Characteristics of the Earth's Structure (Cambridge (CIE) IGCSE Geography) : Revision Note

Structure of the Earth

• The Earth is composed of various layers

  • Inner core: About 1400km in diameter, a solid and dense layer composed of iron and nickel with temperatures of about 5000 - 6000°C

  • Outer core: About 2100km thick, a semi-molten metal layer with temperatures between about 4000 and 6000 °C

  • Mantle: About 2900km thick, a semi-molten layer which is less dense than the outer core this is split into two parts 

    • Asthenosphere

    • Lower mantle

• The lithosphere is made up of two types of crust together with the rigid upper part of the mantle. The thickness varies.

• The two types of crust are:

  • Oceanic

  • Continental

Crust 

• There are two types of crust and they are part of the lithosphere

  • The oceanic crust is thinner (5-10km) but heavier, and denser and is composed mainly of basalt 

  • The Continental crust is thicker (25-100km) but is older and less dense and is composed mainly of granite

• Oceanic crust is continually being created and destroyed as a result of plate movement where it is denser and so subducts under the continental crust

Mantle

• The mantle is composed of the:

  • The upper mantle has two layers

    • The rigid upper layer which is part of the lithosphere 

    • The Asthenosphere is a partially molten plastic-type layer which moves under high pressure and is between 35 and 250 km

  • The lower mantle is solid and the largest of the Earth's layers (2900 km) 

Tectonic plates

• The Earth's crust is broken into 15 large tectonic plates and several smaller ones

• The places where the plates meet are called plate boundaries

Tectonic plate movement

• The heat within the Earth (geothermal), which is estimated to reach between 5 and 6000is generated by radioactive decay

• Heat rises from the Earth's core and creates convection currents in the mantle

  • The tectonic plates which make up the crust (lithosphere) move on top of the semi-molten mantle below

• The current theory is called slab pull theory 

  • This suggests that in addition to the convection currents, plate movement is a result of the weight of the denser oceanic plates subducting and dragging the rest of the plate along

Mantle plumes

• Within the mantle, there are also mantle plumes, which are upwellings of superheated rock

• These can lead to either:

  • Divergent plate boundaries

  • Hot spots located at random points on a tectonic plate

Types of plate boundary

• Volcanic eruptions and earthquakes most commonly occur at or near plate boundaries

Earthquake distribution

• Earthquakes occur at all types of plate boundaries

  • Most (approximately 90%) occur around the rim of the Pacific Ocean

  • This zone is called the 'Ring of Fire'

Volcano distribution

• Most volcanoes occur at divergent (constructive) and convergent (destructive) plate boundaries

• Like earthquakes, the majority of active volcanoes (approximately 75%) are located around the Pacific 'Ring of Fire'

• Volcanoes also occur at hotspots away from plate boundaries and are plumes/columns of magma which escape through the Earth's crust

• There are four main types of plate boundaries:

  • divergent (constructive)

  • convergent (destructive)

  • collision

  • transform (conservative)