Lavaka
Lavaka, the Malagasy word for "hole", is a type of erosional feature common in Madagascar. They are most abundant in the central highlands of Madagascar, where there are deep laterites developed on in steep terrain in a monsoonal climate. Lavakas form where hard laterites overlie thick (tens of meters) saprolite, on steep (35 to 55 degree) slopes, in areas that have a hot dry season and a warm wet season.
Lavakas are not landslides. They are a type of gully, formed via groundwater sapping. Associated erosion is usually rapid, producing a sediment yield on the order of 8000 cubic metres over several months.
Although human activities—such as deforestation, overgrazing, and road creation—can contribute to lavaka formation, lavakas are caused by natural processes. Air photos reveal remnants of ancient lavakas in recently-deforested areas, showing that those areas were eroded by lavakas before the rainforests grew; and C14 dating indicates that some lavakas are up to 20,000 years old, meaning they were present in the Malagasy landscape before the arrival of humans (human arrival in Madagascar is <2000 years before present).
Lavaka image database at Williams College Digital Collections: Team Lavaka image collection
REFERENCES
Wells, N.A., Andriamihaja, B., and Rakotovololona, H.F.S., 1991, Patterns of development of lavaka, Madagascar's unusual gullies: Earth Surface Processes and Landforms, v. 16, p. 189-206.
Wells, N. A., and Andriamihaja, B., 1993, The initiation and growth of gullies in Madagascar: are humans to blame?: Geomorphology, v. 8, p. 1-46.
