Chapter 4: Earth’s Structure
An earthquake begins deep underground. Rock that has been locked under strain for years finally slips, and the shock spreads outward in every direction as waves travelling through solid rock. Thousands of kilometres away, in a quiet room, an instrument is waiting. A seismograph traces ground movement as a wavering line on a slowly moving chart, and this morning the line has been almost flat. Then the pen jumps. The first set of waves has arrived — the primary waves, the faster of the two. The line steadies, and the operator waits for the second set, the secondary waves, which always follow. They never arrive. Nothing is broken. The instrument is working, the earthquake was real, and stations on the near side of the earth recorded both sets clearly. Yet here, on the far side, only one kind of wave has come through. Something the waves had to cross on the way let the first set pass and stopped the second. No one has ever been inside the earth. What we know about its interior we have read off records like this one, and off the material volcanoes throw up from deep below. Those records say the earth is built in layers.
Diagram
Fig. 4.A1 Caption: Waves from an earthquake spread through the earth, but only one kind reaches the far side. Brief: Cutaway circle of the earth showing concentric layers unlabelled at this stage; arrows from a surface focus fanning through the interior, one set of arrows crossing the whole earth to the far side, a second set stopping short partway in; label only ‘primary waves’ and ‘secondary waves’ and the recording station on the far side.
4.1 How We Know About The Earth’s Interior
Almost everything else in Geography can be visited. A river, a plateau, a coastline and a volcano can all be walked up to and measured. The inside of the earth cannot. No excavation and no exploration has ever reached the deep interior, and nothing has ever been brought back from it by hand.
So the knowledge in this chapter is not built on digging. It is built on reading signals that arrive at the surface from below, and on working backwards from them. That is an ordinary way of knowing in science, and it is worth understanding before you meet the layers themselves.
The chapter draws on four lines of evidence, and each one answers a different question about the interior. (a) The study of seismic waves tells scientists about the density, pressure and temperature of the material the waves have passed through. (b) The molten material thrown up by volcanoes tells them what the interior is actually made of. (c) Meteorites, read alongside the theories of the origin of the earth, tell them what materials the whole earth is likely to be built from. (d) The heat the earth gives out tells them how hot the interior is.
Flowchart
Fig. 4.A2 Caption: Four kinds of evidence reach the surface, and each one answers a different question about the interior. Nodes: what reaches the surface → seismic waves recorded by instruments → density, pressure and temperature inside; → molten material from the magma chamber → the materials the interior is made of; → meteorites falling on the earth → the earth is made of the same materials; → heat given out by the earth → the temperature deep inside
Waves That Travel Through The Earth
Seismic waves are the waves sent out through the earth by an earthquake, and they are the single most useful signal we have. They start at one point, spread out in all directions, and pass through whatever lies in their way before some of them emerge again at the surface.
Two kinds matter here. The primary waves, written P-waves, reach the surface of the earth. The secondary waves, written S-waves, do not pass through liquids. That single fact does a great deal of work in this chapter, because it turns a missing signal into information: wherever S-waves fail to get through, something along their path must be liquid.
Scientific instruments record these waves as they arrive. From those records — which waves came, which did not, and how they behaved on the way — scientists have been able to work out the density, the pressure and the temperature of the interior of the earth. Nobody measured those three things directly. They were read off the waves.
Material Brought Up From Below
The second line of evidence arrives on its own. The molten material thrown up by volcanoes rises to the surface of the earth from the magma chamber, a store of molten rock lying deep inside the earth.
That material is a sample. It began below and ended up where it can be picked up, weighed and analysed. From studying it, scientists have come to know about the materials that constitute the interior of the earth. It is the closest thing to a direct look that we have.
Meteorites And The Earth’s Recipe
The third line of evidence falls out of the sky. Keeping in mind the theories of the origin of the earth, scientists study meteorites, pieces of rock and metal from space that fall on the surface of the earth.
The reasoning runs like this. If the earth formed alongside these bodies out of the same material, then what a meteorite is made of should also be what the earth is made of. On that basis the earth is said to be constituted of the same materials as the meteorites. Notice that this is an argument rather than an observation, and it depends on the theory of origin being right.
The Heat Inside
The fourth line of evidence is warmth. The heat given out by the earth comes from radioactive elements deep inside it — elements that break down slowly over very long periods and release heat as they do so.
The study of this heat in interior areas helps us to estimate the temperature of the interior of the earth. The word estimate is doing honest work in that sentence, and you will meet the estimated figures for each layer later in the chapter.
