Three Worlds to Explore

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Three Worlds to Explore
Look Up? (Astronomy)
Look at Surface? (Geology)
Look Down? (Geophysics)
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The Earths Interior
Deep wells and boreholes (4 to 12 km)
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The Earth Whats it made of?
Astronomical Observations
Gravity
Seismology
N
S
Magnetism
Heat Flow
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Density of Water 1 gm/cm3
Crust 2.6-3.1 gm/cm3
Mantle 3.3-5.7 gm/cm3
Outer Core 10-12 gm/cm3
Inner Core 13-14 gm/cm3
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P and S Wave Paths
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• Relatively simple experiment to estimate some
  basic properties of the Earths interior
• Mass M
• Radius R
• Average Density D
• Rough Estimate of Variation of Density in Interior

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Newtons Law of Gravitation
r
F
F
m
M
Force of attraction (F) is proportional to the
masses, and is inversely proportional to the
square of the distances between the masses.
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Newtons Law of Gravitation
r
F
F
m
M
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Henry Cavendish (1731-1810) determined the
universal constant of gravitation G in 1798.
torsion fiber
massive lead spheres
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F
m
R
M
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F
m
R
?
Measure the force
M
Can measure in lab
Use a known mass
If we could measure R, we could determine M.
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How can we measure R?
Greek ScientistEratosthenes (276-194 BCE)
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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
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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.
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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)
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The Earth Whats it made of?
Astronomical Observations
Gravity
Seismology
N
S
Magnetism
Heat Flow
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F
m
Eratosthenes
R
Measure the force
M
Can measure in lab
Use a known mass
M 6x1027 gm 6,000,000,000,000,000,000,000,000,
000 gm
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R
M
Average Density 5.5 gm/cm3
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Average density of the Earth 5.5 gm/cm3
Average density of crustal rocks 2.7 gm/cm3
Dense Material?
Density must increase with depth.
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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
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Earths 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.
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Precession of the Earth causes the North Pole
to point to different parts of the sky during a
26,000 year cycle.
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Precession of the Earth is similar to the
motion of a spinning top.
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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.
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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.
Lower moment of inertia
Higher moment of inertia
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Density of Water 1 gm/cm3
Crust 2.6-3.1 gm/cm3
Mantle 3.5-5.7 gm/cm3
Outer Core 10-12 gm/cm3
Inner Core 13-14 gm/cm3
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The Earth Whats it made of?
Astronomical Observations
Gravity
Seismology
N
S
Magnetism
Heat Flow
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(No Transcript)
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Earthquake in Japan Magnitude 8.0 September 25,
2003 1950 UTC
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Izmit Turkey Seismogram
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P and S Wave Paths