51°26' N, 157°07' E, summit elevation 1578 m; Last Eruption 1901
Iliinsky volcano is located at the NE shore of the Kurile Lake, in the crater (caldera) of the Late Pleistocene pre-Iliinsky volcano. The collapse of the pre-Iliinsky edifice presumably was related to the catastrophic Kurile Lake caldera eruption ab. 7600 14C yr BP (Ponomareva et al., submitted; Zaretskaia et al., 2001).
Pyroclastic deposits from Illinsky directly overlie the Kurile Lake caldera ignimbrite, so the volcano probably started to form soon after the caldera collapse. The initial eruptions of Illinsky between 7600 and~6500 14C yrs BP produced lava and moderate tephra ranging from basaltic andesite to dacite. Following a 1700 year repose period, Iliinsky started violent explosive activity ~4800 14C yrs BP, which became especially intense ~ 4000 14C yrs BP. The volcano reached its modern shape and altitide about 1900 14C yrs BP, and then entered almost 2000-yrs-long repose period. The latest eruptions, preceding the repose, produced sequence of tephra layers and thick andesitic lava flows. In 1901, a phreatic eruption formed a large crater on the eastern slope of the volcano, and may herald the start of a new period of activity (Ponomareva et al., 2001).
Composition of the Iliinsky erupted products ranges from basalts to dacites and perfectly fits into the range of the tephra from the Kurile Lake caldera eruption, so it is likely related to the same magma system.
The largest tephra erupted from Iliinsky exceeds 1 km3, both silicic and mafic tephras are spread over distances of more than 50 km from the volcano. Silicic and mafic varieties may occur in the same layer and could have been produced during a single eruption.
While eastern slope of the volcano hosts only one large flank crater, western slope and foot of the edifice are dotted with many flank vents, some of them located as far as 6 km from the volcano's summit.
Important marker ash layers helpful for deciphering the complicated Iliinsky stratigraphy are: KSht1 (~1000 14C yr BP) from Ksudach volcano, DG II (4300 14C yr BP) and DG III (1600 14C yr BP) from Dikii Greben' volcano, KHD (~2500 14C yr BP) from Khodutkinsky crater. These ashes differ from Iliinsky tephras in composition and grain size. Some pumice layers from Iliinsky, e.g. layer ZLT (also known as "upper coconuts", ~4800 14C yr BP), also serve as good local markers.