The Structure of the Earth
Senior 1 · Geography
The Structure of the Earth
Lesson 2 of 3
22 min
The deepest hole ever drilled, in Russia, reached about 12 km down. The centre of the Earth is 6,371 km down, so the drill went less than one five-hundredth of the way. Yet scientists can describe the inside of the Earth layer by layer. In this lesson you will learn what the layers are, and how we know.
Figure 1. The inside of the Earth. The mantle, outer core and inner core are drawn in true proportion; the crust is far too thin to see at this scale, so it has been drawn thicker.
Definition
Crust: the thin, solid, rocky outer layer of the Earth. Continental crust, under the land, is 30 to 70 km thick and made mainly of light rocks such as granite (rich in silica and aluminium, sometimes called sial). Oceanic crust, under the oceans, is only 5 to 10 km thick and made of heavier basalt (silica and magnesium, sima). Example: the granite and quartzite hills of Rwanda are continental crust.
Definition
Mantle: the thick layer of very hot rock beneath the crust, reaching down to about 2,890 km. It is solid, but so hot that it flows extremely slowly, a few centimetres a year, like very stiff toffee. Its upper part, just below the lithosphere, is softer; the plates of the lithosphere slide over it.
Definition
Core: the centre of the Earth, made mainly of iron and nickel. The outer core (2,890 to 5,150 km down) is liquid; the inner core (5,150 km to the centre) is solid, because the enormous pressure there stops the metal from melting even at over 5,000 °C.
Definition
Discontinuity: a boundary where the rock or metal changes suddenly, so that earthquake waves change speed. The boundary between the crust and the mantle is the Mohorovičić discontinuity (the "Moho"); between the mantle and the core is the Gutenberg discontinuity.
| Layer | Depth (km) | State | Main materials | Temperature |
|---|---|---|---|---|
| Crust | 0 to 5–70 | Solid | Granite (continents), basalt (ocean floor) | From surface temperature to several hundred °C at its base |
| Mantle | to 2,890 | Solid, but flows very slowly | Rocks rich in iron and magnesium | About 1,000 °C near the top to about 3,700 °C at the bottom |
| Outer core | 2,890 to 5,150 | Liquid | Iron and nickel | About 4,000 to 5,000 °C |
| Inner core | 5,150 to 6,371 | Solid | Iron and nickel | About 5,000 to 5,400 °C, as hot as the surface of the Sun |
Notice two patterns: going down, both the temperature and the density (heaviness) increase. The heaviest materials, iron and nickel, sank to the centre when the young Earth was molten.
Worked example 1: how thin is the crust?
Imagine the Earth shrunk to the size of a football with a radius of 11 cm (110 mm). How thick would continental crust, about 35 km thick, be on this ball?
That is about as thick as six pages of an exercise book. The liquid outer core would begin mm below the surface of the ball, nearly halfway to the centre.
Worked example 2: getting hotter with depth
In the crust, temperature rises on average by about for every kilometre you go down. A borehole is drilled 2 km deep where the surface temperature is . Estimate the temperature at the bottom.
Deep mines have to be cooled with chilled air so that miners can work. Where hot rock lies close to the surface, the heat can be used to make electricity (geothermal power).
Important
Case study: the crust beneath Rwanda
Most of Rwanda stands on old continental crust made of granite, quartzite and schist, rocks more than a billion years old. These old rocks hold the tin, tantalum (coltan) and tungsten ores that Rwanda mines and exports. In the far west the picture changes: along the Albertine Rift the crust is being stretched and thinned. Where it is thin and cracked, molten rock from the mantle can rise, which is why the Virunga volcanoes, the hot springs by Lake Kivu and frequent small earthquakes are all found in the west of the country and not in the east. You will study this in the next lesson.
Three minutes to the centre of the Earth, by the BBC (3.5 min). As you watch, list the layers in order and note the temperature and state (solid or liquid) of each.
No one has ever drilled through the crust, let alone reached the core. How can scientists be confident that the outer core is liquid but the inner core is solid? Which piece of evidence do you find most convincing, and why?
Warning
The mantle is not a sea of lava. Many people imagine liquid rock under the crust. The mantle is almost entirely solid; it only flows over millions of years. Molten rock (magma) forms in small pockets where pressure drops or water is added, as happens under rift valleys and volcanoes.
Tip
Think of a boiled egg: the shell is the thin crust, the white is the mantle and the yolk is the core. Then remember: the outer core is the liquid one; the inner core is squeezed solid by the weight of everything above it.
Practice
Remember
Sources and credits
The lesson text, cut-away diagram, table, worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
