Land bridge
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A land bridge, in biogeography, is an isthmus or other land connection between what at other times are separate land masses which allows animals and plants to cross and colonise new lands. Land bridges are commonly created by regression, in which sea levels fall exposing previously submerged sections of continental shelf. Land bridges are also formed by upthrust at the edge of continental plates.
The most recent significantly low sea levels were about 20,000 years ago (during the Upper Paleolithic) when worldwide sea levels were about 120 meters below today's level. By 10,000 years ago, the sea level had risen to 20 meters below today's level. For causes of sea level changes, see sea level rise.
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[edit] Examples
The best-known example is the Bering land bridge, which joined present-day Alaska and eastern Siberia at various times during the Pleistocene ice ages, enabling humans to migrate from Eurasia to the Americas (see Models of migration to the New World). Another land bridge may have connected Great Britain to Europe at around the same time.
A land bridge surviving to the present day is the Sinai, connecting North Africa with Southwest Asia. Hominids and humans probably migrated out of Africa through the Sinai.
A land bridge that rose from the sea floor because of upthrust at the edge of a continental plate is Central America. The Cocos Plate, an oceanic tectonic plate off the west coast of Central America, is being subducted in a convergent boundary under the North American Plate to the north and the South American Plate to the south. This caused first an island arc and then continuous land to emerge.
[edit] Land bridge theory
Before the theory of plate tectonics, it was believed that land bridges could explain the occurrence of species in separate continents and the resemblances of geologic formations on different continents. Many land bridges were thought up, cross-crossing large areas of ocean, the most prominent of them being Lemuria. However, when the sea bed of the Atlantic Ocean was mapped using echo sounding between 1924 and 1927, no remains of land bridges could be found. Though this was an strong argument for Alfred Wegener's theory of continental drift, it would take about 50 more years until mainstream geology fully acknowledged the motion of continents.
[edit] Other meanings
- Air rights
The term land bridge sometimes refers to covering transportation facilities with bridges for non-transportation uses, such as parks, housing, or offices. For this use, see air rights.
- Containers on railways
In the railroad industry, land bridge refers to the transport of containers by rail between ports on either side of a land mass, such as North America.
- Saudi Arabia
In 2005 Saudi Arabia initiated a 3,000 km project of railway construction to link cities, ports and mines within the country, and potentially with adjacent countries.
- Wildlife
The term land bridge is sometimes used for wide bridges built over highways for wildlife and park users to cross. A natural crossing of a waterway is known as a natural arch or natural bridge.
