Plate Boundaries and Associated Hazards

Now that you understand the structure of the Earth and the theory of plate tectonics, this lesson examines what happens at the edges of tectonic plates — the plate boundaries. It is at these boundaries that the vast majority of earthquakes and volcanic eruptions occur. The Edexcel B specification requires you to understand four types of plate boundary, the processes that occur at each, and the hazards they produce. Each boundary type creates a distinctive set of landforms and hazards.

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The Four Types of Plate Boundary

There are four main types of plate boundary, defined by the direction of plate movement:

Boundary Type	Plate Movement	Also Known As	Key Processes
Constructive	Plates move apart	Divergent	Sea-floor spreading, volcanic eruptions
Destructive	Plates move together	Convergent	Subduction, volcanic eruptions, earthquakes
Conservative	Plates slide past each other	Transform	Earthquakes (no volcanoes)
Collision	Two continental plates move together	Convergent (continent-continent)	Fold mountains, earthquakes (no volcanoes)

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Constructive (Divergent) Boundaries

At a constructive boundary, two tectonic plates move apart from each other. Magma from the mantle rises to fill the gap, creating new oceanic crust. This process is called sea-floor spreading.

How It Works

1. Convection currents in the mantle pull two plates apart.
2. The lithosphere thins and cracks as it is stretched, creating rift valleys and faults.
3. Magma (molten rock) from the asthenosphere rises through the cracks to the surface.
4. The magma solidifies to form new oceanic crust, creating a mid-ocean ridge — an underwater mountain chain.
5. As new crust is continuously formed, the plates gradually move apart at rates of 2–5 cm per year.

Key Example: The Mid-Atlantic Ridge

The Mid-Atlantic Ridge is the world's longest mountain range, stretching approximately 16,000 km from the Arctic Ocean to the southern tip of Africa. It runs down the centre of the Atlantic Ocean and is where the North American Plate and the Eurasian Plate are moving apart (in the north) and the South American Plate and the African Plate are diverging (in the south).

• Iceland is one of the few places where the Mid-Atlantic Ridge rises above sea level. The island is being pulled apart at approximately 2.5 cm per year, creating volcanic activity and geothermal energy that Iceland uses to heat 90% of its homes.
• The eruption of Eyjafjallajökull in Iceland in 2010 occurred at this constructive boundary, disrupting European air travel for weeks.

Hazards at Constructive Boundaries

Hazard	Details
Volcanic eruptions	Generally effusive (runny lava flows rather than explosive eruptions) because the basaltic magma has low silica content and low viscosity
Earthquakes	Generally shallow and of low to moderate magnitude because the crust is thin and the plates are moving apart rather than being forced together
Fissure eruptions	Lava can erupt along long cracks rather than from a central vent

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Destructive (Convergent) Boundaries

At a destructive boundary, two plates move towards each other. There are two main types of destructive boundary:

Oceanic-Continental Convergence

When a dense oceanic plate meets a less dense continental plate, the oceanic plate is forced beneath the continental plate in a process called subduction. This creates a subduction zone — one of the most hazardous geological settings on Earth.

1. The dense oceanic plate descends into the mantle at an angle, forming a deep ocean trench (e.g., the Peru-Chile Trench, up to 8,065 m deep).
2. As the oceanic plate descends, friction and the increasing temperature cause it to melt, generating magma.
3. The magma is less dense than the surrounding rock, so it rises through the continental crust.
4. Where the magma reaches the surface, volcanoes form. A chain of volcanoes parallel to the trench is called a volcanic arc (e.g., the Andes Mountains in South America).
5. The process of subduction also causes powerful earthquakes, including the most powerful earthquakes ever recorded.

Oceanic-Oceanic Convergence

When two oceanic plates converge, the older, denser plate is subducted beneath the younger plate. This creates:

• An ocean trench (e.g., the Mariana Trench — 10,994 m deep, the deepest point on Earth).
• A curved chain of volcanic islands called an island arc (e.g., Japan, the Philippines, the Mariana Islands, Indonesia).

Key Example: The Pacific Ring of Fire

The Pacific Ring of Fire is a horseshoe-shaped zone of intense tectonic activity encircling the Pacific Ocean. It is defined by the subduction of the Pacific Plate (and other oceanic plates) beneath surrounding continental and oceanic plates.

Statistic	Value
Length	~40,000 km
Active volcanoes	~452 (75% of world total)
Earthquakes	~90% of world's earthquakes
Countries affected	Chile, Peru, Ecuador, Colombia, Mexico, USA (west coast), Canada, Russia, Japan, Philippines, Indonesia, New Zealand, and more

Hazards at Destructive Boundaries