Tectonic Plate Movement (DP IB Geography)

Revision Note

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 

Diagram illustrating Earth's layers: continental and oceanic crust, mantle, asthenosphere, lower mantle, outer core, and inner core. Depths and key terms are labeled.
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

A world map highlighting tectonic plates including Eurasian, North American, South American, African, Australian, Pacific, and others, with the equator marked.
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

Cross-section of Earth's layers with labels: inner core, outer core, mantle, asthenosphere, and lithosphere. Shows constructive and destructive plate boundaries and slab pull.
Diagram of convection currents

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

Diagram showing the Hawaiian Hotspot and Pacific Plate movement beneath islands with progressively older volcanoes from Hawai'i to Ni'ihau.
Intra-plate volcano formation

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

Examiner Tip

Make sure you can explain the contribution of each of the main processes that drive the movement of tectonic plates. Remember, it is a combination of all of these factors that cause the plates to move, leading to tectonic hazards.

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