Bathymetric image of the East Pacific Rise, a mid-oceanic ridge located along the floor of the Pacific Ocean. (Image courtesy of Haymon et al., NOAA-OE, WHOI)
The mid-ocean ridge is a continuous range of undersea volcanic mountains that encircles the globe almost entirely underwater. It is a central feature of seafloor terrain that is more varied and more spectacular than almost anything found on dry land, and includes a collection of volcanic ridges, rifts, fault zones, and other geologic features.
At nearly 60,000 kilometers (37,000 miles) long, the mid-ocean is the longest mountain range on Earth. It formed and evolves as a result of spreading in Earth’s lithosphere—the crust and upper mantle—at the divergent boundaries between tectonic plates. The vast majority of volcanic activity on the planet occurs along the mid-ocean ridge, and it is the place where the crust of the Earth is born. The material that erupts at spreading centers along the mid-ocean ridge is primarily basalt, the most common rock on Earth.
Because this spreading occurs on a sphere, the rate separation along the mid-ocean ridge varies around the globe. In places where spreading is fastest (more than 80 millimeters, or 3 inches, per year), the ridge has relatively gentle topography and is roughly dome-shaped in cross-section as a result of the many layers of lava that build up over time. At slow- and ultra-slow spreading centers, the ridge is much more rugged, and spreading is dominated more by tectonic processes rather than volcanism.
Scientists study the physics, chemistry, and biology of mid-ocean ridges gain insight into how Earth works in very fundamental and often surprising ways.