Galán has erupted mainly potassium-rich dacitic to rhyolitic rocks that are often called rhyodacitic, and which reflect a calc-alkaline suite. Each ignimbrite has usually a uniform composition but there is some variation between individual ignimbrites; for example older rocks contain amphibole and younger rocks instead sanidine. Minerals contained in the eruption products include allanite, apatite, biotite, hornblende, ilmenite, magnetite, orthopyroxene, plagioclase, quartz, sanidine and zircon. Hydrothermal alteration has left calcite in some rocks. Trace element patterns are distinct in the Galán ignimbrite in comparison to the Toconquis Group rocks.