This is the first multidisciplinary study on the critical zone of the French volcanic island of Mayotte to address chemical weathering rates. Here we present the first estimation of chemical weathering rates using element fluxes transported in surface and subsurface waters, ranging from 49 to 306 t/km2/yr for rivers and 113 to 1,382 t/km2/yr for groundwater. These results are consistent with the weathering pattern of other tropical volcanic islands and support the hypothesis that subsurface waters transport more solutes to the ocean than surface waters. In addition, a helicopter-borne resistivity survey was correlated to borehole geological data to understand the hydrogeological functioning of the “deep critical zone”.