Cosmology, Earth & Geoscience Comprehensive Notes

Cosmology & Universe Formation

  • Timeline of Cosmic History

    • 13.77 billion years13.77\,\text{billion years} ago: Big Bang expansion commences.
    • 10−36 s10^{-36}\,\text{s} after Big Bang: Inflation driven by vacuum energy; quantum fluctuations stretched to macroscopic scales.
    • 3.75×105 yrs3.75\times10^{5}\,\text{yrs}: Photons decouple → “afterglow light pattern” (Cosmic Microwave Background, CMB); end of “Dark Ages” begins.
    • 4×108 yrs4\times10^{8}\,\text{yrs}: First stars ignite, re-ionising the intergalactic medium.
    • Present: Accelerated expansion attributed to dark energy.

  • Competing Theories on the Origin of the Universe

    • Creation Theory
    • Universe intentionally designed by a Creator; six “days” (or epochs) of ordered creation.
    • Steady-State Theory (Bondi, Gold, Hoyle)
    • Continuous creation of matter maintains constant average density as universe expands.
    • No beginning or end; “Perfect Cosmological Principle.”
    • Big Bang Theory (Standard Model)
    • Originates from an initial singularity—infinitely hot, dense point.
    • Space itself inflates; not an ordinary explosion but expansion of spacetime.
    • Key evidence & milestones:
      • 1920s – Georges Lemaître: “Primeval atom / cosmic egg” hypothesis.
      • 1929 – Edwin Hubble: Galaxies recede; farther ones recede faster (red shift); balloon analogy illustrates metric expansion.
      • 1948 – Ralph Alpher: Predicts relic radiation (CMB).
      • 1949 – Fred Hoyle coins term “Big Bang” during BBC broadcast.
      • 1965 – Penzias & Wilson accidentally detect CMB.
      • CMB analysis ⇒ Universe age 13.8 Ga13.8\,\text{Ga}.

Solar System Formation

  • Location: Solar System resides in one of the spiral arms of the Milky Way.

  • Sun

    • Age ≈ 4.6 Ga4.6\,\text{Ga}; comprises 99.86%99.86\% of system’s mass; dominant gravitational influence.

  • Formation Theories

    • Nebular Hypothesis (Kant–Laplace)
    • Diffuse cloud (nebula) collapses → accretion disk → protostar-Sun + protoplanetary disk → planets.
    • Planetesimal Hypothesis (Chamberlin & Moulton)
    • Near-pass of another star raises solar tides; ejected filaments condense into planetesimals and planets.
    • Tidal Hypothesis (Jeans & Jeffreys)
    • Variant of planetesimal idea: massive star’s tidal pull draws out filament which fragments into planets.
    • Protoplanet (Modified Nebular) Theory
    • Nebular disk develops whirlpools (eddies) → protoplanets; integrates modern knowledge of turbulence & angular momentum conservation.

Earth: A Unique Planet

  • Age: 4.54 Ga4.54\,\text{Ga} (radiometric dating).
  • Origin: Accretion of dust particles within solar nebula.
  • Motions
    • Rotation period (sidereal) 23 h 56 m 4.1 s23\,\text{h}\,56\,\text{m}\,4.1\,\text{s}; solar day 24 h24\,\text{h}.
    • Revolution period ≈ 365.25 days365.25\,\text{days}.
    • Axial tilt 23.5∘23.5^{\circ} → seasons; Tropic latitudes mark limits of sub-solar point.
  • System Classification: Closed system for matter, open for energy (solar irradiance & terrestrial radiation).
  • Habitability Factors
    • Atmosphere with life-supporting gases; magnetic field; presence of liquid water; location in circumstellar “habitable zone.”
    • Currently only known planet with stable surface water and biosphere.

