Geology 101 - Lecture 2

I. Formation and Evolution of Earth

• Inner planets: Mercury, Venus, Earth, Mars. Rocky planets. Volatiles removed
  
• Outer planets: Jupiter, Saturn, Uranus, Neptune. Mostly hydrogen and helium.

II. Evolution of scientific knowledge

How did we figure out the age of the Earth?

• Earth's landscape had been shaped by many catastrophies. So mountains & canyons produced by sudden and worldwide disasters (unknowable causes)
  
• Why? Fit rate of earth processes to biblical age of Earth
• Based on a work by James Ussher, an Anglican bishop, mid 1600s. By doing painstaking review of the Bible, determined that Earth was formed in 4004 BC.
  Very widely accepted
  
  The only way the Earth could look like it does was if it had been changed by great catastrophies.

• Idea from James Hutton (Scottish)
  "Physical, chemical, and biologic processes operating today also operated in past"
• Charles Lyell (1830-1872) made it accessible to the public
• Means that you have to accept a very long history for the Earth.
  (i.e., Marine fossils on top of Mt. Everest)
• This has been unifying theme of geology ever since; EXCEPT for meteorite impact theory for extinction of dinosaurs

III. Geologic Time

• Eons, Eras, Periods, Epochs
  
• Click here for an interactive session with the geologic time scale
  
  
  Magnitude is hard to realize. How long is 4.6 billion years? Count to 4.6 billion, 1 second at a time, would take 150 years.

IV. Earth Evolution

• Core formation

  All planets formed about 4.6 billion years ago. Earth was a homogeneous planet immediately after formation
  
  But then, a profound reorganization as Earth initially heated up due to
  
  1. collision with other rocky bodies
  2. heat of compression
  3. Radioactive decay of elements
  
  Earth heated up so much that much of the interior melted, causing dense elements like iron and nickel to sink to the core and lighter elements like silicon and aluminum float to the crust. Resulted in a stratified Earth - core, mantle, crust
  
  

• Distribution of Elements

  More than 100 elements in entire Earth, but 99% of Earth's mass is made up of only 8 elements
  
  Whole Earth:
  Fe>O>Si>Mg>Ni>S>Ca>Al (others constitute < 1%)
  
  Earth's crust
  O>Si>Al>Fe>Mg>Ca>K>Na (other constitute <1%)
  
  

• Formation of atmosphere

  • Initial atmosphere - H2, He, N
  • Current atmosphere - N2 (78%) and O2 (21%), Others (Ar, CO2, etc)
  • Problem - How do we get from initial to current atmosphere?
  • 4.6 b.y.a. - H2 and He escape into space, volcanoes provide source of other gases
  • 3.5 b.y.a. - O2 appears: Appearance of single-celled organisms. CO2 + sunlight changes to C (in form of sugar) + O2
  • 0.6 b.y.a. - Atmosphere similar to today (multi-celled organisms appear)
  
  

• Formation of oceans

  • Oceans consist of water (96.5%) and salt (3.5%, mostly dissolved halite, Na+, Cl-)
  • Sources of water in the oceans include volcanic eruptions (water is the major gas released) and water from comets (bodies of ice and rock that collided with the Earth)
  • Source of salt = chemical weathering (dissolution) of rock Earth's oceans probably reached the current amounts of salt and water by 3.5 b.y.a

V. Earth Structure

Compositional layers

• core - Fe,Ni.
• mantle - compositionally homogenous, made up of peridotite rock
• crust - compositionally heterogeneous, lots of rock types

Mechanical layers

• inner and outer core
• mesosphere (lower mantle)
• asthenosphere - warm, ductile, weak, mantle beneath lithosphere
• lithosphere - cold, brittle, strong, uppermost crust and mantle

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