Cambridge IGCSE0680

Earthquakes and volcanoes

Environmental Management 0680 Chapter Notes

What this chapter covers

Earthquakes and volcanoesTropical cyclonesFloodingDroughtThe impacts of natural hazardsManaging the impacts of natural hazardsOpportunities presented by natural hazards
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In this chapter

  1. 1The Earth's Structure and Plate Tectonics
  2. 2Plate Boundaries: Where Plates Meet
  3. 3Earthquakes: Causes and Measurement
  4. 4Volcanoes: Formation and Features
  5. 5Managing Tectonic Hazards

1. The Earth's Structure and Plate Tectonics

The Earth is made of four main layers: the inner core, outer core, mantle, and crust. The crust and the very top part of the mantle form a rigid layer called the lithosphere. This isn't one solid piece; it's broken up into massive slabs called tectonic plates. These plates 'float' on the semi-molten layer below, the asthenosphere. The immense heat from the Earth's core generates convection currents in the mantle. These currents are slow-moving circular motions that drag the plates above them, causing them to move, separate, and collide. The vast majority of the world's earthquakes and volcanoes occur along the edges of these plates, known as plate boundaries, where all the action happens.

Key term

Plate Tectonics: The theory that the Earth's outer shell is divided into several large, slowly moving plates whose interactions cause earthquakes and volcanic activity.

Examiner insight

Marks are often awarded for clearly linking the mechanism of convection currents in the mantle to the movement of tectonic plates on the surface.

Fun fact

The Pacific Plate is moving northwest at about 5 to 10 cm per year – roughly the same speed that your fingernails grow.

Worked example 14 marks

Using Figure 6.3 (a map of tectonic plates), describe the global distribution of earthquakes and volcanoes. [4 marks]

  1. 1

    Step 1: Identify the general pattern. State that the distribution is not random; earthquakes and volcanoes are concentrated in specific areas.

  2. 2

    Step 2: Link the pattern to plate boundaries. Explain that most occur in narrow bands along the margins of tectonic plates. A good example is the 'Pacific Ring of Fire'.

  3. 3

    Step 3: Provide a specific example of this link. For instance, mention the line of volcanoes and earthquakes along the west coast of South America, where the Nazca Plate meets the South American Plate.

  4. 4

    Step 4: Mention exceptions. Note that some volcanoes (like those in Hawaii) occur away from plate boundaries over 'hotspots', and some earthquakes can occur along old fault lines within plates.

Recap

  • The Earth's lithosphere is broken into tectonic plates.
  • Convection currents in the mantle are the driving force behind plate movement.
  • Most earthquakes and volcanoes are found at plate boundaries.
  • The Pacific Ring of Fire is the world's most active zone for earthquakes and volcanoes.

Quick check

  1. What is the name of the rigid outer layer of the Earth composed of the crust and upper mantle?1 mark
  2. What process in the mantle causes tectonic plates to move?1 mark

2. Plate Boundaries: Where Plates Meet

Where tectonic plates interact, we get different types of boundaries, each with unique features and hazards. There are three main types:

  1. Destructive (Convergent): Two plates move towards each other. If an oceanic plate meets a continental plate, the denser oceanic plate is forced down (subducted) into the mantle, forming an ocean trench and explosive composite volcanoes. This friction also causes powerful earthquakes. If two continental plates collide, neither can subduct, so they buckle and push upwards to form huge fold mountains, like the Himalayas.
  2. Constructive (Divergent): Two plates move away from each other. Magma from the mantle rises to fill the gap, creating new crust. This happens at mid-ocean ridges (like the Mid-Atlantic Ridge) and can form shield volcanoes. Earthquakes here are frequent but less powerful.
  3. Conservative (Transform): Two plates slide past each other, either in different directions or at different speeds. No crust is created or destroyed. The plates can get stuck, building up immense pressure. When they suddenly slip, it releases a huge amount of energy as a powerful earthquake. The San Andreas Fault in California is a famous example.

Key term

Subduction Zone: A region where one tectonic plate is forced to sink beneath another at a destructive boundary, often resulting in strong earthquakes and explosive volcanoes.

