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)
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 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)
As the hot magmas erupt through vertical fractures the eruptions produced in this manner are typically fissure eruptions and the erupting basalt can cover vast submarine lava fields. Typically, the lava is quenched quickly against the seawaters to produce characteristic bulbous shapes called pillow basalt or pillow lava.
Pillow basalt from the south Pacific. (NOAA)
Black smoker from the mid-Atlantic Ridge (image: Peter Rona, NOAA).
Similar to fumaroles and hot springs on subaerial volcanoes, volcanism on mid-ocean ridges is also evident from the occurrence of numerous hydrothermal vents called black smokers. They form from surface water that seeps downward through cracks where it heated by hot rocks lying above the magma chambers. These hot thermal waters then ascend back through the overlying crust, where they leach out silica and numerous metals from the basaltic lava. The hot springs created at the surface are called black smokers because they are readily identified by billowing dark clouds composed of metal-rich fluids.