Land bridge
From Free net encyclopedia
Land bridge is essentially a historical term; it refers to dry land exposed during periods of low sea level (see regression), connecting what are now separate continents or islands. The best-known is the Bering land bridge, which connected North Asia and Alaska during the last ice age, enabling humans to migrate from Eurasia to the Americas by walking. Another land bridge connected Great Britain to Europe at around the same time. A historic land bridge surviving to the present day is the Sinai, connecting North Africa with Southwest Asia; across this land bridge hominids and humans have migrated out of Africa. A land bridge does not have to be narrow enough to be called an isthmus.
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Occurrence
Land bridges generally occur on continental shelves: the Bering Strait, where the Bering Land Bridge was during the last glaciation, is part of the edge of the North American plate. For causes of sea level changes, see Sea level change.
A land bridge that rose from the sea floor because of upthrust at the edge of a continental plate is Central America. Where 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, first an island arc, and then continuous dry land have been created. 12,000 years ago was when the first humans crossed over to North and South America
Tectonic Plates
Before tectonic plate theory, it was believed that land bridges could explain the occurrence of species in separate continents. Many land bridges were thought up, criss-crossing large areas of ocean. They also apparently disappeared without a trace. Continental drift made this theory negatable.
- Bering land bridge connected North Asia and Alaska.
- Sinai Peninsula connects North Africa (Egypt) and Southwest Asia.
- Isthmus of Panama connects North America (Panama) and South America (Colombia).
- There was a land bridge between the British Isles and the continental Europe (France).
- There were land bridges between Japan (including Okinawa), Sakhalin Island, Korean Peninsula, and China. At the time, the Yellow Sea was completely land, and the Sea of Japan was a large lake.
- There was a land bridge between Iberian Peninsula (Spain) and North Africa (Morocco). The Mediterranean Sea became a large lake and eventually dried up completely.
- There were land bridges between Malay Peninsula and Indonesia. The combined subcontinent is called Sundaland.
- There were land bridges between New Guinea, Australia, and Tasmania. The combined continent is called Sahul.
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.