Tectonic Hazards: Plate Tectonics and Processes

Tectonic hazards originate deep inside the Earth. Understanding the planet's layered structure explains why plates move and why hazards concentrate where they do.

Layer	Approximate depth	State	Key properties
Crust	0–70 km	Solid	Two types: oceanic (dense, thin, 5–10 km, basalt) and continental (less dense, thicker, 30–70 km, granite)
Mantle	70–2,890 km	Semi-molten (plastic)	Can flow slowly over millions of years; convection currents drive plate movement
Outer core	2,890–5,150 km	Liquid	Iron and nickel; generates Earth's magnetic field
Inner core	5,150–6,371 km	Solid	Iron and nickel; extremely high pressure keeps it solid despite intense heat

The lithosphere is the rigid outer shell — the crust and the uppermost mantle together. It is fractured into approximately 15 major sections and several smaller ones called tectonic plates.

The density difference between oceanic and continental crust is fundamental. When the two collide, oceanic crust sinks because it is more dense — not heavier. Always use the term density in exam answers.

Plates move because of convection currents in the mantle. Radioactive decay in the Earth's interior generates intense heat, which drives a continuous circulation:

1. Mantle material heats up, expands and becomes less dense → rises toward the base of the lithosphere
2. It spreads out sideways, dragging the plates above it
3. The material cools, becomes denser, and sinks back down
4. It is reheated and the cycle continues

Plates move at between 2 and 5 cm per year — roughly the speed a fingernail grows. Over millions of years this produces dramatic changes in continental position (continental drift), the opening and closing of ocean basins, and the building of mountain ranges.

The places where plates interact are plate margins (also called plate boundaries). The vast majority of the world's earthquakes and volcanic eruptions occur at or near plate margins because that is where tectonic energy is concentrated and released.

Earthquakes and volcanoes are not randomly distributed — they cluster in predictable zones that correspond to plate margins.

The "Ring of Fire": A horseshoe-shaped belt encircling the Pacific Ocean. It accounts for roughly 75% of the world's volcanoes and approximately 90% of the world's earthquakes. The Pacific Plate is subducting beneath surrounding plates along much of this margin.

The Alpine-Himalayan belt: Running from southern Europe through Turkey, Iran, Pakistan, and into Southeast Asia. This is a zone of ongoing continental collision between the African, Eurasian, and Indian plates — responsible for earthquakes in Turkey, Iran, Pakistan, and Nepal.

Mid-ocean ridges: Largely underwater constructive margins running through every ocean. Iceland is a rare example of a mid-ocean ridge rising above sea level, making it regularly volcanically active.

The map below shows how earthquakes and volcanoes trace plate margins: the Pacific Ring of Fire, the Alpine-Himalayan collision belt, and the Mid-Atlantic Ridge.

In a map question, describe distribution using compass directions and specific locations, not just country names: "Earthquakes cluster in a belt running from western North America, around the Pacific Ocean to East Asia and into Southeast Asia." Vague answers ("they are found in many places") are unlikely to score marks.

At a constructive margin, two plates move apart from each other (diverge).

Process:

1. Plates pull apart, creating a gap in the lithosphere
2. Reduced pressure at the surface allows mantle material to partially melt
3. Magma rises through the gap and erupts onto the surface
4. Lava cools to form new oceanic crust
5. This continuous creation of new crust builds mid-ocean ridges

Hazards produced:

• Volcanic eruptions: effusive rather than explosive — lava flows freely because low silica content gives it low viscosity. Less explosive and generally less dangerous than destructive margin volcanoes.
• Earthquakes: shallow-focus (near the surface), typically low to moderate magnitude as plates pull apart.

Landforms: mid-ocean ridges, rift valleys (where divergence occurs on land), shield volcanoes

Examples:

• Iceland — sits directly on the Mid-Atlantic Ridge; Eyjafjallajökull erupted in 2010, disrupting European air travel for weeks but causing no deaths
• East African Rift Valley — Africa is slowly splitting along a constructive margin; Lake Tanganyika and Lake Malawi occupy parts of the rift

At a destructive margin, two plates converge — move toward each other. When oceanic crust meets continental crust, the denser oceanic plate is forced down beneath the lighter continental plate in a process called subduction.

