Download Composition and Internal Structure of Earth

Composition and Internal
Structure of Earth
Source: NASA
Density of Earth
„
Density = Mass/Volume
„
What is the volume (or diameter) of the Earth?
„
How much does the Earth weigh?
Diameter of the Earth
7.2/360 = 787/C
C=39,350 km (now 40,030 km)
Diameter = 12,525 km (now 12,742 km)
Alexandria
7.2º
light rays
from sun
7.2º
Syene
1
Mass of the Earth
Cavendish (1798) using Newton’s Laws
Force = mass × acceleration (Newton’s 2nd Law)
F = mbody × g = G ×
mEarth × mbody
r2
(Newton’s Law of Universal Gravitation)
G is universal gravitation constant (6.672 × 10-11 m3kg-1s-2)
g is acceleration due to gravity on earth, measured by timing
how long it takes a mass to fall in a vacuum (9.8 ms-2)
Mass of Earth = 5.9742 × 1024 kg
Average Density of Earth
Diameter of the Earth = 12,742 km
(7,920 miles )
„ Mass of the Earth = 5.974 × 1024 kg
g
(13.15 × 1024 lb)
„ Density = 5.5 g/cm3
„
Earth is actually not a perfect sphere: it is slightly oblate (squished)
being fatter across the equator ( 12756 km diameter) than at the
North and South Poles (12714 km diameter)
Mean Atomic Wt of Planets
Anderson and Kovach
(1967)EPSL
2
Internal structures of
terrestrial planets
1. Drilling the Earth!
Well.. Only 1/3 of crust depth can be explored
2. Xenolith from Deep Earth (Mantle)
:“Peridotite”
(play video clip >>)
>>)
3
3. Seismic Data
Analogy of Air/Water
Play Animation>>
Earth Interior
W. W. Norton
Seismic Data give strong evidence of
Existence of Discrete Layers
Int.--C06ab
Int.
W. W. Norton
4
What about the deeper interior?
Int.--C.10a
Int.
W. W. Norton
… another evidence of liquid outer core
Int.--C.09
Int.
W. W. Norton
Another layer? -> Yes, Inner Core!
5
Compositional Terms
Physical Property Terms
(Butter!)
(Liquid)
Inner Core
Mantle
Outer Core
(Liquid)
Crust
Chemical compositions
Crust -> from rocks near the surface
Mantle -> from mantle xenolith
„ Core -> from comparison with meteorite
„
„
6
Crust
1) Continental Crust
– Thicker (~40 km)
– Lower Density
– Mainly composed of
Granite comp’n
comp n
2) Oceanic Crust
– Thinner (~7km)
– Higher Density
– Mainly composed of
Basalt comp’n
Mantle
„
„
„
„
Lithosphere: Rigid
Asthenosphere (~100(~100-400 km): less Rigid
(butter-- or wax
(butter
wax--like, semimolten
semimolten))
Transition Zone (~400
(~400( 400-700 km)
Lower
Mesosphere (below ~700 km)
Mantle
„
Compositional difference between Crust and
Mantle is significant, but not as large as the
difference with Core
Core..
„
Two types of high P rocks
Upper
Mantle
– Eclogite
– Peridotite
„
Which one is the dominant rock type in
mantle?
– Needs to satisfy seismic data
– Needs to satisfy petrologic data
„
Peridotite is the dominant rock type
7
Core Composition?
How do we know?
„
Outer Core
– Fe + Ni
– Lower Density (compared to Inner Core)
– Liquid
qu d (in
( motion)
o o )
„
Inner Core
– Fe + Ni
– Higher Density
– Solid
Clue comes from Meteorite!
„
It is believed that a certain type of meteorites has
primordial chemical composition of the solar system.
It is probably a valid assumption that the
chemical composition of such meteorite is
very similar to the composition of the
Earth.
Earth
„ Then, which element is enriched or
depleted in the crust/mantle?
„ Furthermore, what would be the
composition of the Earth’s core?
„
8
/
In Chondrite
Fe2O3: 30%
NiO: 2.1%
Zn: 0.05%
Cr: 0. 4%
Major
Core Constituents!
Important Terms
„
Refractory elements
– High condensation T
– Ca, Al, U, Ti, REEs
„
Volatile elements
– Low
L
condensation
d
ti T
– K, Rb
„
Incompatible Elements (Kd
(Kd << 1)
– LILE (Large Ion Lithophile Element)
ƒ K, Rb
Rb,, Ce,
Ce, Sr,
Sr, Ba
– HFSE (High Field Strength Element)
ƒ Zr
Zr,, Nb,
Nb, Hf,
Hf, REEs, Th,
Th, U, Ta
What caused such layering?
“Rain--Out Model”
“Rain
Need Energy!!
(for this process
+ plate tectonics
tectonics))
9
Heat (Energy) Source #1:
Bombardment
1. During the Early Stage of Solar System Formation
2. Primordial Process
3. Enough to melt the whole planet (Magma Ocean)
4. Earth and other Planets were very Hot!
Heat (Energy) Source #2:
Differentiation
Sinking of FeFe-Ni fraction to the center of
the Earth (and other Planets) release
gravitational potential energy
„ During the Early Stage of Solar System
Formation
„ Primordial Process
„
Heat (Energy) Source #3:
Tidal Force
When a body1 act on the gravity on
body2, the gravitational field may not
even on the body2
„ Distortion is commonly induced
„ Internal Friction results in dissipation of its
rotational kinetics into Heat
„
10
Heat (Energy) Source #4:
Radioactive Decay
„
„
„
Spontaneous changes (decay) in atomic nuclei
– Parent: an unstable radioactive isotope
– Daughter: the isotopes resulting from the
decay of a parent
Provides important info to Geologists
– (1) age dating
– (2) heat budget calculation
Still occurring
11