The marginal seas have many more islands than the ocean itself. The Mid-Atlantic Ridge, the most striking feature on the floor of the Atlantic, is a submarine mountain range that extends from north of Iceland to the Antarctic Ocean. The ridge forms the boundary between the tectonic plates, rigid slabs of earth's crust that support the continents and make up the seafloor. New seafloor is created in the rift by seafloor spreading . In this process, the plates move away from the rift.
Melted rock rises into the rift and hardens to form the new seafloor.
This great mountain range is the site of submarine earthquakes, volcanos, and hydrothermal vents . These vents occur when water seeps through cracks in the seafloor and is then heated by underlying volcanic molten rock. The water then rises back to the ocean floor as mineral-rich hot water springs. The Mid-Atlantic Ridge rises above the ocean surface in several places, forming islands. These islands include Ascension, the Azores, Iceland, and St. Helena.