Process:

1. Oceanic plate descends at the subduction zone, forming a deep ocean trench
2. Heat and pressure melt the descending plate, forming magma
3. Magma is less dense than surrounding rock → rises through the crust
4. Erupts at the surface as a composite volcano (also called a stratovolcano) — steep-sided, built from alternating layers of lava and ash

The cross-section below shows subduction at a destructive margin: the dense oceanic plate descending beneath the continental plate, the ocean trench, and magma rising to feed a composite volcano.

Hazards produced:

• Explosive volcanic eruptions: high silica content makes magma viscous, trapping gases; pressure builds and eruptions can be catastrophic
• Earthquakes: generated at all depths as the plate descends; include some of the world's most powerful earthquakes

Landforms: ocean trenches, fold mountains, composite volcanoes, island arcs

Examples:

• Nazca Plate subducting beneath South America → the Andes mountain chain, Chilean volcanoes, devastating earthquakes in Chile, Peru, and Ecuador
• Pacific Plate subducting beneath Japan → the Japanese archipelago itself is a chain of volcanic islands; Japan experiences ~1,500 earthquakes per year

Where two oceanic plates converge, one subducts beneath the other, creating chains of volcanic islands called island arcs — Japan, the Philippines, and the Caribbean islands formed this way.

At a conservative margin, two plates slide past each other horizontally. Crust is neither created nor destroyed.

Process:

1. Plates move in the same direction but at different speeds, or in directly opposing directions
2. Friction prevents smooth sliding — the plates lock together and stress accumulates over decades or centuries
3. Eventually the locked section gives way suddenly — energy releases as a powerful, shallow earthquake
4. No magma is involved at any stage

Hazards produced:

• Earthquakes: often very powerful and shallow (close to the surface), making them particularly destructive at ground level
• No volcanic activity — there is no subduction, so no melting, so no magma

Landforms: transform faults, offset landscape features visible from the air

Examples:

• San Andreas Fault, California — the Pacific Plate moves northwest relative to the North American Plate at ~6 cm per year. The 1906 San Francisco earthquake (magnitude ~7.9) destroyed much of the city; the 1989 Loma Prieta earthquake (M6.9) killed 63 people during the World Series
• Caribbean Plate and North American Plate — a conservative margin runs through the northern Caribbean; the 2010 Haiti earthquake occurred on a fault associated with this boundary

1. Using "heavier" instead of "denser"

Oceanic crust subducts because it is more dense than continental crust, not because it is heavier. Density is mass per unit volume; weight depends on the total mass of an object. The correct scientific term in the context of subduction is always density.

2. Claiming conservative margins produce volcanoes

They do not. No subduction = no melting = no magma = no volcanic activity. This is one of the most common errors on AQA tectonic questions. If a question asks you to describe hazards at a conservative margin, earthquakes only.

3. Confusing constructive and destructive margins

A reliable way to remember: constructive = plates APART = new crust is CREATED = mild eruptions. Destructive = plates TOGETHER = old crust is DESTROYED = violent eruptions and powerful earthquakes. The name of the margin tells you what happens to the crust.

4. Describing distribution without referencing plate margins

"Earthquakes are found in South America, Asia, and California" describes locations without explaining why. The required answer connects location to process: "Earthquakes cluster along the boundaries of the Pacific Plate, particularly where oceanic crust subducts beneath surrounding continental plates."

5. Forgetting that tsunamis are earthquake-generated

Tsunamis are caused by the sudden vertical displacement of the ocean floor during a submarine earthquake at a subduction zone — not by surface storms. They travel at hundreds of kilometres per hour across the open ocean and build to enormous heights as they reach shallow coastal waters. Understanding their tectonic origin is necessary for explaining why monitoring and early warning systems focus on seismic detection.