The Structure of the Earth and Plate Tectonics

The Earth is not a solid, static sphere. Beneath the thin surface we live on, the planet is layered, dynamic and constantly changing. The movement of enormous slabs of rock — tectonic plates — across the Earth's surface is responsible for earthquakes, volcanic eruptions, mountain building and the shape of the continents themselves. Understanding the Earth's structure and the theory of plate tectonics is essential for explaining why tectonic hazards occur where they do.

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The Internal Structure of the Earth

The Earth has a layered structure, with each layer having different physical properties, temperatures and compositions. Scientists have determined this structure primarily through the study of seismic waves — vibrations that travel through the Earth during earthquakes.

The Four Main Layers

Layer	Depth	Thickness	State	Composition	Temperature
Crust	0–70 km	5–70 km	Solid	Silicate rocks (lighter minerals)	Up to ~1,000°C at base
Mantle	70–2,900 km	~2,830 km	Semi-molten (plastic/viscous) in upper part; solid in lower part	Silicate rocks rich in iron and magnesium	1,000–3,700°C
Outer core	2,900–5,100 km	~2,200 km	Liquid	Iron and nickel	3,700–4,400°C
Inner core	5,100–6,371 km	~1,270 km radius	Solid (despite extreme heat, immense pressure keeps it solid)	Iron and nickel	Up to ~5,500°C

Exam Tip: Remember that the mantle is not fully liquid. The upper mantle (called the asthenosphere) is semi-molten and can flow very slowly (a few centimetres per year), which allows tectonic plates to move. The lower mantle is solid but can deform under extreme pressure over geological timescales.

Two Types of Crust

A critical distinction for understanding plate tectonics is the difference between oceanic crust and continental crust:

Property	Oceanic Crust	Continental Crust
Thickness	5–10 km (thin)	25–70 km (thick)
Density	~3.0 g/cm³ (dense)	~2.7 g/cm³ (less dense)
Age	Relatively young (mostly <200 million years)	Very old (up to 4 billion years)
Composition	Basalt (dark, iron-rich)	Granite (lighter, silica-rich)
Can be subducted?	Yes — its higher density means it sinks beneath continental crust	No — too buoyant to be subducted

This difference in density is crucial: when oceanic and continental plates collide, the denser oceanic plate is always forced beneath the continental plate in a process called subduction.

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Evidence for Continental Drift

The idea that the continents move was first proposed by German meteorologist Alfred Wegener in 1912. He called his theory continental drift and suggested that all the continents were once joined together in a single supercontinent he named Pangaea (meaning "all lands"). Pangaea began to break apart approximately 200 million years ago, and the fragments gradually drifted to their current positions.

Wegener's evidence included:

Evidence	Explanation
Continental fit	The coastlines of South America and Africa fit together like jigsaw pieces — particularly the eastern bulge of South America into the Gulf of Guinea in West Africa
Fossil evidence	Identical fossils of the freshwater reptile Mesosaurus were found in both South America and Africa. This organism could not have crossed the Atlantic Ocean, suggesting the continents were once joined
Rock evidence	Matching rock types and mountain chains are found on continents now separated by oceans. The Appalachian Mountains (USA) and Caledonian Mountains (Scotland/Scandinavia) are the same age and composition
Glacial evidence	Glacial deposits and scratched rocks (striations) from the same ice age (~300 million years ago) are found in South America, Africa, India, Antarctica and Australia — regions now in very different climate zones. This makes sense if they were once grouped near the South Pole as part of Pangaea
Coal deposits	Coal (formed from tropical swamp forests) is found in places that are now cold, such as Antarctica and northern Europe, suggesting these landmasses were once in tropical latitudes

Wegener's theory was rejected by most scientists during his lifetime because he could not explain the mechanism — what force could move entire continents? The answer came decades later with the discovery of convection currents in the mantle.

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The Theory of Plate Tectonics

The modern theory of plate tectonics was developed in the 1960s, building on Wegener's work and incorporating new evidence from sea-floor exploration. It explains that the Earth's outer shell (the lithosphere) is divided into large, rigid sections called tectonic plates that float on the semi-molten asthenosphere below.

Key Evidence That Proved Plate Tectonics

Evidence	Explanation
Sea-floor spreading	In the 1960s, Harry Hess proposed that new oceanic crust is continuously created at mid-ocean ridges where magma wells up from the mantle. The new crust pushes older crust outward on both sides, like a conveyor belt
Magnetic striping	The Earth's magnetic field reverses periodically. As new rock forms at mid-ocean ridges, iron minerals align with the current magnetic field and are frozen in place as the rock solidifies. This produces symmetrical stripes of alternating magnetic polarity on either side of the ridge — proving that new rock is continuously formed and pushed outward
Age of ocean floor	Rock samples from the ocean floor show that the youngest rock is at the mid-ocean ridges and the oldest rock is furthest from the ridges (near the continents). This confirms sea-floor spreading
Earthquake and volcano distribution	Earthquakes and volcanoes are concentrated along narrow belts that correspond precisely to the edges of tectonic plates. This pattern would be impossible if the Earth's crust were a single, unbroken shell

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What Drives Plate Movement?

Tectonic plates are moved by convection currents in the mantle. These are slow-moving circular flows of semi-molten rock driven by heat from the Earth's interior:

1. The core heats the base of the mantle. Hot mantle rock becomes less dense and rises slowly towards the surface.
2. When the rising rock reaches the base of the lithosphere, it spreads outward horizontally, dragging the overlying plate with it through friction.
3. As the rock moves away from the heat source, it cools, becomes denser and eventually sinks back down towards the core — this is called a convection current.
4. The cycle repeats continuously, driving plate movement at rates of 1–15 cm per year.

Additional driving forces include:

Force	Explanation
Ridge push	New, elevated crust at mid-ocean ridges pushes older crust outward under gravity
Slab pull	At subduction zones, the dense, sinking oceanic plate pulls the rest of the plate behind it — this is thought to be the strongest driving force
Mantle drag	Friction between the convection currents and the base of the plates drags plates along

graph TD
    A["Hot mantle rock rises<br/>near mid-ocean ridge"] -->|"Spreads outward"| B["Drags plate horizontally<br/>(mantle drag)"]
    B -->|"Rock cools and becomes dense"| C["Cold, dense rock sinks<br/>at subduction zone<br/>(slab pull)"]
    C -->|"Returns to deep mantle"| D["Reheated by core"]
    D -->|"Rises again"| A
    E["New crust forms at ridge<br/>(ridge push)"] -->|"Pushes older crust outward"| B

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The Major Tectonic Plates

The Earth's lithosphere is divided into approximately 15 major plates and several smaller ones: