Global Hazards

What you'll learn

This revision guide covers the Global Hazards topic within OCR GCSE Geography. You'll explore the distribution, causes and impacts of natural hazards including earthquakes, volcanic eruptions and tropical storms. You'll also examine how climate change affects hazard frequency and severity, plus strategies to manage and reduce hazard risks in different parts of the world.

Key terms and definitions

Hazard — A naturally occurring event that poses a threat to people, property and the environment.

Tectonic plate — A rigid section of the Earth's crust that moves due to convection currents in the mantle beneath.

Epicentre — The point on the Earth's surface directly above the focus of an earthquake where seismic waves are strongest.

Tropical storm — A large rotating storm system that forms over warm tropical oceans with wind speeds exceeding 119 km/h (also called hurricanes, typhoons or cyclones depending on location).

Primary effect — The immediate impact of a hazard event, such as buildings collapsing during an earthquake or flooding from a storm surge.

Secondary effect — The indirect consequences that occur after the initial hazard event, such as disease outbreaks, unemployment or economic recession.

Mitigation — Actions taken to reduce the severity or likelihood of future hazard impacts through preparation and long-term planning.

Immediate response — Short-term actions taken in the hours and days after a hazard event to save lives and meet basic needs.

Core concepts

Distribution and causes of tectonic hazards

Earthquakes and volcanic eruptions occur at plate boundaries where tectonic plates interact. The Earth's crust comprises around 15 major plates floating on the semi-molten mantle.

Destructive (convergent) plate boundaries:

• Two plates move towards each other
• Oceanic plate subducts beneath continental plate due to higher density
• Friction causes earthquakes; melting oceanic crust creates magma leading to explosive volcanic eruptions
• Example: Pacific Plate subducting beneath South American Plate along the Peru-Chile Trench
• Forms deep ocean trenches, fold mountains and composite volcanoes

Constructive (divergent) plate boundaries:

• Two plates move apart
• Magma rises from the mantle to fill gaps
• Creates new oceanic crust as magma cools
• Causes relatively gentle volcanic eruptions and minor earthquakes
• Example: Mid-Atlantic Ridge separating the Eurasian and North American plates
• Forms submarine mountain ranges and rift valleys

Conservative (transform) plate boundaries:

• Two plates slide horizontally past each other
• No volcanic activity occurs
• Friction between plates builds up stress, released as earthquakes
• Example: San Andreas Fault in California where Pacific Plate moves northwest relative to North American Plate

Hotspots are localised areas of volcanic activity away from plate boundaries, caused by mantle plumes of rising magma. Hawaii sits above a Pacific hotspot, creating a chain of volcanic islands.

Structure and impacts of earthquakes

Earthquakes release energy through seismic waves that radiate from the focus (point of rupture underground). The epicentre on the surface experiences the strongest shaking.

Earthquake magnitude:

• Measured using the Moment Magnitude Scale (formerly Richter Scale)
• Logarithmic scale — each whole number increase represents 10 times more ground movement
• Magnitude 6.0+ can cause significant damage; 7.0+ considered major earthquakes

Primary effects of earthquakes:

• Ground shaking causing buildings and infrastructure to collapse
• Ground rupture creating cracks and displacement
• Soil liquefaction where saturated soil behaves like liquid
• Immediate deaths and injuries from structural collapse

Secondary effects of earthquakes:

• Tsunamis generated by submarine earthquakes displacing ocean water
• Landslides triggered on unstable slopes
• Fires from ruptured gas lines and electrical systems
• Disease outbreaks due to contaminated water and damaged sewerage
• Economic costs from destroyed infrastructure and business disruption
• Psychological trauma affecting survivors

Volcanic eruptions and their characteristics

Volcanic eruptions vary in explosivity depending on magma composition and gas content.

