The forested Sumaco stratovolcano rises above the jungles of the western Amazon basin, east of Antisana volcano, and occupies an isolated position far to the east of the main Andes volcanic axis. Constructed over Cretaceous sedimentary rocks, it forms a relatively symmetrical cone in an area of heavy rainfall and erosion. Sumaco has produced alkaline tephritic, basanitic, and phonolitic rocks distinct from those of the main Andean chain. The volcano has a broad summit crater, 300 x 400 m wide, containing a central cone. Reports of historical eruptions are somewhat ambiguous; an eruption is inferred in the 18th or early-19th century on the basis of changes in crater morphology.