684 



BIOLOGY OF THE SEAS OF THE U.S.S.R. 



as a fault in the earth's crust and an advance of the mainland massif on the 

 ocean bed, leading to mountain formations. 



Numerous volcanoes are situated on the outer side of the line of faults 

 (the Kuril Islands arc) ; on its inner side, towards the mainland, the earth- 

 quake epicentres descend deeper and deeper into the earth's crust, and under 

 the Sea of Okhotsk they reach a depth of 600 km. 



The narrowness of the Kuril-Kamchatka trench is one of the most char- 

 acteristic features of the bottom topography thereabouts. The trench framed 



by the 9,000 m isobath extends to 

 550 km ; its width, however, is no more 

 than 5 km. The 6,000 m isobath is 

 200 km long. The 5,000 m isobath 

 connects the northeastern part of 

 the Kuril-Kamchatka trench with the 

 northwestern end of the Aleutian 

 trench. The great development of 

 tectonic forms in its bottom topography 

 is also most characteristic of the 

 northern part of the Kuril-Kamchatka 

 trench. Faults (sometimes many 

 hundreds of metres long), submarine 

 landslides and the outcrop of ancient 

 main rocks sometimes lead to the 

 formation of a complex bottom profile. 

 The shores of Eastern Kamchatka, 

 except for their northern part, are made 

 of volcanic rock of different ages (V. 

 Zenkovitch, 1960). There are many 

 coastal features, such as the wide and fairly shallow inlets (Avachinsky, 

 Kronotsky and Kamchatsky Bays), and the peninsulas (Shipunsky, Kronot- 

 sky and Kamchatsky) which do not protrude far to the seaward. The shores 

 of Kamchatka, with its sandy beaches, are greatly affected by the swell. 



The monotony of the coastline is broken by the wide Avachinsky Bay and 

 by the presence of coastal features of the fjord type. Wide areas of dry sand 

 or mud are often formed inside the bays and fjords. The regular, semi- 

 diurnal tides on the shores of Kamchatka reach a height of 2-5 m. 



The slopes of the outer Kuril submarine range (the Vityaz range), the steep 

 slopes of the abyssal and submarine elevations on the edge of the ocean 

 bed are characterized by rocky outcrops (P. Bezrukov, 1955). Many of these 

 sites, especially in the Kuril Straits and on the slopes of the Kuril Islands, have 

 a gravel-pebble floor. At certain points there is in the deposits a considerable 

 admixture of the products of submarine eruptions — pumice, lapilli and vol- 

 canic slag. 



Sand floors are greatly developed on the slopes of the coast of Kamchatka, 

 and in the region of the Kuril Islands (down to a depth of 3,000 m), while 

 diatomaceous oozes are accumulated in the trenches. In general the north- 

 western part of the Pacific, and the Sea of Okhotsk, are exceptionally rich in 



Fig. 325. Block-diagram of the 

 Kuril-Kamchatka trench (Udintzev, 

 1955). 1 Sea of Okhotsk; 2 Kuril 

 Islands; 3 Pacific Ocean; 4 Sub- 

 marine Vityaz range; 5 Kuril- 

 Kamchatka trench; 6 Submarine 

 volcanoes ; 7 Earthquake epicentres. 



