The structure of the Earth
FoundationThe Earth is like an egg: crust (shell), mantle (white), core (yolk). Crust plus upper mantle = the lithosphere — the layer broken into plates.
- Three layers: crust, mantle, core
- Continental crust thick; oceanic thin
- Lithosphere = crust + upper mantle = where plates are
§Text Passage
1The Earth is not a solid lump of rock; like a soft-boiled egg it comes in layers: the crust, the mantle and the core. This lesson takes you from the surface down to the centre, looking at each layer's state, temperature and material — and at why the 'solid Earth' is always on the move.
2Why the layers? When the young Earth was molten, heavy iron and nickel sank to the centre to form the core, while lighter rock floated outwards to form the crust. The mantle makes up more than 80% of Earth's volume, and its slow-flowing asthenosphere is the engine that drives the plates.
3The examples are striking: the deepest hole humans have ever drilled, Russia's Kola Superdeep Borehole, reached only 12 km — not even through the crust. Yet we can 'X-ray' the planet with seismic waves, and that is exactly how scientists discovered that the outer core is liquid.
1Summary 重點摘要
The Earth is like an egg: crust (shell), mantle (white), core (yolk). Crust plus upper mantle = the lithosphere — the layer broken into plates.
2Vocabulary cards Vocabulary
The thin, solid outer layer of rock — like an eggshell.
The thick middle layer; its rock can flow very slowly.
The innermost layer, mostly iron and nickel; liquid outer core, solid inner core.
Crust plus uppermost mantle; broken into plates.
The softer, slowly flowing layer beneath; plates move over it.
The crust beneath continents, thick at about 30–70 km.
The crust beneath the oceans, thin at about 5–10 km.
Waves released by earthquakes; scientists use them to probe the Earth's interior.
3Step by step Steps
- The three-layer eggInside, the Earth is like a three-layer egg: a thin crust outermost, the thick mantle in the middle, and the core at the centre. The deeper one goes, the hotter it gets and the denser the material. The core is made mostly of iron and nickel; the mantle consists of hot rock and alone takes up more than 80% of the Earth's volume; the crust is the thinnest layer beneath our feet. The egg analogy makes the order and thickness of the layers easy to remember.
- The crust is unevenThe crust comes in two kinds and is far from uniform: continental crust is thick — about 30 to 70 km — and granitic, while oceanic crust is thin — about 5 to 10 km — and basaltic. Crust plus the uppermost mantle together form the lithosphere, which is broken into plates floating on the softer asthenosphere. Remembering the thickness and rock types of the two crusts lays a solid foundation for explaining plate movement.
- The lithosphere movesBeneath the lithosphere lies the asthenosphere, softer and capable of slow flow; the whole lithosphere floats on it, broken into large plates that creep along at a few centimetres a year. This slow flow is the engine that drives the plates, and plate tectonics starts from exactly this point. In other words, the ground under our feet, though it seems still, is always moving — too slowly for anyone to feel in a single lifetime.
- How do we know?No one has ever seen the deep Earth directly: the deepest drilling of all — Russia's Kola Superdeep Borehole — reached only about 12 km, not even through the crust. Scientists instead take X-rays of the planet with seismic waves: after an earthquake, the waves travel through the interior to stations on the far side, and by analysing how their speeds change through each layer, scientists work out the thickness and state of every layer — that is how the liquid outer core was found.
- Two waves, two jobsSeismic waves come in two main kinds: P-waves travel fast and pass through solids and liquids alike, while S-waves are slower and can cross only solids. After a quake, stations on the far side of the Earth receive no S-waves, from which scientists deduced that the outer core is liquid; combined with the way wave speeds change in different materials, they mapped the boundaries between crust, mantle and core. The two waves divide the work, and together they are the pair of eyes that sees inside the planet.
- The core's heat and magnetismThe deeper one goes the hotter it gets, and the core reaches some 5000–6000°C, about as hot as the surface of the sun. The liquid iron-nickel of the outer core keeps churning and generates the magnetic field that wraps the whole planet: compasses point north and migrating birds find their way thanks to this invisible shield, which also deflects the charged particles streaming from the sun and protects the atmosphere and life. Unreachable though it is, the core shapes life at the surface at every moment.
4Diagrams Diagrams
5Worked example Example
Example: order mantle, core and crust from outside in.
- Outermost: crust — where we stand
- Middle: mantle — the thickest layer
- Innermost: core — mostly iron and nickel
- Memory aid: hotter and denser inward — the order never reverses
- Answer: From outside in: crust → mantle → core. The crust is the thin outer shell we stand on; the mantle is the thickest layer of hot rock; the core at the centre is mostly iron and nickel, the hottest and densest of all. Remember 'hotter and denser inward' and the order can never be reversed — a quick sketch helps in exams.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is the Earth's outermost layer called?
Q2Which two metals make up the core?
Q3Which two parts make up a plate?
Q4What is special about S-waves, and what did they help scientists discover?
Q5Which rock dominates continental crust and which dominates oceanic crust, and which crust is thicker?
8Recap Recap
- Three layers: crust, mantle, core
- Continental crust thick; oceanic thin
- Lithosphere = crust + upper mantle = where plates are
- The outer core is liquid; the inner core is solid
- The deeper you go, the hotter and denser it gets