The world's highest historically active volcano, Llullaillaco sits astride the Chile-Argentina border. The summit is formed by a smaller well-preserved cone that was constructed on an older Pleistocene edifice. A major debris-avalanche deposit produced by collapse of the older volcano about 150,000 years ago extends eastward into Argentina and diverges around the N and S sides of the older Cerro Rosado stratovolcano 17 km to the E. Construction of several lava domes and flows was associated with growth of the modern cone. The two most prominent flows contain distinct flow levees and ridges and extend down the N and S flanks. These two extremely youthful-looking dacitic flows were initially considered to be of Holocene age, but more recent Ar/Ar dating indicates that they are of late Pleistocene age (Richards and Villeneuve, 2001). Two explosive eruptions and another that may have included lava effusion were reported in the 19th century.