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GE 11a, 2014, Lecture 5
Spherical structure of the earth
The earth, ca. 1800
Nevil Maskelyne and the Schiehallion experiment (1774)
Schiehallion (‘Sidh Chailleann’) Scotland
Nevil Maskelyne doing his impression of Ben Franklin

F
d
Ms
m .g
F = m.g.tan() = G.m.Ms/d2
ME = (RE2/d2).(Ms/tan()) ~ 6.1024 kg
m.g = G.m.ME/RE2
RE = 6.37.106 m; VE = 1.1.1021 m3
 ~ 5.5 g/cm2 (initially found ~ 4.5)
Densities of common substances (all in g/cc)
Ice
0.917
Water
1.000
Seawater 1.025
Graphite 2.200
Granite ~2.70
Titanium 4.507
Iron
7.870
Copper 8.960
Mercury 13.58
Gas:
proportional to P/RT
Two options: sub-equal mix of metal and rock or…
an ideal gas, w/ high density at high P (B. Franklin)
Mass distribution in earth’s interior
Period of precession
Moment of inertia
Period of spin
Torque (sun and moon trying to pull
earth’s tidal bulge into plane of
ecliptic)
I = i mi.ri2
ri
mi
Higher
Earth has I much less than
expected for homogeneous sphere
Lower
Kraemer, 1902
View combining known density, moment of inertia,
oblateness, rigidity of surface rocks, and topography
Note bad for a bunch of turn-of-the-century quacks!
Focus
“sample” outer ca. 200 km,
but most energy in upper 10 km
Mantle
Core
S P
Seismograph
A mechanical seismograph
Anatomy of a seismic signal
Minutes
0
10
20
30
40
50
Surface waves
P
S
‘Primary’ (first to arrive)
‘Secondary’ (second to arrive)
P
S
Amplitude
=23 mm
S-wave interval = 24 seconds
…and the time
interval between
the P- and S-waves
(I.e., the distance
from the epicenter.
Interval between
S and P waves (s)
Distance (km)
P-wave
Richter
magnitude
measure the
amplitude of the
largest seismic
wave…
Amplitude
(mm)
Connect the points
to determine the
Richter magnitude.
Real data is more complicated…
Look at your notes, John!
Look at your notes, John!
Look at your notes, John!
Mg2SiO4 in upper mantle
Mg2SiO4 in lower mantle
The core’s density is less than that of pure Fe. Requires a low-mass
Alloying agent. S? O? H? ???
The events in early earth history that controlled core/mantle segregation
Robin Canup’s movie
Animation of Theia forming in Earth's L5 point and then drifting into impact. The animation progresses in one-year steps making Earth appear not to move.
Evidence for the ‘magma ocean’ stage of the earth is essentially erased by later
convection, plate formation, subduction, etc. But the moon’s crust preserves
evidence planets are largely liquid early in their history.
And today Io, a moon of Jupiter, seems to possess a partially crystallized magma
ocean, supported by tidal heating