INTRO
A. STUDY the movement of earthquake(body) waves through earths interiorB. PATHS of seismic waves are bent(refracted) as they moveC. ABRUPT changes in velocities at discontinuities (boundaries)
Layers Defined by Composition
A. CRUST1. Oceanic crusta. 3-6 miles thickb. Density = 3.0 g/cm2.Continentala. 12-50 miles thickb. Density=2.7g/cmB.MANTLE1. Extends to 1800 miles2.Composed of peridotite3. Average density=4.5 g/cmC. MOHO-CRUST mantle boundaryD. CORE1. Iron-nickel alloy2. Radious-2150 miles
Layers Defined by Physical Properties
A.LITHOSPHERE(crust and uppermost brittle mantle)1. Cooler, rigid layer2.Averages 60 miles in thickness, but may be up to 150 miles below older parts of continentsB.ASTENOSPHERE(low velocity zone of the mantle)1. Soft,weak layerC. MESOSPHERE1. Lower mantle (solid)2. Rocks hot and capable of very gradual flowD. OUTER CORE1. Behaves as a liquid 2. 1350 miles thick3.Average density=11 g/cm4. Convective flow of iron within zone generates Earths magnetic fieldE. INNER CORE1. Behaves as a solid2. Radius 750 miles3.Average density=12.8 g/cm4.Rotates faster than rest of Earth
Discovering Earths Major Layers
A.CRUST / mantle boundary (1909)-Velocity of P-waves increases abruptly below 30 milesB. CORE-mantle boundary1. Located at about 2900 km2. P-Wave shadow zone (1914)a. Area where direct P-waves are absent at seismic stations between 105-140 degrees from an EQb. P-waves are slowed and bent, so that few P-waves occur in shadow zoneC. 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 EQD. 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