Natural Hazards & Plate Tectonics

This lesson introduces the concept of natural hazards and explores the theory of plate tectonics — the foundation for understanding earthquakes and volcanic eruptions. A solid grasp of plate theory is essential for the rest of this topic and will appear in almost every GCSE Geography exam paper.

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What Is a Natural Hazard?

A natural hazard is a natural event that has the potential to cause loss of life, injury, or damage to property and the environment. Not every natural event is a hazard — it only becomes a hazard when it threatens human life or activity.

Term	Definition
Natural event	A physical process that occurs in the natural world (e.g. an earthquake in an uninhabited desert)
Natural hazard	A natural event that poses a risk to people, property, or the environment
Natural disaster	A natural hazard that actually causes significant loss of life, economic damage, or disruption
Vulnerability	The degree to which a population is susceptible to the effects of a hazard
Resilience	The ability of a community to recover from a hazard event

Types of Natural Hazard

Natural hazards can be grouped into categories:

• Tectonic hazards — caused by the movement of tectonic plates (earthquakes, volcanic eruptions, tsunamis)
• Weather hazards — caused by atmospheric processes (tropical storms, extreme rainfall, heatwaves)
• Climate hazards — caused by long-term changes to climate patterns (drought, sea-level rise)
• Geomorphological hazards — caused by land processes (landslides, avalanches, flooding)

Exam Tip: The AQA specification focuses on three main areas: tectonic hazards, weather hazards, and climate change. Make sure you can classify any given hazard into the correct category.

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Factors Affecting Hazard Risk

The level of risk from a natural hazard depends on several interacting factors:

1. Vulnerability — poor communities with low-quality buildings and limited access to healthcare are more at risk.
2. Capacity to cope — wealthier countries can invest in prediction, protection, and emergency services.
3. Nature of the hazard — some hazards are more frequent, intense, or widespread than others.
4. Urbanisation — as more people move into cities, the potential number of people affected increases.
5. Climate change — rising global temperatures are increasing the frequency and intensity of some weather hazards.
6. Poverty — people in poverty are more likely to live in hazard-prone areas and less able to prepare or recover.

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The Structure of the Earth

To understand plate tectonics, you need to know the internal structure of the Earth.

Layer	Depth	State	Key Facts
Crust	0–70 km	Solid	Two types: oceanic (thin, dense) and continental (thick, less dense)
Upper mantle	70–700 km	Semi-molten	Contains the asthenosphere where convection currents flow
Lower mantle	700–2,900 km	Solid but flows	Extremely high pressure keeps it solid despite high temperatures
Outer core	2,900–5,100 km	Liquid	Made of iron and nickel; generates Earth's magnetic field
Inner core	5,100–6,371 km	Solid	Solid iron and nickel; temperatures reach ~5,500 °C

Oceanic vs Continental Crust

Feature	Oceanic Crust	Continental Crust
Thickness	5–10 km	25–70 km
Density	Dense (about 3.0 g/cm³)	Less dense (about 2.7 g/cm³)
Age	Younger (up to 200 million years)	Older (up to 3.8 billion years)
Composition	Basalt	Granite
Can be subducted?	Yes	No (too buoyant)

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Plate Tectonics Theory

The Earth's crust is broken into large pieces called tectonic plates. These plates float on the semi-molten rock of the upper mantle and are constantly, though very slowly, moving.

Key Evidence for Plate Tectonics

• Continental fit — the coastlines of Africa and South America appear to fit together like a jigsaw (first noted by Alfred Wegener in 1912).
• Fossil evidence — identical fossils of Mesosaurus have been found in both South America and Africa.
• Rock evidence — matching rock types and mountain chains are found on continents now separated by oceans.
• Sea-floor spreading — magnetic stripe patterns on the ocean floor show that new crust is being created at mid-ocean ridges.

What Drives Plate Movement?

Convection currents in the mantle are the main driving force:

1. Radioactive decay in the core and mantle produces intense heat.
2. Hot, semi-molten rock rises towards the crust.
3. It spreads out sideways beneath the plates, dragging them along.
4. The rock cools, becomes denser, and sinks back down.
5. This circular movement creates a continuous cycle.

Other contributing forces include ridge push (gravity pushing plates away from elevated mid-ocean ridges) and slab pull (the weight of a subducting plate pulling the rest of the plate with it).

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Types of Plate Boundary

There are four main types of plate boundary. Each produces different hazards.

The following diagram summarises the four plate boundary types and their key features:

graph TD
    A[Plate Boundaries] --> B[Constructive]
    A --> C[Destructive]
    A --> D[Conservative]
    A --> E[Collision]
    B --> B1[Plates move apart]
    B --> B2[New crust formed]
    C --> C1[Plates converge]
    C --> C2[Subduction occurs]
    D --> D1[Plates slide past]
    D --> D2[No crust created or destroyed]
    E --> E1[Two continental plates meet]
    E --> E2[Fold mountains form]

1. Constructive (Divergent) Boundary

• Plates move apart from each other.
• Magma rises to fill the gap, creating new crust.
• Produces gentle volcanic eruptions and shallow earthquakes.
• Example: Mid-Atlantic Ridge (where the Eurasian and North American plates diverge).

2. Destructive (Convergent) Boundary

• Plates move towards each other.
• The denser oceanic plate is forced beneath the continental plate — this is called subduction.
• Creates a deep ocean trench, fold mountains, and explosive volcanic eruptions.
• Produces powerful earthquakes at varying depths.
• Example: Nazca Plate subducting beneath the South American Plate (the Andes).

3. Conservative (Transform) Boundary

• Plates slide past each other horizontally.
• No crust is created or destroyed.
• Friction causes plates to lock; when pressure is released, it causes violent earthquakes.
• No volcanic activity occurs.
• Example: San Andreas Fault, California (Pacific and North American plates).

4. Collision Boundary

• Two continental plates move towards each other.
• Neither plate can be subducted (both are too buoyant), so the crust crumples upwards to form fold mountains.
• Produces powerful earthquakes but no volcanic activity.
• Example: Himalayas (Indian and Eurasian plates).

Exam Tip: You must be able to draw and label a diagram of each plate boundary type. Practice sketching these from memory — the exam may ask you to annotate a cross-section diagram.

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Plate Boundaries and Hazard Distribution

The global distribution of earthquakes and volcanoes is not random. They are concentrated along plate boundaries, particularly around the Pacific Ring of Fire, which accounts for about 75% of the world's active volcanoes and 90% of earthquakes.

Some hazards do occur away from plate boundaries — for example, hotspot volcanoes like those in Hawaii, where a mantle plume of hot rock rises through the middle of a plate.

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Summary

• A natural hazard is a natural event that threatens people and property.
• The Earth has a layered structure: crust, mantle, outer core, and inner core.
• Tectonic plates float on the semi-molten mantle and are moved by convection currents.
• The four plate boundary types are: constructive, destructive, conservative, and collision.
• Earthquakes and volcanoes are concentrated along plate boundaries.

Exam Tip: When answering questions on plate boundaries, always name a specific real-world example. Generic answers without place names will not reach the top mark bands.