Processes and Features of Earthquakes & Volcanoes (Cambridge (CIE) IGCSE Geography): Revision Note

Plate boundary processes

• Different processes happen at each type of plate boundary

• These processes can lead to earthquakes and the formation of volcanoes

  • As the plates move, earthquakes can occur at any of the boundaries

  • Volcanoes only form at constructive (divergent) and destructive (convergent) boundaries

Constructive (divergent) plate boundary

• At the constructive (divergent) boundary, the plates are moving apart

  • As they move apart, magma rises to fill the gap

  • This leads to the formation of volcanoes and eruptions

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

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

Destructive (convergent) plate boundary

• At a destructive (convergent) plate boundary, the plates are moving together

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

  • This leads to friction and heat, which melts the crust and forms magma

  • Magma rises to the surface through cracks in the crust

  • It erupts on the surface as lava and forms a volcano

• 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

Collision boundary

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

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

  • This process forms fold mountains such as the Himalayas

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

Conservative (transform) boundary

• At a conservative (transform) 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

Earthquake characteristics

• 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

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

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

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

Types and classification of volcano

Types of volcano

• There are three main types of volcanoes:

  • Composite (strato-volcano)

  • Shield volcano

  • Cinder volcano

• The type of volcano which forms depends on the type of lava which erupts

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

Cinder volcano

• Cinder volcanoes are:

  • circular and cone-shaped

  • usually less than 1000 feet high

  • composed of hardened ash, tephra and lava, which is forcefully ejected from the volcano and cools whilst in the air, falling as cinder fragments

Classification of volcanoes

• 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
      

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 7 (Tambora 1815)

Volcanic hazards

• Volcanic eruptions may include a range of features

• Some of these hazards are localised or can have a global impact

Tephra

• Tephra refers to all the material that is ejected during an eruption, including ash and volcanic rocks

• 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

  • The ash fall can cover large areas, disrupting transportation, damaging infrastructure and affecting animal/human health

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

  • These are between 60 mm and 5 m in diameter

Pyroclastic flow

• 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

  • Can cover large areas and lead to total destruction

Lahars

• 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

  • The lahars can cover large areas, destroying infrastructure, buildings and crops

Lava flows

• 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

  • Lava can cover extensive areas leading to the destruction of buildings and infrastructure

Earthquakes

• These 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

Toxic gases

• Gases released during an eruption include carbon dioxide, sulphur dioxide and hydrogen sulphide

• The gases can travel long distances depending on the wind patterns

• They pose a health hazard to humans and animals