Tectonic Landscapes and Hazards

What you'll learn

This topic examines the Earth's structure, plate tectonic theory, and the formation of hazards including earthquakes and volcanic eruptions. You'll study how tectonic processes create distinctive landscapes, the impacts of tectonic hazards on people and places, and how societies respond to and manage these risks. The WJEC specification requires knowledge of named examples from contrasting economic contexts.

Key terms and definitions

Plate tectonics — the theory that Earth's lithosphere is divided into large slabs (plates) that move slowly over the asthenosphere, driven by convection currents in the mantle.

Conservative plate boundary — where two plates slide horizontally past each other; crust is neither created nor destroyed, but friction causes earthquakes (e.g. San Andreas Fault, California).

Subduction zone — the area where an oceanic plate is forced beneath a continental or another oceanic plate at a destructive boundary, creating deep ocean trenches and volcanic activity.

Primary effects — the immediate impacts of a tectonic hazard, directly caused by the event itself (e.g. buildings collapsing from ground shaking, lava flows destroying property).

Secondary effects — the after-effects of a tectonic hazard that occur later as indirect consequences (e.g. disease outbreaks, economic recession, unemployment).

Magnitude — the measurement of energy released by an earthquake, typically recorded on the Moment Magnitude Scale (MMS) or Richter Scale.

Epicentre — the point on the Earth's surface directly above the focus where an earthquake's effects are usually strongest.

Viscosity — the thickness or stickiness of magma/lava; affects eruption style (low viscosity = runny basaltic lava; high viscosity = thick rhyolitic lava with explosive eruptions).

Core concepts

Structure of the Earth

The Earth consists of distinct layers with different properties:

Inner core (solid) — extremely hot (5500°C), composed mainly of iron and nickel under immense pressure. Approximately 1200 km radius.

Outer core (liquid) — molten iron and nickel, temperatures between 4000-5000°C. Approximately 2300 km thick. Movement here generates Earth's magnetic field.

Mantle — the thickest layer (2900 km), divided into lower (solid) and upper sections. The upper mantle includes the asthenosphere (semi-molten, plastic-like), where convection currents occur.

Lithosphere — Earth's rigid outer shell, comprising the crust and uppermost mantle. Broken into tectonic plates (approximately 100 km thick).

Crust — two types:

• Continental: thicker (30-70 km), older, less dense, mainly granite
• Oceanic: thinner (5-10 km), younger, more dense, mainly basalt

Plate boundaries and tectonic processes

Constructive (divergent) boundaries

Plates move apart, allowing magma to rise and create new oceanic crust. Characteristics:

• Effusive volcanic eruptions with low viscosity basaltic lava
• Frequent but low magnitude earthquakes (shallow focus)
• Formation of mid-ocean ridges and rift valleys
• Example: Mid-Atlantic Ridge (Iceland sits on this boundary)

Destructive (convergent) boundaries

Two types exist:

Oceanic-continental collision:

• Denser oceanic plate subducts beneath continental plate
• Creates deep ocean trenches
• Explosive volcanic eruptions (andesitic magma, high viscosity)
• Major earthquakes at various depths
• Example: Nazca Plate subducting under South American Plate (Andes Mountains)

Oceanic-oceanic collision:

• Older, denser plate subducts
• Forms volcanic island arcs
• Example: Pacific Plate subducting under Philippine Plate

Conservative (transform) boundaries

Plates slide past each other horizontally. Characteristics:

• No volcanic activity (no magma production)
• Powerful earthquakes caused by friction and sudden release of built-up pressure
• Example: San Andreas Fault, California (Pacific Plate and North American Plate)

Collision boundaries

Two continental plates converge:

• Neither subducts (similar density)
• Crust buckles and folds upward creating fold mountains
• Earthquakes but no volcanoes
• Example: Indo-Australian Plate colliding with Eurasian Plate (Himalayas)

Convection currents and plate movement

Heat from radioactive decay in the Earth's core creates convection currents in the semi-molten asthenosphere. Hotter, less dense material rises, cools at the base of the lithosphere, becomes denser and sinks, creating a circular motion. These currents drag plates along at rates of 2-5 cm per year. Additional mechanisms include:

• Slab pull: weight of subducting plate pulls the rest of the plate down
• Ridge push: at constructive boundaries, newly formed crust slides downslope away from mid-ocean ridges

Earthquakes: causes, measurement and impacts

Formation

Earthquakes occur when stress builds up in rocks along fault lines. When stress exceeds rock strength, sudden movement releases energy as seismic waves. The focus (or hypocentre) is the point underground where the earthquake originates; the epicentre is directly above on the surface.

