Earth's Structure
Journey to the centre of the Earth — explore the layers beneath our feet, understand how tectonic plates move, and discover why earthquakes and volcanoes occur where they do.
The Layers of the Earth
The Earth is structured in concentric layers, like an onion. Scientists study the Earth's interior using seismic waves from earthquakes — these waves travel at different speeds through different materials, allowing us to map the interior without drilling there.
Cross-Section of the Earth
| Layer | Thickness | State | Key Facts |
|---|---|---|---|
| Crust | 5–70 km | Solid | Thinnest layer; oceanic crust (basalt, 5–10 km) and continental crust (granite, up to 70 km) |
| Mantle | ~2900 km | Solid but can flow slowly | Largest layer; convection currents in upper mantle drive plate movement; rock can flow over millions of years |
| Outer Core | ~2200 km | Liquid | Mainly iron and nickel; movement of liquid metal generates Earth's magnetic field |
| Inner Core | ~1200 km (radius) | Solid | Solid iron and nickel under enormous pressure; temperature ~5000–6000°C (hotter than surface of Sun) |
Plate Tectonics
The Earth's crust and upper mantle (together called the lithosphere) is broken into large pieces called tectonic plates. These plates float on the semi-molten upper mantle (asthenosphere) and move very slowly — about 2–10 cm per year — driven by convection currents in the mantle.
Divergent Boundaries
Plates move apart. New crust forms as magma rises and solidifies (seafloor spreading). Creates mid-ocean ridges. Example: Mid-Atlantic Ridge. Can create rift valleys on land (e.g. East African Rift).
Convergent Boundaries
Plates move towards each other. Denser oceanic crust sinks beneath lighter continental crust (subduction). Creates deep ocean trenches, volcanic arcs, and mountain ranges. Example: Himalayas (two continental plates colliding).
Transform Boundaries
Plates slide past each other horizontally. Creates fault lines. Frequent earthquakes but little volcanic activity. Example: San Andreas Fault (California, USA).
Evidence for plate tectonics: (1) The shapes of continents fit together like puzzle pieces (e.g. South America and Africa). (2) Matching fossils on separated continents. (3) Matching rock types and mountain ranges across oceans. (4) Seafloor spreading and paleomagnetism data. Alfred Wegener first proposed "continental drift" in 1912.
Earthquakes and Volcanoes
Earthquakes
Earthquakes occur when stress builds up along fault lines and is suddenly released, sending seismic waves through the Earth. The point underground where the earthquake originates is the focus (hypocentre). The point on the surface directly above it is the epicentre.
- P-waves (Primary): Longitudinal waves; fastest; travel through solids and liquids; arrive first.
- S-waves (Secondary): Transverse waves; slower; travel only through solids; cannot pass through outer core (liquid).
- Richter scale: Measures earthquake magnitude. Logarithmic — magnitude 6 is 10× more powerful than magnitude 5.
- Most earthquakes occur along tectonic plate boundaries, especially convergent and transform.
Volcanoes
Volcanoes form where magma (molten rock) from the mantle reaches the surface. Once magma erupts from a volcano, it is called lava. Volcanoes are most common at:
- Subduction zones: Water from subducting oceanic crust lowers the melting point of the mantle, producing magma (e.g. Ring of Fire around the Pacific).
- Divergent boundaries: Magma rises through the gap as plates separate (e.g. Iceland).
- Hot spots: Mantle plumes of exceptionally hot rock melt through the overlying plate (e.g. Hawaiian Islands). Not at plate boundaries.
Australia is one of the most geologically stable continents because it sits in the middle of the Indo-Australian plate, far from plate boundaries.
Key Vocabulary
Lithosphere
The rigid outer layer of the Earth, made up of the crust and the upper solid part of the mantle. It is broken into tectonic plates that move relative to each other.
Subduction
The process where a denser tectonic plate (usually oceanic) is forced beneath a less dense plate at a convergent boundary. Subducting crust melts and can form volcanoes.
Seismic Waves
Vibrations that travel through the Earth following an earthquake or explosion. P-waves (longitudinal) travel through solids and liquids; S-waves (transverse) travel only through solids.
Convection Current
The circular movement of heated material (like mantle rock) rising, spreading, cooling, and sinking. Convection currents in the mantle are the driving force behind tectonic plate movement.
Worked Examples
Explain why the outer core is liquid but the inner core is solid, even though the inner core is hotter.
The inner core is at a similar or higher temperature (~5000–6000°C) but is under immense pressure from the weight of all the layers above it.
This extreme pressure raises the melting point of the iron-nickel mix to above its actual temperature, keeping it solid.
The outer core is at slightly lower pressure, so its melting point is lower, allowing it to remain in a liquid state. The liquid outer core is what generates Earth's magnetic field through convection.
Why are there so many volcanoes around the edge of the Pacific Ocean (the "Ring of Fire")?
The Pacific Ocean floor is made of dense oceanic crust on the Pacific Plate.
At the edges, this dense plate is being subducted (forced under) the surrounding, less dense continental plates.
As the oceanic crust descends, it carries water into the mantle, which lowers the melting point of mantle rock, creating magma. This magma rises through the continental plate, forming chains of volcanoes. Japan, the Philippines, and the Andes all sit on the Ring of Fire.
List three pieces of evidence that support the theory of plate tectonics.
1. Continental fit: The coastlines of South America and Africa match like puzzle pieces, suggesting they were once joined.
2. Fossil evidence: Identical fossils of the same species (e.g. Mesosaurus, Glossopteris) are found on continents now separated by thousands of kilometres of ocean.
3. Paleomagnetism / seafloor spreading: Matching patterns of magnetic reversals in rock on either side of mid-ocean ridges show that the seafloor is spreading outward from central ridges, exactly as plate tectonics predicts.
Knowledge Check
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Key Concepts Summary
- ✓ The Earth has four main layers: crust (thin, solid), mantle (thick, can flow), outer core (liquid iron/nickel), inner core (solid, extremely hot).
- ✓ Tectonic plates are large sections of the lithosphere that move 2–10 cm/year, driven by mantle convection currents.
- ✓ Plate boundaries: divergent (spreading apart), convergent (collision/subduction), transform (sliding past).
- ✓ Earthquakes originate at the focus; the point directly above on the surface is the epicentre.
- ✓ Volcanoes form at subduction zones, divergent boundaries, and hot spots.
- ✓ Evidence for plate tectonics includes continental fit, matching fossils, and paleomagnetism.