Interior of the Earth

Earth has three concentric layers: Crust, Mantle, Core. Crust is thin (5-30 km; oceanic crust ~5-10 km made of SIMA—silica+magnesia, basaltic; continental crust ~30-70 km of SIAL—silica+alumina, granitic). Mantle extends to 2900 km. Core extends to 6371 km centre; outer core is liquid, inner core solid (iron-nickel, NIFE). Memory aid for discontinuities (top to bottom): 'Con-Moho-Rep-Gut-Lehmann.' Conrad (within crust), Mohorovicic/Moho (crust-mantle), Repetti (upper-lower mantle), Gutenberg (mantle-core), Lehmann (outer-inner core). The asthenosphere (upper mantle, 100-400 km) is the source of magma. S-waves cannot pass through the liquid outer core, proving its liquid state—a shadow zone forms between 105 and 142 degrees.

Earthquake (body) waves: Primary (P) and Secondary (S). P-waves are longitudinal/compressional, fastest, travel through solids, liquids and gases. S-waves are transverse, travel ONLY through solids—this is the key UPSC fact. Surface waves (L-waves) are most destructive. Shadow zones: P-wave shadow zone lies between 105 and 142 degrees from the epicentre; S-wave shadow zone is everywhere beyond 105 degrees (a much wider zone). The S-wave shadow zone existence proves the outer core is liquid; P-waves bend (refract) on entering the liquid core, creating their narrower shadow zone. Mnemonic: 'S Stops in Solids only.'

Temperature increases with depth (geothermal gradient ~1 degree C per 32 m near surface, but the rate slows sharply deeper). Sources of internal heat: radioactive decay (uranium, thorium, potassium), primordial heat from Earth's formation, and gravitational/tidal friction. Density rises with depth—from ~2.7 g/cc (crust) to ~13 g/cc (inner core). Pressure also rises with depth. Despite high temperature, the inner core stays solid due to immense pressure. Example: deepest mine (Mponeng, South Africa, ~4 km) and deepest borehole (Kola Superdeep, Russia, ~12.3 km) only scratched the crust, confirming our interior knowledge relies mainly on indirect (seismic) evidence.

Cover the answer, recall, then check. 12 cards on the must-know facts about Earth's interior for UPSC Prelims.

Q1. Name the three concentric layers of the Earth from outside to inside.
A1. Crust (outermost, thin), Mantle (middle, thickest by volume), and Core (innermost, densest). The core is divided into a liquid outer core and a solid inner core.

Q2. What is the Mohorovicic (Moho) discontinuity?
A2. The boundary separating the crust from the mantle. Other key discontinuities: Conrad (upper–lower crust), Gutenberg (mantle–core), and Lehmann (outer–inner core).

Q3. Differentiate SIAL and SIMA.
A3. SIAL = continental crust, rich in Silica + Aluminium, granitic and lighter. SIMA = oceanic crust, rich in Silica + Magnesium, basaltic and denser.

Q4. Which seismic wave can pass through liquids, and which cannot?
A4. Primary (P) waves — longitudinal — pass through solids, liquids and gases. Secondary (S) waves — transverse — travel only through solids, which is how we know the outer core is liquid.

Q5. What is the S-wave shadow zone and what does it prove?
A5. S-waves are not recorded beyond 105° from the epicentre — a shadow zone across the far side. Because S-waves cannot cross liquids, this proves the outer core is liquid.

Q6. What is the P-wave shadow zone?
A6. The zone between 105° and 142° from the epicentre where P-waves are not directly received, caused by refraction at the mantle–core boundary.

Q7. What is the asthenosphere?
A7. A semi-molten, plastic layer in the upper mantle just below the lithosphere. It is the main source of magma for volcanic eruptions and enables plate movement.

Q8. Of what material is the Earth's core (NIFE) chiefly composed?
A8. Nickel and Iron (hence NIFE). The core is the densest layer; the inner core is solid due to immense pressure despite very high temperature.

Q9. Distinguish focus (hypocentre) from epicentre of an earthquake.
A9. The focus is the point inside the Earth where the rupture originates; the epicentre is the point on the surface directly above the focus.

Q10. What are the direct and indirect sources of information about Earth's interior?
A10. Direct: rocks from mining and deep drilling (e.g. Kola Superdeep Borehole, ~12 km). Indirect: seismic waves, density, pressure, temperature, gravity, magnetic field and meteorites.

Q11. What is the geothermal gradient?
A11. The rate at which temperature rises with depth — on average about 1°C for every 32 metres near the surface. It confirms the interior gets progressively hotter.

Q12. Which scale measures earthquake magnitude, and which measures intensity?
A12. The Richter scale measures magnitude (energy released); the Mercalli scale measures intensity (observed damage/effects).