Viedma like other volcanoes of the Austral Volcanic Zone has erupted andesite and dacite. Phenocrysts include amphibole, biotite, hypersthene and plagioclase; orthoclase, plagioclase and pyroxene also occur as xenoliths. The rocks form a calcalkaline suite, but there is also an adakitic signature. The xenoliths may reflect a crustal contamination of the magma erupted at Viedma.