Shield volcanoes:

• Form at constructive boundaries and hotspots
• Runny basaltic lava with low silica content
• Gentle eruptions allowing lava to flow long distances
• Wide base with gentle slopes (e.g. Mauna Loa, Hawaii)

Composite (stratovolcanoes):

• Form at destructive boundaries
• Viscous andesitic/rhyolitic lava with high silica content
• Explosive eruptions with pyroclastic flows, ash clouds and volcanic bombs
• Steep-sided cone shape (e.g. Mount Fuji, Japan; Soufrière Hills, Montserrat)

Primary effects of volcanic eruptions:

• Lava flows destroying property and vegetation
• Pyroclastic flows (superheated gas and rock) travelling at 700 km/h
• Volcanic ash blanketing areas, collapsing roofs and contaminating water
• Volcanic gases (sulphur dioxide, carbon dioxide) causing suffocation

Secondary effects of volcanic eruptions:

• Lahars (volcanic mudflows) when ash mixes with water from ice melt or rainfall
• Agricultural damage from ash covering farmland
• Respiratory illnesses from inhaling ash particles
• Climate cooling if ash reaches the stratosphere, blocking sunlight
• Tourism disruption from aviation restrictions

Formation and structure of tropical storms

Tropical storms require specific conditions to form:

Formation requirements:

• Sea surface temperature above 27°C to provide energy through evaporation
• Location between 5° and 30° latitude (Coriolis effect too weak at equator)
• Low wind shear allowing vertical development
• Ocean depth of at least 70 metres
• Atmospheric instability encouraging rising air

Development sequence:

1. Warm ocean heats air above, causing evaporation and rising
2. Rising air creates low pressure at surface
3. Trade winds spiral inwards due to Coriolis effect
4. Condensation releases latent heat, powering further uplift
5. Air cools and descends in central eye, creating calm conditions
6. System intensifies as cycle continues, forming distinctive structure

Tropical storm structure:

• Eye: Central area of descending air with calm, clear conditions (30-50 km diameter)
• Eyewall: Ring of most intense winds and heaviest rainfall surrounding the eye
• Rainbands: Spiral bands of cloud and precipitation extending outwards
• Wind speeds decrease with distance from eyewall
• Storms dissipate over land or cool water as energy source removed

Impacts and responses to tropical storms

Primary effects:

• Strong winds exceeding 119 km/h destroying buildings, uprooting trees
• Storm surge raising sea level by several metres, flooding coastal areas
• Heavy rainfall (300-400 mm in 24 hours) causing immediate flooding
• Deaths from drowning, flying debris and structural collapse

Secondary effects:

• Landslides triggered by saturated ground on slopes
• Freshwater contamination from saltwater intrusion and sewage
• Disease outbreaks (cholera, typhoid) due to poor sanitation
• Agricultural losses reducing food security
• Unemployment as businesses destroyed
• Infrastructure damage isolating communities

Responses vary between high-income countries (HICs) and low-income countries (LICs):

HICs typically have:

• Advanced forecasting technology and early warning systems
• Comprehensive evacuation plans and emergency shelters
• Building codes requiring hurricane-resistant construction
• Well-funded emergency services and rescue operations
• Insurance schemes spreading financial risk
• Example: USA responses to hurricanes include National Hurricane Center monitoring

LICs often experience:

• Limited monitoring equipment and communication infrastructure
• Densely populated coastal slums with poor-quality housing
• Inadequate emergency services and healthcare facilities
• Dependency on international aid for recovery
• Slow reconstruction due to limited financial resources
• Example: Haiti struggled with Hurricane Matthew (2016) response

Climate change and hazard risk

Evidence that climate change affects hazard patterns:

• Rising global temperatures increase ocean heat content
• Warmer seas provide more energy for tropical storm formation
• Studies show increasing intensity (not frequency) of tropical storms
• Changed atmospheric circulation patterns affecting storm tracks
• Sea level rise exacerbates storm surge flooding impacts

Projected changes:

• More Category 4 and 5 hurricanes with higher wind speeds
• Increased rainfall intensity causing worse flooding
• Expanded tropical zones bringing storms to new regions
• Higher storm surges due to elevated baseline sea levels
• Greater economic losses in coastal cities

