For example in the Philippine Sea Plate the West Marianas Ridge has formed due to diffuse rifting in the south of the Marianas trough. What does it mean?
If memory serves rifting is defined as either discrete or diffuse. Discrete rifting means that the crust is coming apart in one narrow area, usually in the form of a single defined split in the crust such as at divergent plate boundaries. Diffuse rifting is often seen in backarc spreading and involves many small localised extensional breaks in the crust, each break is small but with multiple breaks spread over a large area the total extension can be quite large. For example discrete rifting can be looked at as a 2 metre break along a single axis while a zone of diffuse rifting would involve several 1 metre breaks each with its own axis, all of which are parallel to each other and perpendicular to the direction of rifting.