IGCSEGeographyCIERevision Notes2. The Natural Environment2.1 Earthquakes & Volcanoes2.1.1 Main Features of Earthquakes & Volcanoes

Main Features of Earthquakes & Volcanoes (CIE IGCSE Geography)

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Main Features of Volcanoes

  • A volcano is formed when magma erupts onto the Earth's surface as lava through a vent in the Earth's crust
  • The magnitude of a volcanic eruption is measured on the Volcanic Explosivity Index (VEI)
    • The scale is open ended but the highest in recorded human history was an 8 (Tambora 1815)
  • All volcanoes have features in common, as shown in the diagram below:

features-of-a-volcano

  • Due to the type of lava erupted the formation of a volcano varies
  • Composite (strato-volcanoes) 
    • Steep-sided
    • Sticky (viscous) lava 
    • More explosive eruptions
    • Formed from alternating layers of ash and lava
    • Tend to form on convergent (destructive) plate boundaries

F8U9n2SN_composite-strato-volcano

  • Shield volcanoes
    • Gently sloping sides
    • Runny/thin lava
    • Less explosive - gentle eruptions
    • Tend to form on divergent (constructive) plate boundaries or hot spots
    • Frequent eruptions

shield-volcano

  • Volcanoes may be active, dormant or extinct
    • Active
      • The volcano has recently erupted and is likely to erupt again
    • Dormant
      • Has erupted in the last 2000 years and may possibly erupt again
    • Extinct
      • Shows no evidence of eruption in historic times and there is no evidence of a magma reservoir

Volcanic eruption features

  • Eruptions may include a range of features as shown in the table below:

Feature Characteristics
Lava When magma erupts to the surface it is known as lava. The lava can be thin and runny or thick and slow moving. This depends on the composition of the magma
Ash Pulverised solid lava which measures less than 2mm in diameter. Ash is ejected into the atmosphere and can travel thousands of kilometres
Pyroclastic flow Fast moving, very hot clouds of poisonous gases mixed with ash. Average speeds of about 100km/h but can move at up to 700km/h
Lahars When volcanoes erupt snow and ice on the peak melts and combines with the ash. This creates fast moving mudflows or lahars
Earthquakes Magma rising to the surface through the vents in the volcano increases pressure on the Earth's crust leading to earth tremors
Volcanic bombs Fragments of molten rock which are ejected from the volcano. These are between 60mm and 5m in diameter

Worked example

Shield volcanoes are often formed on constructive (divergent) plate boundaries.

Describe two characteristics of a shield volcano.

[2]

  • Answer: Any two of the following
    • Fluid/thin/runny lava [1]
    • Low (height to width ratio) [1]
    • Gentle slopes [1]
    • Gentle eruption [1]
    • Frequent eruptions [1]

Main Features of Earthquakes

  • An earthquake is the sudden, violent shaking of the ground
  • Earthquakes are the result of pressure building when tectonic plates move
  • The epicentre is the point on the Earth's surface directly above the focus
  • The focus is the point at which the earthquake starts below the Earth's surface
  • The magnitude (amount of energy released) by earthquakes is measured on the Moment Magnitude Scale which replaced the Richter scale
  • The damage caused by earthquakes is measured on the Mercalli Scale
  • Earthquakes can occur anywhere but mostly occur at or near plate boundaries
  • Earthquakes happen at all plate boundaries - constructive (divergent), destructive (convergent), collision and conservative (transform)
  • At a constructive (divergent) plate boundary, earthquakes tend to be weaker as the plates are moving apart
  • At destructive (convergent), collision and conservative (transform) plate boundaries earthquakes tend to be stronger

Earthquake sequence

  • The sequence of an earthquake is the same regardless of the boundary at which it happens:
    • As the tectonic plates move, they can get stuck
    • Pressure builds as the plates continue to try to move
    • Eventually the plates jolt free, and the pressure is released as energy
    • The point at which the earthquake starts is the focus
    • The epicentre is the point directly above the focus on the earth's surface
    • The energy passes through the Earth's crust as waves which is the earthquake

O1GttXaQ_features-of-an-earthquake

Features of an Earthquake

  • Earthquakes can also happen as a result of human activity such as drilling into the crust or mining

Worked example

Tick the one statement in the table below which is the correct definition of an earthquake.

[1]

A crack in the earth's crust caused by a volcano  
A vent between two plates of the earth's crust through which molten magma escapes  
Movement of large slabs of rock in the mantle  
The shaking of the ground caused by plate movement  

  • Answer:
    • The shaking of the ground caused by plate movement [1]

Exam Tip

When describing the processes which lead to an earthquake or volcanic eruption, it is helpful to write the formation down as a sequence of steps. This will make the process easier to remember.

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    Bridgette

    Author: Bridgette

    After graduating with a degree in Geography, Bridgette completed a PGCE over 25 years ago. She later gained an MA Learning, Technology and Education from the University of Nottingham focussing on online learning. At a time when the study of geography has never been more important, Bridgette is passionate about creating content which supports students in achieving their potential in geography and builds their confidence.