The Bahamas, composed of 700 islands and more than 2500 cays, stretch for 650 miles, from the vicinity of southeastern Florida to eastern Cuba. The islands and cays are essentially the high points of carbonate banks, formed by the deposition of marine shells which have compacted into limestone and dolomite; these carbonates are up to 5 km thick, with the deepest layer dating back to the Jurassic.
The carbonate banks, deposited in warm, shallow ocean waters, sit on the basement rock of the North American Plate, which rifted from Africa as the Atlantic began to open, some 180 million years ago; this rifting occurred east of the original Pangean suture line and the segments that sit beneath Florida (and likely the Bahamas) were originally part of the African Plate. Though the exact origin of the plate segment beneath the Bahamas remains uncertain, it may have broken from the Florida Platform as the southern edge of the North American Plate subducted beneath the Caribbean Plate.
Over the eons, ocean currents, changing sea levels and surface erosion have molded the carbonate banks which, today, harbor 5% of our planet's coral reefs. The islands themselves, composed of porous limestones and dolomites, are rich in karst topography (few surface streams but many underground streams, caverns and springs). Native vegetation and wildlife either spread up from Cuba and Hispaniola during periods of low sea level (e.g. in concert with Pleistocene glaciation), drifted in on ocean currents or sprouted from the droppings of mainland birds. Since humans colonized Cuba about 7000 years ago, it seems likely that the first Bahamians appeared soon thereafter.
