Divergent Boundary
A divergent boundary is a place where two tectonic plates move apart, allowing magma to rise and form new crust at mid-ocean ridges or rift valleys.
Definition
A divergent boundary is where two plates pull away from each other, and melted rock rises to fill the gap and hardens into new crust. As plates move apart, mantle material rises to fill the space, creating new lithosphere: in ocean basins this forms mid-ocean ridges with seafloor spreading, and on continents it forms rift valleys that may eventually split the continent and open a new ocean. Divergent boundaries have shallow earthquakes and basaltic volcanism that is usually gentle. They are zones of lithospheric extension where normal faulting, dike injection, and decompression melting create new crust; mid-ocean ridges have a magma chamber or melt lens beneath the axis and a sheeted-dike complex, and spreading rate controls morphology. Continental rifting proceeds from broad extension and normal faulting to focused rifting, often with flood basalt volcanism from a mantle plume, and finally to seafloor spreading, a sequence preserved in the passive margins on both sides of the Atlantic. Slow-spreading ridges expose serpentinized mantle peridotite along detachment faults.
Example
Iceland sits right on top of a divergent boundary and is slowly being pulled in two. The Mid-Atlantic Ridge separates the North American and Eurasian Plates, the East African Rift is a continental divergent boundary slowly splitting Africa apart, and the Red Sea is a rift that has already become a young ocean. The Triassic rift basins of North Carolina (Durham, Sanford, Dan River) are failed rifts from the breakup of Pangaea, filled with red beds and cut by CAMP diabase dikes. The Afar Triangle is a triple junction where the Red Sea, Gulf of Aden, and East African Rift meet, and in 2005 a $60\ \text{km}$ dike opened there in days.
Key Insight
Divergent boundaries build new ground; they are the only place on Earth where new crust is made. They are where oceans are born, since the Atlantic started as a rift valley in Pangaea about $200$ million years ago, and the East African Rift may be the Atlantic of the future. Not every rift becomes an ocean. Failed rifts (aulacogens) such as the Midcontinent Rift under Lake Superior or the Reelfoot Rift under the New Madrid seismic zone remain as weak zones in the continent, and they are where intraplate earthquakes tend to concentrate.