Taken together, these four lines of evidence agree with one another, and that agreement is why the picture in the rest of this chapter is trusted. When waves, lava, meteorites and heat all point to the same arrangement of material, the arrangement is unlikely to be wrong.
4.2 Earth’s Structure
The earth is made up of several concentric layers. Concentric means one inside another, all sharing the same centre, in the way the layers of an onion do. No layer sits beside another; each one wraps completely around the one below it.
The outer layer is the earth’s crust, which is also called the lithosphere. This is the shell of solid rock we stand on, and it is the only layer any of us ever touches.
Not all of that shell is dry land. Where the crust lies beneath the oceans and the seas, the water resting on it forms the hydrosphere, the earth’s covering of water. Extending upwards from the surface, the earth is enveloped by a blanket of gases which make up the atmosphere. Layers of rock below, water across much of the surface, gases above — that is the whole arrangement, and Figure 4.1 shows it in one drawing.
Diagram
Fig. 4.1 Caption: Layers of the earth. Brief: Reproduce as printed — concentric circles showing core at the centre, mantle around it, crust as the outer shell, with the ocean layer and the atmosphere outside; label crust (lithosphere), mantle, core, ocean (hydrosphere), atmosphere, Moho Discontinuity and Gutenberg Discontinuity exactly as printed.
The rest of this chapter works inwards from that outer shell. First it asks what the crust is chemically made of, and then it divides the whole interior into its three main layers and looks at each one in turn.
4.3 Chemical Composition
Chemical composition means what a layer is made of — which elements and minerals are present in it, and in what proportion. It is a different question from how thick a layer is or how hot it is, and answering it first turns out to explain a great deal about how the earth is arranged.
What The Crust Is Made Of
The lithosphere, the outermost crust of the earth, is the thinnest of all the layers. Thin here is relative: it is thin compared with the mantle and the core beneath it, not thin compared with anything on the surface.
It is made up of rocks with a large content of silicates, feldspar and mica. Silicates are minerals built mainly around silicon and oxygen, and feldspar and mica are two of the commonest rock-forming minerals found in them. The important point is that the crust is built from comparatively light materials.
Sial And Sima
The lithosphere can be divided into two broad divisions, called SIAL and SIMA. Both names are made from the chemical symbols of the elements that dominate each one. (a) The name Sial is taken from Silica (Si) and Aluminium (Al). (b) The name Sima is named after Silica (Si) and Magnesium (Mg).
The two are told apart by density, which is how much matter is packed into a given volume of a substance. Sial has the lower density of the two, and it therefore appears to float above Sima, which is the denser layer lying beneath it. Because Sial rides on top, it is the Sial layer that forms the continents, while Sima makes up the ocean floor. Both are part of the same crust and both contain silica; what separates them is density, and what follows from that density is where each one ends up.
Figure 4.2 shows this in cross-section, with a continent standing on the Sial and the Sima running on beneath it.
Diagram
Fig. 4.2 Caption: Chemical composition of crust. Brief: Reproduce as printed — cross-section showing a continent rising above sea level with the sea level line marked, the Sial mass forming the continent, and the Sima layer extending beneath it and out under the ocean; label Continent, Sea Level, SIAL and SIMA exactly as printed.
Nife At The Centre
The same reasoning about density carries all the way to the middle of the earth. The core of the earth is called Nife, a name made from Nickel (Ni) and Iron (Fe), the two metals it is made up of.
Since these are heavy metals with a high density, they are found deep inside the earth. That is the pattern the whole chapter rests on: the lighter material ends up near the surface and the heaviest material ends up at the centre.
Flowchart
Fig. 4.A3 Caption: Moving inwards from the surface, each layer named by its chemistry is denser than the one above it. Nodes: Sial — silica and aluminium, forms the continents → Sima — silica and magnesium, forms the ocean floor → Nife — nickel and iron, forms the core → density increasing at every step inwards
4.4 Internal Composition
Based on the chemical composition described above, the earth’s interior is divided into three layers. (a) The core lies at the centre and is made of the heaviest material. (b) The mantle lies above the core and takes up most of the earth’s volume. (c) The crust is the thin outer shell we live on.
Between each pair of layers there is a boundary, and each boundary has a name and a discoverable depth. The layers and their boundaries together make up the picture in Fig. 4.A4, which you should keep in view for the rest of the chapter.
