<P> The Caribbean's floor is composed of sub-oceanic sediments of deep red clay in the deep basins and troughs . On continental slopes and ridges calcareous silts are found . Clay minerals likely having been deposited by the mainland river Orinoco and the Magdalena River . Deposits on the bottom of the Caribbean Sea and Gulf of Mexico have a thickness of about 1 km (0.62 mi). Upper sedimentary layers relate to the period from the Mesozoic to the Cenozoic (250 million years ago to present) and the lower layers from the Paleozoic to the Mesozoic . </P> <P> The Caribbean sea floor is divided into five basins separated from each other by underwater ridges and mountain ranges . Atlantic Ocean water enters the Caribbean through the Anegada Passage lying between the Lesser Antilles and Virgin Islands and the Windward Passage located between Cuba and Haiti . The Yucatán Channel between Mexico and Cuba links the Gulf of Mexico with the Caribbean . The deepest points of the sea lie in Cayman Trough with depths reaching approximately 7,686 m (25,220 ft). Despite this, the Caribbean Sea is considered a relatively shallow sea in comparison to other bodies of water . </P> <P> The pressure of the South American Plate to the east of the Caribbean causes the region of the Lesser Antilles to have high volcanic activity . There was a very serious eruption of Mount Pelée in 1902 which caused many casualties . </P> <P> The Caribbean sea floor is also home to two oceanic trenches: the Cayman Trench and Puerto Rico Trench, which put the area at a high risk of earthquakes . Underwater earthquakes pose a threat of generating tsunamis which could have a devastating effect on the Caribbean islands . Scientific data reveals that over the last 500 years the area has seen a dozen earthquakes above 7.5 magnitude . Most recently, a 7.1 earthquake struck Haiti on January 12, 2010 . </P>

Where does the caribbean sea end and the atlantic ocean begin