Environmental Processes and Hazards: Formation, Structure, and Composition of Earth

Formation, Structure and Composition of Earth

Recommended Literature

"Understanding Earth" by Grotzinger & Jordan (2014). Older editions are also acceptable (Press and Sievers).
"Earth's Climate: Past and Future" by Ruddiman (2013 or 2008).

Formation of Earth

Earth formed approximately 4.6 billion years ago, likely from a supernova.
First chemical elements originated during the Big Bang, about 13.8 billion years ago.

Origin of Elements (Nucleosynthesis)

Primordial period (Big Bang nucleosynthesis) - approximately 3 minutes after the Big Bang.
Stellar nucleosynthesis - approximately 200 million years after the Big Bang.
Supernova nucleosynthesis - approximately 9.2 billion years after the Big Bang.

1. Primordial Period: Formation of the Universe

"Big Bang": Occurred approximately 13.8 billion years ago.
- The universe expanded from a primordial point in a cataclysmic explosion.
- As the universe expanded and cooled, physical laws (gravity, magnetism) developed, and matter and energy formed and differentiated.
Theory: Based on the observed expansion of the Universe.

“Big Bang” Nucleosynthesis

Cosmos underwent an "inflation," expanding from the size of an atom to a melon in $10^{-34}$ seconds.
Post-inflation: Cosmos filled with hot particles (quarks, leptons, photons, neutrinos) that formed the first matter.

Formation of Protons and Neutrons

Rapid cooling (10-6 seconds after “Big Bang”) allowed quarks to form protons and neutrons.
- Proton (Hydrogen): Composed of 2 up quarks and 1 down quark.
- Neutron: Composed of 1 up quark and 2 down quarks.

Formation of First Atomic Nuclei

Occurred during the first 2 minutes of the Universe.
- Hydrogen ( $1H$ ).
- Deuterium ( $2H$ or D).
- Tritium ( $3H$ or T).
- Helium ( $3He$ and $4He$ ).
Unstable nuclei like $5Li$ , $5He$ , and $8Be$ formed during the first 3 minutes but decayed back to stable $4He$ .
Stable hydrogen atoms formed approximately 700,000 years after the Big Bang.

2. Stellar Nucleosynthesis

Nebulas of Hydrogen and Helium atoms gravitationally collapsed to form the first stars and galaxies about 200 million years after the Big Bang.
This process generated heat and energy, fueling further fusion reactions.

Helium Fusion / Triple-alpha process

Helium accumulated in the cores of stars.
Further nuclear fusion reactions formed Beryllium (Be), Carbon (C), and Oxygen (O).
This released energy, powering further fusion.
Lighter elements (Hydrogen) diffuse to the rim, while heavier elements (Helium) concentrate in the center. This process begins at 170 million degrees Celsius.
White Dwarfs: Most smaller stars do not form other elements at this stage.
Larger Stars (700 million degrees Celsius): Heavier elements concentrate in the center, while lighter elements (gas) diffuse to the rim.
- Magnesium and Oxygen form if the star is more than eight times as massive as the Sun.
Larger Stars (2 billion degrees Celsius)
Larger Stars (>3 billion degrees Celsius): Silicon and Sulphur atoms fuse into Iron (lasts about a day).
- Iron cannot be fused any further due to its atomic structure.
Larger Stars (>100 billion degrees Celsius): Gravitational collapse occurs, and the star detonates in a cataclysmic explosion (Supernova).

3. Supernova Nucleosynthesis

Explosion of a giant star generates high temperatures and pressures, causing nuclear fusion reactions that create elements with atomic numbers 27-92 (greater than Iron).
Supernovas produce interstellar clouds of gas and dust containing a wide range of elements due to nuclear fusion.

Formation of the Solar System (4.6 Billion Years Ago)

A diffuse, slowly rotating nebula begins to contract.
The enveloping disk of gas and dust forms grains that collide and clump together into planetesimals.
Matter concentrated at the center becomes the Protostar (our Sun).
Planetesimals, approximately 1 km in size, form.
A flat, rapidly rotating disk forms.

Formation of Planets

Solid Planets: Mercury, Venus, Earth, Mars (Inner Planets).
Gas Giants: Jupiter, Saturn (primarily hydrogen and helium).
Outer Planets: Uranus, Neptune, Pluto.
Asteroid belt.

Earth’s Structure and Composition

Proto-Earth (4.6 billion years ago) differentiated into layers to form Earth as a zoned planet.
Iron sank to the center, and lighter material floated upward.
- Crust: 0-40 km.
- Mantle: 40-2890 km.
- Liquid iron outer core: 2890-5150 km.
- Solid iron inner core: 5150-6370 km.

Today’s Earth: Heat and Energy Balance

Earth’s outer core is slowly cooling down (100 degrees Celsius per billion years).

Earth's Layers

Layers subdivided based on seismic waves.
Crust: 0-100 km thick.
Lithosphere: Crust and uppermost solid mantle.
Asthenosphere: Top layer of the mantle consisting of hot, weak rock that is easily deformed.
Mantle: 2900 km.
Outer core: Liquid, 5100 km.
Inner core: Solid, 6378 km.

Note: Diagrams presented are not to scale.

Evidence from Seismic Studies

Earthquakes produce P (primary) and S (secondary) waves that penetrate Earth’s layers with different velocities depending on material density.
Moho (Mohorovicic Discontinuity): Separates the crust from the underlying mantle.
Different densities indicate different material/chemical compositions.

