Earth’s vital statistics
Shape:
almost spherical
Size: 6400km in radius
Average density: 5.5gm/cc; surface: 3gm/cc or less;
centre may be 1010-15gm/cc
Temperature:
core: 22002200-2750oc
Pressure:
core: 33-4 million times the
atmospheric pressure at sea level
Origin and probable age: thought to have condensed
and congealed from a nebula of gas, dust, and icy
comets about 4.6 billion years ago.
The oldest rock: 3.96 billion yrs old: Earth was
forming continental crust nearly 400 billion
years ago!!
As Earth solidified – gravity sorted materials by density: heavier
substances such as Iron gravitated slowly to the centre and
lighter elements such as silica slowly welled upward to the
surface to be concentrated on the crust.
Result: interior is in concentric circles – each of distinct
chemical composition and temperature
Heat from the interior migrates outward from the centre by
conduction and by convection through the different layers
– some of which are more fluid/ plastic
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More than a quarter century after humans
first set foot on Moon, the deepest boreholes
have penetrated barely 12km into Earth’s
crust: that’s less than 1/500th of the distance
to the centre of Earth!!
However, it is established that
the interior of the Earth is layered!
Evidences of the internal structure is obtained by
using Earthquake waves, called the Seismic waves
The operating Principle: The speed of an earthquake wave is
proportional to the density of the materials through which it
travels.
The denser the material – the faster the speed
Seismic waves also change direction under certain circumstances
through refraction (bending) and reflection
Plastic zones simply do not transmit some seismic waves: they
absorb them!
These distinctive ways help Seismologists to deduce the
structure of Earth’s interior
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Since seismic waves spread out from their
source just like ripples on a pond, they get
weaker the further you get from the earthquake.
http://www.seismo.unr.edu/htdocs/links.html
http://www.seismo.unr.edu/ftp/pub/louie/class/100/effectskobe.html
A Seismograph
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What do the waves tell us?
•P waves travel through solid as well as liquid
•S waves travel only through solid
•Waves are reflected or refracted according to the
density of the material through which they travel
•P waves are faster than S waves
•There is a distinct S waves shadow zone
•There is also a P wave shadow zone where they are
significantly weaker
•P waves appear on the
other side of the earth,
but arrive much later
How the waves travel:
the X-Ray act
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What can we interprete
from these?
P, S waves do not travel in
straight lines inside the Earth
P, S waves get curved as they
gradually travel inward – speed
changes with depth – indicating
changing density
Shadow zone: Earth’s interior not
uniform: Mantle – 2900 Km thick
Radius of Core: 3470 Km
S waves do not travel through liquid: there is a S wave shadow
zone: Earth’s core is liquid
Faint traces of P waves in the antipode shadow zone: the speed
increases: refracted 4 times: must have travelled through DENSE
MASS: Inner Core is very heavy, dense material: Nickel and Iron
Earth’s Interior has an onion-like structure!
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Solid Inner Core:
Radius: 1220 Km
Composition: Nickel and Iron
(NIFE) in SOLID state
(Because pressures are
enormous: melting point
temperatures are higher than
prevailing temperatures)
Liquid Outer Core:
Thickness: 2250 Km
Composition: Essentially same as Inner Core but pressures are
less: in LIQUID State
Density of Inner and Outer Core: 12.5 g/cm3 (accounts for 5.5
g/cm3 for the planet
Composition of the Core:
Av. Density of Earth: 5.5 g/ cm3
Av. Density of Crust: 2.7 – 3 g
Av. Density of Mantle:
moderately higher than crustal
rocks
CORE must be VERY DENSE!!
Why IRON??
• Almost the right density
• In LIQUID state at the estimated temperature and pressure of
the Core
• Abundant on Earth
• Magnetic qualities
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Composition of the Core:
Av. Density of Earth: 5.5 g/ cm3
Av. Density of Crust: 2.7 – 3 g
Av. Density of Mantle:
moderately higher than crustal
rocks
CORE must be VERY DENSE!!
Why NICKEL??
• Meteorites contain IRON nad NICKEL
• The Core is actually a little less dense
The INNER Solid Core: Crystalline Iron and Nickel
The Outer Liquid Core: Iron, Nickel, some sulphur, silicon
Composition of Mantle:
Lower Mantle:
Thickness: 2230 Km
State: Solid/ Rigid (Oxides of
Iron, Magnesium, Silicon)
Upper Mantle:
Thickness: 670 Km
Not that well understood!!!
State: Partially molten/ plastic
(different from Lower Mantle)
• It is capable of flowing slowly
• Contains pockets of molten rock: feeds magma chambers
• The uppermost part is solid. This solid uppermost part and the
Crust on top is called LITHOSPHERE
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So what’s the result of the X-Ray act ?
30 Km
Crust
Rocky/ Solid
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Lithosphere:
This is the Earth’s outer layer
Very important
Thickness: of uneven thickness
Rigid
Plastic
Some places: below 5 km
Some places: below 40 km
Lithosphere can move over Asthenosphere
Heat sources deep inside Mantle keep Asthenosphere in motion:
causes movements in the crustal layer
Continental crust: lowest density: Silica and Aluminium (SIAL)
Granite – a common rock – same density as landmasses
Oceanic crust: higher density: Silica and Magnesium (SIMA)
Basalt – combined density: 3g/ cm3
The Crust:
Varies in thickness
Has cracks
Is fragmented into segments
We call these crustal PLATES
Continental Plates: largely made of Granite
(lighter)
Oceanic Plates: largely made of Basalt
(heavier)
All these plates move over the plastic
Asthenosphere
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