Lascar is formed by two irregularly shaped truncated cones that extend east-west, on a trend that includes Aguas Calientes. Six craters are located on the volcano, but sometimes only five craters are counted, in which case the central crater is considered to be the active one. The extinct western cone (also known as Apagado) is composed of layers of lava and pyroclastics. Its large crater is filled by another cone, which forms the highest summit of the Lascar volcano. Immediately east of it lies the eastern cone, which is contiguous with the western cone. The eastern cone (also known as Activo) is capped off with three distinct craters which are delimited by arcuate fractures. Measurements made from 1961 to 1997 determined that the eastern crater is wide and deep and thus the largest, the central crater is wide and deep, and the western crater is wide and deep, increasing to depth in 2005-2006. The craters show evidence that activity has migrated westward. The westernmost of these three eastern craters is the currently active one, surrounded by rims that reach heights of . In 1985, a hot spot in this crater was observed in satellite images. In the centre of the westernmost crater lies a smaller crater, deep and wide. There are many fumaroles along the rim of the inner crater.