evidence: Location - The Comoros archipelago consists of four volcanic islands – Ngazidja (Grande Comore), Moheli, Anjouan and Mayotte. These are extended to the east by several emergent (Glorieuses, Geyser) and submerged (Zélée, Leven, Cordelière) reef-banks, which are described in the next section on the "Iles Éparses" of the Mozambique channel. Geologically, the islands and banks have complex origins. Hypotheses include classic plate movement over the Comoros hotspot over the last 15 my, versus alkaline basaltic magma extrusion along north-west fractures remaining from southward movement of Madagascar relative to the African mainland, periodically active during the Tertiary. The age of the Comorian islands increases from west to east. The formation of Mayotte started with submarine volcanism from 15 to 10 mya, then sub-aerial island formation around 8 mya. Ngazidja is the youngest, formed c. 130,000 ya by two active volcanoes, Karthala which is still active, and La Grille, now dormant and to the north. West of Ngazidja, and rising to 10 m below the surface is the Banc Vailheu, likely the youngest seamount being produced by the Comoros hotspot.