Being about high over its base, Isluga has five craters, a main crater wide at the end of snowcovered summit ridge and underwent caldera collapse. The volcano itself is constructed from lava domes and flows on top of Miocene ignimbrites. Several stages of eruptive activity are recognized, some of them exposed through erosion. A northwestern debris avalanche has been found on Isluga, on top of which the recent volcano is constructed. The fumarolically active crater region has been buried with surge deposits from phreatomagmatic activity, and the northern ridge is covered by Holocene lavas. A glaciated dacitic lava flow is dated 0.096±0.006 Ma by K-Ar analysis. North of Isluga lies the dissected Quimsachatas volcano, which has been dated at 0.566±0.017 Ma. Both the summit crater and the area below the crater on the southern flank are faintly fumarolically active, with yellow sulfur deposits observed. The fumaroles appear to produce water vapour.