Department of Mines and Energy, Government of Newfoundland and Labrador, 95 Bonaventure Avenue, St. John's, Newfoundland, Canada

The stages of continental collision are explained in terms of the forces driving plate motions, which are dominated by the downward pull of subducting oceanic lithosphere and, to a lesser extent, by the outward push of spreading oceanic ridges.

The Taconic stage marks attempted subduction of continental crust. The buoyancy of continental crust offsets the negative buoyancy of subducting oceanic lithosphere and other driving forces so that plate motion is halted. Orogeny involves vertical buoyancy forces and is concentrated along the narrow belt of plate overlap at the subduction zone.

In a major collision the Taconic stage destroys a substantial proportion of the earth's subducting capacity. It is an event of such magnitude that it has global consequences, reducing sea-floor spreading and the rate of convection. This results in retention of heat within the earth and a consequent increase in the forces driving the plates. The orogenic pause represents the time taken for these forces to become strong enough to overcome the obstruction of buoyant continental crust and renew subduction at the collision zone.

The Acadian stage of collision occurs when renewed subduction is achieved by detachment of continental crust from its underlying lithosphere. As the subcrustal lithosphere is subducted, the crust moves horizontally. The result is crustal shortening with widespread deformation and generation of anatectic granitic magma, as well as subduction related volcanism.

The effects of continental collision on the rate of sea-floor spreading can be related to eustatic changes in sea level, glaciations, and mass extinctions. There may also be connections, through changes in the rate of mantle convection, to the earth's magnetic polarity bias and rotation rate.