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Lavaka ({{IPA-mg, ˈlavakə̥), the Malagasy word for "hole", usually found on the side of a hill, is a type of erosional feature common in
Madagascar Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Africa ...
. However, lavakas have also been found in
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. It is bounded to the south by of coastline that stretch along the Atlantic Ocean, South Atlantic and Indian Oceans; to the ...
, the
Democratic Republic of the Congo The Democratic Republic of the Congo (french: République démocratique du Congo (RDC), colloquially "La RDC" ), informally Congo-Kinshasa, DR Congo, the DRC, the DROC, or the Congo, and formerly and also colloquially Zaire, is a country in ...
, and
South Carolina )'' Animis opibusque parati'' ( for, , Latin, Prepared in mind and resources, links=no) , anthem = " Carolina";" South Carolina On My Mind" , Former = Province of South Carolina , seat = Columbia , LargestCity = Charleston , LargestMetro = ...
, and similar landforms have been found in
Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
, the Great Plains of the U.S., and Eswatini. 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
monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal osci ...
al 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 Groundwater sapping is a geomorphic erosion process that results in the headward migration of channels in response to near constant fluid discharge at a fixed point. The consistent flow of water displaces fine sediments which physically and chemical ...
. 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 Overgrazing occurs when plants are exposed to intensive grazing for extended periods of time, or without sufficient recovery periods. It can be caused by either livestock in poorly managed agricultural applications, game reserves, or nature res ...
, road creation, and grassland burning—can contribute to lavaka formation, lavakas can also develop by purely natural processes. Work by Neil Wells and colleagues showed that 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 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 less than 2000 years before present). Among the natural controls on lavaka formation are the amount of
seismic activity An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...
in the region, the
topographic relief Terrain or relief (also topographical relief) involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The Latin w ...
(or slope), and hydraulic conductivity of materials in the saprolite. For instance, earthquakes in the region can cause cracks in the hard, upper layer of laterite, which allows water to seep into the more porous layer of saprolite underneath. This causes chemical reactions within the saprolite to leach certain minerals from the rock and if the hydraulic conductivity of that rock is high enough, the water can carry those minerals away, which can cause the infrastructure of that rock to collapse and erode. Lavakas can often cause a lot of damage to nearby communities. During the monsoon season, heavy rains carry away all the eroded material from the lavaka, which can destroy surrounding crops and infrastructure. This is the basis for much of the recent research that has been conducted on the variables involved in lavaka formation (especially in Madagascar, where lavakas pepper the Central Highland landscape). The term "lavaka" entered the international geography / geology vocabulary following the work of Riquier (1954).


References

* Cox, R., Bierman, P., Jungers, M.C. and Rakotondrazafy, A.F.M., 2009, Erosion rates and sediment sources in Madagascar inferred from 10Be analysis of lavaka, slope, and river sediment. ''Journal of Geology'', v. 117, p. 363-376. * Cox, R., Zentner, D.B., Rakotondrazafy, A.F.M., and Rasoazanamparany, C.F., 2010, Shakedown in Madagascar: Occurrence of lavakas (erosional gullies) associated with seismic activity. ''Geology'', v. 38, p. 179-182. *Rakotondrazafy, A.F.M. et al., 2012, Relation Between Bedrock Geology, Topography and Lavaka Distribution in Madagascar. ''
South African Journal of Geology The ''South African Journal of Geology'' is a quarterly peer-reviewed scientific journal published by the Geological Society of South Africa that was established in March 1896 as the ''Transactions of the Geological Society of South Africa'', obta ...
'' 115.2: 225-250. GeoRef. Web. 9 Oct. 2013.
Raveloson, A., Visnovitz, F., Székely, B., Molnár, G., Udvardi, B., 2012, A multidisciplinary study on lavaka (gully erosion) formation in Central Highlands, Madagascar. European Geoscience Union General Assembly April 22-27, ''Geophysical Research Abstracts'', v. 14, EGU2012-12483-1
*Riquier, Jean, Etude sur les « lavaka », Mémoires de l'Institut Scientifique de Madagascar, Tananarive, Série D : Sciences de la Terre, 1954, p. 169-189.
Udvardi, B., Raveloson, A., Visnovitz, F., Szabó, Cs., Kovács, I., Székely, B., 2012, Sedimentological features of lateritic and saprolitic horizons in a mid-slope lavaka, Central Highlands, Madagascar. European Geoscience Union General Assembly April 22-27, ''Geophysical Research Abstracts'', v. 14, EGU2012-4365
*Voarintsoa, N.R., Cox, R., Madison, R.M.O, Rakotondrazafy, A.F.M., 2012, Lithologic controls on lavaka occurrence in Madagascar. ''South African Journal of Geology'', v. 115, p. 225-250. *Wells, N.A., Andriamihaja, B., Rakotovololona, H.F.S., 1991, Patterns of development of lavaka, Madagascar's unusual gullies: ''
Earth Surface Processes and Landforms ''Earth Surface Processes and Landforms'' is a peer-reviewed scientific journal published by John Wiley & Sons on behalf of the British Society for Geomorphology. It covers geomorphology and more in general all aspects of Earth sciences dealing w ...
'', v. 16, p. 189-206. *Wells, N.A., Andriamihaja, B., 1993, The initiation and growth of gullies in Madagascar: are humans to blame?: '' Geomorphology'', v. 8, p. 1-46.


External links


More about lavakas
*Lavaka image database by
Rónadh Cox Rónadh Cox (born 1962) is an Irish geologist who is the Edward Brust Professor of Geology and Mineralogy at Williams College, Massachusetts. Her research considers the impact of storms on coastal boulders. She is a Fellow of the Geological Soc ...
a
Williams College Digital Collections: Team Lavaka image collection
Landforms of Madagascar Erosion landforms Geology of Madagascar