Tectonic Plate Movement (SL IB Geography)

Earth’s structure

• The Earth is composed of layers:
• Inner core: About 1400km in diameter, a solid and dense layer composed of iron and nickel with temperatures of about 5500°C
• Outer core: About 2100km thick, a semi-molten metal layer with temperatures between about 5000–5500°C
• Mantle: About 2900km thick, a semi-molten layer that is less dense than the outer core 
• The upper mantle has two layers:
• The rigid layer above the asthenosphere, which, together with the crust, makes up the lithosphere
• The asthenosphere is a semi-molten, plastic-type layer, which moves under high pressure
• Crust: The thickness varies, and is made up of two types of crust 

Earth's structure

• The Mohorovičić discontinuity – also known as Moho – is the boundary between the crust and the mantle

Crust

• There are two types of crust:
• The oceanic crust, which is thinner (5–10km) but heavier and denser
• The continental crust, which is thicker (25–90km) but is older and lighter
• Oceanic crust is continually being created and destroyed as a result of plate movement:
• The oceanic crust is denser and so subducts under the continental crust
• The continental crust is not destroyed and so is much older than the oceanic crust

Plate tectonics

• The crust is broken into several tectonic plates

Distribution of the major tectonic plates

• The tectonic plates move on top of the semi-molten mantle below
• A plate boundary or margin is where two plates meet

Convection currents

• In the past, the theory of convection currents was used on its own to explain tectonic plate movement
• The heat from radioactive decay in the core moves upwards into the mantle, creating convection currents
• More recently there are the ridge push and slab pull theories:
• Ridge push theorises that, as new crust is formed, it is pushed up into the spreading mid-ocean ridge, which forces them further apart
• Slab pull theory suggests that movement is the result of the weight of the denser oceanic plates subducting and dragging the remainder of the plate along

Diagram of convection currents

• Intra-plate volcanism is when volcanoes occur away from plate boundaries
• These are also known as hot spots
• At a hot spot the tectonic plate passes over a plume of magma:
• The magma rises to the surface through cracks in the crust
• As the tectonic plate moves slowly over the magma plume, a line of islands may form e.g. Hawaii

Intra-plate volcano formation

Subduction

• Convection currents in the mantle drag the overlying lithosphere towards each other
• A subduction zone is formed when two plates meet and the heavier, denser plate subducts under the lighter, less dense plate:
• As the oceanic crust cools, it becomes denser and heavier, and gravity forces the lithosphere down into the subduction zone 
• As it sinks, it drags or pulls the plate with it
• This is known as slab pull

Rifting

• Rift zones are areas where the plates are moving apart from each other
• Most rift zones are beneath the oceans but a few are found on land, such as the Central African Rift
• As the plates move apart, magma rises to fill the space