The Atlantic Ocean is the second largest ocean of the world and occupies an elongated basin extending from the Arctic Ocean in the north to the Southern Ocean in the south. Its bottom relief is highly varied and consists of continental margins, deep ocean basins, submarine ridges, trenches, plateaus, seamounts, and abyssal plains. The present configuration of the Atlantic Ocean floor is mainly the result of plate tectonic movements, volcanic activity, sedimentation, and erosion over geological time. The continental shelf forms the submerged extension of the continents and is generally wider along the eastern coasts of North America and South America as well as the western coasts of Europe and Africa. These shelves are important fishing grounds because they receive abundant nutrients and sunlight. The continental shelf is followed by the continental slope, which descends steeply toward the deep ocean floor. Between the continental shelf and the abyssal plain lies the continental rise, where sediments carried by rivers and underwater currents accumulate. One of the most remarkable features of the Atlantic Ocean floor is the Mid-Atlantic Ridge. It is a continuous submarine mountain chain running almost through the centre of the ocean from north to south. It divides the Atlantic into eastern and western basins. The ridge is formed