Flowchart
Fig. 4.A4 Caption: Working inwards from the surface, each layer is separated from the next by a named boundary. Nodes: crust → Mohorovicic Discontinuity → mantle → upper mantle → lower mantle → Gutenberg Discontinuity → core → outer core → inner core → centre of the earth
4.4.1 Core
The core is the most inaccessible part of the earth, lying right at its centre. Nothing from it has ever been reached directly, and everything said about it here has been worked out from the kinds of evidence described in Section 4.1.
Its main characteristics are these. (a) It is the densest part of the earth, which is what puts it at the centre rather than anywhere else. (b) It is divided into an outer core and an inner core. (c) The entire core is about 7000 km in diameter. (d) The temperature ranges from about 4400 °C to about 6000 °C. (e) It is generally believed that the core is composed of iron and nickel.
That last point has a consequence you can measure at the surface with a compass. The iron and nickel of the core are responsible for the earth’s magnetism, and the magnetic field they produce is oriented towards the North and South Poles.
Why One Core Is Solid And The Other Is Not
The outer and inner core are made of the same materials but are not in the same state. The evidence for the difference is the behaviour of the waves. The S-waves disappear in the outer core, and since secondary waves cannot pass through liquids, this suggests the outer core is in a molten state.
The inner core is deeper still, and there the pressure and density are higher than anywhere else in the earth. The solid state of the inner core is due to that high density and pressure, which have compressed the molten rock material into a solid.
4.4.2 Mantle
The mantle is the layer lying below the crust and above the core. Its average thickness is 2900 km, and it makes up 84 per cent of the earth’s total volume, so in sheer bulk it is most of the earth.
The mantle is divided into two parts, the upper mantle and the lower mantle, and the difference between them is one of depth, temperature and state. (a) The upper mantle extends from the Moho Discontinuity down to a depth of 700 km, and it is cooler and made of solid rocks. (b) The lower mantle extends from 700 km down to 2900 km, and it is hotter and denser than the upper mantle.
Within the upper part there is a zone that behaves differently again. At a depth of around 100 to 410 km the mantle is partially molten, and this partially molten zone is known as the asthenosphere. Fig. 4.3 shows it lying directly beneath the crust, with the solid mantle below.
Diagram
Fig. 4.3 Caption: Asthenosphere. Brief: Reproduce as printed — block diagram showing a continent and an oceanic ridge above sea level, the crust as the top slab, the asthenosphere as the band beneath it and the mantle below that; label Continent, Oceanic ridge, Sea level, CRUST, ASTHENOSPHERE and MANTLE exactly as printed.
It would be reasonable to expect that the deeper you go the more molten the rock becomes, since it is getting hotter all the way down. That is not what happens. The lower mantle is solid despite being hotter than the asthenosphere above it, and it is solid because of high pressure. Pressure, not temperature alone, decides the state of the material.
The reason pressure matters so much is that it does not stay constant. As the pressure increases towards the interior of the earth, the density also increases, so every step inwards means material packed more tightly than the step before. Across the mantle as a whole the temperature varies between about 1000 °C and around 3,700 °C.
Fig. 4.4 sets out the whole interior with depths marked from the surface, and it is worth reading slowly.
Diagram
Fig. 4.4 Caption: Structure of the earth. Brief: Reproduce as printed — a wedge cut from the globe opened out to show the layers with depths marked from the surface; label Lithosphere (Crust and upper most solid mantle), Asthenosphere, Crust 0–100 km, Mantle, Outer Core, Inner Core, Core, and the depth markers 2900 km, 5100 km and 6378 km exactly as printed.
Every number on Fig. 4.4 is a depth below the surface, not a thickness. The centre of the earth sits at 6378 km, the top of the core at 2900 km and the boundary within the core at 5100 km. Because the figures are depths, a thickness has to be worked out by subtracting one from another, and the core’s own diameter can be checked the same way.
The result is worth pausing on. A radius of 3478 km out of 6378 km means the core alone accounts for more than half the distance from the centre of the earth to its surface. The layer we live on, at 60 km at its thickest, would not be visible as a separate band on a drawing of the earth made to true scale — which is exactly why Fig. 4.1 and Fig. 4.4 both have to exaggerate it.
4.4.3 Gutenberg Discontinuity
A discontinuity is a boundary inside the earth where the material changes sharply rather than gradually. The boundary between the mantle and the core — the mantle-core interface — is known as the Gutenberg Discontinuity, and it begins at a depth of 2800 km.
