Continental rifting is a tectonic process that creates oceans and influences the arrangement of continents on Earth’s surface. By analyzing xenoliths (samples of the lithosphere from ~100 km depth) in lavas from southeastern Uganda, we suggest localized melting of metasomatic lithosphere is sufficient to encourage continental rift formation and propagation. The xenoliths contain evidence for C-rich fluid metasomatism and extensive infiltration by silicate liquids (magmas), suggesting a key link between fluid movement and crustal weakening.