Origin & Evolution of the Earth and Crust

The question of how the Earth and the solar system came to be has produced a long succession of hypotheses, evolving from 18th-century ideas of a cooling gas cloud to the modern solar nebular theory, set within the cosmological frame of the Big Bang. Fix the storyline: the Sun and planets condensed from a rotating cloud of gas and dust (the solar nebula); the early Earth then heated, melted and differentiated into a layered body — core, mantle and crust — while outgassing and condensation built the atmosphere and hydrosphere. The outermost shell, the crust, is itself divided into a lighter continental (SIAL) and a denser oceanic (SIMA) type, organised into cratons, shields, platforms and mobile belts, and endlessly recycled through the rock cycle.

1. Early monistic theories

• Kant's Gaseous Hypothesis (1755): the Sun and planets formed from a slowly rotating cloud of cold, hard particles that collided, generated heat and became a hot rotating gas, from which rings condensed into planets. (Flawed — the source of initial rotation was unexplained.)
• Laplace's Nebular Hypothesis (1796): a hot, rotating gaseous nebula cooled and contracted; as it spun faster it shed successive rings at its equator, each condensing into a planet, the central mass becoming the Sun. (Elegant, but fails on angular momentum — the Sun has most of the mass but little of the angular momentum.)

2. Dualistic (two-body) theories

• Chamberlin–Moulton Planetesimal Hypothesis (1904): a passing star raised huge tongues of matter from the Sun; this matter cooled into tiny solid planetesimals that accreted into planets.
• Jeans–Jeffreys Tidal Hypothesis (1917–29): a large passing star's gravity drew out a cigar-shaped filament of gas from the Sun; the filament broke into planets (biggest in the middle — explaining why Jupiter/Saturn are largest).
• Binary / Companion-star theories (Russell, Lyttleton): the Sun once had a companion star; material from it (or from its disruption) formed the planets.
• Otto Schmidt's Interstellar Dust Hypothesis (1943): the Sun, moving through a cloud of interstellar dust and gas, captured it; this material formed a disc that accreted into planets — a forerunner of the modern view.

3. Modern solar nebular theory & the Big Bang context

• The accepted modern solar nebular theory holds that ~4.6 billion years ago a fragment of a giant molecular cloud collapsed (perhaps triggered by a nearby supernova) into a spinning protoplanetary disc; the centre became the proto-Sun, and dust grains in the disc accreted into planetesimals and then planets — rocky planets near the hot Sun, gas giants farther out. This sits within the Big Bang cosmology (~13.8 billion years ago), which describes the origin of the universe, space, time and the elements themselves.

4. Evolution of the Earth: accretion & differentiation

• The young Earth grew by accretion of planetesimals and then heated (impacts, compression, radioactivity) until it largely melted. By differentiation, heavy elements (iron, nickel) sank to form the core, lighter silicates rose to form the mantle, and the lightest material formed a thin crust. Outgassing from volcanism released gases that formed the early atmosphere; as the Earth cooled, water vapour condensed to form the hydrosphere (oceans). (The Moon is thought to have formed from a giant impact.)

5. The Earth's crust: SIAL, SIMA and structural units

• The crust has two types: continental crust — thick (~30–70 km), old, light, granitic, rich in silica & aluminium = SIAL (density ~2.7); and oceanic crust — thin (~5–10 km), young, denser, basaltic, rich in silica & magnesium = SIMA (density ~3.0). They are separated by the Conrad discontinuity, and the crust sits on the mantle across the Mohorovicic (Moho) discontinuity.
• Structural units of continents: cratons (ancient, stable continental cores), their exposed Precambrian parts being shields (e.g. Canadian, Baltic, Indian/Deccan shield); sediment-covered stable areas are platforms; and the belts of active folding/faulting are mobile (orogenic) belts (e.g. the Himalaya).

6. The rock cycle

• Crustal rocks are continuously recycled through the rock cycle: igneous rock (cooled magma) → weathered & eroded to sediments → compacted/cemented into sedimentary rock → altered by heat & pressure into metamorphic rock → melted back to magma → igneous again. The cycle links endogenetic (melting, uplift) and exogenetic (weathering, deposition) processes over geological time.

Tested on matching each theory to its author, the monistic/dualistic split, Laplace's angular-momentum flaw, differentiation, and SIAL/SIMA & crustal units.

Optional Paper I / GS Paper I. The examiner rewards comparing hypotheses and explaining differentiation & crustal structure.