Tectonic Hazards » Plate Boundary Types

What you'll learn this session

Study time: 30 minutes

The theory of plate tectonics and how it explains Earth's structure
The four main types of plate boundaries (constructive, destructive, conservative and collision)
Tectonic processes occurring at each boundary type
Landforms created at different plate boundaries
Real-world examples and case studies of each boundary type

Introduction to Plate Tectonics

Our Earth is constantly on the move! The ground beneath your feet is actually part of a giant jigsaw puzzle made up of pieces called tectonic plates. These plates float on a semi-molten layer called the asthenosphere and are constantly moving - albeit very slowly (about the same speed as your fingernails grow).

Key Definitions:

Tectonic plates: Large sections of the Earth's crust that float on the semi-molten mantle beneath.
Plate boundaries: Areas where tectonic plates meet and interact with each other.
Plate tectonics: The scientific theory explaining how the Earth's crust is divided into plates that move across the planet's surface.
Convection currents: The circular movement of heat in the mantle that drives plate movement.

The Earth's Structure

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 layer where convection currents occur
Crust: Thin outer layer (5-70km thick) divided into tectonic plates

Types of Plate Boundaries

There are four main types of plate boundaries, each creating different landforms and tectonic hazards. Let's explore each one!

🌋 Constructive (Divergent) Boundaries

At constructive boundaries, plates move away from each other. As they separate, magma rises from the mantle to fill the gap, creating new crust. This process is called sea-floor spreading.

Features and landforms:

Mid-ocean ridges
Rift valleys
Shield volcanoes
Shallow earthquakes

Example: Mid-Atlantic Ridge

The Mid-Atlantic Ridge runs through Iceland, where you can actually stand with one foot on the North American plate and one on the Eurasian plate at Thingvellir National Park!

🌋 Constructive Boundary Diagram

At constructive boundaries:
1. Plates move apart
2. Magma rises from the mantle
3. Magma cools to form new crust
4. Creates a ridge along the boundary

The movement is driven by convection currents in the mantle. Hot material rises, spreads out, cools and then sinks back down, creating a circular motion that pulls the plates apart.

🔥 Destructive (Convergent) Boundaries

At destructive boundaries, plates move towards each other. When an oceanic plate meets a continental plate, the denser oceanic plate is forced beneath the continental plate in a process called subduction.

Features and landforms:

Composite volcanoes (steep-sided)
Deep ocean trenches
Fold mountains
Violent earthquakes (often deep)

Example: Pacific Ring of Fire

The Pacific Ring of Fire contains 75% of the world's active volcanoes, formed by subduction zones around the Pacific Plate.

🔥 Destructive Boundary Diagram

At destructive boundaries:
1. Oceanic plate subducts under continental plate
2. Oceanic plate melts as it descends into the mantle
3. Magma rises through the continental crust
4. Explosive volcanoes form on the continental plate

The friction between the plates causes earthquakes, while the melting of the subducting plate creates magma that rises to form volcanoes.

🔀 Conservative (Transform) Boundaries

At conservative boundaries, plates slide past each other horizontally. No crust is created or destroyed, but friction between the plates causes them to stick, building up pressure until they suddenly slip.

Features and landforms:

Shallow but powerful earthquakes
Fault lines
No volcanic activity
Displaced features (e.g., roads, fences)

Example: San Andreas Fault

The San Andreas Fault in California is where the Pacific Plate slides past the North American Plate, causing frequent earthquakes in the region.

🔀 Conservative Boundary Diagram

At conservative boundaries:
1. Plates move parallel to each other
2. Friction causes plates to stick
3. Pressure builds up
4. Plates suddenly slip, causing earthquakes

The movement is jerky rather than smooth, which is why earthquakes occur. The longer the plates are stuck, the more pressure builds up and the stronger the eventual earthquake.

⚠ Collision Boundaries

At collision boundaries, two continental plates move towards each other. Since both plates have similar density, neither can subduct. Instead, they crumple and fold upwards, creating mountain ranges.

Features and landforms:

Fold mountains
Powerful earthquakes
Little or no volcanic activity
Metamorphic rocks

Example: Himalayan Mountains

The Himalayas were formed when the Indian Plate collided with the Eurasian Plate. Mount Everest continues to grow taller by about 4mm each year as the collision continues!

⚠ Collision Boundary Diagram

At collision boundaries:
1. Two continental plates collide
2. Neither plate can subduct (too buoyant)
3. Crust is compressed and forced upwards
4. Creates fold mountains

The collision process is ongoing and very slow. The Himalayan mountains have been forming for about 50 million years and continue to rise today.

Case Studies of Plate Boundaries

🌋 Iceland (Constructive)

Iceland sits directly on the Mid-Atlantic Ridge where the North American and Eurasian plates are moving apart at about 2cm per year. This creates:

Frequent volcanic eruptions (e.g., Eyjafjallajökull in 2010)
Geothermal activity (hot springs, geysers)
Visible rift valleys

Iceland harnesses this geothermal energy for electricity and heating, with 85% of homes heated by geothermal energy.

🔥 Japan (Destructive)

Japan sits where the Pacific Plate subducts beneath the Eurasian Plate, creating:

Frequent earthquakes (2011 Tōhoku earthquake, magnitude 9.0)
Tsunami hazards
Active volcanoes (e.g., Mount Fuji)

Japan has developed advanced earthquake-resistant buildings and tsunami warning systems to cope with these hazards.

⚠ Himalayas (Collision)

The Himalayas formed when India collided with Asia about 50 million years ago:

World's highest mountains (Mount Everest)
Devastating earthquakes (2015 Nepal earthquake)
Landslide hazards

The Indian plate continues to push northward at about 5cm per year, making the Himalayas grow taller and causing ongoing seismic activity.

Case Study: The 2004 Indian Ocean Tsunami

On 26 December 2004, a massive undersea earthquake occurred at a destructive plate boundary where the Indian Plate subducts beneath the Burma Plate:

Magnitude: 9.1-9.3 (third-largest ever recorded)
Epicentre: Off the west coast of northern Sumatra, Indonesia
Cause: Sudden vertical displacement of the seafloor by about 10 meters
Effects: Triggered tsunamis up to 30 meters high affecting 14 countries
Impact: Over 230,000 people killed, making it one of the deadliest natural disasters in recorded history

This disaster led to the development of the Indian Ocean Tsunami Warning System, which now helps protect coastal communities.

Comparing Plate Boundaries

Understanding the differences between plate boundaries helps us predict and prepare for tectonic hazards:

Boundary Type	Plate Movement	Landforms	Hazards
Constructive	Moving apart	Mid-ocean ridges, rift valleys, shield volcanoes	Gentle eruptions, minor earthquakes
Destructive	Oceanic plate subducts under continental plate	Composite volcanoes, ocean trenches	Violent eruptions, major earthquakes, tsunamis
Conservative	Sliding past each other	Fault lines	Powerful shallow earthquakes
Collision	Continental plates collide	Fold mountains	Major earthquakes, landslides

Exam Tips for Plate Boundaries

When answering questions about plate boundaries in your iGCSE Geography exam:

Always use the correct terminology (e.g., subduction, convection currents)
Be specific about the direction of plate movement for each boundary type
Link the processes to specific landforms and hazards
Use named examples and case studies to support your answers
Include simple diagrams to illustrate plate movements when appropriate
Remember to explain how the processes work, not just describe them

🧠 Test Your Knowledge!