This document discusses the composition and structure of Earth's interior based on seismic wave data. It describes key discontinuities like the Mohorovicic discontinuity that separates the crust from the mantle. The mantle contains a low-velocity zone corresponding to the asthenosphere where rocks are close to their melting point. Below the mantle is an outer liquid core and inner solid core based on the detection of seismic shadow zones. Heat flow in Earth occurs mainly through convection in the liquid outer core. Paleomagnetic evidence from ancient rocks indicates Earth has maintained a liquid outer core and magnetic field for billions of years.
1. 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
2. Geologic Discontinuities
1. seismic waves reflect and refract at discontinuities
or boundaries between geologic materials (ie.
rocks)
2. velocity relationships
4. 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
5. Composition and Structure of the Earths Interior
2. Isostasy
i. continental crust floats on top of oceanic curst
6. 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
7.
8. 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 (?)
9.
10. 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
11.
12. 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
13.
14. 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
15.
16.
17.
18. Magnetic Field
1. William Gilbert (1600) realized the earth is a big
magnet
2. supports theory that the outer core is liquid
19. 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
20. 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
