Transcript Earth`s Internal Structure I

Look Up?
(Astronomy)
Three
Worlds
to
Explore
Look at Surface?
(Geology)
Look Down?
(Geophysics)
Deep wells
and
boreholes
(4 to 12 km)
The Earth’s
Interior
Astronomical
Observations
The
Earth:
What’s it
made of?
Gravity
Seismology
N
S
Magnetism
Heat Flow
Density of Water
1 gm/cm3
Mantle
3.3-5.7 gm/cm3
Outer Core
10-12 gm/cm3
Crust
2.6-3.1 gm/cm3
Inner Core
13-14 gm/cm3
P and S Wave Paths
Relatively simple experiment to estimate
some basic properties of the Earth’s
interior:
Mass = M
Radius = R
Average Density = D
Rough Estimate of Variation of Density in Interior
Newton’s Law of Gravitation
r
F
M
F
m
Force of attraction (F) is proportional to the
masses, and is inversely proportional to the
square of the distances between the masses.
Newton’s Law of Gravitation
r
F
F
M
Mm
F = G r2
m
Henry Cavendish (1731-1810) determined the
universal constant of gravitation G in 1798.
torsion fiber
massive
lead
spheres
F m
R
Mm
F = G 2
R
M

F R2
M =
Gm
F m
?
R
Measure the force
M
2
FR
M =
Gm
Can measure in lab
If we could measure R,
we could determine M.
Use a known mass
How can we measure R?
Greek Scientist:
Eratosthenes
(276-194 BCE)
Eratosthenes (276-194 BCE)
Observed the
angles of the
noonday Sun in
two Egyptian
cities that were
roughly north
and south of
each other.
Syene (presently Aswan) and Alexandria
Eratosthenes (276-194 BCE)
The angles differed
by 7 degrees (or
1/50 of a complete
circle).
Circumference of
the Earth must be
50 times the
distance between
the cities.
Eratosthenes (276-194 BCE)
The cities are 788 km
apart.
Circumference of the
Earth must be
50 x 788 km
= 39,400 km
R = 39,400/2p = 6,271 km (modern value = 6,371 km)
Astronomical
Observations
The
Earth:
What’s it
made of?
Gravity
Seismology
N
S
Magnetism
Heat Flow
F m
R
M
Eratosthenes
Measure the force
2
FR
M =
Gm
Can measure in lab
Use a known mass
M = 6x1027 gm =
6,000,000,000,000,000,000,000,000,000 gm
4 3
Volume = pR
3
R
M

Mass
Density =
Volume

Average Density = 5.5 gm/cm3
Average density
of the Earth =
5.5 gm/cm3
Dense
Material?
Average density
of crustal rocks
= 2.7 gm/cm3
Density must
increase with
depth.
Density must
increase with
depth.
But, how can we
obtain a more
detailed picture
of the variation
of density with
depth?
Dense
Material?
Astronomical
Observations
Earth’s axis of
rotation points
towards different
stars at different
times.
Gravitational forces
from the Sun and the
Moon cause the Earth
to twist and turn in its
orbit around the Sun.
“Precession” of the
Earth causes the
North Pole to point
to different parts
of the sky during a
26,000 year cycle.
“Precession” of the Earth is similar to the
motion of a spinning top.
From precession
of the Earth it
is possible to
measure the
moment of
inertia of the
Earth.
Moment of inertia is a
measure of how hard it is
to twist an object.
Moment of inertia is a
measure of how hard it is
to twist an object.
The more that the mass
is concentrated towards
the center of an object,
the easier it is to twist
the object.
Higher moment
of inertia
Lower moment
of inertia
Density of Water
1 gm/cm3
Mantle
3.5-5.7 gm/cm3
Outer Core
10-12 gm/cm3
Crust
2.6-3.1 gm/cm3
Inner Core
13-14 gm/cm3
Astronomical
Observations
The
Earth:
What’s it
made of?
Gravity
Seismology
N
S
Magnetism
Heat Flow
Earthquake in Japan
Magnitude 8.0
September 25, 2003
19:50 UTC
Izmit Turkey Seismogram
P and S Wave Paths