Measurement

• Richter Scale: logarithmic scale measuring magnitude (0-10+); each whole number increase represents 10× more ground shaking and approximately 32× more energy
• Moment Magnitude Scale (MMS): more accurate for larger earthquakes, measures total energy released
• Mercalli Scale: measures intensity (I-XII) based on observed damage and human perception

Factors affecting impact severity

• Magnitude and depth: shallow focus earthquakes (0-70 km) cause more surface damage
• Population density: more casualties in densely populated areas
• Time of day: affects number of people in vulnerable locations
• Building design: earthquake-resistant construction significantly reduces casualties
• Level of development: wealthier countries typically have better preparation, infrastructure, and emergency response
• Geology: soft sediments amplify shaking; solid rock transmits waves efficiently but buildings withstand better

Volcanic eruptions: types and impacts

Eruption styles

Determined primarily by magma viscosity and gas content:

Effusive eruptions:

• Low viscosity basaltic lava flows easily
• Gases escape readily
• Shield volcanoes with gentle slopes
• Regular, predictable activity
• Example: Kilauea, Hawaii

Explosive eruptions:

• High viscosity andesitic/rhyolitic lava traps gases
• Pressure builds until violent eruption
• Produces pyroclastic flows, ash clouds, volcanic bombs
• Composite (stratovolcanoes) with steep sides
• Example: Mount St Helens, USA (1980)

Volcanic hazards

• Lava flows: destroy everything in path but usually slow-moving (people can evacuate)
• Pyroclastic flows: extremely fast (700 km/h), hot (1000°C) avalanches of gas, rock and ash; deadly
• Ash clouds: disrupt aviation, contaminate water, collapse roofs, respiratory problems
• Lahars: volcanic mudflows when ash mixes with water (rain/melted ice); travel fast down valleys
• Volcanic gases: carbon dioxide, sulfur dioxide (acid rain); can be toxic

Managing tectonic hazards

Monitoring and prediction

• Seismometers: detect earthquake tremors
• GPS satellites: measure ground deformation/plate movement
• Tiltmeters: detect changes in volcano slope angle
• Gas analysis: increased sulfur dioxide indicates rising magma
• Thermal imaging: detects heat changes in volcanic areas

Protection strategies

Earthquake preparation:

• Earthquake-resistant buildings: flexible materials, shock absorbers, deep foundations, cross-bracing
• Automatic shutoffs for gas/electricity to prevent fires
• Land-use zoning: avoiding building on fault lines
• Education: drop, cover, hold drills
• Emergency supplies and evacuation plans

Volcano protection:

• Exclusion zones around active volcanoes
• Diversion channels for lava flows
• Spraying water on lava to solidify it
• Early warning systems and evacuation procedures
• Ashfall shelters

Responses after events

Immediate (hours to days):

• Search and rescue
• Emergency medical treatment
• Temporary shelter, food, water
• Restore communications

Short-term (days to weeks):

• Reconnect utilities
• Clear debris
• Prevent disease outbreaks
• Foreign aid and donations

Long-term (months to years):

• Rebuild infrastructure
• Rehouse population permanently
• Economic recovery
• Improve future preparedness

Worked examples

Example 1: 4-mark question

Explain the formation of a volcano at a destructive plate boundary. [4]

Model answer:

At a destructive boundary, the denser oceanic plate subducts beneath the less dense continental plate [1]. As the oceanic plate descends into the mantle, friction and contact with hotter surrounding rock causes it to melt [1]. This melting produces magma, which is less dense than the surrounding rock [1]. The magma rises through weaknesses in the continental crust and may erupt at the surface, forming a volcano [1].

Examiner note: Each mark requires a distinct point. Link cause and effect clearly. Use proper terminology (subduct, magma not lava inside Earth).

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Example 2: 6-mark question

Compare the primary and secondary effects of an earthquake you have studied. [6]

Model answer using Nepal earthquake (2015):

The Nepal earthquake (magnitude 7.8, April 2015) had severe primary effects. Nearly 9,000 people died, mainly from building collapse caused directly by ground shaking [1]. Over 20,000 people were injured by falling debris [1]. Historic temples in Kathmandu, including Dharahara Tower, were destroyed instantly [1].

