1.2A and B

• Plate Tectonics

  Earth's tectonic plates move at a speed of 2-5 cm per year. 

  There are seven very large plates (e.g. African, Pacific) and smaller minor plates (Nazca, Philippine Sea) and dozens of small microplates. 

  They all fit together in a constantly moving jigsaw of rigid lithosphere. 

  Each plate is about 100 km thick. (With thinner oceanic/continental crust on top.)

  Its lower part consists of upper mantle material whilst its upper part is either oceanic or continental crust. 

  It's still a theory because scientists have not yet directly observed the interior of the Earth. 
• Key Discoveries

  Alfred Wegener's Continental Drift hypothesis in 1912 that postulated that now-separate continents had once been joined. 

  The ideas of Arthur Holmes in the 1930s that Earth's internal radioactive heat was that driving force of mantle convection that could move tectonic plates.

  The discovery in 1960 of the asthenosphere, a weak, deformable layer beneath the rigid lithosphere, on which the latter moves.

  The discovery in the 1960s of magnetic strips in the oceanic crust of the sea bed; these are paleomagnetic signals from past reversals of the Earth's magnetic field and prove that new ocean crust is created by the process of sea-floor spreading at mid-ocean ridges. (seafloor spreading and palaeomagnetism occur at constructive margins, where new crust is being made)

  The recognition of transform faults by Tuzo Wilson in 1965. 
• Subduction: the process of one plate sinking beneath another at a convergent plate boundary. Earthquakes at subduction zones occur at a range of focal depths from 10 km to 400 km, following the line of the subducting plate. 

  ​These occur at destructive margins, where an oceanic plate subducts under a continental plate. 
• Gravitational sliding: Constructive margins have elevated altitudes because of the rising heat between them, which creates a 'slope' down which oceanic plates slide. This occurs at destructive margins. 
• Slab pull: Cold, dense oceanic plate is subducted beneath less dense continental plate; the density of the oceanic plate pulls itself into the mantle - this is slab pull.  This occurs at destructive margins. 
• Mantle Convection

  All tectonic hazards are caused by the Earth's internal heat engine.

  Radioactive isotopes such as uranium-238 and thorium-232 in the Earth's core and mantle generate huge amounts of heat which flow towards the Earth's surface. 

  This heat flow generates convection currents in the plastic mantle. 

  The interior of the Earth is therefore dynamic rather than static. 
• Most tectonic hazards occur at or near tectonic plate boundaries. These represent the locations of ascending (divergent (constructive) plate boundaries) and descending (convergent (destructive and collision) plate boundaries) arms of mantle convection cells. 
• The Earth's Structure:

  Crust

  Mantle - a solid, but is deformable and capable of a very slow flow because of the very high temperatures. 

  Outer core - liquid

  Inner core - solid   
• ​Transform Zones

  Conservative plate boundaries consist of transform faults. These faults 'join up' sections of constructive plate boundary as they traverse the Earth's surface in a zig-zag pattern.

  ​In some locations, long transform faults act like a boundary in their own right, most famously in California where the fault zone (including the San Andreas fault) creates an area of frequent earthquake activity. 
• Earthquakes along conservative boundaries often have shallow focal depths, meaning high magnitude earthquakes can be very destructive. Volcanic activity is absent.