THE EARTHS INTERIOR

• Seismic Wave Review

1. p-waves and s-waves

2. velocity = Distance / time

3. seismic waves travel at different speeds through different materials

Geologic Discontinuities

1. seismic waves reflect and refract at discontinuities or boundaries between geologic materials (ie. rocks)

2. velocity relationships

Geologic Discontinuities

3. Paths of Seismic Waves

i. homogeneous composition = straight

ii. non-homogeneous composition = curved

Composition and Structure of the Earths Interior

1. Crust: Oceanic and Continental Crust

i. Continental Crust

(1) felsic (granitic): 6 km / sec

(2) 35 km thick

ii. Oceanic Crust

(1) mafic (basalt): 7 km / sec

(2) 5-10 km thick

(3) denser than continental crust

Composition and Structure of the Earths Interior

2. Isostasy

i. continental crust floats on top of oceanic curst

3. Mantle

i. Andrija Mohorovi i (1909

(1) discovered a discontinuity at a depth of about 30 km

(2) Mohorovičić Discontinuity (Moho)

(a) p-waves below travel at 8km / sec

(b) p-waves above travel at 6.5km / sec

(3) the Moho separates the crust from the mantle

3. Mantle

ii. low-velocity zone

(1) 100 - 250 km deep

(2) p- and s-wave velocities decrease

(3) corresponds to the asthenosphere

(a) a layer in which the rocks are close to there melting point and are less elastic

(b) magmas are produced here(?)

(c) discontinuous (?)

4. Core

i. 1906 R.D. Oldham

(1) discovered that seismic waves arrived later than expected at seismic stations more than 130 degrees from an earthquake focus

(2) he postulated the existence of a core that transmits seismic waves at a slower rate than shallower depths

(3) only very weak p-waves between 103-143 degrees from an earthquake focus: P-wave shadow zone

ii. Inge Lehman (1936)

(1) postulated that the Earth has a solid inner core

(2) explains the existence of weak p-waves in the shadow zone

4. Core

iii. Harold Jeffreys (1926)

(1) discovered that s-waves were no just simply slowed by the core but were blocked by it

(2) large and complete s-wave shadow at locations greater than 103 degrees

Internal Heat and Heat Flow

1. heat is due to meteor impact and radioactive decay

2. earth cools by conduction and convection

i. conduction

(1) hot particles transfer heat to cool particles

(2) very inefficient and cannot explain the solid mantle (must be another mechanism)

ii. convection

(1) heated liquid expands and rises since it is less dense than cool material around it

Magnetic Field

1. William Gilbert (1600) realized the earth is a big magnet

2. supports theory that the outer core is liquid

Magnetic Field

3. paleomagnetism

i. the earths magnetic field is recorded in cooling rocks that contain material that will align to a magnetic field - thermoremanent magnetism

Magnetic Field

3. paleomagnetism

iii. since rocks that are 3.5 billion years old record a magnetism, the earth had a magnetic field at that time - fluid core

iv. magnetic stratigraphy

(1) about ½ of the rocks on earth contain a magnetic field opposite to today's field

(2) patterns have been recognized and we can use it as a stratigraphy (a sequence) to tell approximate age