Fig.1

Fig.2

Fig.3

Fig.4

Fig.5

Fig.6

Fig.7

Fig.8

Fig.9

Fig.10

Fig.11

Kurile Lake (Fig.1) is located in the southernmost part of Kamchatka, at the southern end of the subduction-related Eastern volcanic front. Five Late Pleistocene-Holocene volcanoes (from north to south: Zheltovsky; Iliinsky, Dikii Greben'; Koshelev, and Kambalny) located only 12-18 km one from another surround Kurile Lake (Figs. 1, 2). Kurile Lake has an area of about 76 km² and drains into the Ozernaya River, which flows westward to the Sea of Okhotsk. Water level in the lake is only 81 m a.s.l. Widespread outcrops of white pumice, more than 150 m high, surround Kurile Lake and evidence voluminous Plinian eruption in the past. Our studies show that this eruption took place about 7600 ¹⁴C yr BP and likely resulted in the formation of under-water Kurile Lake caldera and probably also of a large crater now enclosing Iliinsky volcano (Braitseva et al., 1995, 1997; Ponomareva et al., submitted; Zaretskaia et al., 2001).

The Kurile Lake caldera eruption (coded KO) was preceded by more than 1500 yrs long period of quiescence in the region recorded by a thick paleosol (Ponomareva et al., 2001). The eruption began with phreatoplinian activity, presumably through a proto-Kurile Lake, which deposited a finely-grained rhyolitic vitric ash and a sequence of minor pyroclastic flows (Fig. 3). As the vent emerged above the water level the eruption changed to a sustained plinian column, generating silicic pumice fall deposits (Fig. 4). The later part of the KO activity is characterized by flow and surge deposits. Pyroclastic flows ponded the valleys in all the directions over distances of about 50 km, and also covered plateaus and ridges with a thinner blanket (Figs. 5-7). The ignimbrite was probably welded close to the source (Fig. 9), and demonstrated transition from rhyolite composition to mafic one, and again to rhyolite during the eruption (Figs. 6-8). Fine ash derived from the ignimbrite constituted at least half of distal fallout. The ash was likely entrained by a strong wind across the Sea of Okhotsk and farther to the northwest to Asia mainland (Figs. 10, 11) but also covered a large part of the Kamchatka peninsula (Melekestsev et al., 1991; Braitseva et al., 1997b). KO marker ash was used in paleovolcanological reconstructions (Braitseva et al., 1998; Melekestsev et al., 1990, 1994, 1999; Seliangin, Ponomareva, 1999), in marine research (Gorbarenko et al., 2000, 2002), environmental studies (Anderson et al., 1998), and it has obviously a huge tephrochronological potential since it allows to correlate various depositional successions over a large area from Kamchatka to Asian mainland over the Sea of Okhotsk. The ash dispersal pattern suggests that the KO ash can be also found in the Pacific Ocean bottom sediments east of Kamchatka and Northern Kurile islands.

The Kurile Lake caldera eruption produced at least 140-170 km³ of tephra or 70-80 km³ of magma that makes it the largest Holocene eruption in the Kurile-Kamchatka volcanic arc and ranks it with the largest Holocene explosive eruptions on the Earth such as Tambora, Kikai, Baitoushan, and Crater Lake. Kurile Lake caldera eruption was the most important constituent of "a century" of volcanic catastrophes (6600-6400 BC). During this interval two calderas and numerous other eruptions with a total volume of erupted products of at least 245-292 km³ occurred in Kamchatka (Braitseva et al., 1995; Melekestsev et al., 1998).

The eruption likely resulted in the formation of a large Kurile Lake-Iliinsky depression (Fig. 2). It could have originated as a combination of a caldera collapse and associated flank failure of the pre-Iliinsky edifice. Underwater caldera is bounded by sub-vertical ring faults (Bondarenko, 1991) and is best approximated by a plate collapse process (Lipman, 1997).

Post-caldera activity included the eruption of Iliinsky and Dikii Greben’ volcanoes and several extrusive domes within the Kurile Lake. Composition of the Iliinsky erupted products perfectly fits into the range of the caldera tephra, so it is likely related to the same magma system. Serdtze Alaida (“Alaid's Heart”) extrusive dome rises from the depth of more than 300 m and forms an isolated island in the middle of caldera (Fig. 1). It is composed of dacite close to dacite from the KO tephra.

Vera Ponomareva

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