A large eruption occurred approximately 4200 years ago. Block-and-ash flow deposits (classified as CB1) found around the caldera have been interpreted as indicating that a lava dome was erupted prior to the caldera collapse at Cerro Blanco, although it is not clear by how much this eruption predates the main eruption. Deposits from this lava dome-forming episode consist of blocks which sometimes exceed sizes of embedded within ash and lapilli.