The initiation and growth of gullies in Madagascar: are humans to blame?

doi:10.1016/0169-555X(93)90002-J

Geomorphology

Volume 8, Issue 1, September 1993, Pages 1-46

https://doi.org/10.1016/0169-555X(93)90002-J Get rights and content

Abstract

Lavaka (Madagascar's unusual gullies) require thick lateritic regolith on smoothly convex hills, and they are initiated locally by a variety of cultural and natural proximate causes, such as paths, roads, defensive trenches around hillforts, steep fields, unchanneled to slightly concentrated storm run-off, springs, mass movements, etc. Growth continues by rain attack, incision by run-off, earthfalls caused by wetting and drying cycles, and especially basal sapping by diffuse groundwater flow above bedrock, somewhat in that order.

Lavaka are less directly permitted and promoted by (1) hardening of exposed laterite surfaces, which creates a hard-soft-hard profile from bedrock to laterite, which favors incision relative to lateral erosion, (2) the superimposition of concave run-off profiles onto convex hills, and, locally, (3) re-equilibration of watersheds after stream piracy and faulting. Run-off and hardening are interrelated with devegetation, because hardening results from exposure following devegetation, because both hardening and devegetation increase run-off relative to infiltration, and because hardening, rain attack, and run-off impede vegetation. Devegetation is certainly worsened by harmful human activities (such as range burning, forest cutting, and/or overgrazing), but tectonism and natural climatic aridification are probably ultimately more important, given that many lavaka are simply a natural part of the landscape's evolution and that some lavaka clearly predate primary (uncut) rain forest.

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These common erosional features populate the rolling topography of the central highlands. In their mature phase, lavaka are inverse teardrop-shaped gullies with steep-sided walls exceeding 15 m, broadening uphill with narrow outlets (Cox et al., 2010; Rabarimanana et al., 2003; Wells and Andriamihaja, 1993) (Fig. 1A). The abundance of open grassland on the island and the presence of these impressive erosion features has led to the narrative whereby a once fully forested island would have been gradually deforested upon the arrival of humans by 2000 cal.
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The initiation and growth of gullies in Madagascar: are humans to blame?

Abstract

Quat. Res.

Quat. Res.

Monographe hydrologique de l'Ikopa et de la Betsiboka

Stades d'évolution des raviments sur les plateaux sableux du Haut-Shara

Arroyo-cutting and filling

J. Geol.

Les phénomènes anciens de rajeunissement des sols ferallitiques à la latitude de Tananarive

C. R. Sémaine Géol. Malgache (Ann. Géol. Madagascar)

Piping as an erosion process in the uplands of Sarawak

J. Trop. Geogr.

Spring sapping and valley network development, with case studies by Kochel, R.C., Baker, R.V., Laity, J.E., and Howard, A.D.

Man and the environment in Madagascar

Some erosional features due to piping and subsurface wash with special reference to the Sudan

Geogr. Ann.

Contribution à l'ðude de l'érosion des sols à Madagascar: Introduction

Doc. Bur. Géol.

Deux exemples d'érosion accélérée à Madagascar

Doc. Bur. Géol.

Précis de géologie malgache

Ann. Géol. Madagascar

The development of discontinuous gullies in a pumice catchment

Am. J. Sci.

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Earth Surf. Process. Landforms

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Rapport d'Etude Bureau Pedologique, # 121

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Mém. ORSTOM

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Bull. Soc. Géol. France

Sublaterite cave systems and other pseudokarst phenomena in the humid tropics: the example of the Kasewe Hills, Sierra Leone

Z. Geomorphol.

Gémorphologie de l'Antsihanaka et de l'Antanosimboangy: les formes normales du relief—les phénomènes d'érosion accélérée

Doc. Bur. Géol.

Essai sur la morphologie de quelques reliefs latéritiques à Madagascar

Bull. Acad. Malgache

Erosion and deposition in the loessmantled Great Plains, Medicine Creek drainage basin, Nebraska

U.S. Geol. Surv. Prof. Pap.

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Geogr. J.

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Pre-settlement vegetation changes at Lake Tritrivakely, Madagascar

Paleoecol. Afr.

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Cause, effect, and megafaunal extinction in Holocene Madagascar — pollen anda charcoal influx at Lake Kavitaha

Am. J. Phys. Antropol.

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Soil slips, debris flows, and rainstorms in the Santa Monica Mountains and vicinity, southern California

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The Piedmont landscape of Maryland: a new look at an old problem

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Recent extinctions in Madagascar: the loss of the subfossil fauna

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The climatology of Madagascar

Les cyclones tropicaux des mers malgaches

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