Earth's Interior and Plate Tectonics Lecture Notes

Earth’s Interior

Course Information

• Lecture 4, EAOS111, Monday 3 March 2025
• Readings: The Blue Planet, pages 61-63, 125-132 (Today); pages 23-26, 111-125 (Tuesday)

Plate Tectonics

• The unifying theory of geology.
• The outer layer of the Earth (lithosphere) consists of separate plates that move with respect to one another.

Today’s Learning Outcomes

• Describe the differences in composition and strength of Earth’s layers and how they influence the tectonic cycle.
• Define the 3 types of plate boundaries and their sense of plate movement.

The Big Bang

• Occurred 13.8 billion years ago (13.8 \times 10^9 years).
• The universe cooled, allowing protons, neutrons, and electrons to combine into hydrogen atoms.
• Big bang nucleosynthesis: nuclear fusion formed hydrogen (H) and helium (He).

Stars: Element Factories

• Gravity pulls together material to make stars.
• Stellar nucleosynthesis: heavier elements created (carbon, nitrogen, oxygen).
• Supernovae: create elements heavier than iron (Fe).

Planet Formation

• Gravity pulls together material to form new stars and a surrounding accretionary disk. Dense and hot conditions initiate fusion reactions.
• Accretionary disk starts to coalesce to make planetesimals, which collide to make protoplanets, and eventually planets.
• Differentiation occurs, leading to the formation of an iron core. Gravity makes planets round.
• Process occurred around 4.57 billion years ago (4.57 \times 10^9 years).

Solar System

• Consists of the Sun, planets, moons, asteroids, and comets.
• Two groups of planets:
  • Terrestrial (Earth-like): Small, dense, rocky.
  • Jovian (Jupiter-like): Large, low-density, gas giant.

Earth’s Interior

• Earth has internal layering that originated during planetary differentiation (after condensation, accretion).
• The layers are distinguished based on composition and strength.

Density of Chemical Elements

*Formula: Density = \frac{mass}{volume}

• Densest material is at the center of the Earth, while the least dense material is at the surface.
• Examples of densities of elements:
  • Hydrogen (H): 0.0899 g/cm3
  • Lithium (Li): 0.535 g/cm3
  • Beryllium (Be): 1.848 g/cm3
  • Helium (He): 0.1789 g/cm3
  • Sodium (Na): 0.968 g/cm3
  • Magnesium (Mg): 1.738 g/cm3
  • Aluminum (Al): 2.7 g/cm3
  • Silicon (Si): 2.33 g/cm3
  • Sulfur (S): 1.96 g/cm3

Seismic Waves

Primary Waves (P-waves, Compressional Waves)

• Travel most quickly; the first earthquake waves to arrive at a receiver.
• Travel through solids and liquids.
• Travel faster through cold material and slower through hot material.

Secondary Waves (S-waves, Shear Waves)

• Travel slower than P-waves; the second earthquake waves to arrive at a receiver.
• Cannot travel through liquids.

Seismic Waves and Earth's Interior

• P-waves are refracted by changes in density, creating a P-wave “shadow zone.”
• S-waves cannot travel through liquids, creating an S-wave “shadow zone.”

Earth’s Interior: Compositional Layers

• Continental crust:
  • Granite (average composition)
  • 35-40 km thick
  • Density ~2.8 g/cm3
• Oceanic crust:
  • Basalt (average composition)
  • 7-10 km thick
  • Density ~3 g/cm3
• Mantle:
  • Peridotite (solid rock)
  • Density ~3.3 g/cm3
• Core:
  • Fe-Ni alloy
  • Liquid outer core
  • Solid inner core

Earth’s Interior: Strength Layers

• Lithosphere:
  • 0-100 km deep
  • Strong, cool, rigid (crust + upper mantle)
• Asthenosphere:
  • 100-350 km deep
  • High temperature
  • Weak, easily deformed, solid (mantle)
• Mesosphere:
  • 350-2883 km deep
  • High temperature & pressure
  • Strong, solid (mantle)

Heat Transfer

• The driver of plate tectonics.
• Misconception Alert: The mantle is solid rock that deforms in a ductile (plastic) way when at high pressures and temperatures (>1300°C).
• Heat transfer occurs through convection and conduction.

The Scientific Method

1. Observe (where the evidence comes in)
2. Hypothesize (develop a reasonable explanation for the observation)
3. Predict (what are the other implications of the hypothesis?)
4. Test predictions (look for new evidence to support/refute predictions)
5. Repeat, repeat, repeat…

"Contracting Earth" Hypothesis

• Early 20th-century hypothesis to explain mountains and valleys.
• Predicted that Earth's surface elevation would have a normal distribution.
• Predictions not supported by tests; the hypothesis was discarded.

Elevation Data

• Highest point: Mt. Everest, 8,848 m above sea level
• Average Land Height: 797 m
• Average Ocean Depth: 3,686 m
• Deepest point: Marianas Trench, 10,911 m below sea level

Alfred Wegener (1880-1930)

• German meteorologist and climatologist.
• Published his hypothesis of Continental Drift in The Origin of Continents and Oceans (1915).
• Observed ancient seafloor in mountains and uplift during earthquakes.
• Proposed that continents were originally connected as a single supercontinent (Pangaea), then drifted apart.
• Suggested that plants and animals spread freely, and noted parallel coastlines where continents once fit together.

Evidence Supporting Continental Drift

Fit of the Continents

Fossil Distribution

• Mesosaurus: freshwater reptile.
• Glossopteris: woody, seed-bearing plant.

Matching Geology

Matching Paleoclimate

• The geologic record preserves evidence of changes in environment and climate through time.
• Wegener’s model correctly predicts the distribution of climate belts as preserved in Permian-aged rocks.
  • Rock types and their environments:
    • Coal: Swamps and jungles in tropical regions
    • Limestone, coral reef: Shallow seas
    • Salt deposits, sandstone: Desert

Pangaea Reconstruction

• Glacial till of the same age (Permian, ~250 Ma) found on four continents, some now near the Equator.
• Distribution is best explained if all continents were part of Pangaea, centered at the South Pole.

Criticism of Wegener’s Hypothesis

• Wegener provided no mechanism for how or why the continents move, so his hypothesis was met with skepticism.
• "What force could possibly be great enough to move the immense mass of a continent?"
• Some called his ideas "delirious ravings."

Arthur Holmes (1890-1965)

• British geologist.
• First to suggest a mechanism for plate tectonics in 1928: Convective current mechanism for continental drift.
• Pioneer of geochronology – performed the first accurate radiometric dating of a rock (uranium-lead).
• Produced the first quantitative geological time scale.
• Suggested continents are carried by the flow of the mantle on which they sit.

Lithospheric Plates

• 9 major and many minor plates

Plate Boundaries

Divergent Margins

• Continental divergent margin (continental rift)
• Oceanic divergent margin (oceanic rift)

Convergent Margins

• Ocean-ocean convergent margin (subduction zone)
• Continent-continent convergent margin (collision zone)
• Continent-ocean convergent margin (subduction zone)

Transform Margins

• Transform fault margin (continental crust)