Earth's Interior, Rocks and Plate Tectonics

The layers of the Earth

• The Mohorovičić (Moho) discontinuity separates the crust and the mantle. The Gutenberg discontinuity separates the mantle and the core.
• We know this mainly from seismic waves. S-waves cannot pass through liquids, which tells us the outer core is liquid.

Rocks

Plate tectonics

The lithosphere is broken into plates that move a few cm per year.

• The Pacific "Ring of Fire" has most of the world's active volcanoes and earthquakes.
• Alfred Wegener proposed continental drift (1912), with a supercontinent Pangaea surrounded by the ocean Panthalassa.

Earthquakes and volcanoes

The focus (hypocentre) is where an earthquake starts inside the Earth. The epicentre is the point directly above it on the surface. Magnitude is measured on the Richter / moment magnitude scale, and intensity on the Mercalli scale.

Background: how do we know what is inside the Earth?

No one has drilled deeper than about 12 km (the Kola borehole in Russia), so we rely on indirect sources:

• seismic waves from earthquakes;
• meteorites, which are made of similar material;
• gravity and magnetic surveys;
• volcanic eruptions, which bring up deep material.

P-waves (primary, compressional) travel through solids, liquids and gases. S-waves (secondary, shear) travel only through solids. S-waves do not pass through the outer core – this creates an S-wave shadow zone beyond about 103° from the epicentre. Together with how P-waves bend, this proves the outer core is liquid.

Discontinuities: Mohorovičić (Moho) – crust/mantle; Gutenberg – mantle/core; Lehmann – outer/inner core.

Rocks and the rock cycle

• Igneous (from magma): intrusive (cooled slowly underground – granite, large crystals) and extrusive (cooled fast on the surface – basalt, as in the Deccan Traps).
• Sedimentary (layers of deposits): sandstone, shale, limestone, coal. They contain fossils and hold most of the world's oil and gas.
• Metamorphic (changed by heat and pressure): limestone → marble; shale → slate; granite → gneiss; sandstone → quartzite; coal → graphite.

The rock cycle connects them all: any rock can be transformed into another over geological time.

From continental drift to plate tectonics

• Wegener (1912) proposed continental drift: a supercontinent, Pangaea, surrounded by Panthalassa. Evidence: the "jig-saw fit" of South America and Africa, matching fossils (Mesosaurus, Glossopteris), tillite (glacial) deposits and placer gold. His weakness was that he could not explain the force behind the movement.
• Convection current theory (Arthur Holmes, 1930s) offered a mechanism in the mantle.
• Sea-floor spreading (Harry Hess, 1960s): new crust forms at mid-ocean ridges; rocks are youngest at the ridge and older away from it; magnetic stripes show symmetrical patterns (palaeomagnetism).
• Plate tectonics (late 1960s): the lithosphere is broken into about seven major plates and many minor ones, which move a few centimetres a year.

Effects: earthquakes and volcanoes

• Earthquakes: energy released along faults. Focus (hypocentre) is the point of origin underground; epicentre is the point directly above on the surface. Magnitude is measured by the Richter/moment magnitude scale (each whole number ≈ 32 times more energy); intensity by the Mercalli scale (effects). India's seismic zone map (BIS) has zones II to V; zone V includes the north-east, J&K, Himachal, Uttarakhand, Kutch and the Andaman and Nicobar Islands.
• Volcanoes: about 75% of active volcanoes lie in the "Ring of Fire" around the Pacific. Types: shield (gentle, basaltic – Hawaii), composite/strato (explosive – Fuji, Vesuvius), calderas, and flood basalts (the Deccan Traps).
• Tsunamis: caused by undersea earthquakes that displace water (the 2004 Indian Ocean tsunami from the Sumatra–Andaman earthquake). INCOIS in Hyderabad runs India's tsunami warning centre.

Case study: the Himalaya – still growing, still risky

The Indian plate still moves north at about 4–5 cm/year. The Himalaya rise by a few millimetres a year and store strain that is released as earthquakes (Nepal 2015, magnitude 7.8). Scientists warn of a possible "central seismic gap" between the 1905 Kangra and 1934 Bihar–Nepal earthquakes. The lesson for policy: earthquake-resistant building codes, retrofitting of schools and hospitals, and careful planning of hill towns (see the land subsidence at Joshimath, 2023).

Common misconceptions

• "The Earth's core is liquid throughout." – Only the outer core is liquid; the inner core is solid.
• "Volcanoes occur only at plate boundaries." – Hotspots create volcanoes in the middle of plates (Hawaii; the Réunion hotspot linked to the Deccan Traps).
• "Richter magnitude 6 is twice as strong as 3." – The scale is logarithmic: magnitude 6 releases roughly 32,000 times more energy than magnitude 3.

How UPSC asks it

Prelims asks about seismic waves, layers and discontinuities, rock examples, plate boundary types, and volcanic features. Mains themes include how plate tectonics explains landforms and disasters, and the distribution of earthquakes and volcanoes.

Mains answer framework

Q-type: "How does the theory of plate tectonics explain the formation of the Himalaya and the occurrence of earthquakes in the region?"

• Introduction: the evolution from continental drift → sea-floor spreading → plate tectonics.
• Body: Gondwana break-up; northward drift; the closure of the Tethys; continent–continent collision; folding into three ranges; thrust faults (MCT, MBT, HFT).
• Earthquakes: stress build-up; past examples; the seismic gap.
• Conclusion: implications for planning and disaster preparedness.

Evidence for the earth's interior

Direct evidence (mines, drilling, volcanic material) reaches only a few kilometres. Most knowledge comes from seismic waves: P-waves travel through solids and liquids, while S-waves cannot pass through liquids. The S-wave shadow zone shows that the outer core is liquid.

Practice question

Q. How does plate tectonics explain the formation of the Himalayas? The Indian plate moved north and collided with the Eurasian plate; the oceanic crust between them was consumed and the sediments of the Tethys Sea were folded and uplifted. The collision continues, causing earthquakes in the region.

Check your understanding

1. What are the three types of rocks? Igneous (from cooled magma), sedimentary (from deposited sediments) and metamorphic (changed by heat and pressure).
2. What happens at a convergent boundary? Plates collide, forming mountains, trenches and volcanic arcs.