A peninsula is literally an 'almost-island', and nature builds them many ways
The word peninsula comes from Latin for 'almost island': land wrapped in water on most sides but still tied to the mainland. The largest is the Arabian Peninsula. Some were torn off by drifting plates, some scraped out by glaciers, some poured from volcanoes, and some can briefly become true islands when water levels rise.
The term reached English in the 16th century. The surrounding water need not be sea: land in a tight river bend or between two rivers can count, as with New Barbadoes Neck in New Jersey. The link to the mainland is sometimes a narrow isthmus, like the one at Corinth joining the Peloponnese to central Greece.
Plate tectonics shapes the grandest examples. Arabia is a rift peninsula, formed where plates pulled apart, while colliding plates produced Gibraltar and the Indian subcontinent. Volcanoes erupting near shallow water can build peninsulas too, as along the Alaskan Peninsula. Rivers make delta peninsulas by dumping sediment where they meet the sea, and marine sediment can create limestone ones. Florida owes its outline to several of these at once: continental drift, marine sediment and shifting sea levels.
Ice is another sculptor. Glaciers scour softer rock and leave harder rock standing, as happened at Michigan's Keweenaw Peninsula. Meltwater dammed behind moraines can surround land, and heaps of glacial drift near the coast can become peninsulas themselves; Cape Cod formed this way about 23,000 years ago.
Their outlines are not fixed. High water can cut a peninsula off into an island, wet weather makes them look smaller and dry weather larger, and sea level rise from global warming will permanently shrink some. They have also mattered to people for a very long time: they sheltered prehistoric humans and Neanderthals, offering hunting on land and at sea, and the Malay Peninsula, where the Indian Ocean meets the China Seas, became a hinge of east-west trade.
Source: Peninsula
