Chapter 11: Geophysical techniques continued

1
Chapter 11 Geophysical techniques (continued)
from http//www.csr.utexas.edu/grace/gallery/grav
ity/03_07_GRACE.html
2
gravitational attraction
3
Earths gravity field

• force exerted on a mass at the Earths surface
  arises from --gravitational attraction combined
  with rotation
• i.e. gravity
• Earth is a non-rigid rotating sphere
• --it is flattened at poles and bulging at
  equator
• 0.3 flattening yields
• Rpolar 6357 km
• Requator 6378 km
• Earths shape is an oblate spheroid
• -- acceleration of gravity is 0.5 less at
  equator than at poles
• topography and density variations within Earth
• -- gravity anomalies.

4
gravity measurements

• gravity measurements done using
• --gravimeter can be either relative or
  absolute types
• relative instruments record local variations
  in little g
• little g acceleration of gravity at the
  surface of the Earth
• --affected by tides, atmospheric pressure,
  temperature
• absolute gravimeters measure free fall
  acceleration of a mass can be used to derive
  big G
• gm GM/r2
• --satellites to measure orbital perturbations
• surface gravity field does NOT yield a unique
  solution for density distribution within the
  Earth.

5
relative gravity meters response to density
variations
gravity high
gravity low
low density
high density
high density adds gravity--positive anomaly
(difference) low density subtracts
gravity--negative anomaly (difference)
6
high relief topography--compensated with low
density crustal root first observed in India in
the 1850s mountains underlain by thick
crust --suggests isostasy is correct
7
depth of compensation
if mountains do not have light crustal
roots. positive (adds mass) gravity anomaly
from dense mantle below think about depth of
compensation and mass in column above
8
depth of compensation
negative anomaly less mass than expected
9
gravity field derived from satellite observations
Source NASA GRACE Mission
10
Earths magnetic field

• geomagnetic dynamo
• rotating, liquid metallic outer core gives rise
  to Earths magnetic field
• magnetic reversals
• Earths magnetic field
• magnetic north pole not always aligned with
  geographic north pole (spin axis).
• field also changes in strength
• magnetic anomalies
• local, shallow variations in the concentration of
  magnetic minerals within rock bodies change
  strength of magnetic field (can also happen with
  synthetic materials)

11
Earths magnetic field
magnetic north is offset from true north
bar magnet at angle to Earths rotation axis
similar to bar magnet
from http//www.geo.lsa.umich.edu/crlb/COURSES/2
70
orientation of field on surface depends where you
are. inclination (depends on latitude)
northern hemisphere vector points down southern
hemisphere vector points up
declination (depends on longitude)
east or west of magnetic north (compass must
be setremember from lab)
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US magnetic declination map
13
Earths magnetic field changes through time.
change in magnetic north relative to true
north since 1580 (declination)
but, magnetic field also reverses. north
pole becomes south and south pole becomes
north
S
N
both from http//www.geo.lsa.umich.edu/crlb/COUR
SES/270
14
change in magnetic north is function of rotation
in Earths outer core
1831-2001
daily change
Image source http//www.geolab.nrcan.gc.ca/geomag
/
15
how do rocks acquire magnetism?
magnetite common in basalt
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rocks/minerals that form at high temperatures and
subsequently cool acquire magnetism of
Earths magnetic field at the
time of their formation
pass through Curie point (temperature at
which magnetism of rock/mineral is set)
above Curie point random
below Curie point in external magnetic field
alignment
from http//www.geo.lsa.umich.edu/crlb/COURSES/2
70
17
rocks thus record reversals of magnetic field
through time.
from http//www.geo.lsa.umich.edu/crlb/COURSES/2
70
18
flood basalt sequence in Brazil
photo credits David W. Peate
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magnetic field reversals recorded by lava flows
20
create time-scale for magnetism
black normal polarity (north is north)
blue reverse polarity (south is north)
Mid Cretaceous Quiet Zone - period when
magnetic field remained constant for gt25 million
years.
21
what happens during reversals? (computer
simulations)
reversed
normal
transitional
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magnetic anomalies in shallow crust
magnetic material below adds magnetism
23
removal of magnetic material reduces magnetism
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heat within the Earth

• geothermal gradient
• how is it estimated?
• shallow boreholes, mines, mantle xenoliths
• does it vary with depth?
• heat flow
• inferred from temperature gradients
• sources of heat in shallow crust
• magma
• uranium-rich granitic rocks
• differences between ocean basins and continents

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Earths geothermal gradient
25C/km
1C/km
shallow gradient 25C/km cannot be sustained to
great depths, entire interior of the Earth
would be molten
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heat sources must be in shallow crust for crustal
gradient
basaltic magmas have temperatures gt1150C
(liquid) while crustal temperatures at 10 km
would be 250C, yielding a ?T (excess
T) of 900C that must be dissipated.
granites rich in U, Th, and K, whose decay
generates substantial heat
27
lateral variations in heat flow
28
convection in the mantle
from http//www.geo.lsa.umich.edu/crlb/COURSES/2
70
observed heat flow warm near ridges cold over
cratons
from http//www-personal.umich.edu/vdpluijm/gs20
5.html
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mantle convection animation