1. Structure of Earth
Chapter 2

2. Most simply
Crust –
cold, rigid, thin
Mantle –
warmer, more
dense; outer part
rigid and inner
part plastic
(deformable)
Outer core –
transition zone
then thick liquid
zone
4. Inner core –
solid but warm,
very dense, rich
in magnetic
materials (Ni, Fe)
The earth is layered & density stratified

4. How do we know this? All we see is the crust!
Deepest drill-hole – 12,063 m (7.5 miles)
Still crustal
Deepest ocean drilling – 2 km (1.2 miles)
Studies of the earth’s orbit – gave an idea of mass
Surface rocks predicted lower total mass if the earth were homogeneous

6. Mohorovicic “Moho” discontinuity
Density discontinuity – P waves arrived at seismic station before they should have in an homogeneous earth
Boundary between the crust and mantle
Discovered by Croatian geophysicist based on observations of seismic waves generated by earthquakes.
Fun fact – there was an effort to drill a “Mohole” but failed due to lack of $$ and technology

7. Evidence for layering Mainly we know depend on seismology
Seismic waves generated from earthquakes
“Primary” P-waves (compression waves; longitudnally propagated waves; oscillate in same direction as movement like sound waves)
“Secondary” S-waves (transverse waves; horizontally propagated; oscillate perpendicular to movement like water waves)
1900 – identified P & S waves on a seismograph (Oldham)
Waves were passing through the earth faster than predicted
Wave speed increases with increasing density!
Waves were being refracted (bent so they changed direction)
Hypothesized that there were areas of Earth with different densities
1906 – no S-waves passed through the earth
Shadow zone – no S-waves
P-waves took longer than expected

8. Why are these
waves important?
We can detect
these waves
independently
They behave
differently passing
through different
media

9. Prediction of earthquake waves passing through a planet of
Point of origin of seismic source.
Prediction of earthquake waves
passing through a planet of
regularly changing density.
Prediction of earthquake waves
passing through a homogeneous
planet.

10. What P waves do in & around liquid outer core (bend) – see book.
What S waves do around liquid
outer core (do not penetrate).
P-wave
shadow zone
142o
P-wave
shadow zone
142o

11. Seismology
Changes in travel time and path tell us about the earth’s structure
Refraction of waves led to discovery of earth’s core and Moho
Travel time of waves led to discovery of layers
Now we use changes in travel time and path tell us about location of disturbances (earthquakes or bombs)

13. Earth’s functional layers
Crust – we know most about it; continental crust is less dense
Moho – a density discontinuity that separates crust from the mantle
Depth varies under continents and oceans
First thought that this was layer where crust moved relative to earth’s interior BUT, outer layer of mantle moves with crust!
Lithosphere – crust plus rigid mantle (not totally rigid but, movements cause things like earthquakes and volcanoes
Asthenosphere – plastic layer of mantle; lithosphere floats on asthenosphere
Mantle includes part of lithosphere, asthenosphere and solid mesosphere

14. Chemical composition
of layers:
Crust – lightweight (0.4% mass/1% volume of earth) – ocean crust (basalt – O, Si, Mg & Fe) is denser than continental crust (granite – O, Si, Al)
Mantle – denser (68% mass/83% volume of earth) - Si, O, Fe & Mg
Core – densest (31.5% mass/16% volume of earth) - mainly Fe & Ni with some Si, S and heavy elements

15. Typical Densities of Earth Materials
TABLE I
Typical Densities of Earth Materials
Substance
Density*
Sea Water
1.02
Limestone **
Granite **
Sandstone **
Slate **
Basalt **
Average Density of Continents
2.7
Average Density of SiMa (Mantle Material)
3.3
* Actual densities vary slightly, depending on chemical composition.
(** Source: Handbook of Chemistry and Physics)

16. Physical responses Lower mantle Core 2900 – 6370 km ~3400
Dense, viscous liquid
Solid inner core

17. Classifying layers
By composition

18. Isostatic equilibrium and rebound
This concept helps us understand the “floating” of lithosphere on asthenosphere

19. Isostacy
Ocean basins and continents “float” on asthenosphere at equilibrium so that total pressure at depth in mantle is everywhere the same.
Depending on density, things will float at a certain height and displace a different amount of water
Most mass is below the surface, what sticks out of the fluid is supported by bouyancy of displaced fluid below the surface
Examples – icebergs, ships, blocks of wood of different densities in water

20. What does this mean?
Mountains have roots that are deeper than surface expression
As erosion removes mass from the top of a mountain, the roots shrink upward or the asthenosphere “rebounds”
Example: younger (higher) Rockies have deeper roots than older Appalacians
Example: continental rebound from glaciers (Great Lakes & Long Island Sound examples); sea level decreases even though more water!

22. Take home points Layers of the earth – density stratification
How do we know earth’s structure – seismology and the role of S and P waves
Moho, lithosphere and asthenosphere
Isostacy; Isostatic equilibrium