Common pitfall

Students frequently confuse collision boundaries (continental-continental) with subduction boundaries (oceanic-continental). Collision creates fold mountains, while subduction creates volcanoes and trenches.

Worked example 16 marks

Explain why both earthquakes and volcanoes occur along destructive plate boundaries, such as where the Nazca Plate meets the South American Plate. [6 marks]

  1. 1

    Step 1: Identify the plate boundary type and movement. State that this is a destructive boundary where the oceanic Nazca plate moves towards the continental South American plate.

  2. 2

    Step 2: Explain volcano formation. The denser Nazca plate is subducted (forced under) the South American plate. The heat and pressure cause the oceanic plate to melt.

  3. 3

    Step 3: Continue the volcano explanation. This molten rock (magma) is less dense than the surrounding mantle, so it rises through cracks in the continental plate and erupts at the surface to form explosive composite volcanoes, creating the Andes mountains.

  4. 4

    Step 4: Explain earthquake formation. As the Nazca plate subducts, it doesn't slide smoothly. It gets stuck due to friction.

  5. 5

    Step 5: Continue the earthquake explanation. Stress and pressure build up over time. When the pressure is too great, the plates jolt past each other, releasing a huge amount of energy as seismic waves, causing a powerful earthquake.

  6. 6

    Step 6: Conclude by linking both processes. Therefore, the process of subduction at this destructive boundary is responsible for both volcanic activity and major earthquakes.

Recap

  • Destructive boundaries involve plates moving together, causing volcanoes and strong earthquakes.
  • Constructive boundaries involve plates moving apart, creating new crust and shield volcanoes.
  • Conservative boundaries involve plates sliding past each other, causing powerful earthquakes but no volcanoes.
  • Fold mountains form at continental-continental collision boundaries.
  • Ocean trenches and composite volcanoes form at oceanic-continental subduction boundaries.

Quick check

  1. What type of plate boundary creates the Himalayas?1 mark
  2. At which type of plate boundary is crust neither created nor destroyed?1 mark

3. Earthquakes: Causes and Measurement

An earthquake is the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere. This usually happens when tectonic plates get stuck at a fault line. As the plates continue to try to move, strain builds up like a stretched elastic band. When the strain overcomes the friction holding them, the plates suddenly slip, releasing the stored energy as seismic waves. The point inside the crust where the pressure is released is called the focus. The point on the surface directly above the focus is the epicentre, which is where the strongest shaking is felt. The energy released is measured by its magnitude on the Richter scale, which is logarithmic. This means a magnitude 7 earthquake is 10 times more powerful than a magnitude 6, and 100 times more powerful than a magnitude 5.

Key term

Epicentre: The point on the Earth's surface directly above the focus, where the earthquake's shaking is typically strongest.

Examiner insight

High-scoring answers differentiate between magnitude (Richter scale - energy released) and intensity (Mercalli scale - observed effects and damage).

Worked example 14 marks

The 2015 Nepal earthquake had a magnitude of 7.8, while the 2011 Japan earthquake had a magnitude of 9.0. The Japan earthquake caused 21,000 deaths, while the Nepal earthquake caused 9,000. Explain two reasons why a higher magnitude earthquake might not always cause more deaths. [4 marks]

  1. 1

    Step 1: Identify a relevant factor. One key factor is the level of economic development and preparation. Japan is a high-income country (HIC) with strict building codes and advanced management strategies.

  2. 2

    Step 2: Explain the first factor. Japan's earthquake-resistant buildings, tsunami warning systems, and well-drilled population helped to reduce the death toll despite the quake's immense power. Nepal is a low-income country (LIC) with less stringent building codes and fewer resources for disaster management, making its population more vulnerable.

  3. 3

    Step 2: Identify a second factor. Another factor is the location of the epicentre and secondary hazards. The Japan earthquake occurred offshore, triggering a devastating tsunami which caused most of the deaths. The Nepal earthquake was inland, with major impacts from landslides in mountainous terrain.

