THE EARTHS INTERIOR
- Seismic Wave Review
- p-waves and s-waves
- velocity (distance/time)
- 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)
- velocity relationships
- 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
- felsic (granitic): 6 km / sec
- 35 km thick
- Oceanic Crust
- mafic (basalt): 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
- Internal Heat and Heat Flow
- heat is due to meteor impact and radioactive decay
- earth cools by conduction and convection
- conduction
- hot particles transfer heat to cool particles
- very inefficient and cannot explain the solid mantle (must be another mechanism)
- convection
- heated liquid expands and rises since it is less dense than cool material around it
- examples
- Magnetic Field
- William Gilbert (1600) realized the earth is a big magnet
- supports theory that the outer core is liquid
- paleomagnetism
- the earths magnetic field is recorded in cooling rocks that contain material that will align to a magnetic
field - thermoremanent magnetism
- ancient magnetism is called paleomagnetism
- since rocks that are 3.5 billion years old record a magnetism, the earth had a magnetic field at that time -
fluid core
- magnetic stratigraphy
- about ½ of the rocks on earth contain a magnetic field opposite to today's field
- patterns have been recognized and we can use it as a stratigraphy (a sequence) to tell approximate
age
- implications?