Main Features of Earthquakes & Volcanoes (Cambridge International IGCSE Geography)

Main Features of Volcanoes

Spec point — 2.1.1 · Describe the main types & features of volcanoes

• A volcano forms when magma erupts onto the Earth's surface as lava through a vent in the Earth's crust

  • When magma erupts onto the surface, it is called lava

• 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 7 (Tambora 1815)

• All volcanoes have common features, as shown in the diagram below:

Common features of a volcano

• The type of volcano which forms depends on the type of lava which erupts
• Composite and shield volcanoes are two of the most common forms of volcanoes

Composite (strato-volcano)

• Composite volcanoes, also known as strato-volcano, have:

  • steep-sides
  • sticky (viscous) lava
  • more explosive eruptions
  • alternating layers of ash and lava

• They tend to form on convergent (destructive) plate boundaries

Composite (strato) volcano

Shield volcanoes

• Shield volcanoes have:

  • gently sloping sides
  • runny/thin lava
  • less explosive—gentle eruptions
  • frequent eruptions

• They tend to form on divergent (constructive) plate boundaries or hot spots

Shield volcano

• Volcanoes may be active, dormant or extinct

  • Active

    • The volcano has recently erupted and is likely to erupt again

  • Dormant

    • Has not erupted for many years but there is evidence of a magma reservoir

  • 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

• 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 is pulverised solid lava which measures less than 2 mm in diameter

  • Ash is ejected into the atmosphere and can travel thousands of kilometres

• Pyroclastic flow is fast-moving, very hot clouds of poisonous gases mixed with ash

  • Average speeds of about 100 km/h but can move at up to 700 km/h

• Lahars occur when volcanoes erupt and snow and ice on the peak melt

  • The meltwater combines with the ash
  • This creates fast-moving mudflows or lahars

• Earthquakes are caused by magma rising to the surface through the vents in the volcano

  • This increases pressure on the Earth's crust, leading to earth tremors

• Volcanic bombs are fragments of molten rock which are ejected from the volcano

  • These are between 60 mm and 5 m in diameter

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

Describe two characteristics of a shield volcano.

[2 marks]

Final answer: Answer

Any two of the following:

• Fluid/thin/runny lava [1]
• Low (height to width ratio)[1]
• Gentle slopes [1]
• Gentle eruption [1]
• Frequent eruption [1]

Main Features of Earthquakes

Spec point — 2.1.1 · Describe the main types & 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

Where do earthquakes happen?

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

• Earthquakes happen at all plate boundaries:

  • constructive (divergent)
  • destructive (convergent)
  • collision
  • 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

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 that is the correct definition of an earthquake.

[1 mark]

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

Final answer: Answer

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

Exam Hint When describing the processes that 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.