What marks it out is the behaviour of the waves passing through. Here there is an abrupt change in the seismic waves generated by earthquakes or explosions as they travel through the earth. (a) Primary seismic waves, the P waves, decrease in velocity as they cross it. (b) Secondary seismic waves, the S waves, disappear completely.
Below this boundary the material is dense, probably because of a greater percentage of iron. Notice how the argument runs: nobody saw the change of material, but two different waves changed their behaviour at the same depth, and one of them stopped altogether.
4.4.4 Crust Or Lithosphere
The crust is called the lithosphere because it is a sphere of solid rocks, from the word Lithos, meaning stones or rocks. Two facts about it sit oddly together and are worth holding at once: the crust is only 1 per cent of the earth’s mass, and it contains all known life in the universe.
It is also genuinely thin, and unevenly so. (a) Below high mountains the crust is about 60 km thick. (b) Below the oceans it is just 5 to 10 km thick. (c) Considering the size of the earth, the crust is in every case a relatively thin layer.
Conditions change quickly as you go down through it. The temperature of the upper part of the oceanic crust has been estimated at 0 °C, but it gradually starts rising towards the interior. The weight and pressure of the rocks also start increasing with depth, which is the beginning of the same increase that eventually holds the lower mantle and the inner core solid.
4.4.5 Mohorovicic Discontinuity
The discontinuity between the crust and the mantle is known as the Mohorovicic Discontinuity, usually shortened to the Moho Discontinuity. It was discovered by a Yugoslavian scientist, Andreaja Mohorovicic, in 1909.
Its chief features are these. (a) It forms the boundary between the crust and the mantle. (b) Like the crust itself, it does not exist at a uniform depth, and is found at about 8 km beneath the oceans and about 32 km beneath the continents. (c) It has the characteristics of both the layer above it, the crust, and the layer below it, the mantle.
The contrast with the Gutenberg Discontinuity is worth fixing. Both are boundaries and both were found from the behaviour of seismic waves, but the Moho lies between crust and mantle at a few tens of kilometres and shares the characteristics of both, while the Gutenberg lies between mantle and core at 2800 km and is where the S waves stop altogether.
A Crust That Does Not Stay Still
Because of the continuous changes in the earth’s crust due to the forces of nature, the thickness of the lithosphere is very fragile compared with the nature and composition of the earth as a whole. The layer we live on is the layer that alters.
Two theories describe how it alters. The Theory of Continental Drift, put forward by Wegener in the early 20th century, and the Theory of Plate Tectonics, put forth in the 1960s, both suggest that the earth’s crust undergoes structural changes. Besides this, the lateral movements of the plates give the earth a new shape, though very slowly, over millions of years.
4.5 Summary
- Our knowledge of the earth’s interior does not come from excavation or exploration, but from four indirect lines of evidence.
- Seismic waves, recorded by scientific instruments, reveal the density, pressure and temperature inside.
- Molten material thrown up by volcanoes from the magma chamber shows what the interior is made of.
- Meteorites, read alongside the theories of the origin of the earth, suggest the earth is made of the same materials.
- The heat given out by radioactive elements deep inside allows the interior temperature to be estimated.
- Primary waves, the P-waves, reach the surface of the earth, while secondary waves, the S-waves, do not pass through liquids.
- The earth is made up of several concentric layers, one inside another.
- The outer layer of solid rock is the crust, also called the lithosphere.
- The water lying over the crust beneath the oceans and seas forms the hydrosphere.
- The blanket of gases extending upwards forms the atmosphere.
- By chemical composition, the lithosphere is the thinnest layer and is made of rocks with a large content of silicates, feldspar and mica.
- The lithosphere has two broad divisions, named after the elements that dominate them.
- Sial, from Silica and Aluminium, has the lower density, appears to float above Sima, and forms the continents.
- Sima, from Silica and Magnesium, is denser and makes up the ocean floor.
- The core is called Nife, from Nickel and Iron. These are heavy metals of high density, which is why they are found deep inside the earth.
- On the basis of this chemical composition, the interior is divided into three layers — Core, Mantle and Crust.
- The core is the most inaccessible and the densest part of the earth, about 7000 km in diameter, with temperatures from about 4400 °C to about 6000 °C.
- It is divided into an outer core and an inner core, and is believed to be composed of iron and nickel, which is responsible for the earth’s magnetism, oriented towards the North and South Poles.
- The S-waves disappear in the outer core, which suggests it is molten.
- The inner core is solid because high density and pressure have compressed the molten rock material.
