A continental edge that lacks tectonic activity is called a passive margin. At this point the continental margin ceases to be a plate boundary. As the edges of the continents move away from the heat associated with active spreading, the thinned continental crust cools and drops in elevation, eventually dropping below sea level. If rifting continues, the continent splits into two pieces and a narrow ocean basin forms as seafloor spreading takes place. The entire continental crust thins as it is pulled apart and elevation of the central rift decreases over time. Continued rifting causes decompression melting and the resulting magma from the mantle may solidify beneath the surface or may erupt from volcanoes and long fissures on the surface. The down dropped blocks may form basins that can trap sediment and water. The next stage of continental rifting is progressive stretching that causes large crustal blocks to drop down along faults, forming a continental rift. But the uplift is short lived because the crust is stretched and pulled at a divergent boundary so it will begin to thin. Increased heating causes increased expansion which translates to further uplift. That magma can melt parts of the continental crust which produces additional magma. The initial stage of continental rifting commonly includes broad uplift of the surface as mantle-derived magma ascends into crust. If continental rifting progresses it can lead to seafloor spreading and the formation of a new ocean basin. When you have divergence within a continent it is called continental rifting and it creates a continental rift. What is Continental Rifting? Most divergent boundaries are beneath oceans but a divergent boundary can form within a continent. The elevation of the seafloor decreases away from the ridge because the rock cools and contracts. Mid-ocean ridges are elevated above the surrounding seafloor because they consist of hotter, less dense materials and the underlying asthenosphere is thinner and bulging right beneath the ridges. These eruptions create new ocean crust that is incorporated into the oceanic plates as they move apart. Much of the magma solidifies at depth but some erupts on the seafloor forming submarine lava flows. The magma rises along narrow conduit, accumulates in magma chambers beneath the rift and eventually becomes part of the oceanic lithosphere. As the plates pull apart, the load on the asthenosphere decreases which decreases the pressure in the asthenosphere and leads to decompression melting. As the plates move apart, an opening is created which allows the asthenosphere to rise toward the surface to fill that space. That movement causes faulting which results in frequent small to moderate-sized earthquakes. The rift forms because large blocks of crust slip down as spreading occurs. A narrow trough, or rift runs along the axis of most mid-ocean ridges. Mid-ocean ridges are also called spreading centers because of the way the plates spread apart. This is where new oceanic lithosphere forms. When a divergent boundary occurs between two oceanic plates a mid-ocean ridge. What is a Divergent Boundary? A divergent boundary is a type of plate boundary where two plates move apart or diverge relative to one another.

Divergent plate boundary