Lesson 2: Interior of the Earth

Rule 1: Why the inside is hot — The Earth formed about 4.6 billion years ago as a hot gaseous ball. It cooled by radiation from the outside towards the centre. Like the cream on boiled milk, the outer crust cooled and became solid first, while the inside is still hot and semi-liquid. The outer planets are even now gaseous.

In the figure : In the figure indirect evidence about the interior: volcanic eruptions, the rising temperature in deep mines, seismic waves and bore holes (which reach only a tiny part).

In the figure : In the figure, like the cream on boiled milk, the outer layer of the Earth, the crust, cooled first and became solid, while the inside is still hot.

• Easy : Easy: Cream on milk cools first, like the crust.
• Medium : Medium: The Earth is 4.6 billion years old, so 4600 million years.
• Exam-type : Exam: Why is the inside still hot?

Rule 2: Four clues — (1) Volcanic eruptions throw out hot magma, gases and steam; cooled lava forms igneous rocks. (2) Temperature rises as we go deep into a mine. (3) Seismic waves of earthquakes travel through the interior; their direction and speed tell us what is inside. (4) Bore holes: a 7 km hole is being dug in the Koyna–Varna region, but the radius of the Earth is 6371 km, so it reaches a tiny part: 7 ÷ 6371 × 100 = 0.11%. We cannot dig through the Earth because of extreme heat, pressure and molten parts.

• Easy : Easy: Lava cools into igneous rock.
• Medium : Medium: A 3 km mine is hotter at the bottom than at the top.
• Exam-type : Exam: Name two indirect evidences.

Rule 3: Three layers — While going from the surface to the centre, the temperature and density change greatly. On this basis the interior is divided into crust, mantle and core.

• Easy : Easy: There are 3 layers.
• Medium : Medium: Crust is the outermost; core is the innermost.
• Exam-type : Exam: On what basis are layers made?

Spot the difference

Rule 1: Thickness — The average thickness is 30 to 35 km. It is 16 to 45 km below the continents, about 40 km below the mountain ranges and less than 10 km below the oceans.

In the figure : In the figure the crust is thickest below the mountain ranges (about 40 km), about 30 km thick below the continents and only 7 to 10 km below the oceans.

• Easy : Easy: Crust is thinnest below the oceans.
• Medium : Medium: Mountain crust 40 km − ocean crust 10 km = 30 km thicker.
• Exam-type : Exam: Where is the crust thickest?

Rule 2: Continental and oceanic crust — Continental crust: silica and aluminium, earlier called Sial; about 30 km thick; density 2.65–2.90 g/cm³; mainly granite. Oceanic crust: silica and magnesium, earlier called Sima; 7–10 km thick; density 2.9–3.3 g/cm³; mainly basalt and gabbro. So oceanic crust is thinner but denser. The Conrad discontinuity lies between them.

In the figure : In the figure continental crust: silica and aluminium (Sial), granite, about 30 km. Oceanic crust: silica and magnesium (Sima), basalt and gabbro, 7–10 km.

• Easy : Easy: Sial is continental crust.
• Medium : Medium: A 30 km continental crust is 30 − 10 = 20 km thicker than a 10 km oceanic crust.
• Exam-type : Exam: Compare the two crusts in thickness and density.

Rule 3: Temperature and the water-rock link — As we go deeper the temperature increases; then the rate of increase reduces, and in the core it increases again. Crust rocks keep changing form through the rock cycle: igneous, sedimentary, metamorphic.

In the figure : In the figure the temperature rises fast near the surface, more slowly through the mantle and fast again in the core (about 5000 °C in the outer core).

• Easy : Easy: Rocks change form in the rock cycle.
• Medium : Medium: Igneous rock can become sedimentary after weathering.
• Exam-type : Exam: Name the three rock types.

Spot the difference

Rule 1: Upper and lower mantle — The mantle has an upper and a lower part. The upper mantle is more liquid, has the magma chambers and is also called the asthenosphere (दुर्बलावरण); deep-seated earthquakes are usually found here. Endogenetic movements in the mantle cause mountains, rifts, volcanoes and earthquakes. The Moho discontinuity lies between the crust and the mantle.

In the figure : In the figure the upper mantle (asthenosphere) holds the magma chambers; the crust and the uppermost mantle form the lithosphere, about 100 km thick.

• Easy : Easy: The lithosphere is about 100 km thick.
• Medium : Medium: If the crust is 35 km, the mantle part of the lithosphere is 100 − 35 = 65 km.
• Exam-type : Exam: Why is the mantle the centre of earthquakes and volcanoes?

Rule 2: Density and temperature — At 2400–2900 km depth the temperature is about 2200 °C to 2500 °C, and the material changes suddenly. The mantle's average density is 4.5 g/cm³ and increases with depth because pressure increases; the lower mantle has about 5.7 g/cm³.