  4. 4

    Step 4: Explain the second factor. The nature of the secondary hazard (tsunami vs landslide) and the preparedness for it can be more critical than the initial magnitude in determining the final death toll. Other factors like population density near the epicentre and time of day also play a role.

Recap

  • Earthquakes are caused by the sudden release of pressure at a fault line.
  • The focus is the origin point underground; the epicentre is the point on the surface above it.
  • Seismic waves carry the energy released during an earthquake.
  • The Richter scale measures magnitude and is logarithmic.
  • Earthquake impact depends on magnitude, depth, development level, and population density.

Quick check

  1. What is the difference between the focus and the epicentre of an earthquake?2 marks

4. Volcanoes: Formation and Features

A volcano is a vent or opening in the Earth's crust through which magma, volcanic ash, and gases escape. There are two main types of volcano, determined by the type of lava they erupt.

  1. Shield Volcanoes: Found at constructive boundaries or hotspots (like Hawaii). They erupt thin, runny basaltic lava which can flow for long distances before cooling. This creates wide volcanoes with gentle, low-angle slopes. Eruptions are frequent but gentle and effusive (non-explosive).
  2. Composite Volcanoes (or Stratovolcanoes): Found at destructive (subduction) boundaries. They erupt thick, viscous andesitic lava that doesn't flow far. They are built up from alternating layers of sticky lava and ash from explosive eruptions. This creates classic tall, steep-sided, cone-shaped volcanoes (like Mount Fuji). The viscous magma can trap gas, leading to a huge build-up of pressure and violent, dangerous eruptions.

Key term

Composite Volcano: A tall, cone-shaped volcano built up of alternate layers of lava and ash, typically found at destructive plate boundaries and known for explosive eruptions.

Common pitfall

A common mistake is to assume all volcanoes are steep, explosive cones. Students must remember shield volcanoes, which have gentle slopes and effusive eruptions.

Fun fact

The largest volcano in our solar system is Olympus Mons on Mars. It's a shield volcano that's nearly 25 km high, almost three times the height of Mount Everest.

Worked example 16 marks

Draw a simple, labelled cross-section diagram of a composite volcano and explain how its characteristics are formed. [6 marks]

  1. 1

    Step 1: Draw the diagram. Sketch a tall, steep-sided cone shape. Label the magma chamber at the base, a central vent/conduit leading to the top, and a crater at the summit. Show alternating layers of 'ash' and 'lava' making up the cone. You could also add a secondary cone on the side.

  2. 2

    Step 2: Explain the shape. State that the steep sides are due to the eruption of thick, viscous (sticky) lava that cools and solidifies quickly, not travelling far from the vent.

  3. 3

    Step 3: Explain the layers. Explain that the name 'composite' or 'strato' comes from the fact it is built of alternating layers (strata). These layers are formed by successive eruptions of ash and then lava.

  4. 4

    Step 4: Explain the eruption style. Link the viscous lava to explosive eruptions. The thick magma traps gases, causing pressure to build up in the magma chamber, which is then released in a violent explosion that produces ash clouds, followed by lava flows.

  5. 5

    Step 5: Link to plate tectonics. Mention that these volcanoes are typically formed at destructive (subduction) plate boundaries where melting oceanic crust creates this type of andesitic magma.

Recap

  • Shield volcanoes have gentle slopes, runny lava, and are found at constructive boundaries.
  • Composite volcanoes have steep sides, viscous lava, and are found at destructive boundaries.
  • The viscosity of lava determines the shape of the volcano and the style of eruption.
  • Composite volcanoes are built from alternating layers of ash and lava.
  • Explosive eruptions occur when viscous magma traps high-pressure gas.

