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

Main 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 8 (Tambora 1815)

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

• 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

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

• 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

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

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

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