In the figure : In the figure the density increases with depth: crust 2.65–3.3, mantle 4.5–5.7, outer core 9.8 and inner core 13.3 g/cm³.

• Easy : Easy: Density increases with depth.
• Medium : Medium: Lower mantle 5.7 − average 4.5 = 1.2 g/cm³ more.
• Exam-type : Exam: Why does density increase with depth?

Spot the difference

Rule 1: Outer and inner core — The core is about 3471 km thick (6371 − 2900). The outer core (2900–5100 km) is liquid or semi-liquid, rich in iron, density 9.8 g/cm³, about 5000 °C. The inner core (5150–6371 km) is solid because of extreme pressure, density about 13.3 g/cm³. Its main elements are iron (Fe) and nickel (Ni), so it is called Nife. Its temperature is almost equal to the surface of the Sun. The Gutenberg discontinuity lies between the mantle and the core.

In the figure : In the figure the Earth has three main layers: the crust (30–35 km), the mantle (up to 2870 km) and the core, with a liquid outer core (2900–5100 km) and a solid inner core (5150–6371 km).

• Easy : Easy: Outer core is liquid.
• Medium : Medium: Inner core: 6371 − 5150 = 1221 km thick.
• Exam-type : Exam: Why is the inner core solid though hottest?

Rule 2: Thickness of each layer — Mantle about 2870 km; outer core 5100 − 2900 = 2200 km; inner core 6371 − 5150 = 1221 km; crust very thin. Main elements: crust – silicon and aluminium; mantle – silicon and magnesium; core – nickel and iron.

In the figure : In the figure the mantle is about 2870 km thick, the outer core about 2200 km and the inner core about 1221 km; the crust is very thin.

• Easy : Easy: The mantle is the thickest layer.
• Medium : Medium: Outer core 2200 km is almost twice the inner core 1221 km (2 × 1221 = 2442).
• Exam-type : Exam: Give the main elements of each layer.

Spot the difference

Rule 1: P and S waves — The velocity of P waves increases with depth up to 2900 km, where it is about 12 km/s; on entering the core it reduces to about 8 km/s, a drop of 12 − 8 = 4 km/s. S waves stop at about 2900 km and are absorbed, so scientists believe the outer core is liquid or semi-liquid.

In the figure : In the figure P waves pass through solids and liquids (their speed falls from about 12 to 8 km/s at 2900 km); S waves stop at 2900 km because the outer core is liquid.

• Easy : Easy: S waves cannot pass through liquids.
• Medium : Medium: A P wave at 8 km/s crosses the 2200 km outer core in 2200 ÷ 8 = 275 s.
• Exam-type : Exam: How do S waves prove a liquid outer core?

Rule 2: Discontinuities — A discontinuity (विच्छिन्नता) is a change in the trend of the wave-velocity graph; it shows a change of material at that depth. Conrad: continental and oceanic crust. Moho: crust and mantle. Gutenberg: mantle and core. Gravity increases to a certain depth and then reduces; at the centre it is zero, so weight there would be zero.

In the figure : In the figure discontinuities: Conrad (between the continental and the oceanic crust), Moho (crust and mantle) and Gutenberg (mantle and core).

• Easy : Easy: Moho is between crust and mantle.
• Medium : Medium: Gutenberg lies at about 2900 km.
• Exam-type : Exam: What does a discontinuity show?

Rule 3: Geo-dynamo — The temperature difference between outer and inner core gives rise to vertical currents in liquid iron. Earth's rotation gives them a spiral motion. Electric currents develop in these spirals and produce the magnetic field: the geo-dynamo (भू-डायनॅमो).

In the figure : In the figure the geo-dynamo: a temperature difference makes vertical currents in the liquid iron, the rotation makes them spiral and the electric currents give the magnetic field.

• Easy : Easy: Rotation spirals the currents.
• Medium : Medium: The dynamo works only because the outer core is liquid.
• Exam-type : Exam: Explain the geo-dynamo in 3 steps.

Rule 4: Magnetosphere — The Earth behaves like a giant magnet. Its magnetic field spreads far out and forms a cover called the magnetosphere (चुंबकावरण), the fifth sphere after the lithosphere, hydrosphere, atmosphere and biosphere. It protects the atmosphere from solar winds, streams of charged particles from the Sun.

In the figure : In the figure the magnetic field forms the magnetosphere, which protects the atmosphere of the Earth from the solar winds coming from the Sun.

• Easy : Easy: Magnetosphere is the fifth sphere.
• Medium : Medium: Four spheres come before it.
• Exam-type : Exam: How does the magnetosphere protect us?

Spot the difference