It is not clear how long after the 4.2 ka eruption this activity occurred, but it has been grouped as the CB3 unit (the domes are classified as CB31). This activity also generated block-and-ash deposits (unit CB32) on the caldera floor. The domes are of rhyolitic composition, the block-and-ash deposits consist of ash and lapilli and appear to have formed when domes collapsed. As lava domes grow, they tend to become unstable as their vertical extent increases until they collapse. Additionally, internally generated explosions appear to have occurred at Cerro Blanco as lava domes grew and sometimes completely destroyed the domes.