The mid-ocean ridge is the world's longest mountain range, although it is submerged. It is a continuous ocean ridge that stretches through all the world's oceans, including the Pacific, Atlantic, Indian, Arctic, and Southern Ocean. It exists along the boundary of tectonic plates. In fact, the only reason that the mid-ocean ridge exists is due to weaknesses in the Earth's crust at plate interfaces which permit volcanic activity. The volcanic activity generates magma, which cools to form this continuous submarine mountain range.
Consisting of one continuous mountain range with four prominent branching ranges, the mid-ocean ridge has a continuous length of 40,400 mi (65,000 km). The total length of the system is 49,700 mi (80,000 km). The mid-ocean ridge has an average height of about a mile. Because it generally exists only in the darkest and deepest parts of the oceans, the mid-ocean ridge was not discovered until the 1950s. The first segment to be discovered was the Mid-Atlantic Ridge, which remains the most famous portion of the mid-ocean ridge. At first, it was thought that a long continuous submerged mountain range was only an Atlantic phenomenon, until further surveys revealed portions of the ridge worldwide. Previously, the longest known mountain range in the world was the Andes mountains in South America, with a length of 4,400 miles (7,000 km).
Now, scientists know that the mid-ocean range is continuously being formed by a phenomenon known as ocean spreading, where convective currents of magma in the mantle push volcanic material up through divergent boundaries between tectonic plates. Ocean spreading pushes the margins of oceanic tectonic plates beneath continental plates, a process known as continental subduction. The margins of these plates are subducted into the mantle, where they melt. Because of this process, the oceanic tectonic plates are all relatively young, under 100 million years in age. By radiocarbon dating different portions of exposed oceanic plate, we can determine the age of the plate and the magnitude and direction of spreading.