Explain the difference between a sea-level transgression and regression.

Marine regression means that, submerged sea floor are exposed. It exposes the sea bottom while the transgression may result into flooding. A sea level transgression or marine transgression means when the water level or sea level arises in land, it also means that the sea level will move towards higher land. 

That result from subduction of oceanic plates beneath lighter continental plates. most of the earth's volcanoes are located around the pacific ring of fire because that the location of most of the earth's subduction zones.