Sea of Japan - Geography and Geology

Geography and Geology

The Sea of Japan was once a landlocked sea when the land bridge of East Asia existed.

Onset of formation of Japan Arc was in Early Miocene. Early Miocene period also corresponds to incipient opening of the Japan Sea, and the northern and southern parts of the Japanese archipelago that were separated from each other. During the Miocene there was expansion of Sea of Japan. The northern part of the Japanese archipelago was further fragmented at later periods until the orogenesis of the northeastern Japanese archipelago began in the later Late Miocene. The southern part of the Japanese archipelago remained as a relatively large landmass. The land area had expanded northward in the Late Miocene. The orogenesis of high mountain ranges in the northeastern Japan started in Late Miocene and it lasts in Pliocene also.

Nowadays it is bound by the Russian mainland and Sakhalin island to the north, the Korean Peninsula to the west, and the Japanese islands of Hokkaidō, Honshū and Kyūshū to the east and south. It is connected to other seas by five straits: the Strait of Tartary between the Asian mainland and Sakhalin; La Pérouse Strait between the islands of Sakhalin and Hokkaidō; the Tsugaru Strait between the islands of Hokkaidō and Honshū; the Kanmon Straits between the islands of Honshū and Kyūshū; and the Korea Strait between the Korean Peninsula and the island of Kyūshū. The Korea Strait is composed of the Western Channel and the Tsushima Strait, on either side of Tsushima Island. The straits were formed in recent geologic periods. The oldest of them are the Tsugaru and Tsushima straits. Their formation had interrupted the migration of elephants into the Japanese islands at the end of the Neogene Period (about 2.6 million years ago). The most recent is La Perouse Strait. Its formation about 60,000 to 11,000 years ago had closed the path used by the mammoths which had earlier moved to the northern Hokkaidō. All the straits are rather shallow with a minimal depth of the order of 100 meters or less. This hinders water exchange thereby isolating the water and aquatic life of the Sea of Japan from the neighboring seas and oceans. The sea has a surface area of about 978,000 km² (377,600 sq mi), a mean depth of 1,752 m (5,748 ft) and a maximum depth of 3,742 m (12,276 ft). It has a carrot-like shape, with the major axis extending from southwest to northeast and a wide southern part narrowing toward the north. The coastal length is about 7,600 km with the largest part (3,240 km) belonging to Russia. The sea extends from north to south for more than 2,255 km and has a maximum width of about 1,070 km. It has three major basins: the Yamato Basin in the southeast, the Japan Basin in the north and the Tsushima Basin (Ulleung Basin) in the southwest. The Japan Basin is of oceanic origin and is the deepest part of the sea, whereas the Tsushima Basin is the shallowest with the depths below 2,300 meters. On the eastern shores, the continental shelves of the sea are wide, but on the western shores, particularly along the Korean coast, they are narrow, averaging about 30 kilometers.

There are three distinct continental shelves in the northern part (above 44°N). They form a staircase-like structure with the steps slightly inclined southwards and submerged to the depths of 900–1400, 1700–2000 and 2300–2600 meters. The last step sharply drops to the depths of about 3,500 meters toward the central (deepest) part of the sea. The bottom of this part is relatively flat, but has a few plateaus. In addition, an underwater ridge rising up to 2,300 meters runs from north to south through the middle of the central part.

The Japanese coastal area of the sea consists of Okujiri Ridge, Sado Ridge, Hakusan Banks, Wakasa Ridge and Oki Ridge. Yamato Ridge is of continental origin and is composed of granite, rhyolite, andesite and basalt. It has uneven bottom covered with boulders of volcanic rock. Most other areas of the sea are of oceanic origin. Seabed down to 300 meters (1,000 ft) is of continental nature and is covered with a mixture of mud, sand, gravel and fragments of rock. The depths between 300 and 800 meters (1,000–2,600 ft) are covered in hemipelagic sediments (i.e., of semi-oceanic origin); these sediments are composed of blue mud rich in organic matter. Pelagic sediments of red mud dominate the deeper regions.

There are no large islands in the sea. Most of the smaller ones are located near the eastern coast, except for Ulleungdo (South Korea). The most significant islands are Moneron, Rebun, Rishiri, Okushiri, Ōshima, Sado, Okinoshima, Ulleungdo, Askold, Russky and Putyatin. The shorelines are relatively straight and are lacking large bays or capes; the coastal shapes are simplest for Sakhalin and are more winding in the Japanese islands. The largest bays are Peter the Great Gulf, Sovetskaya Gavan, Vladimira, Olga, Posyet in Russia, East Korea Bay in North Korea and Ishikari (Hokkaidō), Toyama and Wakasa (Honshū) in Japan. Prominent capes include Lazareva, Peschanyi (sandy), Povorotny, Gromova, Pogibi, Tyk, Korsakova, Crillon, Sōya, Nosappu, Tappi, Nyuda, Rebun, Rishiri, Okushiri, Daso and Oki.