THE INTERIOR OF THE EARTH
Seismic Waves - produced by earthquakes
p-waves and s-waves
velocity
seismic waves travel at different speeds through different materials
Geologic Discontinuities
seismic waves reflect and refract at discontinuities or boundaries between geologic materials (ie. rocks)
Paths of Seismic Waves
homogeneous composition = straight
non-homogeneous composition = curved
Composition and Structure of the Earths Interior
Crust: Oceanic and Continental Crust
- Continental Crust
- Oceanic Crust
- 7 km / sec
- 5-10 km thick
- denser than continental crust
- Isostasy
- continental crust floats on top of oceanic curst
Mantle
- Andrija Mohorovii (1909)
- discovered a discontinuity at a depth of about 30 km
- Mohorovii Discontinuity (Moho)
- p-waves below travel at 8km / sec
- p-waves above travel at 6.5km / sec
- the Moho separates the crust from the mantle
- low-velocity zone
- 100 - 250 km deep
- p- and s-wave velocities decrease
- corresponds to the asthenosphere
- a layer in which the rocks are close to there melting point and are less elastic
- magmas are produced here(?)
- discontinuous (?)
Core
- 1906 R.D. Oldham
- discovered that seismic waves arrived later than expected at seismic stations more than 130 degrees from an earthquake focus
- he postulated the existence of a core that transmits seismic waves at a slower rate than shallower depths
- only very weak p-waves between 103-143 degrees from an earthquake focus: P-wave shadow zone
- Inge Lehman (1936)
- postulated that the Earth has a solid inner core
- explains the existence of weak p-waves in the shadow zone
- Harold Jeffreys (1926)
- discovered that s-waves were no just simply slowed by the core but were blocked by it
- large and complete s-wave shadow at locations greater than 103 degrees