Tectonic Plate Movement (DP IB Geography)

Convection Currents

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 

• 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

• 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

Magma Plumes

• 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

Subduction & Rifting

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