A rift is a place where the Earth's crust and lithosphere are being pulled apart. Rift features are a central linear downfaulted segment, called a graben, with parallel normal faulting and rift-flank uplifts on either side forming a rift valley, where the rift remains above sea level. The axis of the rift area commonly contains volcanic rocks, and active volcanism is a part of many, but not all active rift systems. Major rifts occur along the central axis of mid-ocean ridges, where new oceanic crust and lithosphere is created along a divergent boundary between two tectonic plates.

Mid-ocean ridge.

A mid-ocean ridge is an underwater mountain system that consists of various mountain ranges (chains), typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading center, which is responsible for seafloor spreading. A mid-ocean ridge demarcates the boundary between two tectonic plates, and consequently is termed a divergent plate boundary. Global mid-oceanic ridge system is part of every ocean, making the mid-oceanic ridge system the longest mountain range in the world. The continuous mountain range is 65,000 km and the total length of the oceanic ridge system is 80,000 km long.