Photograph by Nathalie Bakoariniaina
Lavaka
The word means "hole" or "gully" in Malagasy, and it has become the accepted international term for the spectacular erosional features that characterise the highlands of Madagscar.
Lavakas are gullies formed by groundwater flow, with steep or vertical sides and flat floors. They have an inverted teardrop shape, with a wide headwall, and narrow, deeply incised outfall. Average size is ≈80 m long, 40 m wide, and 15 m deep (based on field measurements of 350 lavakas).
Lavakas form in areas where:
- Slopes are steep and convex;
- Regional elevation is ≈1000m;
- The bedrock geology is crystalline rock with a high proportion of feldspars and other weatherable minerals;
- Saprolite (material that is completely weathered but unmixed) forms a layer 10s of metres thick above the bedrock;
- The saprolite is overlain by 0.5-1 m of hard laterite (highly oxidised and weathered tropical soil);
- The climate is monsoonal, with a long dry season and a long torrential rainy season;
- The average temperature in the coolest month is no less than 10°C; and
- There is substantial low-grade earthquake activity
The details of how lavakas form are not well understood, but they appear to develop by groundwater sapping and subterranean erosion in porous and friable saprolite beneath a baked lateritic duracrust.
A simplistic way of putting this would be thus: steeply-piled, loose, weathered material, through which groundwater has been running, will fall down if you shake it when it's wet and soggy.
Lavakas are widespread in Madagascar, and densities reach 25/km2 in some areas (Wells and Andriamihaja, 1993*). Lavaka erosion damages infrastructure, removes hillslope pasturage, and causes debris flows that devastate agricultural land in the valleys.
For more information see Team Lavaka's recent conference abstracts:
Cox, Bierman, Jungers, Rakotondrazafy, and Finkel, 2006
Rasoazanamparany, Cox, Macklin, and Rakotondrazafy, 2006
Gress, Cox, and Rakotondrazafy, 2005
Cox, Rakotondrazafy, and Rakotondramazava, 2004
Cox, Rakotondrazafy and Bakoariniaina, 2003
*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.
Photograph by Rónadh Cox
