The Early Earth and Related Geological Concepts Study Notes

The Early Earth

Origin and Formation of the Universe

  • Big Bang (13.7 Ga)

  • Formation of Nuclei (≤3 min following the B.B.)
      - Quarks form nuclei of Hydrogen (H) and Helium (He) (only protons).

  • Formation of Atoms (700,000 years after the Big Bang)
      - Expansion of the universe lowers temperature (T°C).   - Electrons attach to nuclei of H and He, creating a "soup" of gas (matter).

  • Formation of Stars and Galaxies (≥200 Ma after B.B.)
      - Gravitational collapse of matter (H, He) forms stars & galaxies.

  • Solar Nebula (6 Ga)
      - A diffuse mass of gas and dust forms a rotating gaseous and dusty disk.

Formation of the Protosun

  • Accretion of Matter
      - Occurs at the center of the solar nebula (disk).   - Leads to an increase in temperature (T°C) and pressure (P).   - Nuclear fusion occurs (multiple nuclei join to form a heavier nucleus, releasing energy).

Formation of Planetesimals

  • Formation of Small Solid Bodies
      - Gradual cooling of the disk leads to the accretion of matter.   - Birth of the sun (~4.55 Ga) and first planets.

  • Hypotheses Regarding Planet Formation
      - Previous hypotheses:     - Coaccretion from the solar disk
        - Fission from part of the Earth due to centrifugal force
        - Capture from an asteroid.   - Actual Hypothesis:     - Giant Impact Hypothesis       - Collision between the Earth and Theia (Mars-sized with chaotic orbit, 4.526 Ga).       - Ejected material forms the Moon.

Composition and Differentiation of the Moon and Earth

  • Similar Composition   - Perhaps a mix/melting of both bodies during the collision.

  • Energy from Impact
      - Causes an ocean of magma to form on the Earth and moon.

  • Differentiation
      - Moon develops crust, mantle, and core structures.

  • Changing Distance Between Earth and Moon
      - 22,500 km to 384,000 km over time.
      - Historical tidal effects were stronger and more frequent.   

Effects of the Moon on Earth's Rotation

  • Slowing of Earth’s Rotation:   - At 4.5 Ga: 1 day = 6 hours, one year = 1434 days.   - At 2.5 Ga: 1 day = 12 hours, one year = 714 days.   - At 400 Ma: 1 day = 22 hours.

Early Earth's Differentiation and Magnetic Shield

  • Heat Sources:   - Impacts increase thermal energy.   - Abundance of radioactive elements contributes to heat.

  • Differentiation Process (4.45 Ga)   - Magmatic ocean separates:     - Dense elements (Fe, Ni) sink to form metallic core.     - Less dense silicates move towards the surface to form the mantle.

  • First Magnetic Field Evidence (3.364 Ga)   - Found in Jack Hills Zircon, Australia.

  • Liquid Metal Core
      - Caused electrical currents leading to magnetic fields.

Early Atmosphere and Water Presence on Earth

  • Formation of the Atmosphere   - Magnetic shield deflects solar winds.   - Retains light elements (H, O, N).

  • Water Evidence (~4.4 Ga)   - Possible origins:     - Hypothesis 1: Meteorites and Comets
          - Celestial bodies with high water content strike Earth.       - Hypothesis debated due to isotopic composition differences.     - Hypothesis 2: Degassing of the Mantle
          - Water stored in the mantle as hydrated silicates & gases.       - Eruptions release water vapor into the atmosphere.

  • Early Atmosphere Composition (4.4 - 3.5 Ga):   - Water vapor, methane (CH4), ammonia (NH3), hydrogen (H2), nitrogen dioxide (N2), carbon dioxide (CO2), sulfur dioxide (SO2), etc.   - Atmospheric density >250 times today; greenhouse gases significantly higher leading to a mean temperature of +60°C, compared to +15°C today.

Earliest Rocks and Signs of Life

  • Oldest Rocks (4.03 - 4.3 Ga)

  • Signs of Life
      - Oldest fossils: stromatolites from Greenland (3.7 Ga).   - Biogenic Graphite (3.7 Ga):
        - Found in metasedimentary rocks.     - Isotopic signature depleted in 13C, indicates life.

  • Fossilized Micro-organisms
      - Tube and filament-shaped structures in metasedimentary rocks (3.8 to 4.3 Ga).

Internal Structure of the Earth

Introduction to the Earth's Interior

  • Layers Based on Composition:   1. Crust (<2%)
         - Continental crust (thick ~40-70 km, less dense, covers ~30% of Earth's surface) and oceanic crust (thin ~6-10 km, more dense, covers ~70% of Earth's surface).   2. Mantle (81%)
         - Upper mantle (plastic) and lower mantle (solid).   3. Core (17%)
         - Outer core (liquid) and inner core (solid).

Crust Composition

  • Continental Crust
      - Felsic to intermediate composition (↓Mg, Fe).

  • Oceanic Crust
      - Mafic composition (↑Mg, Fe).

Mohorovicic Discontinuity (Moho)

  • The boundary between the crust and mantle characterized by a change in density and chemical composition.

  • Density Contrast:
      - Crust: low to medium density, mostly felsic;
      - Mantle: high density, ultramafic.

Elements in the Crust and Mantle

  • Continental Crust Main Elements:   - O (45.5%), Si (26.8%), Al (8.4%), Fe (7.06%), Ca (5.3%), Mg (3.2%), Na (2.3%), K (0.9%), Ti (0.5%).

  • Mantle Composition:   - O (44.8%), Si (21.5%), Mg (22.8%), Fe (5.8%), Al (2.2%), Ca (2.3%), Na (0.3%).

Mantle Dynamics and Convection

  • Mantle Convection   - Heat transport towards lithosphere causes rock to flow; main causes include slab-pull at subduction zones.

