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