Sumaco is an active strato-volcano forming a lofty, symmetrical cone rising to 3828 m from a low elevation on the Amazonian plain. It has a crater within which lies a central cone with a smaller crater of 300x400 m and a depth of 100 m (in 1925); there is a small crater lake. The last eruption was probably in February 1933 when there was explosive activity within the central crater. The feldspathoidal lavas are unique amongst Andean volcanoes. Olivine basalts do occur but most of the rocks studied by Colony and Sinclair (1928) proved to be tephrites with phenocrysts of andesine/labradorite, pale green pyroxene, probably aegirine-augite, occasional basaltic hornblende, very rare olivine, magnetite and apatite with a groundmass containing plagioclase of a less calcic composition, alkali feldspar, pyroxene, nepheline and hauyne. In one sample very abundant hauyne formed the only phenocryst phase.