At the mid-ocean ridges, two oceanic plates move apart. As a consequence - or as the cause ( this in an ongoing discussion),- hot mantle material from the asthenosphere wells upward underneath. The rise of this material causes partial melting, because decreasing pressure lowers the temperature point, where partial melting of the mantle material first occurs.
The Mid-Atlantic Ridge, which splits nearly the entire Atlantic Ocean north to south, is probably the best-known and most-studied example of a divergent-plate boundary. (USGS)
The greater the temperature of the rising mantle and the greater the pressure drop, the more melt is produced. The generated magmas are basalts. Most of them intrude into fractures of the stretched (and thinned) lithosphere, but some may erupt on the sea floor to create new oceanic crust and a series of volcanoes along the mid-ocean ridges.
The evidence for sea-floor spreading came with the discovery that oceanic crust is youngest near the ridge and becomes becomes progressively older away from the spreading center. This age progression could only be explained by the continuous formation of new oceanic crust at the ridges and gradual spreading-apart of the plates over time.
Age of the Atlantic oceanic crust. The crust near the continental margins (blue) is about 200 million years old. It gets progressively younger toward the mid-Atlantic ridge, where oceanic crust is forming today. (NOAA)
Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge. This submerged mountain range, which extends from the Arctic Ocean to beyond the southern tip of Africa, is but one segment of the global mid-ocean ridge system that encircles the Earth. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million years. This rate may seem slow by human standards, but because this process has been going on for millions of years, it has resulted in plate movement of thousands of kilometers. Seafloor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that exists today. (USGS)
The extensive mid-Atlantic ridge is colour-coded in this image for topography. Warm colours (yellow to red) are at higher elevations than the cooler colours (green to blue). The ridge crest increases in elevation toward Iceland, which is thought to be a hotspot underlain by a plume of hot mantle. The hot mantle thus powers abundant volcanism at Iceland and provides thermal bouyancy for the northern ridge segment (courtesy of NOAA).
The crest of the mid-oceanic ridge of the south Pacific is apparent in this false-color image dipicting topography. The warm colors (yellow to red) are at higher elevations than the cool colors (blue). The ridge is offset by numerous fracture zones, the most obvious of which occur near the center of the image (courtesy of NOAA).