Gulf of Mexico numerous bays and smaller inlets. A number of rivers empty into the gulf, most notably the Mississippi River and Rio Grande in the northern gulf, and the Grijalva and Usumacinta rivers in the southern gulf. The land that forms the gulf's coast, including many long, narrow barrier islands, is almost uniformly low-lying and is characterized by marshes and swamps as well as stretches of sandy beach. The Gulf of Mexico is an excellent example of a passive margin. The continental shelf is quite wide at most points along the coast, most notably at the Florida and Yucatán Peninsulas. The shelf
Gulf of Mexico basin subsidence in the central region, as well as heavy sedimentation around the Florida Platform and Northern Gulf of Mexico. While a few intrusive igneous formations found as far south as Mexico City are dated to the Late-Jurassic or Early Cretaceous, the Gulf of Mexico during that time was relatively devoid of igneous activity. It wasn't until the Late Mesozoic that low-volume yet widespread volcanism occurred along the Western and Northwestern margins of the Gulf of Mexico, leaving the Yucatan Peninsula with an extremely alkali volcanic field in the subsurface. The volcanism is believed to have been activated along ancient plate