The Cerro Tuzgle stratovolcano is located in Argentina about 120 km E of the main volcanic arc of the central Andes. Many youthful-looking flank lava flows were erupted from the well-preserved summit crater. Schwab and Lippolt (1976) obtained a Potassium-Argon date of 0.1 million years ago on what they believed to be the youngest Tuzgle lava. However, de Silva and Francis (1991) and González-Ferrán (1995) considered the latest activity to be of Holocene age, and Coira and Kay (1993, Fig. 2B) placed the youngest flow at the Pleistocene-Holocene boundary. Activity began with the eruption of a rhyodacitic ignimbrite, followed by construction of a lava dome complex on the rim of an existing caldera. Andesitic lava flows covered much of the dome complex and later partially filled the crater. Several edifice-collapse events have occurred. The youngest flows were erupted on the SE and SW flanks.