Causes of Earthquakes & Volcanoes (Cambridge (CIE) O Level Geography): Revision Note

Causes of Earthquakes & Volcanic Eruptions

The structure of the Earth

• The Earth is composed of four main layers

  • Inner core - about 1400km in diameter, a solid and dense layer composed of iron and nickel with temperatures of about 5500^oC

  • Outer core - about 2100km thick, a semi-molten metal layer with temperatures between about 5000-5500^oC

  • Mantle - about 2900km thick, a semi-molten layer which is less dense than the outer core 

  • Crust - the thickness varies and it is made up of two types of crust 

Structure of the Earth

Crust

• There are two types of crust

  • Continental crust is thick (25-90km), old and not as dense as oceanic crust

  • Oceanic crust is thinner (5-10km), heavier and denser than continental crust

• 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

Plate tectonics

• The crust is broken into a number of tectonic plates

Tectonic Plates

• These plates move on top of the semi-molten mantle below.

• The movement of the plates is in part due to the convection currents within the mantle

• A plate boundary or margin is where two plates meet

Types of plate boundary

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

• There are four main types of plate boundaries:

  • Divergent (constructive)

  • Convergent (destructive)

  • Collision

  • Transform (conservative)

Divergent (constructive) plate boundary

• At a divergent boundary the plates are moving apart

• The Mid Atlantic Ridge is an example of a divergent plate boundary

• Both volcanic eruptions and earthquakes can occur at this type of plate boundary

Divergent (constructive) plate boundary

Convergent (destructive) plate boundary

• At a convergent (destructive) plate boundary the plates are moving towards each other

• The denser, heavier oceanic plate subducts under the lighter, less dense continental plate

• The boundary between the Nazca plate and the South American plate is an example

• Both volcanic eruptions and earthquakes occur at this type of plate boundary

Convergent (destructive) Plate Boundary

Collision boundary

• At a collision boundary two plates of similar density move towards each other

• Neither is dense enough to subduct, so the land is pushed upwards

• This forms fold mountains such as the Himalayas

• Earthquakes are the main hazard at this type of plate boundary

Collision Boundary

Transform (conservative) boundary

• At a transform (conservative) boundary the plates move passed each other in opposite directions or in the same direction at different speeds

• Earthquakes are the only hazard at this type of boundary

Transform (conservative) Boundary

The Structure of the Earth & Plate Boundaries

• Volcanoes occur at divergent (constructive), convergent (destructive) plate boundaries and hot spots

• Volcanoes do not occur at collision boundaries or transform (conservative) boundaries

Volcanoes at constructive boundaries

• At a divergent (constructive) the tectonic plates are moving away from each other:

  • Divergent plate boundaries often occur under the sea/ocean

  • The lava escapes through the gap left as the plates move apart

  • The lava cools and hardens forming a new crust

• At divergent plate boundaries the lava tends to be runny, and eruptions are less explosive

• These types of eruption form shield volcanoes which have gently sloping sides

Volcanoes at destructive boundaries

• At a destructive (convergent) boundary the tectonic plates are moving towards each other:

  • The heavier, denser oceanic plate subducts under the lighter continental plate

  • In the subduction zone the two plates come together causing friction

  • Friction causes heat and the plate material melts forming magma

  • The magma rises to the surface through cracks in the crust

  • The cooling lava and ash build up forming a volcano

• At destructive plate boundaries the lava tends to be sticky and produces explosive eruptions

• These eruptions tend to form composite or stratovolcanoes 

Volcanoes at 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

Causes of Earthquake Hazards

Earthquakes and plate boundaries

• Earthquakes can occur anywhere but mostly occur at or near plate boundaries

• Earthquakes happen at all plate boundaries - divergent (constructive), convergent (destructive), collision and transform (conservative)

• At a divergent (constructive) plate boundary, earthquakes tend to be weaker as the plates are moving apart

• At convergent (destructive), collision and conservative (transform) plate boundaries earthquakes tend to be stronger