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

Hazard	Details
Volcanic eruptions	Often explosive because the magma is silica-rich (andesitic/rhyolitic), making it viscous. Gas builds up under pressure and erupts violently
Powerful earthquakes	The friction between the subducting plate and the overriding plate generates enormous stress. When this stress is released, it produces powerful earthquakes (e.g., the 2011 Tōhoku earthquake, Mw 9.1)
Tsunamis	Submarine earthquakes at destructive boundaries can displace the sea floor, generating devastating tsunamis (e.g., the 2004 Indian Ocean tsunami)

Exam Tip: The key difference between eruptions at constructive and destructive boundaries is the type of magma. Constructive boundary magma is basaltic (runny, low silica) = gentle eruptions. Destructive boundary magma is andesitic/rhyolitic (viscous, high silica) = explosive eruptions. This is a commonly examined point.

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Conservative (Transform) Boundaries

At a conservative boundary, two plates slide past each other horizontally. No crust is created or destroyed — hence the name "conservative".

How It Works

1. The two plates move in the same or opposite directions, but at different speeds.
2. Friction between the plates causes them to lock together, building up stress over time.
3. When the stress exceeds the strength of the rock, the plates suddenly jerk past each other, releasing energy as an earthquake.
4. Because no magma is produced, there are no volcanoes at conservative boundaries.

Key Example: The San Andreas Fault

The San Andreas Fault in California is the world's most famous conservative boundary. It marks the junction between the Pacific Plate (moving north-west at ~6 cm/year) and the North American Plate (moving west at ~1 cm/year).

Notable Earthquakes on the San Andreas Fault
1906 San Francisco earthquake	Mw 7.9; caused devastating fires; approximately 3,000 deaths
1989 Loma Prieta earthquake	Mw 6.9; 63 deaths; $6 billion damage; collapsed the Cypress Freeway
Future "Big One"	Scientists estimate a 60% probability of a Mw 6.7+ earthquake in the San Francisco Bay Area by 2043

Hazards at Conservative Boundaries

Hazard	Details
Earthquakes	Can be very powerful (up to Mw 8+); often shallow (within 15 km of the surface), making them highly destructive
No volcanoes	No magma is produced because no subduction or rifting occurs
Surface rupture	The fault line may be visible at the surface, with ground displacement of several metres during major earthquakes

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Collision Boundaries

At a collision boundary, two plates carrying continental crust move towards each other. Because both plates are made of low-density continental crust, neither plate can be subducted — both are too buoyant to sink into the mantle.

How It Works

1. The two continental plates converge, and neither can sink.
2. The crust is compressed, crumpled and folded upwards, creating fold mountains.
3. Layers of sedimentary rock that accumulated on the former ocean floor between the two continents are buckled and thrust upwards to form mountain ranges.
4. The process produces powerful earthquakes but no volcanic eruptions (because there is no subduction and therefore no magma generation).

Key Example: The Himalayas

The Himalayan mountain range was formed by the collision of the Indo-Australian Plate and the Eurasian Plate, which began approximately 50 million years ago and continues today.