Lavaka

Lavaka, the Malagasy word for "hole", usually found on the side of a hill, is a type of erosional feature common in Madagascar. However, lavakas have also been found in South Africa, the Democratic Republic of the Congo, and South Carolina, and similar landforms have been found in Brazil, the Great Plains of the U.S., and Swaziland. They are most common in tropical regions between the Cancer and Capricorn latitudes, especially the Central Highlands of Madagascar, where approximately one metre thick laterites develop on steep terrains in a monsoonal climate. Lavakas form where these hard laterites overlie thick (tens of metres) 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. They are usually shaped like a tear-drop with a steep, round headwall that narrows downhill into a shallow outlet channel.

Although human activities—such as deforestation, overgrazing, road creation, and grassland burning—can contribute to lavaka formation, lavakas can also develop by purely natural processes. Air photos reveal remnants of ancient lavakas in recently deforested areas, showing that those areas were eroded by lavakas before the rain forests grew; and C14 dating (radiocarbon 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).