Pierre Wandji

The Tombel graben (West Cameroon): a recent monogenetic volcanic field of the Cameroon Line

The Tombel graben is a recent monogenetic volcanic field located in the continental part of the Cameroon Line. Magmatic activity began with fissure eruptions emitting large lava flows, and continued with explosive episodes generating cinder cones. Tombel lava flows are made up of three major rock types. Ankaramite yields phenocrysts of Fe–Ti oxides, forsterite olivine and diopside-augite clinopyroxene. In addition to this assemblage, basalt and hawaiite (olivine basalt, doleritic basalt and trachybasalt according to their modal compositions) contain phenocrysts of labradorite-andesine plagioclase and scarce kaersutite. The suite of lava flows, ranging from ankaramite to hawaiite, define an incomplete sodic alkaline series. Moreover, basanite, rich in incompatible elements, is locally found as late ejecta. Geochemical data share some oceanic island basalt (OIB) characteristics. It is suggested that the ankaramite–hawaiite series could be due to differing degrees of partial melting of a source at the asthenosphere–lithosphere boundary, followed by low rates of fractional crystallization.

Geohazards (floods and landslides) in the Ndop plain, Cameroon volcanic line

Abstract The Ndop Plain, located along the Cameroon Volcanic Line (CVL), is a volcano-tectonic plain, formed by a series of tectonic movements, volcanic eruptions and sedimentation phases. Floods (annually) and landslides (occasionally) occur with devastating environmental effects. However, this plain attracts a lot of inhabitants owing to its fertile alluvial soils. With demographic explosion in the plain, the inhabitants (143,000 people) tend to farm and inhabit new zones which are prone to these geohazards. In this paper, we use field observations, laboratory analyses, satellite imagery and complementary methods using appropriate software to establish hazard (flood and landslide) maps of the Ndop Plain. Natural factors as well as anthropogenic factors are considered. The hazard maps revealed that 25% of the area is exposed to flood hazard (13% exposed to high flood hazard, 12% to moderate) and 5% of the area is exposed to landslide hazard (2% exposed to high landslide hazard, 3% to moderate). Some mitigation measures for floods (building of artificial levees, raising foundations of buildings and the meticulous regulation of the flood guards at Bamendjing Dam) and landslides (slope terracing, planting of trees, and building retaining walls) are proposed.