ge 11a, 2014, lecture 5 spherical structure of the earth
TRANSCRIPT
GE 11a, 2014, Lecture 5Spherical structure of the earth
The earth, ca. 1800
Nevil Maskelyne and the Schiehallion experiment (1774)
Schiehallion (‘Sidh Chailleann’) ScotlandNevil Maskelyne doing his impression of Ben Franklin
Ms
F = m.g.tan() = G.m.Ms/d2
F
m.g
d
m.g = G.m.ME/RE2
ME = (RE2/d2).(Ms/tan()) ~ 6.1024 kg
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 0.917Water 1.000Seawater 1.025Graphite 2.200Granite ~2.70Titanium 4.507Iron 7.870Copper 8.960Mercury 13.58Gas: 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)
Period of precession
Period of spin
Torque (sun and moon trying to pull earth’s tidal bulge into plane of ecliptic)
Moment of inertia
I = i mi.ri
2 miri Higher
Lower
Earth has I much less thanexpected for homogeneous sphere
Mass distribution in earth’s interior
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
Mantle
SeismographCoreS P
“sample” outer ca. 200 km,but most energy in upper 10 km
A mechanical seismograph
Minutes
Surface waves
0
P S
10 20 30 40 50
‘Primary’ (first to arrive)
‘Secondary’ (second to arrive)
Anatomy of a seismic signal
Amplitude=23 mm
Richtermagnitude
Amplitude(mm)
Inte
rval b
etw
een
S a
nd
P w
aves (
s)
Dis
tan
ce (
km
)
P S
S-wave interval = 24 secondsP-wave
measure the amplitude of the largest seismic wave…
…and the timeinterval between the P- and S-waves (I.e., the distance from the epicenter.
Connect the points to determine theRichter 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-massAlloying 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. The view is of the south pole.
Evidence for the ‘magma ocean’ stage of the earth is essentially erased by laterconvection, 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