Enjeux géologiques propres aux territoires tropicaux

A majority of France’s Overseas Territories are governed by a tropical climate that is typically hot and humid. The vegetation, generally lush in such climatic conditions, and plays an active role in the weathering of the geological substrate, and notably in the development of a lateritic crust that may accumulate to thicknesses of several tens of meters. According to the nature of the weathered substrate, an old craton for French Guiana, an ophiolitic sheet for New Caledonia or an island arc for the Antilles, the diversity of the soils is accompanied by a diversity in fertility, but also in the hazards to which they are subject (landslides, coastal erosion, flooding). Laterite formations may also be a source for economically viable mineral resources, like the New Caledonian nickel deposits. This cohabits with another resource, water, which unavoidably leads to conflicts over use. Guiana’s example is quite different. Lying atop an old granite craton with relatively unfertile soils, its wealth reposes on gold mining. However, the lateritic cover does represent a major stake that must be managed by the public authorities. Despite heavy rainfall, the distribution of water resources is still an issue, and landslide risks present an obstacle for infrastructure projects. These are just so many examples of the geological diversity in the overseas territories, in which researchers in the geosciences have the opportunity to gain knowledge and expertise applicable to tropical or subequatorial nations that is international in scope.

Le climat tropical confère des caractéristiques physico-chimiques singulières aux formations géologiques de l’outre-mer français. En fonction de la nature du substrat, les sols latéritiques peuvent être à l’origine de gisements métallifères… ou d’instabilités de terrain. Nous prendrons les exemples de la Nouvelle-Calédonie et de la Guyane, deux territoires dont la géologie est très dissemblable, mais où le fort développement des latérites entraîne des problématiques communes propres aux territoires tropicaux.