Plate tectonics is the theory explaining the movemement , formation anddestruction of the plates that make up the earths crust.

The Crust

The crust forms the outer surface of the Earth. The crust:

• Is cool enough to behave like a rigid shell
• Is upto 90km thick (compared with the total radius of the earth of 6400km)
• Floats on hotter, semi molten rock (the mantle) below
• Is sub-divided into plates: seven large plates, and 12 smaller plates

Plates

There are two types of plate:

• Oceanic
• Continental

Oceanic plates are:

• Composed of basaltic rock
• 5-10km thick
• Denser than continental plates
• Frequently destroyed beneath the continental plates
• Younger than continental plates

Continental plates are:

• Mainly composed of granite
• 25-90km thick
• less dense and therefore can float on the top of oceanic plates

The plates are moved slowlyby the convection currents in the mantle.

Plate Magins

Were two plates meet is called a plate margin. Plates move in different directions and at different speeds, which causes stress and friction along the plate margins. Stress and friction are mostly responsible for:

• Earthquakes and volcanic activity
• The creation of mountains

Plate margins can be divided into 3 types:

1. Destructive Margins (or conpretional or convergant margins) either with subduction zoes or collision zones.
2. Constructive Margins (or extensional or divergant margins)
3. Conservative Margins (or tensional or transform margins)

You can read more into the different plate margins by clicking here.
This Dynamic Earth is a fantastic publication explaining the perspectives and developments of the theory, to access the online version click here.