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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Today’s Class
•  Lecture:
–  Recap of Uranium-Lead Geochronology
–  Reading: Lowrie 4.1.3 - 4.1.4
–  Recommended: concordia module from:
http://ansatte.uit.no/webgeology/
webgeology_files/english/upb.swf
–  Geomagnetism
5/19/2015
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Presentation Do’s
•  Be prepared!
–  Try out technology before presentation
–  Practice presentation and timing several times
•  Create a presentation of an appropriate level for an upper
division Geophysics class for majors.
•  Stick to the 8-12 minute length: you will be timed.
•  Think about how to phrase a strong TRUE/FALSE question.
•  Look over every single slide with a critical eye: no typos!
5/19/2015
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GSC307
Introduction to Global Geophysics
Presentation Don’t’s
UraniumLead
•  Do not put too much focus on any one detail.
•  Do not incorporate movies of several minutes in length in
a 8-12 minute presentation.
•  Do not put too much text on the slides and proceed to
read them literally during your presentation.
Two isotopes of U decay to Pb with different half lives, so they
are two independent geochronometers:
[206Pb]now = [238U]now(eλ238t – 1)
[207Pb]now = [235U]now (eλ235t - 1)
5/19/2015
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5/19/2015
Cal Poly Pomona
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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
238U
Radioactive
Decay of U &
Growth of Pb
5/19/2015
Compared
to 235U
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GSC307
Introduction to Global Geophysics
5/19/2015
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GSC307
Introduction to Global Geophysics
Concordia Diagram
Concordia
Diagram
Concordia diagram: plot of 206Pb/238U vs. 207Pb/235U, both of
these ratios are proportional with time
⇒ plot of 238U–206Pb “age” against 235U–207Pb “age”
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Concordia consists of points where 238U–206Pb age equals 235U–
207Pb age: ages are concordant
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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Concordia
Diagram
•  Rocks in a closed system move along the concordia as they age.
•  Any rock or mineral not on concordia yields discordant dates:
the two lead ages do not agree.
•  What was the isotopic composition of the 2500 Ma old mineral shown
1000 Ma years ago?
5/19/2015
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Open System
Behavior:
Pb Loss
•  What will happen during an event of metamorphosis, when
temperature increases, rock deforms, and Pb escapes?
•  Zircon must lose 207Pb and 206Pb in exactly the proportions
they exist in zircon because they are chemically identical: a
zircon will not lose 206Pb in preference to 207Pb or vice versa
•  Suppose 2500 Ma old zircon lost lead in event 500 Ma ago
5/19/2015
GSC307
Introduction to Global Geophysics
During Lead Loss
Pb
Loss
•  If loss was complete: where would zircon plot?
•  And if it would have lost half its Pb?
•  All zircons with Pb loss plot on straight line
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So, different samples of zircon that experience different amounts of
Pb loss during same metamorphic event would plot along straight
line.
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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
After Lead Loss
After metamorphic event, ratios start to increase once more for the
next 500 Ma (for this example) due to Uranium decay.
5/19/2015
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Discordia
Straight line between crystallization & metamorphic age: discordia.
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GSC307
Introduction to Global Geophysics
Geo- and Paleomagnetism
Chapter 5
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5/19/2015
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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Fundamentals: Bar Magnet
Magnetic Field = 3D
•  Magnetic lines of force,
converging at ends of
magnet : magnetic poles
•  Bar magnet is dipole
(poles always come in
pairs: positive +m and
negative –m)
•  Positive pole of compass
points towards N
magnetic pole of Earth
•  Field orientation: tangent to lines of force
•  Spacing of lines indicates strength of magnetic force at that
point: close together -> force strong
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Intensity of the Earth’s Magnetic Field
Earth’s Magnetic Field: Vector
00
0
45°N
45°N
90°E
90°E
135°E
135°E
180°
180°
70°N
70°N
60000
0
55000
00
60000 60
50
0
00
50
0
5000
15°N
15°N
35000
35000
55
50000
40000
0
00
45
0
00
30°N
30°N
40
45000
40000
35000
30000
0°
0°
45°N
45°N
0
00
50
4500
0
0
4000
0
4000
00
0
55000
0
4500
0
4500
30°N
30°N
0
00
15°N
15°N
35000
30000
3535
0000
00
30000
3500
0
4000
0
25000
15°S 40000
15°S
45000
30000
50000
00
0
45
0
50
g
5/19/2015
90°W
90°W
45°W
45°W
30°S
30°S
00
0
60°S
60°S
jh
k
g
35000
00
135°W
135°W
Main
Total
Intensity
(F)
MainField
Field
Total
Intensity
(F)
Contour
1000
nT. nT.
Contourinterval:
interval:
1000
Mercator
Projection.
Mercator
Projection.
