Main Menu >  Plate Tectonics and Volcanoes  >  Mid-Atlantic Ridge >  Heat Below, Mountains Above The Mid-Atlantic Ridge rises as high as 2.5 km (1.6 mi) above the ocean floor. Why are spreading ridges so tall? Because the mantle rocks beneath them are hot and therefore buoyant. As these rocks expand and bulge upward, they elevate the sea floor into a ridge.