Earth

Formed ~4.5 billion years ago.

Earth’s layers

Cooling ⇒ dense materials (Fe) sink to middle; low-density minerals crystallized and floated to the surface.

Differentiation into compositional zones or layers

• Central core: dense and hot
  • Composed of nickel (Ni) and iron (Fe)
• Mantle: thickest zone and surrounds the core
  • Composed of ultramafic and mafic rocks and magma (fe, Mg, Si)
  • Heat from core escapes by convective circulation
• Crust: chemically different from core or mantle
  • Two types of crust: Oceanic (mafic) and Crustal (felsic)
  • Water and atmospheric gases interact only with outermost crust

Whole process completed well before 4 billion years ago…

Most Common Chemical Elements in Earth

Earth’s Early Atmosphere

• The heating and subsequent differentiation of the early earth led to the formation of the atmosphere and oceans
• Many minerals contained water or gases in their crystals and released them during the heating and melting
• As the Earth’s surface cooled, the water could condense to form the oceans and gases form the atmosphere
• Chemically different than today
  • Lacked free oxygen (O2)
  • Dominated by nitrogen (N) and carbon dioxide (CO2)
  • Minor amounts of other gases:
    • Methane (CH4)
    • Ammonia (NH3)
    • Sulfur gases
• No modern pollution
• Barren of life because it lacked oxygen

[! Question] So how did the atmosphere fill with oxygen as it is today?

• Earliest life were anaerobic, surviving off sulfur in an oxygen-free environment
• Cyanobacteria began using water, CO2, and sunlight in the oceans for energy, producing oxygen as a by-product
  • First organisms (and still only bacteria) to conduct oxygenic photosynthesis

Early Life

Signs of oxygen and early life

Banded Iron Ore

• Sedimentary rocks of alternating layers of iron oxides and iron-poor chert
• Evidence of oxygenation of the oceans circa ~2-4 billion years ago

Stromatolites

• Sedimentary formations created by photosynthetic organisms, like cyanobacteria
• Peaked in fossil record about 1.25 billions years ago

Look at Earliest Known Life Forms