CALDERA-FORMING ERUPTIONS OF THE LVINAYA PAST VOLCANO ( <i>Iturup Island, Southern Kurils</i> )

Two large-scale volcanic eruptions occurred in the southern part of Iturup Island (Southern Kurils) in the Late Pleistocene, which resulted in the collapse of the Lvinaya Past caldera (partly flooded later), the largest one in the Kuril Island arc. It is 7 × 9 km wide, with a rim area of ca. 50 km2 and a volume of ca. 25 km3 (including a submarine part of 12.26 km3). Comprehensive geological and geochronological studies have established that these two large-magnitude caldera-forming explosive eruptions (LP-I and LP-II) were separated by a repose period of several hundred years. The age of the first eruption (LP-I) is estimated at ca. 13,500 cal yr BP. The age of the second eruption (LP-II), based on a series of radiocarbon dates, is ca. 12,300 cal yr BP. Both eruptions were of Plinian type and involved the massive ejection of silicic pyroclastic material, which is represented by pyroclastic-flow deposits and tephra. In silica and total alkali contents the pumice from the caldera-forming eruptions corresponds to low-alkali dacites and rhyodacites (SiO2 = 63.4–69.95 wt.%, total alkalies of 3.9–5.5 wt.%), whereas andesitic (SiO2 = 58.3 wt.%, total alkalies of 3 wt.%) and rhyolitic (SiO2 ≈ 74 wt.%, total alkalies of 5.6 wt.%) varieties are scarce. The total volume of erupted material from both events is tentatively estimated at 80–100 km3 (DRE = 35–45 km3), with the LP-II eruption being 30–40% more powerful than the LP-I one. We suggest that the LP-I and LP-II eruptions might have impacted both the regional and global environment.