The southern basin is deeper than the northern basin ( against the of the northern basin) and is a Holocene caldera. The nature of the northern basin is less clear; Bondarenko in 1991 supposed that it was an older separate caldera which he named Ilinsky, but Braitseva et al. 1997 and Ponomareva et al. 2004 consider both to be the Kurile Lake caldera. This caldera has an area of about , or . In that case, the ridge that separates the two basins may be a deposit left when earthquakes preceding the caldera forming eruption caused Ilinsky volcano to collapse. Caldera collapse was controlled by faults that parallel the lake shores. Some islands in the lake formed by slumping and others are volcanic cones; the Heart of Alaid (Serdtze Alaida) is a high lava dome. A number of lava domes and pyroclastic cones are found in the lake. The Severnaya bay may be an explosion crater. Lava flows from Ilinsky extend into the lake.