Transcript 4_Ocean126_2006

Structure of Earth
Chapter 2
Most simply
1. Crust –
cold, rigid, thin
2. Mantle –
warmer, more
dense; outer part
rigid and inner
part plastic
(deformable)
The earth is layered & density stratified
3. Outer core –
transition zone
then thick liquid
zone
4. Inner core –
solid but warm,
very dense, rich
in magnetic
materials (Ni, Fe)
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)
– Still crustal
 Studies of the earth’s orbit – gave an idea of
mass
– Surface rocks predicted lower total mass if the
earth were homogeneous
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
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
Why are these
waves important?
•We can detect
these waves
independently
•They behave
differently passing
through different
media
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.
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 142o
shadow zone
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)
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
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
TABLE I
Typical Densities of Earth Materials
Substance
Density*
Sea Water
1.02
Limestone
2.68-2.76**
Granite
2.64-2.76**
Sandstone
2.14-2.36**
Slate
2.6-3.3**
Basalt
2.4-3.1**
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)
Physical responses
Lower mantle
Core
2900 – 6370 km
~3400
Dense, viscous liquid
Solid inner core
Classifying layers
By composition
Isostatic equilibrium and rebound
 This concept helps us understand the
“floating” of lithosphere on asthenosphere
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
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!
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