Where Things Come Apart At divergent plate boundaries, plates move away from each other. Many of these boundaries occur along the oceanic ridge system and are referred to as oceanic spreading centers. They are the sites where the seafloor spreads Oceanic Spreading Centers As two oceanic plates move apart, the lithosphere is being extended, thinned and fractured. Magma form the asthenosphere upwells into the rift valley. Magma solidifies to form basalt, new material which is added to the plate. This type of boundary is referred to as a constructive plate boundary The new material is hotter and less dense than the surrounding lithosphere causing the resultant mid- oceanic ridge to rise 2-3 km above the ocean floor. In Iceland the ridge rises above sea level Mid-ocean ridges encircle the Earth like a seam on a baseball. These ridges form the longest and widest mountain chain in the world. 65,000 km long And up to 1500 km wide The topography of a ridge is very rugged. Split down the middle by a rift valley, which extrudes lava The Bay of Fundy began as a divergent plate boundary that never completely developed into an ocean Divergent plate boundaries can also occur on continents where continental crust is moved apart forming a rift between the plates. This is know as continental rifting. Continental Rifting If water flows into the rift from an adjoining ocean, a new ocean is born. The Read Sea was formed in this manner. Geological activity associated with divergence includes: Volcanism And earthquake activity Neither of these poses much of a threat to people as the activity generally occurs on the ocean floor Links of Interest