Secondary effects developed over the following days and months. Avalanches triggered by the earthquake killed 19 people at Mount Everest Base Camp [1]. Roads were blocked by landslides, preventing aid reaching remote villages for days [1]. Over 3 million people were left homeless, and with damaged water supplies, there was risk of disease outbreaks like cholera [1]. Tourism, vital to Nepal's economy, declined sharply, causing widespread unemployment [1]. [Allow any 6 valid developed points]

Examiner note: "Compare" requires you to discuss both categories with developed examples. Named location and date strengthen answers.

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Example 3: 9-mark question

'Economic development is the most important factor affecting the impact of tectonic hazards.' To what extent do you agree? [9 + 3 SPaG]

Model answer approach:

Introduction: Define key terms and state your judgement.

Paragraph 1 (supporting the statement): Use case study evidence showing how wealthy countries (e.g., Japan 2011 earthquake) had lower death tolls despite high magnitude due to earthquake-resistant buildings, advanced warning systems, well-trained emergency services, and financial resources for immediate response. GDP enables investment in preparedness.

Paragraph 2 (supporting the statement): Contrast with poorer nation (e.g., Haiti 2010 earthquake, similar magnitude 7.0 but 230,000+ deaths). Weak building codes, limited emergency services, poor infrastructure meant secondary effects (disease, slow reconstruction) were devastating and long-lasting.

Paragraph 3 (alternative factors): However, other factors matter: population density (more deaths in urban areas), time of day (Haiti struck at 16:53 when many indoors), distance from epicentre, earthquake depth. Japan 2011 had huge tsunami (over 15,000 deaths) despite wealth—physical geography matters. Volcanic eruptions in Iceland have minimal casualties due to low population regardless of wealth.

Conclusion: Reach a balanced judgement. Economic development is extremely important but interacts with other factors. Wealthier nations generally cope better, but physical factors (magnitude, location, depth) and human factors (population density, time) also significantly influence impacts.

Examiner note: Extended response requires evaluation (weighing up factors), case study evidence, geographical terminology, and a justified conclusion. SPaG marks awarded for spelling, punctuation, grammar.

Common mistakes and how to avoid them

• Confusing magma and lava: Magma is molten rock beneath the surface; lava is magma that has erupted onto the surface. Use the correct term for the location.

• Vague boundary descriptions: Don't just say "plates move." Specify the boundary type, direction of movement (apart/together/past), which plate type (oceanic/continental), and whether subduction occurs.

• Mixing up primary and secondary effects: Primary effects happen immediately due to the hazard itself (shaking, lava). Secondary effects follow later as consequences (disease, unemployment, tsunamis). Landslides and avalanches triggered by earthquakes are secondary effects.

• Forgetting case study details: Generic answers score lower. Learn specific facts: dates, magnitudes, death tolls, locations, named responses. WJEC examiners expect named examples.

• Ignoring command words: "Explain" requires reasons/causes (use "because," "this leads to"). "Describe" requires characteristics/features (what it's like). "Assess/Evaluate" requires weighing up and reaching a judgement.

• Writing everything you know: Answer the specific question asked. If asked about monitoring, don't write paragraphs about building design. Target your knowledge precisely.

Exam technique for "Tectonic Landscapes and Hazards"

• Learn two contrasting earthquake case studies: one from a wealthier country (e.g., Japan, New Zealand, USA) and one from a poorer country (e.g., Nepal, Haiti). Know primary/secondary effects, immediate/long-term responses, specific facts.

• Command word awareness: "Describe" = what/where (2-3 marks typically 1 mark per valid point). "Explain" = why/how (3-4 marks, need cause-effect linkage). "Assess/Evaluate/To what extent" = extended prose discussing multiple viewpoints with a conclusion (6-9 marks).

• Use accurate diagrams: For 4-6 mark questions on processes (e.g., "Explain how an earthquake occurs at a conservative boundary"), a labelled diagram alongside written explanation can gain full marks if sufficiently detailed.

• Structure extended answers: Introduction stating your line of argument, separate paragraphs for different points/case studies with evidence, conclusion summarising your judgement. SPaG marks available for 9-mark questions—check spelling, especially technical terms.

Quick revision summary

The Earth's lithosphere consists of tectonic plates moving over the semi-molten asthenosphere due to convection currents. At constructive boundaries, plates diverge creating new crust and effusive volcanism. At destructive boundaries, subduction causes explosive volcanoes and earthquakes. Conservative boundaries produce earthquakes through friction. Hazard impacts depend on magnitude, development level, population density, and preparedness. Monitoring, prediction, and protection reduce risks. Responses range from immediate rescue to long-term reconstruction. Wealthy nations generally experience lower casualties and faster recovery than poorer nations facing similar magnitude events.