The mid-oceanic ridge is the longest mountain range in the world (40,400 miles long), and its underwater! Mid-ocean ridges mark the boundary between two tectonic plates that are pulling apart at rates of 1 to 20 cm per year. The rate of spread creates different characteristics in the ridges associated rift valley. The Mid-Atlantic Ridge, for example, spreads very slowly at 2 to 5 cm per year and has a very deep rift valley as a result (1 to 3 km deep). In contrast, the East Pacific Rise spreads much faster at 6 to 16 cm per year, which prevents it from having any rift valley at all.

As the tectonic plates spread apart in a process called sea floor spreading, volcanism causes extremely hot asthenosphere to rise from the center of the earth upward. As the asthenosphere rises it melts and produces basalt magmas which eventually erupt from the surface. This process is called basaltic volcanism. This basaltic lava has been erupting from the surface more or less continuously for millions of years. When it erupts, it cools and attaches itself to the edge of the spreading plates in a hardened basalt layer. All mid-ocean ridges have rift valleys.