Plate tectonics theory

• Early world mapping of continents, mountain ranges and volcanoes, as well as fossil evidence, led Alfred Wegener to suggest, in 1912, that the continents were once joined together and has gradually floated apart
• 20th century seabed mapping provided more evidence for Wegener's continental drift and the development of modern plate tectonic theory
• The Earth's crust is divided into solid tectonic plates up to 100km thick
• There are 2 types of tectornic plate - relatively recently formed, dense thin, basaltic oceanic crust. And older, less dense, thicker, granitic continental crust
• Plates separate (diverge) at constructive margins, collide (converge) at destructive margins and slide by each other at conservative margins
• It is the movement of the plates relative to each other that explains the major features of the Earth's surface (mountain chains, ocean trenches...) and the formation of earthquakes and volcanoes
• Tectonic plates move between 2.5cm to 16cm a year, driven by convection currents and gravitational sliding
• Sinking convection currents coincide with destructive (convergent) margins where one plate subducts below another
• Spreading convection currents explain the presence of rising magma at constructive (divergent) margins

  
• Sinking convection currents coincide with destructive (convergent) margins where one plate subducts below another

  
• Gravitational sliding is more important. Ridge push, at divergent margins, is caused by rising magma 'pushing' the older part of the plate sideways

  
• Slab pull, at convergent margins, is caused by gravity pulling the thick, dense plates downwards