The Challenge of Natural Hazards
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Plate tectonics
The Earth's crust is broken into tectonic plates that float on the semi-molten mantle. Convection currents driven by radioactive decay move the plates a few centimetres per year.
All tectonic hazards occur because of plate movement — the type of margin determines the hazard.
Plate tectonics — Key Knowledge
- Tectonic plates large pieces of crust
- Mantle semi-molten rock beneath the crust
- Convection currents driven by heat from radioactive decay in the core
- Movement rate a few centimetres per year
Constructive margins
Plates move apart. Magma rises to fill the gap, forming new crust.
Also called divergent margins — the least destructive type.
Constructive margins — Key Knowledge
- Divergent movement plates move apart
- New crust formed magma rises from mantle
- Volcanic activity gentle eruptions
- Mild earthquakes, Example: Mid-Atlantic Ridge / Iceland
Destructive margins
Plates move together. Where oceanic meets continental, the denser oceanic plate subducts, forming trenches, fold mountains, and violent eruptions.
Continental-continental collision produces fold mountains and earthquakes but no volcanoes (e.g. Himalayas).
Destructive margins — Key Knowledge
- Convergent movement plates collide
- Subduction denser oceanic plate forced under continental plate
- Deep ocean trenches formed at subduction zone
- Fold mountains formed by compression
- Violent volcanoes and earthquakes, Example: Pacific Ring of Fire / Andes
Conservative margins
Plates slide past each other. No crust is created or destroyed.
Also called transform margins — earthquakes only, no volcanoes.
Conservative margins — Key Knowledge
- Transform movement plates slide past each other
- No volcanic activity, Powerful earthquakes caused by friction and sudden release
- Example: San Andreas Fault
Effects of tectonic hazards
Tectonic hazards have primary effects (direct) and secondary effects (indirect, occurring as a result of primary effects).
Secondary effects can cause more damage than primary effects over time — e.g. cholera outbreaks after earthquakes.
Effects of tectonic hazards — Key Knowledge
- Primary effects buildings collapse, deaths/injuries, infrastructure destroyed, landslides
- Secondary effects tsunamis, fires from ruptured gas pipes, disease from contaminated water, homelessness, economic disruption
Responses to tectonic hazards
Responses are divided into immediate (during/just after the event) and long-term (weeks to years after).
HICs typically respond faster with more resources; LICs rely more heavily on international aid.
Responses to tectonic hazards — Key Knowledge
- Immediate responses search and rescue, emergency shelters, medical aid, distributing food and water
- Long-term responses rebuilding infrastructure, improving building codes, setting up monitoring/warning systems, re-housing people
Tectonic hazard case studies
LIC and HIC examples illustrate how development level affects impacts and responses.
HICs have lower death tolls but higher economic costs; LICs suffer more deaths due to weaker infrastructure and slower response.
Tectonic hazard case studies — Key Knowledge
- Haiti 2010 magnitude 7.0, over 220,000 deaths, 1.5 million homeless, slow international response, cholera outbreak
- L'Aquila 2009 magnitude 6.3, 309 deaths, rapid emergency response, government-funded rebuilding, temporary housing provided quickly
Why people live near tectonic hazards
People continue to live near hazardous plate margins for economic, social, and practical reasons.
The benefits of living near hazards often outweigh the perceived risk — especially if eruptions/earthquakes are infrequent.
Why people live near tectonic hazards — Key Knowledge
- Fertile volcanic soils good farming
- Geothermal energy, Tourism, Mineral resources, Community and family ties, Cannot afford to move
Global atmospheric circulation
The atmosphere circulates in three cells per hemisphere, driven by differential heating at the equator and poles.
These cells explain the distribution of the world's deserts (high pressure at 30°) and tropical rainforests (low pressure at equator).
Global atmospheric circulation — Key Knowledge
- Hadley cell equator to 30°
- Ferrel cell 30° to 60°
- Polar cell 60° to poles
The Hadley cell
Warm air rises at the equator creating low pressure and heavy rainfall. Air moves poleward at altitude, cools, and sinks at about 30°N/S creating high pressure and dry conditions.
The Hadley cell is the most important cell — it drives tropical rainfall and desert formation.
The Hadley cell — Key Knowledge
- Rising air at equator low pressure, heavy rainfall
- Poleward movement at high altitude, Sinking air at ~30°N/S high pressure, deserts form
- Surface winds blow back towards equator
Tropical storms
Tropical storms (hurricanes, cyclones, typhoons) are intense low-pressure weather systems with strong winds and heavy rainfall.
Different names for the same phenomenon — hurricanes (Atlantic), cyclones (Indian Ocean), typhoons (Pacific).
Tropical storms — Key Knowledge
- Formation conditions: warm ocean water above 27°C
- between 5–30°N/S of equator, Coriolis effect needed for spin. Structure: eye calm, clear, low pressure
- eyewall most intense winds and rainfall
- spiral rain bands. Lose energy over land or cooler water cut off from warm water energy source
UK weather hazards
The UK experiences weather hazards including flooding, storms, and extreme temperature events, which are becoming more frequent.
The UK doesn't experience tropical storms but faces significant weather hazards — flooding is the most common.
UK weather hazards — Key Knowledge
- Somerset Levels flooding 2014 wettest January in 250 years, rivers burst banks, villages cut off — Moorland/Muchelney, farms devastated, £10 million damage, dredging carried out afterwards
- Increasing frequency linked to climate change warmer temperatures, more atmospheric moisture, changing jet stream patterns
Evidence for climate change
Multiple lines of evidence show that the global climate is changing.
Scientists use multiple independent lines of evidence — no single source is relied upon alone.
Evidence for climate change — Key Knowledge
- Ice cores trapped air bubbles showing past CO₂ levels and temperatures over 800,000 years
- Rising global temperatures about 1°C since pre-industrial times
- Sea level rise thermal expansion and melting ice
- Retreating glaciers, Earlier spring events flowering, bird migration
Causes of climate change
Climate change has both natural and human causes. The current rapid warming is primarily driven by human activity enhancing the greenhouse effect.
The greenhouse effect itself is not bad — it's the additional human emissions that are causing rapid warming.
Causes of climate change — Key Knowledge
- Natural causes: Milankovitch cycles orbital changes — eccentricity, axial tilt, precession
- volcanic eruptions ash and SO₂ reflect sunlight, short-term cooling
- solar output variations sunspot cycles). Human causes: burning fossil fuels (releases CO₂
- deforestation reduces CO₂ absorption
- agriculture methane from livestock and rice paddies, nitrous oxide
- cement production (releases CO₂). Natural greenhouse effect is essential (without it Earth would be ~33°C cooler) — the problem is the enhanced greenhouse effect
Mitigation and adaptation
Responses to climate change are either mitigation (reducing the causes) or adaptation (adjusting to live with the effects).
Both strategies are needed — mitigation to slow future warming, adaptation to deal with changes already happening.
Mitigation and adaptation — Key Knowledge
- Mitigation: renewable energy wind, solar, tidal
- carbon capture and storage, international agreements Paris Agreement 2015
- reducing deforestation, energy efficiency, electric vehicles. Adaptation: flood defences, drought-resistant crops, changing agricultural practices, managed retreat from coastlines, building on stilts, water conservation
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The Challenge of Natural Hazards
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