Due to changes in sea level, Japan has at various times been connected to the continent by , with continental Russia to the north via the

Sōya Strait Soya may refer to: Food * Soya bean, or soybean, a species of legume native to East Asia, widely grown for its edible bean * Soya sauce, see soy sauce, a fermented sauce made from soybeans, roasted grain, water and salt Places * Sōya District, ...

Sakhalin Sakhalin ( rus, Сахали́н, r=Sakhalín, p=səxɐˈlʲin; ja, 樺太 ''Karafuto''; zh, c=, p=Kùyèdǎo, s=库页岛, t=庫頁島; Manchu: ᠰᠠᡥᠠᠯᡳᠶᠠᠨ, ''Sahaliyan''; Orok: Бугата на̄, ''Bugata nā''; Nivkh: ...

, and the

Mamiya Strait Strait of Tartary or Gulf of Tartary (russian: Татарский пролив; ; ja, 間宮海峡, Mamiya kaikyō, Mamiya Strait; ko, 타타르 해협) is a strait in the Pacific Ocean dividing the Russian island of Sakhalin from mainland Asi ...

, and with the

Korean Peninsula Korea ( ko, 한국, or , ) is a peninsular region in East Asia. Since 1945, it has been divided at or near the 38th parallel, with North Korea (Democratic People's Republic of Korea) comprising its northern half and South Korea (Republic o ...

to the southwest, via the

Tsushima Strait or Eastern Channel (동수로 Dongsuro) is a channel of the Korea Strait, which lies between Korea and Japan, connecting the Sea of Japan, the Yellow Sea, and the East China Sea. The strait is the channel to the east and southeast of Tsushima ...

and

Korea Strait The Korea Strait is a sea passage in East Asia between Korea and Japan, connecting the East China Sea, the Yellow Sea and the Sea of Japan in the northwest Pacific Ocean. The strait is split by the Tsushima Island into the Western Channel and t ...

. Land bridges also connected the Japanese Islands with each other. These land bridges enabled the migration of terrestrial fauna from the continent and their dispersal within Japan.

Geological background

Around 25 million years ago, the

Sea of Japan The Sea of Japan is the marginal sea between the Japanese archipelago, Sakhalin, the Korean Peninsula, and the mainland of the Russian Far East. The Japanese archipelago separates the sea from the Pacific Ocean. Like the Mediterranean Sea, it h ...

began to open, separating Japan from the continent and giving rise to the Japanese island arc system of today. The Sea of Japan as a

back-arc basin A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of ...

was open both to the northeast and to the southwest by 14 Ma, while

marine transgression A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling ...

further contributed to the isolation and insulation of Japan. Due to the level of tectonic activity in the area and significant

subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope move ...

of the Japanese Islands since the

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

, exact quantification of historic sea level changes is problematic.

Northern land bridge

Based on current depths, a reduction in sea level would be sufficient to connect

Hokkaidō is Japan's second largest island and comprises the largest and northernmost prefecture, making up its own region. The Tsugaru Strait separates Hokkaidō from Honshu; the two islands are connected by the undersea railway Seikan Tunnel. The la ...

with the mainland. The and — sometimes referred to jointly as the or Sakhalin land bridge — are thus thought to have been in place during most glacial periods.

Western land bridge

With a minimum depth of and based in part on the appearance in Japan of

Proboscidea The Proboscidea (; , ) are a taxonomic order of afrotherian mammals containing one living family (Elephantidae) and several extinct families. First described by J. Illiger in 1811, it encompasses the elephants and their close relatives. From ...

, the and — sometimes referred to jointly as the Korean land bridge — are understood to have been in place at 1.2 Ma, 0.63 Ma, and 0.43 Ma.

Kuril land bridge

A has been insufficient to connect Hokkaidō with Kamchatka during the

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

. The southern Kuril land bridge that connected

Kunashir , other_names = kz, Kün Ashyr; ja, 国後島 , location = Sea of Okhotsk , locator_map = File:Kurily Kunashir.svg , coordinates = , archipelago = Kuril Islands , total_islands = , major_islands = , area = , length = , width = fr ...

i and the Lesser Kurils to Hokkaidō during the Early Holocene was insufficient with the rising sea level at around 6,000 BP.

Seto land bridges

Honshū , historically called , is the largest and most populous island of Japan. It is located south of Hokkaidō across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyūshū across the Kanmon Straits. The island separa ...

, Shikoku, and

Kyūshū is the third-largest island of Japan's five main islands and the most southerly of the four largest islands ( i.e. excluding Okinawa). In the past, it has been known as , and . The historical regional name referred to Kyushu and its surround ...

are separated by shallow straits that rarely exceed in depth. Consequently, they were frequently connected together as a single land mass.

Tsugaru land bridge

The Tsugaru Strait, with a depth in excess of , represents a more significant faunal boundary, known as

Blakiston's Line The Blakiston Line or Blakiston's Line is a wikt:faunal, faunal boundary line drawn between two of the four largest islands of Japan: Hokkaidō in the north and Honshū, south of it. It can be compared with faunal boundary lines like the Wallace L ...

. The most recent age of the is uncertain.

Ryūkyū land bridge

The Ryūkyū Islands, separated by deeper straits still (the Tokara Gap), have been isolated from the main islands throughout the

. The was sufficient temporarily to connect Miyako-jima with

Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the nort ...

during the late Middle Pleistocene, allowing for the migration of the

Steppe mammoth The steppe mammoth (''Mammuthus trogontherii'', sometimes ''Mammuthus armeniacus'') is an extinct species of Elephantidae that ranged over most of northern Eurasia during the late Early and Middle Pleistocene, approximately 1.8 million-200,000 y ...

(''Mammuthus trogontherii''). During this period, the

Miyako Strait The , also known as the Kerama Gap, is a waterway which lies between Miyako Island and Okinawa Island consisting of a 250km-wide passageway with international waters and airspace. It is the widest strait in the Ryukyu Islands. Political significa ...

was sufficient to prevent the land bridge reaching

Okinawa Island is the largest of the Okinawa Islands and the Ryukyu (''Nansei'') Islands of Japan in the Kyushu region. It is the smallest and least populated of the five main islands of Japan. The island is approximately long, an average wide, and has an ...

References

{{Reflist Landforms of Japan Geology of Japan Historical geology Biogeography