Kuril Islands

The Kuril islands, extending from north to south for 1200 km, are a part of the submarine

uplift located between the Sea of Okhotsk basin and the Kuril oceanic trench*. The archipelago comprises 30 large islands and numerous small ones. Two sectors, different in the history of development and morphology are distinguished within the archipelago (Luchitsky, 1974; Piskunov, 1987). The inner arc, known as the Greater Kuril,

extends from Shumshu island to Kunashir and the outer arc, termed the Lesser Kuril, which includes Shikotan and the small isles of the Habomai group (Figure1).

Tectonics and Environmental Change

Tectonically the Kurils belong to the Kuril-Kamchatka island arc which is related, as all island arcs, to an oceanic trench located 150-170 km from the volcanic front. Pronounced seismic activity is associated with the arc and the Benioff zone can be traced to a depth of 650 km (Tarakanov, 1972). The Greater Kurils and the Lesser Kurils differ in their morphology, geological structure, and history of development.

The Lesser Kurils, as well as the submarine Vityaz uplift to which they are connected and the peninsulas of eastern Kamchatka, are interpreted as an outer non-volcanic arc of the Kuril-Kamchatka system. The Lesser Kurils are formed mainly by late Cretaceous volcanic rocks. Volcanism of a submarine nature, as a result of which the islands emerged, was active in the Campanian (85-80 Ma BP) and the geological complex of this age is represented mainly by basalts. In the Maastrichtian (73-65 Ma BP), volcanic activity faded and flysh with olistostromes accumulated. Volcanic activity of the time was represented mainly by magmatic intrusions into sediments (Piskunov and Sergeev, 1992). At the end of the Cretaceous, volcanism occurred in terrestrial conditions. The completion of the magmatic activity was marked by gabbro intrusions. At present, there are no active volcanoes in the Lesser Kurils. The ancient calderas have been eroded and gentle hills with a height of 150-250 m dominate the islands.

The Greater Kurils are a younger formation where volcanism commenced at about 30 Ma BP in the late Oligocene-early Miocene. There were three major stages of arc development and these are reflected in the sequence of Cenozoic rocks (Fedorchenko et al., 1989; Piskunov and Sergeev, 1992). The most ancient rocks of the lower Miocene age comprise basalts, andesites, rhyolites, and their tuffs. The latter are abundant and can be correlated with the Green Tuff Formation of Japan. During the next stage in the middle Miocene, volcanism occurred mainly under terrestrial conditions and the stratigraphy is characterized by the occurrence of both volcanic rocks and flysh-type sequences.

Clastic material contains rocks which are not found in the Greater Kurils and which have apparently been derived from the dissected terrain in the back-arc area. The composition of these sequences shows that the Sea of Okhotsk basin did not exist at the time, while their inclined position reflects block rotation along faults during rifting and the opening of the Sea of Okhotsk basin at about 15 Ma BP (Zonenshain et al., 1990). The latest stage, during which the modern volcanic cones emerged, began in the late Miocene and is continuing at present. Its geology is a typical cal-calkaline volcanic sequence.

Similarly to Kamchatka, there were major periods of enhanced volcanic activity in the Kurils in the late Pleistocene and early Holocene (e.g., the Medeleev and Golovnin calderas formed 38-40 Ka BP). Over a hundred terrestrial volcanoes occur on the islands at present and 39 of them are active. The most active volcanoes are the tallest Kuril volcano, the Alaid (2339 m) located in the north of the arc and the Sarychev located on Matua island. The last powerful eruption of the Alaid occurred in 1972 when a new slag cone 150 m high formed in the centre of the volcano. The Sarychev is known for its regular activity. The most violent eruptions occurred in 1930 and 1946 when the ashfalls reached central Kamchatka. By contrast, another major volcano, the Golovnin caldera (the southernmost Kuril volcano located in Kunashir island) has not erupted for more than a century although it accommodates geothermal springs and two large lakes formed 640-680 years BP (Razjigaeva et al., 1998).

Typical of the Kurils is low to middle mountainous relief with altitudes of 500-1300 m. Five major morphological types of relief are distinguished: volcanic, seismo-tectonic, erosive-denudational, coastal (of abrasion and accumulation types), and aeolian (Grabkov and Isachenko, 1982). Aeolian forms date back to about 4.5-4.7 Ka BP when an extensive marine regression occurred (Bazarova et al., 1998). Most islands have volcanic relief and the landscape is that of individual volcanoes or volcanic ridges linked by sediment-filled isthmuses. Six to seven marine terraces occur in the coastal zone with altitudes ranging between 2-3 and 200-250 m a.s.l. Both marine terraces and volcanic massifs are strongly dissected which indicates a high rate of erosion and marine abrasion.

The landscapes of the Kuril islands were formed under the influence of two major factors: volcanism and seismic activity, and climate change. Climate is strongly influenced by the physical and chemical properties of the ocean, such as sea surface temperature and salinity, and variability in the paths of the oceanic currents may result in regional climatic change. Periodic changes in ocean circulation patterns due to changes in trade wind intensity in the eastern equatorial North Pacific Ocean and movements of the ocean plate have been discussed in relation to the warm Kuroshio current and climate change in Japan (Taira, 1980; Sawada and Handa, 1998). The islands of Japan and the Kurils are located in close proximity (as early as in the middle Holocene the islands of Hokkaido and Kunashir were connected by a landbridge) and climatic and environmental changes in northern Japan and the southern Kurils are closely correlated (Bazarova et al., 1998). However, the paleooceanography of the Kurils and its impact on the landscape development require further independent investigation.

*Kuril–Kamchatka Trench