of dip
poles
j : Position
: Position
of dip
poles
15°S
15°S
45°S
45°S
65
00
70°S
70°S
180°
180°
65000
65000
00
55000
00
40000
4500
0
6000
0
60000
45
00
0
30
300
000
0
40
50
00
0
55
00
0
35
0
45000 4500
0
00
0
00
40
55
0
60°S
60°S
0
00
35
45°S
45°S
00
50000 5000
0
25
0
0°
0°
5000
0
5555
00000
0
25000
55000
30°S
30°S
0
00
60
Cal Poly Pomona
45°E
45°E
60°N
60°N
55
00
•  Intensity of field strength (F)
0°
0°
60°N
60°N
0
60
–  or: angle between magnetic and
geographic meridian
–  measured clockwise from N
–  varies from 0 to 360 degrees
45°W
45°W
0
00
25
•  Declination d = azimuth of
horizontal component of magnetic
field (from geographic north)
90°W
90°W
3000
0
–  or: angle at which magnetic vector
dips below horizontal
–  negative: F points upward
–  varies from -90 to 90 degrees
US/UK World Magnetic Model -- Epoch 2010.0
US/UK World Magnetic Model - Epoch 2015.0
Main
TotalIntensity
Intensity(F)
(F)
Main Field
Field Total
135°W
135°W
180°
70°N
70°N
3500 3500
0
0
•  Inclination i = angle between
magnetic field and horizontal (at
Earth’s surface)
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4000 4000
0
0
GSC307
Introduction to Global Geophysics
5/19/2015
55
00
0
5/19/2015
0°
0°
Cal Poly Pomona
45°E
45°E
90°E
90°E
135°E
135°E
70°S
70°S
180°
180°
Map developed
bydeveloped
NOAA/NGDC
CIRES
Map
by&NOAA/NGDC
& CIRES
http://ngdc.noaa.gov/geomag/WMM
http://ngdc.noaa.gov/geomag/WMM/
Map reviewed
by NGA
and BGS
Map
reviewed
by NGA/BGS
Published December
2014
Published
January 2010
5
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
The Earth As a Dipole
Most important part of
Earth’s magnetic field at
surface is dipole field,
today inclined ~11.5°
from Earth’s spin axis
•  accounts for more than
90% of intensity
The Earth As a Dipole: Geomagnetic Poles
Intersection of axis of
best-fitting dipole with
Earth’s surface =
geomagnetic poles
(north and south)
•  antipodal (exactly
opposite) to each other
What would D be for a dipole
aligned with the Earth’s rotation
axis?
5/19/2015
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
5/19/2015
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Third Type of Pole: Magnetic Poles
US/UK World Magnetic Model - Epoch 2015.0
Main Field Inclination (I)
135°W
70°N
90°W
45°W
0°
45°E
90°E
135°E
180°
70°N
80
60°N
60°N
80
45°N
45°N
60
60
60
60
30°N
30°N
40
40
40
20
15°N
20
40
0°
0
-20
20
0
-20
-40
Magnetic Poles
15°S
-20
-40
-60
30°S
0
-60
•  Two points at Earth’s
surface where magnetic
field is purely vertical
•  Not antipodal
-6
60°S
-60
-4
45°S
-80
-60
-60
45°S
0°
-20
-40
0
15°S
30°S
15°N
0
20
0
60°S
j
k
-80
-60
70°S
180°
5/19/2015
Cal Poly Pomona
135°W
90°W
Main field inclination (I)
Contour interval: 2 degrees, red contours positive (down); blue negative (up); green zero line.
Mercator Projection.
j : Position of dip poles
5/19/2015
45°W
0°
Cal Poly Pomona
45°E
90°E
135°E
70°S
180°
Map developed by NOAA/NGDC & CIRES
http://ngdc.noaa.gov/geomag/WMM
Map reviewed by NGA and BGS
Published December 2014
6
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
US/UK World Magnetic Model - Epoch 2015.0
Main Field Total Intensity (F)
135°W
70°N
90°W
45°W
0°
45°E
90°E
135°E
180°
70°N
Earth’s magnetic field is
not a perfect dipole
60°N
55
60
50
0
00
00
0
55
0
5000
0
00
40000
35000
35000
0
00
45
0
4000
15°N
45°N
0
00
50
4500
0
0
4500
30°N
60°N
•  stronger at high latitudes
than near equator
•  South Atlantic magnetic
anomaly
00
0
45°N
40
30°N
0
00
15°N
35000
30000
3500
0
0°
30000
3500
0
0°
40000
15°S 40000
30000
4500
0
15°S
5000
0
30°S
5500
0
55
0
35
0
00
50000
30°S
6000
0
55000
35000
00
00
40000
30
0
45°S
45000
25000
25
0
45°S
65
00
00
0
0
40
00
0
60
60°S
0
45
00
60°S
00
0
50
00
70°S
180°
5/19/2015
135°W
Main Field Total Intensity (F)
Contour interval: 1000 nT.
Mercator Projection.
j : Position of dip poles
Cal Poly Pomona
5/19/2015
GSC307
Introduction to Global Geophysics
90°W
45°W
0°
Cal Poly Pomona
45°E
90°E
135°E
70°S
180°
Map developed by NOAA/NGDC & CIRES
http://ngdc.noaa.gov/geomag/WMM
Map reviewed by NGA and BGS
Published December 2014
GSC307
Introduction to Global Geophysics
Secular Variation
Non-Dipole
Field
Total field – field from inclined geocentric dipole = non-dipole
field
5/19/2015
j
k
35000
Cal Poly Pomona
At any particular place on Earth geomagnetic field (both dipole as
well as non-dipole component) is not constant over time
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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Secular
Variation
Magnetic Declination
•  one of few "solid Earth" phenomena that changes
significantly over a human lifetime
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GSC307
Introduction to Global Geophysics
5/19/2015
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GSC307
Introduction to Global Geophysics
Poles (Geomagnetic & Magnetic) Move!
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5/19/2015
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GSC307
Introduction to Global Geophysics
–  next reversal? Lab
exercise
B
o
o
o
o
,ONGITUDEOFPOLE
o7
•  And geomagnetic pole
has drifted westward
A
o
4ILTOFDIPOLEAXIS
•  Variation in both
strength and direction
•  Dipolar field has
decreased in strength
since 1550
$IPOLEMOMENT !M
Secular Variation
of Dipole
C
o7
o7
o7
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9EAR
9