Rising warm current from the mantle, domes, stretches, and thins continental crust above it.
Surface breaks into three-branched fracture.
This inactive branch forms a linear depression known as an aulacogen , or failed arm . The two active branches marked by high heat flow, normal faulting, shallow earthquakes, and basaltic volcanism. When rifting ends, the original edges are no longer plate margins, nor are they seismically or volcanically active. They are passive continental margins . The present seafloor spreading ridges are the lithospheric scars of the breakup of Pangea .