Quick check

  1. Which type of lava, basaltic or andesitic, forms a shield volcano?1 mark
  2. Name one example of a composite volcano.1 mark

5. Managing Tectonic Hazards

Since we cannot prevent earthquakes or volcanic eruptions, we focus on managing their impacts through mitigation. Strategies can be grouped into the '3 Ps': Prediction, Protection, and Preparation. Prediction: This involves monitoring the hazard to try and forecast when it will happen. For volcanoes, scientists monitor for signs like ground swelling, gas emissions (especially sulfur dioxide), and small earthquakes (tremors) around the volcano. Predicting earthquakes is much harder, but scientists can identify high-risk areas based on plate tectonics and historical data. Protection: This involves designing buildings and infrastructure to withstand the hazard. For earthquakes, this includes using reinforced steel frames, shock absorbers at the base of buildings, and automatic window shutters. For volcanoes, protection is more difficult, but can involve building earth barriers to divert lava flows or strengthening roofs to withstand the weight of ash. Preparation: This is about planning for what to do if a hazard strikes. It includes creating evacuation plans, educating the public on what to do (e.g., 'drop, cover, hold on' for earthquakes), setting up warning systems (e.g., tsunami sirens), and stockpiling emergency supplies like water, food, and medicine.

Key term

Mitigation: Actions taken to reduce or lessen the severity, seriousness, or painfulness of the impacts of a hazard.

Examiner insight

Examiners look for a balanced answer that considers a range of management strategies and acknowledges that their effectiveness varies depending on the country's level of development.

Worked example 16 marks

For a named earthquake you have studied, explain the management strategies used to reduce the impact. [6 marks]

  1. 1

    Step 1: Name the case study. For example, Christchurch, New Zealand (2011).

  2. 2

    Step 2: Describe a 'Protection' strategy. New Zealand has strict building codes. After the 2010 quake, many buildings were being strengthened. Post-2011, the Earthquake Recovery Authority (ERA) has enforced even higher standards for rebuilding, ensuring new structures can better withstand shaking.

  3. 3

    Step 3: Describe a 'Preparation' strategy. New Zealand has a well-developed civil defence system. Public education campaigns ensure residents know what to do. Emergency services are well-funded and trained for disaster response, which was seen in the rapid deployment of rescue teams and provision of aid like bottled water and chemical toilets.

  4. 4

    Step 4: Describe a long-term 'Preparation'/'Protection' strategy. The ERA also implemented land-use zoning. Areas worst affected by liquefaction were designated 'red zones', and rebuilding was forbidden. This long-term planning aims to move people and critical infrastructure away from the most hazardous areas.

  5. 5

    Step 5: Link the strategies. These strategies work together. Strong buildings (protection) reduce collapse, while public awareness (preparation) ensures people react correctly, saving lives.

  6. 6

    Step 6: Conclude on effectiveness. Although 185 people died, the toll would have been far higher without these mitigation strategies, demonstrating their importance in a high-risk, high-income country.

Recap

  • Tectonic hazard management involves prediction, protection, and preparation.
  • Predicting volcanoes is more reliable than predicting earthquakes.
  • Protection involves engineering solutions like earthquake-proof buildings.
  • Preparation includes evacuation plans, education, and emergency drills.
  • The effectiveness of management strategies often depends on a country's wealth and governance.

Quick check

  1. Give one example of a 'Protection' strategy against earthquakes.1 mark
  2. Monitoring gas emissions is a method used to predict which type of hazard?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Describe the global distribution of earthquakes and volcanoes. [4 marks]4 marks
  2. Explain the formation of a composite volcano. You may use a diagram in your answer. [6 marks]6 marks
  3. Compare the causes of earthquakes at destructive and conservative plate boundaries. [4 marks]4 marks
  4. Using named examples, explain why some earthquakes with a high magnitude cause fewer deaths than others with a lower magnitude. [8 marks]8 marks
  5. Distinguish between a shield volcano and a composite volcano in terms of their shape, lava type, and eruption style. [6 marks]6 marks
  6. Explain how convection currents in the mantle cause tectonic plates to move. [3 marks]3 marks
  7. Define the terms 'focus' and 'epicentre'. [2 marks]2 marks
  8. Describe three different methods used to manage the impacts of volcanic eruptions. [6 marks]6 marks
  9. Explain why living near a volcano can present opportunities as well as threats. [6 marks]6 marks
  10. For a named earthquake event, describe the primary and secondary impacts. [6 marks]6 marks

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