Transcript lecture3_2012 - Earth and Atmospheric Sciences

Chronology of
geomagnetic
field reversals
magnetic anomaly
“number”
Ocean floor
age, millions of
years (Ma),
determined
largely from
deep sea
drilling
Seafloor spreading is a tape recorder of the geomagnetic field!
Age, Ma
The recorded
reversal chronology
crust
moho
upper mantle
The tape drive
The recording head
of the tape recorder
Ocean Ridge system
Modern view of ocean bathymetry derived from satellite altimetry. see
EXPLORING THE OCEAN BASINS WITH SATELLITE ALTIMETER DATA
Global Bathymetric Prediction for Ocean Modelling and Marine Geophysics
Global bathymetry
Map shown
in next slide
Ship tracks across the East Pacific Rise which obtained the magnetic anomalies
shown in the next slide. The measurements were made in the 1960’s by the Columbia
University research vessel Eltanin.
21
20
19
The famous
Eltanin 19
profile
The Eltanin 19 profile is among the most influential geophysical profiles ever published. It provided the “smoking gun”
evidence for seafloor spreading, evidence that turned a majority of skeptics into a majority of believers. The profile was
published together with three others in 1966 by Pitman and Heirtzler in “Magnetic Anomalies over the Pacific-Antarctic
Ridge” (Science, 154, 1164-1171). The figures above comes from that paper. The track lines (ELT 19-21) of the research
vessel Eltanin are shown together with the correlated magnetic anomalies (numbered dashed lines) and the 2000 fathom
bathymetric contour. The crosses are earthquake epicenters. The inferred active spreading center would be between
anomalies 1 and 1’. The voyage occurred in 1965.
Eltanin profiles of
magnetic anomalies
The four profiles show total
intensity anomalies and
bathymetry (ocean depth in
km) along the four tracks
shown on the previous map.
Note that track 20 crosses the
ridge system twice.
The vertical scale for total
intensity anomaly, DF, is
shown in “gammas”. This is
the same as nanoTeslas or
nT. The horizontal lines are
at zero anomaly; the scale is
thus minus 500 to plus 500
nT.
The incredible symmetry of the Eltanin 19 profile
+500
ESE
WNW
0
-500
+500
WNW
0
ESE
-500
total intensity anomaly calculated from model
mirror image of measured
profile to show symmetry
measured profile of total
intensity anomalies
magnetic field intensity,Fobs
intensiy anomaly, DF
Smoothly varying global field plus small, short
wavelength effects of crustal magnitizations
0
distance along
ship track
subtract global field to yield total intensity
anomaly
0
distance along
ship track
Strike of spreading ridge
direction of cross section
(perpendicular to axis of
spreading)
thickness of
magnetized prisms
Typical model for
seafloor spreading type
of anomaly: very long
rectangular prism (2-D
model)
perpendicular to strike
Vertical, downwards
Typical model for
seafloor spreading type
of anomaly: very long
rectangular prism (2-D
model)
thickness, t
width, w
Vertical direction of
cross section, z axis
horizontal direction of
cross section, x axis
strips extend to “infinity”
vertical downwards
magnetization
equivalent to two
parallel strips of
magnetic poles as
shown: top strip of
negative poles and
bottom strip of positive
poles
perpendicular to strike
Vertical, downwards
strips extend to “infinity”
Magnetic field lines for vertically
downwards magnetization in
cross-sectional view
---------------J
+ + + + + + + + + + + + +++
Magnetic field lines for vertically
upwards magnetization
+ ++ + + + + + + + + + + + +
- - - - - - - - - -J- - - - - -
Magnetic field due to
magnetized prism taken
along the surface above
the prism (directions
only)
ocean surface
---------------J
+ + + + + + + + + + + + +++
Earth’s field, He
Vertically downwards
magnetization parallel to
vertical earth’s field
Magnetized prism
field adds to Earth’s
field, DF positive
---------------J
+ + + + + + + + + + + + +++
Earth’s field, He
Magnetic field due to
magnetized prism taken
along the surface above
the prism (directions
only)
Magnetized prism
field adds to Earth’s
field, DF positive
---------------J
+ + + + + + + + + + + + +++
Earth’s field, He
Magnetic field due to
magnetized prism taken
along the surface above
the prism (directions
only)
Magnetized prism
field perpendicular
to He, DF = 0
---------------J
+ + + + + + + + + + + + +++
Earth’s field, He
Magnetic field due to
magnetized prism taken
along the surface above
the prism (directions
only)
Magnetized prism
field subtracts from
He, DF negative
---------------J
+ + + + + + + + + + + + +++
Earth’s field, He
Magnetic field due to
magnetized prism taken
along the surface above
the prism (directions
only)
Magnetic total intensity anomaly from single vertically magnetized prism
DF/2J
width = 10 km
distance, x, along cross section perpendicular to the trend of the rectangular prism
Magnetic total intensity anomaly from single vertically magnetized prism
DF/2J
width = 80 km
distance, x, along cross section perpendicular to the trend of the rectangular prism
Combination of rectangular
prisms
DF/2J
Earth’s
field, He
d
t
axis of spreading
Five rectangular prisms: one central one plus two pairs
symmetrically on either side (seafloor spreading model in
the region near the spreading axis); magnetic anomaly of
each prism is plotted separately
Wo
W
DF/2J
Multiple prisms vertically magnetized in alternate directions:
combined effect
Map of magnetic
anomaly numbers
Deep Sea
Drilling sites
magnetic anomaly number
paleontological age, Ma
Deep sea
drilling in the
South Atlantic
Ocean
Seafloor ages from
deep sea drilling
versus
geomagnetic reversal
chronology
Age (Ma) from geomagnetic reversal chronology extrapolate
in South Atlantic assuming constant rate of spreading
Chronology of
geomagnetic
field reversals
magnetic anomaly
“number”
Ocean floor
age, millions of
years (Ma),
determined
largely from
deep sea
drilling
4600 My
Geologic
time scale
600 My
Age range of modern
ocean floor
http://www.geo.ucalgary.ca/~macrae/timescale/timesc
Transform faults
7000
3500
1500
-500
0
500
-1500
-2500
-3500
-4500
-5500
-6500
South Atlantic Ocean
South Atlantic Ocean
inactive fracture zone
7000
3500
active transform fault
1500
-500
0
500
-1500
-2500
-3500
-4500
-5500
-6500
active ridge crest
active ridge crest
inactive fracture zone
inactive fracture zone
active transform fault
Transform faults and fracture zones
Map pattern of magnetic
anomaly number
Deep sea drilling
calibration of
chronology of
geomagnetic
field reversals
back to 180 Ma.
Map pattern of
ocean floor
age
Age of ocean floor
anomaly no. 5
10.9
6
13
18 21
25
31
34
M10N
M0 M4
M16 M21 M25
120.4
154.3
From Muller, et al., 1997
180
Pacific Ocean
Creation of Atlantic & Indian oceans
and associated “passive margins”
closing of Tethys Ocean: collisional tectonics
of Mediterranean-central and southeast Asia
Interaction of East Pacific Rise with
western North America
Continued subduction beneath
western South America as East
Pacific Rise comes closer