Tectonic Hazards » Plate Boundary Processes

What you'll learn this session

Study time: 30 minutes

The structure of the Earth and its layers
Different types of plate boundaries and their movements
How tectonic processes create landforms and hazards
Case studies of significant tectonic events
How to identify and explain plate boundary processes on maps

The Earth's Structure and Plate Tectonics

Our planet is made up of several layers and the movement of these layers creates the landforms we see today and causes earthquakes and volcanic eruptions. Understanding plate tectonics helps us explain why these hazards occur where they do.

Key Definitions:

Plate tectonics: The theory that Earth's outer shell is divided into plates that move, causing earthquakes, volcanoes and mountain building.
Tectonic plates: Large sections of the Earth's crust and upper mantle that move relative to each other.
Convection currents: The movement of heat within the mantle that drives plate motion.

🌎 Earth's Layers

The Earth is made up of four main layers:

Inner core: Solid ball of iron and nickel (5,500°C)
Outer core: Liquid layer of iron and nickel
Mantle: Semi-solid rock layer where convection occurs
Crust: Thin outer layer (5-70km thick) divided into tectonic plates

🔥 Plate Movement

Plates move because of convection currents in the mantle:

Heat from the core causes material in the mantle to rise
As it reaches the crust, it spreads out and cools
Cooler material sinks back down
This circular movement drags the plates above
Plates move very slowly (2-10cm per year)

Types of Plate Boundaries

Where plates meet, we call these plate boundaries. There are three main types, each creating different landforms and hazards.

👉👈 Convergent Boundaries

Plates move towards each other

When plates collide, one of three things happens:

Ocean-continent: Oceanic plate subducts (goes under) continental plate
Ocean-ocean: One oceanic plate subducts under another
Continent-continent: Both plates crumple up, neither subducts

Creates: Mountains, deep ocean trenches, volcanoes, strong earthquakes

Examples: Andes Mountains, Himalayas, Japan's volcanoes

👈👉 Divergent Boundaries

Plates move away from each other

As plates pull apart:

Magma rises from the mantle to fill the gap
Creates new crust as it cools
Can occur on land or under the ocean

Creates: Rift valleys, mid-ocean ridges, shallow earthquakes, shield volcanoes

Examples: Mid-Atlantic Ridge, East African Rift Valley, Iceland

👉👉 Transform Boundaries

Plates slide past each other

When plates grind alongside each other:

No new crust is created or destroyed
Friction causes plates to stick, then suddenly slip
Energy releases as earthquakes

Creates: Fault lines, shallow but powerful earthquakes

Examples: San Andreas Fault (California), North Anatolian Fault (Turkey)

Landforms Created by Plate Tectonics

Volcanic Landforms

Volcanoes form when magma reaches the Earth's surface. Their shape depends on the type of magma and eruption style.

💣 Shield Volcanoes

Where: Divergent boundaries and hotspots

Features:

Gently sloping sides
Made from runny (low viscosity) lava
Less explosive eruptions
Very large base diameter

Example: Mauna Loa, Hawaii

💥 Composite Volcanoes

Where: Convergent boundaries

Features:

Steep sides
Made from layers of ash and thick (high viscosity) lava
Explosive eruptions
Cone-shaped

Example: Mount Fuji, Japan

Mountain Formation

Mountains form through different processes at plate boundaries:

⛰ Fold Mountains

Form when two continental plates collide and the crust folds upwards like a rug being pushed together.

Examples: Himalayas, Alps, Andes

The Himalayas are still growing by about 5mm per year as India continues to push into Asia!

🌋 Volcanic Mountains

Form when magma erupts and builds up around a volcanic vent over time.

Examples: Mount St. Helens, Mount Etna

These can form in chains along subduction zones, creating "rings of fire" like the one around the Pacific Ocean.

Tectonic Hazards: Earthquakes

Earthquakes occur when pressure builds up at plate boundaries and is suddenly released. This sends seismic waves through the Earth that can cause devastating damage.

🛠 Measuring Earthquakes

Richter Scale: Measures the magnitude (energy released) on a logarithmic scale. Each whole number is 10 times stronger than the previous.

Mercalli Scale: Measures the intensity (observed effects) from I-XII based on damage and human experience.

Key terms:

Focus: The point underground where the earthquake begins
Epicentre: The point on the surface directly above the focus
Seismic waves: Energy waves that travel through the Earth

🏨 Secondary Effects

Earthquakes can trigger other hazards:

Tsunamis: Giant waves caused by underwater earthquakes
Landslides: When shaking destabilizes slopes
Liquefaction: When soil behaves like a liquid during shaking
Fires: From broken gas lines and electrical damage

Case Study: 2004 Indian Ocean Earthquake and Tsunami

Date: 26 December 2004

Location: Off the west coast of Sumatra, Indonesia

Plate boundary type: Convergent (Indo-Australian Plate subducting under the Burma Plate)

Magnitude: 9.1-9.3 (third largest ever recorded)

Impacts:

Triggered a series of devastating tsunamis up to 30m high
Affected 14 countries around the Indian Ocean
Over 230,000 people killed
1.7 million people displaced
Estimated $10 billion in damage

Responses:

Massive international aid response ($14 billion)
Development of the Indian Ocean Tsunami Warning System
Improved building codes in affected areas
Better education about tsunami warning signs

Living with Tectonic Hazards

Despite the dangers, many people live near plate boundaries. Understanding why people live in these areas and how they adapt is an important part of geography.

❓ Why Live in Hazardous Areas?

Fertile soil: Volcanic ash creates excellent farming conditions
Tourism: Volcanoes and hot springs attract visitors
Resources: Geothermal energy, minerals and precious metals
Historical settlement: Cities established before understanding risks
Lack of alternatives: Limited space in countries like Japan

🚧 Managing Tectonic Hazards

Prediction: Monitoring seismic activity and volcanic gases
Protection: Earthquake-resistant buildings, sea walls for tsunamis
Preparation: Evacuation plans, emergency drills, education
Planning: Land use zoning, building codes, insurance
Aid: Emergency response teams, international assistance

Case Study: Japan's Earthquake Preparedness

Japan sits on the "Ring of Fire" where four tectonic plates meet, making it one of the most earthquake-prone countries in the world.

Preparation measures include:

Building technology: Base isolation systems, flexible frames and shock absorbers
Early warning system: Can give people up to a minute's warning before shaking begins
Education: Regular earthquake drills in schools and workplaces
Infrastructure: Automatic shutoffs for gas and high-speed trains
Personal preparedness: Most homes have earthquake kits with food, water and essentials

These measures saved thousands of lives during the 2011 Tohoku earthquake (magnitude 9.0), though the tsunami that followed caused devastating damage and triggered the Fukushima nuclear disaster.

Key Skills: Reading Tectonic Maps

Being able to identify plate boundaries and predict hazards from maps is an essential skill for your iGCSE exam.

Look for these features:

Plate boundary lines
Arrows showing direction of plate movement
Earthquake locations (often shown as dots)
Volcano locations (often shown as triangles)
Ocean trenches (shown as lines with hatch marks)
Mountain ranges

Remember: Clusters of earthquakes and volcanoes usually indicate plate boundaries. The depth of earthquakes can help identify the type of boundary (shallow at divergent and transform, deeper at convergent).

In your exam, you might be asked to identify boundary types, explain the hazards associated with them, or describe the distribution of tectonic features on a map.

🧠 Test Your Knowledge!