Uncertainty factors:

• Natural climate variability makes trends difficult to isolate
• Limited historical tropical storm data in some regions
• Complex interactions between ocean, atmosphere and land
• Regional variations in climate change impacts

Managing hazard risks

Monitoring and prediction:

• Seismometers detect earthquake waves but cannot predict timing accurately
• GPS monitors plate movement and volcano deformation
• Gas emissions and ground temperature indicate volcanic activity
• Satellite imagery tracks tropical storm development
• Computer models forecast storm paths 3-5 days ahead

Protection measures:

For earthquakes:

• Earthquake-resistant buildings with reinforced frames and foundations
• Automatic shut-off systems for gas and electricity
• Tsunami warning systems with ocean buoys detecting wave height
• Land-use zoning restricting development in high-risk areas

For tropical storms:

• Sea walls and levees preventing storm surge flooding
• Mangrove restoration providing natural coastal protection
• Building codes requiring storm shutters and reinforced roofs
• Drainage improvements reducing urban flooding

Planning and preparedness:

• Hazard mapping identifying high-risk zones
• Emergency drills practising evacuation procedures
• Stockpiling supplies (water, food, medical equipment)
• Public education campaigns raising awareness
• Insurance schemes providing financial protection
• International cooperation sharing expertise and resources

Long-term mitigation:

• Retrofitting older buildings to improve resilience
• Relocating vulnerable communities from hazard zones
• Diversifying economies reducing dependency on at-risk sectors
• Climate change adaptation addressing root causes

Worked examples

Example 1: 4-mark question

Explain how destructive plate boundaries lead to volcanic eruptions. [4 marks]

Model answer: At destructive plate boundaries, an oceanic plate and continental plate move towards each other (1). The denser oceanic plate is forced downwards beneath the continental plate in a process called subduction (1). As the oceanic plate descends into the mantle, friction and heat cause it to melt, forming magma (1). This magma is less dense than surrounding rock so rises through weaknesses in the continental plate, eventually erupting at the surface as a volcano (1).

Examiner guidance: This answer gains full marks by explaining the sequence clearly with accurate geographical terminology. Each development point earns one mark.

Example 2: 6-mark question

Compare the primary and secondary effects of earthquakes in a HIC and a LIC. Use named examples. [6 marks]

Model answer: Primary effects in HICs like the 2011 Christchurch earthquake (New Zealand) included building damage despite earthquake-resistant construction, with 185 deaths (1). In contrast, the 2010 Haiti earthquake (LIC) caused catastrophic primary effects with over 220,000 deaths due to poor building standards and densely populated urban slums (1).

Secondary effects differed significantly. Christchurch experienced business disruption and tourism decline, but well-funded emergency services provided immediate aid and reconstruction began quickly with insurance payouts (1). Haiti suffered prolonged secondary effects including a cholera outbreak that killed thousands due to contaminated water supplies and inadequate sanitation (1). Haiti required extensive international aid for reconstruction, whereas New Zealand mobilised domestic resources more effectively (1). Both countries experienced long-term economic impacts, but Haiti's weaker economy took longer to recover with GDP falling by 5% (1).

Examiner guidance: This comparative answer uses specific named examples with accurate data, balancing discussion between both countries and addressing both primary and secondary effects.

Example 3: 9-mark question with SPaG

Assess the effectiveness of different strategies for reducing the impacts of tropical storms. [9 marks + 3 SPaG]

Model answer: Monitoring and prediction systems have proven highly effective in reducing deaths from tropical storms. The USA's National Hurricane Center uses satellite technology, aircraft reconnaissance and computer models to forecast hurricane paths 3-5 days in advance, allowing authorities to issue warnings and evacuate coastal populations. This was demonstrated during Hurricane Irma (2017) when Florida evacuated 6.5 million people, preventing thousands of deaths. However, prediction accuracy decreases beyond 72 hours, and some communities ignore warnings, limiting effectiveness.

