03 The theory of plate tectonics
Isaacs, Oliver and Sykes (1968) stated that the lithosphere is divided into major tectonic plates — continental, oceanic, or both together — which float on the plastic asthenosphere and move very slowly under the effect of the heat convection-current eddies.
Types of plate movement
- Divergent (constructive): tension forces from rising convection currents drive two plates apart.
- Convergent (destructive): compression forces from sinking convection currents collide two plates.
- Transform (conservative): the edge of one plate slides horizontally past the edge of another, producing cutting transform faults.
Results of divergent movement
- Oceanic plate with oceanic plate → a mid-ocean ridge and new oceans, as in the Atlantic Ocean.
- Continental plate with continental plate → a continental rift, as in the rifting of the Red Sea, produced by the Arabian plate moving away from the African plate.
Results of convergent movement
- Continental with continental → huge fold mountain chains, as in the Himalayas.
- Continental with oceanic → the denser oceanic plate is subducted beneath the continental plate and melts in the mantle, forming volcanic mountain chains as in the Andes.
- Oceanic with oceanic → one plate sinks under the other, forming deep ocean trenches and volcanic island arcs.
Results of transform movement
Large deformations and transform faults accompanied by violent volcanoes and earthquakes. Examples: the San Andreas fault and the Gulf of Aqaba.
Relation to geological structures
These movements produce the folds and faults already studied: compression produces folds and reverse faults, while tension produces normal faults.