  • Gutenberg Discontinuity
      - Boundary between the mantle and the outer core marked by major contrast in density.

Core Composition

  • Core Composition
      - Outer core: liquid metallic iron with possible inclusion of nickel and sulfur ( ext{± S, ± O}).   - Inner core: solid iron and nickel.   - Radius of Core: 3486 km.

Earth’s Magnetic Field

  • Definition: Earth acts like a huge bar magnet, producing a magnetic field.

  • Significance of the Field:   - Navigation for humans and animals; deflection of cosmic radiation.

  • Causes of Earth’s Magnetic Field:   - Density differences in the outer core resulting in convection of molten iron which induces electric currents.

Meteorites and Geological Time

Meteorites: Messengers from Space

  • Definitions:   - Meteor: a bright streak in the sky when a meteoroid enters the atmosphere.   - Meteorite: the remnant of the meteoroid after it survives its passage through the atmosphere.

  • Types of Meteorites:
      1. Iron meteorites
      2. Stony-iron meteorites
      3. Stony meteorites.

Relative Dating of Geological Bodies

  • Principle of Superposition: In an undisturbed sedimentary sequence, older strata lie beneath younger strata.

  • Cross-Cutting Relationship: A geological feature that cuts another is younger than what it cuts.

  • Unconformities: Surfaces of erosion or non-deposition between geological layers indicating time intervals without deposition.

Absolute Dating of Geological Bodies

  • Absolute dating attributes specific numerical ages to rocks via radiometric methods (decay of radioactive isotopes).

  • Example: Uranium Decay in Zircon: Zircon incorporates uranium but not lead during crystallization. Uranium decays into lead, allowing for age determination based on the ratio of Pb/U.

Earthquakes: Thunder Underground

Definition and Causes of Earthquakes

  • Definition: Vibration of the Earth due to the sudden release of energy in rocks.

  • Origin: Stress applied to solid rocks causes strain leading to rupture and energy release.

  • Causes:   - Movements of tectonic plates   - Ancient meteorite impacts   - Volcanism (rising magma)   - Isostatic adjustments.

Types of Waves Emitted by Earthquakes

  • Body Waves:   - P waves (Primary) - travel through all materials at 6.2 km/s.   - S waves (Secondary) - only through solids at 3.6 km/s.

  • Surface Waves:   - Love waves (horizontal motion, 3 km/s)   - Rayleigh waves (elliptical motion).

Measuring Earthquakes

  • Mercalli Scale: Qualitative measure based on damage and perception (12-point scale).

  • Richter Scale: Quantitative, logarithmic scale for measuring the magnitude and energy released.

  • Moment Magnitude Scale: Developed to quantify energy released during medium-to-large earthquakes.

Factors Affecting Earthquake Impact

  • Magnitude, Duration, Focus Depth, Distance from Epicenter, Geological Parameters influence affecting shaking and damage.

  • Geological Parameters: Homogeneity of substrate, hardness of rocks, and thickness of sediment cover can significantly impact the intensity of ground shaking.

Rock Deformation

Notion of Strain and Stress

  • Definitions:   - Stress: Force acting on a rock unit altering its shape/volume.   - Resistance: The force preventing deformation.   - Strain: Deformation occurring due to stress exceeding resistance.

Types of Deformation

  1. Elastic: Returns to original shape after stress removal (e.g., rubber band).

  2. Plastic: Permanent deformation once the limit is exceeded (e.g., toffee).

  3. Brittle: Fractures when the limit is reached (e.g., glass).

Stress Types and their Effects

  • Compressional Stress: Converging forces causing shortening.

  • Types of Faults:   - Normal Fault: Hanging wall moves down (extended crust).   - Reverse Fault: Hanging wall moves up (shortened crust).   - Strike-slip Fault: Lateral movement (horizontal displacement).

Plate Tectonics

Theory of Plate Tectonics

  • Overview: Suggests that lithospheric deformation is caused by tectonic plates moving on the asthenosphere.

  • Evidence:   - Fit of continents, fossil distribution, signs of ancient glaciation.

Types of Plate Boundaries

  1. Divergent: Plates move apart; characterized by rifting and oceanic ridge formation.

  2. Convergent: Plates collide; subduction results in oceanic trenches and mountain formation.

  3. Transform: Plates slide past each other without creating/destroying lithosphere.

Earthquake and Volcano Formation at Plate Boundaries

  • Divergence leads to volcanic activity along mid-ocean ridges.

  • Convergence leads to volcanic arcs and mountain formation.

Volcanism

Formation of Magma and Volcanoes

  • Processes:   - Decompression, addition of volatiles, or heat addition can produce magma.

  • Types of Volcanoes:   1. Shield Volcanoes: Formed from low-viscosity basaltic magma (e.g., Hawaii).   2. Stratovolcanoes: Formed from high-viscosity andesitic to rhyolitic magma (e.g., Mount Fuji).   3. Cinder Cones: Formed from explosive eruptions of low-viscosity magma.

Magmatic Hazards

  • Types of eruptions range from low-risk effusive flows to high-risk explosive eruptions causing pyroclastic flows, ash fall, and volcanic gases.

Mass Movements

Overview

  • Defined as downslope movements of rock and soil due to gravitational forces exceeding material resistance.

  • Types: Falls, slides (slumps, rockslides), flows (debris flows, mudflows).

Significant Underlying Factors

  • Conditions of Instability: Material type, slope angle, climate, vegetation, water content all affect mass movement susceptibility.

Common Triggers of Mass Movements

  • Heavy rainfall, earthquakes, rapid snowmelt, and human activities can initiate mass movements.

Conclusion

  • Understanding these geological processes is critical for predicting and mitigating natural hazards.