Earth Chemical Composition: Evidence from Geochemical Studies

Evidence from ore smelting slags and meteorites.
Melting a chondrite (extra-terrestrial rock/meteorite) yields 3 immiscible liquids plus vapor:
- Gas Phase (Atmophile/volatile): H, He, N, Noble gases.
- Silicate Liquid (Lithophile): Alkalis, Alkaline Earths, Halogens, B, O, Al, Si, Sc, Ti, V, Cr, Mn, Y, Zr, Nb, Lanthanides, Hf, Ta, Th, U.
- Sulfide Liquid (Chalcophile): Cu, Zn, Ga, Ag, Cd, In, Hg, Tl, As, S, Sb, Se, Pb, Bi, Te.
- Metallic Liquid (Siderophile): Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Mo, Re, Au, C, P, Ge, Sn.

Composition of the Core

Inner Core (Solid, 1230-1530 km thick, 5430°C):
- Iron (Fe): ~80%.
- Nickel (Ni): ~12%.
- S, O, siderophile elements: ~8%.
Outer Core (Liquid, 2250 km thick, 2730–4230°C):
- Molten Fe: ~85%.
- Some Ni.
- Some Si, S, C, or O.
Convection in the liquid outer core and spin of the solid inner core generate Earth’s magnetic field.
The magnetic field is also evidence for a dominantly iron core.

Earth’s Magnetic Field

Generation: Electrical currents flowing in the slowly moving molten iron of the fluid outer core.
Mechanism: Rotation of Earth on its axis causes these electric currents to form a magnetic field.
Extent: Extends several tens of thousands of kilometers into space.
Protection: Protects Earth from charged particles of the solar wind and cosmic rays (harmful ultraviolet radiation).

Composition of the Mantle

Mantle makes up >2/3 of Earth’s mass (4000-1400°C).
Composition of the upper mantle (Asthenosphere) is approximated by pyrolite, a theoretical igneous rock consisting of about three parts of peridotite and one part of tholeiitic basalt.
- $SiO_2$ : 45 wt%
- $MgO$ : 30-40 wt%
- $FeO$ : 8-13 wt%
- $Al<em>2O</em>3$ : 3 wt%
- $CaO$ : 3 wt%
- High Mn, Cr, Ti concentrations

The Lithosphere

Earth’s hard and rigid outer layer.
Two types of crusts (depending on rock density):
- Lighter, more silicic (felsic) continental crust.
- Heavier, less silicic (mafic) oceanic crust.

Average Composition of the Crust

Oxygen: 47 wt%
Silicon: 28 wt%
Aluminum: 8.1 wt%
Iron: 5.0 wt%
Calcium: 3.6 wt%
Sodium: 2.8 wt%
Potassium: 2.6 wt%
Magnesium: 2.1 wt%
Others: 0.8 wt%

Composition of the Continental Crust

Felsic: $SiO_2$ concentrations between 52 and 77 wt% (composition of igneous rocks “Andesite/Diorite” and “Rhyolite/Granite”).

Composition of the Oceanic Crust

Mafic: $SiO_2$ concentrations between 48 and 52 wt% (composition of igneous rock "Basalt/Gabbro").

Plate Tectonics

The Shape of the Lithosphere

Typical elevation of land surface: 0-1 km.
Typical depth of ocean: 4-5 km.
Examples: Himalayan Mountains, Marianas Trench.

Mantle Convection

Mantle convection causes the break-up of the lithosphere into several tectonic plates.
Stratified Convection Boundary near 700 km separates the two convection systems.

Earth’s Tectonic Plates

7 large plates and several smaller and micro plates.
Composed of both continental and oceanic crust.

Processes at Plate Boundaries

Divergent: Plates drift apart from each other (constructive).
Convergent: Plates collide with each other (destructive).
Transform: Plates slide past one another.

Divergent Boundaries

Oceanic Plate Separation: Mid-Ocean Ridge (MOR).
- Volcanoes and earthquakes concentrate.
- Most Transform faults are found in the ocean basin and connect offsets in the mid-ocean ridges.
Continental Plate Separation: Parallel valleys, volcanoes, and earthquakes.
- Example: East African Rift Valley.

Convergent Boundaries

Ocean-Continent Convergence: Subduction zone.
- A volcanic arc of mountains forms.
- Shallow and deep earthquakes occur.
- Example: Andes Mountains, Peru-Chile Trench.
Ocean-Ocean Convergence: Deep-sea trench, volcanic island arc.
- Deep and shallow earthquakes occur.
- Example: Mariana Islands, Mariana Trench.
Continent-Continent Convergence: Crust crumbles, creating high mountains and a wide plateau.
- Example: Himalayas.
- Main thrust fault, Tibetan Plateau, Eurasian Plate.

The Rock Cycle

Deposition of sediment -> Sedimentary rock -> Burial & Compaction -> Deformation & Metamorphism -> Metamorphic rock -> Melting -> Magma -> Crystallization of Magma -> Igneous rock -> Weathering of rocks at surface -> Erosion & Transport

The Theory of Continental Drift (Plate Tectonics)

Alfred Wegener (German meteorologist, 1880-1930).

Evidence of Past Plate Motions

Fit of the continents and similarity of rock assemblages.
Similarity of geological sequences on the different continents.
The fossil record.
- South America and Africa: Cynognathus, Mesosaurus, Lystrosaurus, Glossopteris.
Evidence of similar climate zones.
- Pioneering paleoclimatic and paleoenvironmental reconstructions.
Hot spots.
- Rate = distance/age difference = $distance/time$ .

Tectonic Plates - Modern Rates of Motion (cm/year)

Rates vary for different plates (e.g., Pacific Plate, North American Plate).

Exercise 2: Past Plate Motion Rates

Calculations based on distances and ages on the map involving the Emperor Seamount Chain and Hawaiian Ridge. This allows past plate motion rates to be determined by $distance/time$ .