- The mantle lies below the crust, has an average thickness of 2900 km and makes up 84 per cent of the earth’s total volume, with temperatures between about 1000 °C and around 3,700 °C.
- The upper mantle runs from the Moho Discontinuity to 700 km and is cooler and made of solid rocks.
- The lower mantle runs from 700 km to 2900 km and is hotter and denser than the upper mantle, but stays solid because of high pressure.
- At a depth of around 100 to 410 km the mantle is partially molten, and this zone is the asthenosphere.
- As pressure increases towards the interior of the earth, the density also increases.
- The Gutenberg Discontinuity is the boundary between the mantle and the core, beginning at a depth of 2800 km, where seismic waves change abruptly — P waves slow down and S waves disappear completely. The material below is dense, probably because of a greater percentage of iron.
- The crust is called the lithosphere because it is a sphere of solid rocks, and although it is only 1 per cent of the earth’s mass, it contains all known life in the universe.
- It is about 60 km thick below high mountains and just 5 to 10 km thick below the oceans.
- The upper part of the oceanic crust has an estimated temperature of 0 °C, rising gradually towards the interior, while the weight and pressure of rocks also increase with depth.
- The Mohorovicic or Moho Discontinuity is the boundary between the crust and the mantle, discovered by the Yugoslavian scientist Andreaja Mohorovicic in 1909. It lies at about 8 km beneath the oceans and about 32 km beneath the continents, and has the characteristics of both the crust above and the mantle below.
- The crust does not stay still. The Theory of Continental Drift, put forward by Wegener in the early 20th century, and the Theory of Plate Tectonics, put forth in the 1960s, suggest that the crust undergoes structural changes, and the lateral movements of the plates slowly give the earth a new shape over millions of years.
Flowchart
Fig. 4.A5 Caption: The whole interior in one chain, from the surface to the centre, with each layer’s depth, material and state. Nodes: surface → crust, 5 to 10 km under the oceans and about 60 km under high mountains, solid rock of Sial and Sima → Mohorovicic Discontinuity, about 8 km deep under the oceans and about 32 km under the continents → upper mantle, Moho to 700 km, cooler and solid, with the partially molten asthenosphere at about 100 to 410 km → lower mantle, 700 km to 2900 km, hotter and denser but solid under pressure → Gutenberg Discontinuity at 2800 km, P waves slow and S waves stop → outer core, molten, iron and nickel → inner core, solid under high density and pressure, about 4400 °C to 6000 °C → centre of the earth at 6378 km
What This Chapter Cannot Tell You
Every figure in this chapter is an estimate arrived at indirectly, and no instrument has ever been placed in the mantle or the core to check one. The temperatures in particular are ranges, not readings.
The drawings cannot be trusted for scale. A crust of 60 km against an earth of 6378 km would be almost invisible if Fig. 4.1 and Fig. 4.4 were drawn honestly, so both exaggerate it to make labelling possible.
The chapter also stops short of explaining what it raises at the end. It states that the crust undergoes structural changes and that plates move laterally, but it does not say what drives that movement, or how fast, or where.
4.6 Check Your Concepts
Study Fig. 4.2. Sial and Sima are both part of the same crust, yet Sial forms the continents and Sima forms the ocean floor. Explain why, using what the chapter says about density.
Fig. 4.4 marks depths measured from the surface: the top of the core at 2900 km, a boundary within the core at 5100 km, and the centre of the earth at 6378 km. Using only these figures, calculate the thickness of the outer core and the thickness of the inner core, showing every step of your working.
The mantle is partially molten at a depth of around 100 to 410 km, but the lower mantle below 700 km is solid even though it is hotter. Explain how this is possible, and state which factor decides whether the material is solid or molten.
The chapter says that the study of heat helps us to estimate the temperature of the interior of the earth. Explain why the word estimate is used rather than measure, and state one thing this kind of evidence cannot tell us.
How do meteorites help scientists to know about the interior of the earth?
Why is the earth’s interior, in most part, found in a solid state despite great heat and pressure?
Distinguish between the Gutenberg Discontinuity and the Mohorovicic Discontinuity.
Describe the layers of the interior of the earth and their chemical composition.
What would happen if the earth’s core no longer remained hot and the magma cooled and hardened inside the earth? Give reasons to support your answer.
4.7 Case-Based Questions
- Look at the figure given beside the question and answer the following:
- Label the parts marked (1), (2) and (3).
- Name the state — solid, liquid or gas — in which each part exists.
- Which part of the earth is suitable for human habitation, and why?