Abstract
The Comores Islands together with the Tertiary volcanic province of N Madagascar form a sublinear trend of alkali-olivine basalt shield volcanoes across the entrance of the Mozambique Channel. K/Ar dating of shield-building lavas confirm an eastward increase in age of volcanism along the chain, consistent with a hotspot origin for the lineament. The velocity of the Somali plate over the magma source is 50 mm/yr. The absolute motion of the Somali plate was tracked for the last 10 m.y., and the relative motion across the East African Rift is calculated by subtracting the Somali plate absolute motion from African plate absolute motion during this period. The model predicts 320 km of total separation across the rift in the past 10 m.y., which is greater than that estimated from geological evidence. The geometry of older portions of the Comores and Reunion trends indicates that there was no significant relative motion between the African and Somali plates prior to approx 10 m.y. ago.