Earth’s Subsystems (Open Systems)

  • Geosphere
    • Lithosphere framework: rocks, minerals, landforms.
    • Internal Layers
    • Crust (thinnest; oceanic & continental).
    • Mantle (thickest; convective flow; Mohorovičić discontinuity at crust–mantle boundary; Repetti & Gutenberg discontinuities deeper).
    • Outer Core (liquid iron–nickel; flow generates geomagnetic field).
    • Inner Core (solid Fe; radius ≈1.255 km\approx1.255\,\text{km}).
  • Hydrosphere
    • 71%71\% of surface covered by water; 97%97\% salt water, 3%3\% fresh (most frozen).
    • Water cycle: evaporation, transpiration, condensation, precipitation, runoff, infiltration.
    • Cryosphere: glaciers, polar caps, permafrost.
  • Atmosphere
    • Multilayer gas envelope; “Earth’s security blanket.”
  • Biosphere
    • Vernadsky: life shapes Earth; biogeochemical cycles (carbon, nitrogen, oxygen, etc.).
    • Gaia Hypothesis (Lovelock & Margulis): Earth functions as a self-regulating organism; living & non-living components co-evolve.
  • Interdependence: Changes in one subsystem propagate to others (matter & energy exchange).

Rocks & Minerals

  • Rock Cycle
    • Igneous → weathering → sediments → lithification → sedimentary → metamorphism → metamorphic → melting → magma → igneous.
  • Rock Types & Formation
    • Igneous
    • Intrusive (plutonic): slow cooling within crust (granite, diorite, pegmatite).
    • Extrusive (volcanic): rapid cooling at surface (basalt, obsidian, pumice).
    • Sedimentary: surface accumulation & lithification (sandstone, limestone, shale).
    • Metamorphic: transformation under heat/pressure (marble, quartzite, gneiss).
  • Minerals
    • Definition: naturally occurring, inorganic, solid, definite chemical formula, crystalline structure, stable at room T.
    • Major crustal elements: O, Si, Al, Fe, Mg, Ca, Na, K.
    • Physical Properties
    • Luster, color, streak, hardness (Mohs scale 1−101{-}10), cleavage & fracture, specific gravity.
    • Examples: halite (fixed formula NaCl\text{NaCl}), quartz (SiO2\text{SiO}_2), diamond hardness 1010.

Exogenic (Surface) Processes

  • Weathering
    • Physical/Mechanical
    • Frost wedging, block disintegration (thermal expansion), exfoliation (sheeting).
    • Chemical
    • Oxidation (formation of Fe oxides/rust), carbonation (acidic H<em>2CO</em>3\text{H}<em>2\text{CO}</em>3 dissolves carbonates → sinkholes), hydration (water addition expands minerals), cation exchange, chelation (organic complexing).
    • Biotic Weathering: plants, microbes, animals enhance breakdown.
  • Erosion (transport)
    • Agents: water & wind.
    • Types: wind erosion (deflation), sheet, rill (cm-scale channels), gully (deep valleys → badlands).
    • Controlling factors: climate, topography, vegetation, tectonics.
  • Mass Wasting (gravity-driven)
    • Falls (free fall of blocks), slides (coherent movement along plane), flows (water-saturated, viscous motion).
  • Deposition & Lithification
    • Sediment settling, compaction, cementation → sedimentary rock.

Endogenic (Internal) Processes

  • Magma Generation
    • Decompression melting: mantle upwelling lowers pressure.
    • Heat transfer (injection of hot basaltic magma into crust melts felsic rocks).
    • Flux melting: addition of volatiles (H<em>2<em>2O, CO</em>2</em>2) lowers melting point at subduction zones.
  • Internal Heat Sources
    • Accretionary heat, core segregation friction, radioactive decay.
  • Tectonic Stresses
    • Compressional, tensional, shear → deformation patterns.
  • Metamorphism Types
    • Burial (lithostatic pressure), dynamic (shear zones), regional (orogenic belts), hydrothermal (fluid-rich).
  • Structural Deformation
    • Folding: monocline (single bend), anticline (arch), syncline (trough) due to compression.
    • Faulting
    • Dip-Slip: normal (hanging wall drops under tension), reverse (hanging wall rises under compression).
    • Strike-Slip/Transform: lateral displacement under shear.
  • Volcanism
    • Magma ascends through conduits/vents; driven by mantle convection & plate motions; volcano emits lava, gases, pyroclasts.

Ethical, Philosophical & Practical Implications

  • Cosmological models frame humanity’s place in the universe—religious (Creation) vs. secular (Big Bang) perspectives.
  • Gaia concept encourages environmental stewardship; Earth systems viewed as interlinked—anthropogenic change in one sphere (e.g., CO2_2 emissions) feeds back to climate, oceans, biosphere.
  • Understanding exogenic & endogenic processes underpins hazard mitigation (earthquakes, landslides, volcanic eruptions) and resource management (minerals, groundwater, soils).