Causes of Earthquakes & Volcanoes (Cambridge International IGCSE Geography)

Causes of Earthquakes and Volcanic Eruptions

Spec point — 2.1.3 · Describe the causes of earthquakes & volcanic eruptions

Causes of Earthquakes & Volcanic Eruptions

The structure of the Earth

• The Earth is composed of four main layers

  • Inner core: about 1400 km in diameter, a solid and dense layer composed of iron and nickel with temperatures of about 5500 ^ºC
  • Outer core: about 2100 km thick, a semi-molten metal layer with temperatures between about 5000 and 5500 ^ºC
  • Mantle: about 2900 km 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

Earth's structure

The Earth's crust

• There are two types of crust

  • Continental crust is thick (25–90 km), old and not as dense as oceanic crust
  • Oceanic crust is thinner (5-10 km) and heavier and denser than the 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 many tectonic plates

The various 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 past each other in opposite directions or in the same direction at different speeds
• Earthquakes are the only hazard at this type of boundary

Conservative (transform) boundary

Worked Example Study Fig. 1, which is a cross-section through a volcano.

[1 mark]

Fig 1

(i) What type of plate boundary is shown in Fig 1?

Final answer: Answer

• Constructive/divergent [1]

(ii) Choose the correct labels for features X and Y shown in Fig 1.

Choose from the list below:

• convection currents
• direction of plate movement
• lava escaping from the volcano
• new crust created
• subduction

Final answer: Answer

• X = convection currents
• Y = direction of plate movement

Exam Hint Draw each of the plate boundaries and add annotations to outline the processes. This will help you to remember what happens at each one.

Causes of Volcanic Eruptions

Spec point — 2.1.3

Causes of Volcanic Eruptions

• 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) boundary, 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 often 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 can form, e.g. Hawaii

Diagram illustrating the formation of hotspot islands

Causes of Earthquake Hazards

Spec point — 2.1.3

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

Diagram showing the main features of an earthquake