka formation. Using GIS analysis, we compared the locations of recent earthquakes (1347 events with magnitudes 3.0-5.4, recorded between 1979 and 1995) with the distribution of zones where lavakas are abundant (as mapped by Henri Besairie in 1957), and found that they are strongly correlated. We cannot say whether individual seismic events are responsible for specific lavakas; but the observation that lavakas are most common in zones that are seismically active implies that earthquakes play a significant role. Repeated mild earth-shaking events might aid in loosening the saprolite and making it more vulnerable to lavaka-forming processes; or individual earthquakes might initiate collapse of slopes already weakened by groundwater sapping. More detailed work into the relationships between individual lavakas and local seismicity is needed to determine which of these mechanisms might be active. Our ongoing GIS analysis will quantify the relative importance of geology, topography, and seismicity in the location of lavaka-prone areas [F. Rasoanazamparana]

Mountain Forum
The last stop permitted observation of the "Lavaka" phenomenon from a scientific view point. It is a spectacular type of erosion that effects the hill slopes of the Malagasy highlands. Its morphology results from three causes: ablation, transport and deposit. The formation process, the different evolution stages, the correlation between the top (generally of a quasi-circular form) and the bottom (flared or looking like a long and narrow draining gully) were evoked. Lavaka formation and evolution are closely related to the contrasting and agressive tropical climate characteristics which act upon regions with steep slopes and with fragile instable vegetation due to human activites and fire.