Structure of the Earth
I. Structure of the Earth
- A. How Do We Know This?
- 1. Earthquake waves
- a. velocity of earthquake waves is proportional to the density
of the material the waves pass through
- i. for the CORE: Iron alloyed with Nickel, or Sulfur, Silicon
or Oxygen
- ii. for the MANTLE: Magnesium-Iron Silicates and Oxides
- 2. mantle xenoliths
- a. pieces of mantle carried to Earth's surface in volcanic
rocks
- 3. Meteorites
- a. Irons
- b. Stones (chondrites, carbonaceous chondrites, achondrites)
- c. Stony irons
- 4. Orbital characteristics
- a. average density of Earth is 5.5 gm/cm3
- b. crustal rocks and upper mantle rocks only 2.5 to 3.3 gm/cm3
- c. deeper parts of the Earth must be significantly denser
- B. Core
- 1. Inner core is solid
- 2. Outer core is fluid
- 3. Deduced from seismic wave behaviour
- C. Mantle
- 1. Subdivided according to seismic velocity
- 2. Reasons for seismic velocity changes
- a. <400 km - olivine + pyroxene
- i. low velocity zone - partially melted (source of oceanic
basalts)
- ii. density 3.3 gm/cm3
- b. 400-700 km - pyroxene becomes olivine and stishovite and
olivine transforms to spinel-like phase
- ii. density 3.5 gm/cm3
- c. >700 km - breakdown of silicates into oxides
- ii. density 3.7 to 5.5 gm/cm3
- D. Crust
- 1. Oceanic crust
- a. magnesium, iron, calcium, aluminosilicates (pyroxene +
feldspar) = basalt/gabbro
- b. derived from partial melting of mantle rocks
- 2. Continental crust
- a. sodium, potassium, calcium, iron, magnesium aluminosilicates
(feldspar + quartz + amphibole) = granite/granodiorite
- b. developed over time by distillation process -> partial
melting of oceanic crust & sediments
- E. Hydrosphere and Atmosphere
- 1. "Out gassed" from mantle during differentiation
- a. mostly Nitrogen, Water, and Carbon dioxide
- i. also some hydrogen and helium and other noble gases
- b. Carbon dioxide was mostly scrubbed from atmosphere by
reaction with water & biologically fixed as limestone, continually
recycled
- i. Gives Earth moderate "greenhouse" effect
- ii. No water on Venus (too close to Sun) - runaway "greenhouse"
effect
- iii. No liquid water on Mars - no recycling of carbon dioxide
- too little "greenhouse" effect
- c. Water condensed to form oceans
- d. Nitrogen is the bulk (79%) of atmosphere
- i. much of the rest (20%) of the atmosphere is Oxygen generated
biologically during photosynthesis
- ii. 1% of the atmosphere is radiogenic Argon
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