The Earth: Origin, Interior and Evolution (NCERT Geography Digest)

Origin of the universe and the solar system

• Big Bang theory (expanding universe): the universe began about 13.7 billion years ago from an extremely dense, hot singularity that expanded and cooled. Edwin Hubble's observation that galaxies are receding (the red shift) supports an expanding universe.
• Nebular hypothesis (Laplace, refined by Otto Schmidt and Carl von Weizsacker): the Sun and planets formed from a rotating disc (nebula) of gas and dust about 4.6 billion years ago. The Sun formed at the centre; the planets accreted from the disc.
• Planets: the four inner (terrestrial) planets (Mercury, Venus, Earth, Mars) are rocky and dense; the outer (Jovian) planets (Jupiter, Saturn, Uranus, Neptune) are giant gas planets. (Pluto is now a dwarf planet.)

Evolution and differentiation of the Earth

• The early Earth was a hot, molten body. As it cooled, differentiation occurred: heavier materials (iron, nickel) sank to form the core, while lighter materials rose to form the crust.
• Degassing: volcanic outgassing released water vapour and gases that built the early atmosphere; condensation of water vapour formed the oceans. Life later changed the atmosphere's composition (oxygen).
• Origin of the Moon: the most accepted view is the giant impact ("Big Splat"), in which a Mars-sized body struck the early Earth and the ejected debris coalesced into the Moon.

Sources of information about the interior

• Direct sources: surface rocks, mining, deep drilling (limited depth), and volcanic eruption material.
• Indirect sources: density, pressure and temperature trends, gravity and magnetic field data, meteorites (thought to resemble Earth's interior), and most importantly seismic (earthquake) waves.

Earthquake waves

• Body waves travel through the interior; surface waves (L-waves) travel along the surface and cause most of the destruction.
• P-waves (primary, longitudinal): fastest, arrive first, and travel through solids, liquids and gases.
• S-waves (secondary, transverse): slower, and travel only through solids. Their inability to pass through the outer core proves the outer core is liquid.
• Shadow zones: the P-wave shadow zone lies roughly between 105 and 145° from the epicentre; the S-wave shadow zone is the entire area beyond 105° (a much wider zone). These shadow zones are the chief evidence for a liquid outer core.

Layered structure of the Earth

The convection of the mantle and the movement of the liquid outer core (which generates the Earth's magnetic field) are central to plate tectonics and geomagnetism.

Volcanoes and earthquakes (introduction)

• Earthquakes result from the sudden release of energy along faults; the point of origin is the focus (hypocentre) and the point directly above on the surface is the epicentre. Magnitude is measured on the Richter scale (energy) and intensity on the Mercalli scale (effects).
• Volcanoes are openings through which magma, gases and ash reach the surface; types include shield, composite (strato), caldera and flood-basalt (the Deccan Traps are a flood-basalt province).

How CAPF asks this

• Which wave proves the liquid outer core (S-wave). Order and state of the layers. The discontinuities (Moho, Gutenberg). The age of the Earth and the universe.
• The composition shorthand (SIAL, SIMA, NIFE) and which is crust versus core.

Authored practice

1. Which seismic wave cannot travel through liquids, thereby proving the outer core is liquid? (a) P-wave (b) S-wave (c) L-wave (d) Rayleigh wave. Answer: (b) S-wave. Authored practice, not a verbatim PYQ.
2. The discontinuity separating the crust from the mantle is called: (Answer: the Mohorovicic / Moho discontinuity.) Authored practice, not a verbatim PYQ.