The mechanisms by which rifting occurs can be divided into two classes on the basis of how the extensional forces occur. The two classes are active and passive rifting.
Active rifting is not caused by the application of direct force to the lithosphere, as the name might suggest, but rather occurs due to thermal erosion of the lower lithosphere due to the upwelling of mantle material from convection or plumes. The erosion of the lithosphere creates a higher gravitational potential for the material above. This causes the material to collapse under gravitational forces and spread, resulting in rifting.
Passive rifting is caused by the direct application of opposing forces to the lithosphere to create extension. The forces may originate from mantle plumes, mantle convection or by trench pull resulting from subduction.