Divergent plate boundaries: spreading-center volcanismSpreading-center volcanism occurs at rift-zones, where two plates are moving apart from each other. Most commonly this is the case at mid-oceanic ridges, where two oceanic plates move apart. The (developing) boundary between two spreading continental plates is known as a continental rift.
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.
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)
Iceland: a mid-ocean ridge exposed above sea level
Map showing the Mid-Atlantic Ridge splitting Iceland and separating the North American and Eurasian Plates. The map also shows Reykjavik, the capital of Iceland, the Thingvellir area, and the locations of some of Iceland's active volcanoes (red triangles), including Krafla. (USGS)
Pillow basalt from the south Pacific. (NOAA)
Black smoker from the mid-Atlantic Ridge (image: Peter Rona, NOAA).