The conical Iliinsky volcano (also spelled Ilyinsky), rising dramatically to 1555 m above the NE shore of Kurile Lake, was constructed beginning about 7600 radiocarbon years ago at the NE margin of Kurile Lake caldera. The modern edifice grew within a 4-km-wide caldera produced by collapse of an earlier volcano creating large debris avalanches at about the time of formation of the adjacent Kurile Lake caldera. A period of strong silicic explosive volcanism during the mid-Holocene lasted about 800 years. A series of youthful lava flows cover much of the northern flanks. Growth of the modern cone was completed about 1900 years ago, after which a long quiescent period began. The only recorded historical eruption, in 1901, produced a large 1-km-wide crater on the NE flank.