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INTRO
- A. STUDY the movement of earthquake(body) waves through earths interior
- B. PATHS of seismic waves are bent(refracted) as they move
- C. ABRUPT changes in velocities at discontinuities (boundaries)
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Layers Defined by Composition
- A. CRUST
- 1. Oceanic crust
- a. 3-6 miles thick
- b. Density = 3.0 g/cm
- 2.Continental
- a. 12-50 miles thick
- b. Density=2.7g/cm
- B.MANTLE
- 1. Extends to 1800 miles
- 2.Composed of peridotite
- 3. Average density=4.5 g/cm
- C. MOHO-CRUST mantle boundary
- D. CORE
- 1. Iron-nickel alloy
- 2. Radious-2150 miles
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Layers Defined by Physical Properties
- A.LITHOSPHERE(crust and uppermost brittle mantle)
- 1. Cooler, rigid layer
- 2.Averages 60 miles in thickness, but may be up to 150 miles below older parts of continents
- B.ASTENOSPHERE(low velocity zone of the mantle)
- 1. Soft,weak layer
- C. MESOSPHERE
- 1. Lower mantle (solid)
- 2. Rocks hot and capable of very gradual flow
- D. OUTER CORE
- 1. Behaves as a liquid
- 2. 1350 miles thick
- 3.Average density=11 g/cm
- 4. Convective flow of iron within zone generates Earths magnetic field
- E. INNER CORE
- 1. Behaves as a solid
- 2. Radius 750 miles
- 3.Average density=12.8 g/cm
- 4.Rotates faster than rest of Earth
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Discovering Earths Major Layers
- A.CRUST / mantle boundary (1909)-Velocity of P-waves increases abruptly below 30 miles
- B. CORE-mantle boundary
- 1. Located at about 2900 km
- 2. P-Wave shadow zone (1914)
- a. Area where direct P-waves are absent at seismic stations between 105-140 degrees from an EQ
- b. P-waves are slowed and bent, so that few P-waves occur in shadow zone
- C. LIQUID behavior of outer core (1926) S-Waves are completely blocked by core creating an S-waves shadow zone (105 to 256 degrees) frm an EQ
- D. EXISTENCE of inner core (1936)- The weak P-waves observed in the P-wave shadow zone are refravted as they move from outer core to inner core
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