Structural protection measures provide varying levels of effectiveness. New Orleans rebuilt levees to withstand Category 3 hurricanes after Hurricane Katrina (2005), successfully protecting the city during Hurricane Ida (2021). However, such engineering solutions are expensive (New Orleans spent $14.5 billion) and may be unaffordable for LICs. Bangladesh has invested in coastal embankments and cyclone shelters, reducing deaths from Cyclone Amphan (2020) to under 100 compared to 300,000 in the 1970 Bhola cyclone, demonstrating that even lower-cost structures save lives when combined with evacuation planning.

Natural protection through mangrove conservation has emerged as a cost-effective strategy. Mangroves reduce wave energy by up to 66% and stabilise coastlines, providing protection during storm surges. Vietnam's mangrove restoration programme saved $7 million annually in dyke maintenance while protecting coastal communities. However, mangroves require years to establish and cannot protect against the most intense storms.

Overall, the most effective approach combines multiple strategies appropriate to local contexts. HICs achieve low death tolls through investment in monitoring, engineering and insurance, while LICs can significantly reduce casualties through early warning systems, cyclone shelters and community preparedness at lower cost. No single strategy eliminates all impacts, but integrated hazard management substantially reduces both deaths and economic losses.

Examiner guidance: This answer reaches Level 3 (7-9 marks) by assessing multiple strategies with specific examples, demonstrating critical evaluation using evidence, and reaching a balanced conclusion. Clear paragraphs, accurate terminology and proper spelling/grammar earn full SPaG marks.

Common mistakes and how to avoid them

• Confusing plate boundary types: Learn the correct hazards for each boundary. Destructive boundaries have earthquakes AND volcanoes; conservative boundaries have ONLY earthquakes; constructive boundaries have gentle volcanoes and minor earthquakes.

• Mixing up primary and secondary effects: Primary effects happen immediately during the hazard event (buildings collapse, people killed by winds). Secondary effects occur afterwards (disease, unemployment, economic recession). Time is the key distinction.

• Using vague statements without specifics: Instead of "many people died," write "over 220,000 people died in the Haiti earthquake." Examiners reward precise facts, figures and place names.

• Describing formation without explaining: When asked to "explain," don't just list what happens. Use connectives (therefore, because, this causes, as a result) to show cause-and-effect relationships clearly.

• Ignoring command words: "Assess" requires judgement about effectiveness/importance, not just description. "Compare" needs similarities AND differences. "To what extent" demands a balanced argument with conclusion.

• Forgetting case study details: Know specific names, dates, locations and statistics for your hazard case studies. Generic answers without examples rarely achieve top marks.

Exam technique for "Global Hazards"

• Command word recognition: "Explain" (4-6 marks) requires reasons using connectives. "Assess/Evaluate" (9 marks) demands weighing up different factors with a judgement. "Compare" needs explicit differences/similarities between two things.

• Case study structure: For 9-mark questions, use the PEEL framework — Point (clear statement), Evidence (specific case study detail with names/dates/figures), Explanation (how evidence supports point), Link (connect to question focus).

• Mark allocation guides timing: Spend approximately one minute per mark. A 4-mark question needs about 4 minutes with four developed points. A 9-mark question requires 10-12 minutes with three detailed paragraphs.

• Always use geographical terminology: Replace everyday words with precise terms — use "epicentre" not "centre," "pyroclastic flow" not "hot ash," "eyewall" not "edge of the storm." This demonstrates geographical knowledge and improves marks.

Quick revision summary

Global hazards occur at predictable locations based on plate tectonics and ocean conditions. Earthquakes and volcanoes concentrate at plate boundaries through destructive, constructive or conservative interactions. Tropical storms form over warm tropical oceans when specific atmospheric conditions align. Hazard impacts vary between HICs and LICs due to infrastructure quality, wealth and preparedness. Primary effects occur immediately; secondary effects develop afterwards. Management strategies include monitoring, protection, planning and mitigation. Climate change intensifies some hazards, particularly tropical storms. Effective hazard reduction requires integrated approaches combining technology, engineering and community preparation.