Volcanoes (DP IB Geography)

Revision Note

Characteristics of a Volcano

  • 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:

    Diagram illustrating volcano features including crater, ash cloud, volcanic bombs, secondary cone, main vent, layers of ash, lava, and magma chamber.
    Volcano characteristics
  • Volcanic eruptions may include a range of features such as the ones listed in the table below

Features of an eruption

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

Ash is pulverised solid lava that measures less than 2mm in diameter. It is ejected into the atmosphere and can travel thousands of kilometres

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

These are fragments of molten rock that are ejected from the volcano. They are between 60mm and 5m in diameter

  • Due to the different types of eruption and lava, the formation of a volcano varies

Composite volcanoes

  • 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

      Diagram of a composite (strato) volcano showing labeled parts: layers of ash and lava, main vent, steep slopes, and magma chamber.
      Composite volcano

Shield volcanoes

  • 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

Diagram of a shield volcano showing gentle sloping sides from runny lava, layers of cooled lava, a main vent, and a magma chamber beneath the surface.
Shield volcano

Cinder volcanoes

  • Cinder volcanoes are:

    • Circular and cone shaped

    • Usually less than 1000 feet high

    • Composed of hardened ash, tephra and lava

      • The lava is forcefully ejected from the volcano and cools whilst in the air, falling as cinder fragments

Secondary Hazards of Volcanoes

  • There are a range of secondary hazards associated with volcanic eruptions

Illustration showing a volcanic eruption with labeled effects: acid rain, ash fallout, ash cloud, pyroclastic flow, lahar, landslide, and earthquakes.
Volcanic hazards
  • Lahars – mudflows that occur when tephra mixes with water, either from rainfall or from melted snow and ice

    • They are fast-flowing and destroy everything in their path

    • E.g. the lahars that occurred when Mount Pinatubo erupted in the Philippines in 1991 caused extensive damage and disruption

    • Lahars are usually associated with composite volcanoes

Map showing lahar paths from Mount Pinatubo to the South China Sea and Manila Bay, with Angeles City marked nearby. Legend indicates lahars in brown.
Map showing the extent of lahars in an eruption of Mount Pinatubo
  • Pyroclastic flow – a super-heated mixture of gas and tephra that flows at speeds of up to 700km per hour:

    • This is one of the deadliest volcanic hazards as the pyroclastic flow can travel long distances and destroy everything in its path

    • E.g. when Fuego volcano erupted in Guatemala in 2018, pyroclastic flows destroyed several nearby towns

    • Pyroclastic flows are usually associated with composite volcanoes

  • Landslides the mass movement of materials down a slope occurring when the rock is no longer able to resist the force of gravity:

  • This can occur during volcanic eruptions due to the movement of the crust

  • Landslides can also cause a tsunami if the debris falls into the sea

Examiner Tip

You may be asked in the exam to examine the relationship between secondary hazards and the type of volcano.  For example, pyroclastic flows are usually the result of highly explosive eruptions from composite volcanoes.

Last updated:

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Bridgette Barrett

Author: Bridgette Barrett

Expertise: Geography Lead

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.

Jenna Quinn

Author: Jenna Quinn

Expertise: Head of New Subjects

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.