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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Additional Examples + Answers: Use to
Practice for Exam Next Week
Today
•  Lecture:
–  Triple Junctions
•  Lab 3 will be due before start of lecture next
Tuesday.
•  Exam 1: Thursday next week!
–  Covers all lecture and lab material
–  Bring: ruler, protractor, calculator, graph paper
4/21/15
Cal Poly Pomona
4/21/15
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Another Example Velocity Diagram
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Cal Poly Pomona
Cal Poly Pomona
Numbers shown are full
spreading rate!
And Its Solution
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Cal Poly Pomona
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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
http://emvc.geol.ucsb.edu/1_DownloadPage/Download_Page.html
Plate Boundaries Change
Western U.S. Plate
Boundaries
Main features:
•  San Andreas Fault
•  Cascadia Subduction Zone
•  Mendocino Fracture Zone
Joined at the Mendocino Triple
Junction
4/21/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
4/21/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Triple Junctions
Triple Junctions
Points where three plate
boundaries meet are called
triple junctions.
Triple junctions may move
(migrate) and/or change
tectonic environment.
4/21/15
Cal Poly Pomona
•  R = Ridge
•  F = transform Fault
•  T = subduction zone
(Trench)
Triple junction is stable if
relative motions of three
plates and their geometry
do not change over time.
4/21/15
Cal Poly Pomona
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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Triple Junctions
Example: RTF Junction
A
Vector diagram of
interplate velocities at
triple junction is closed
triangle
AVB + BVC + CVA = 0
B
!
"6#
#
"
RIDGE
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Cal Poly Pomona
4/21/15
!6"
#6!
TRANSFORM
FAULT
SUBDUCTION
ZONETRENCH
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Triple Junction Migration
•  Triple junctions move
(migrate) along plate
boundaries
•  Because triple junctions
have velocities, much
may be learned by
plotting their motions in
velocity space
Triple Junction Stability
•  Stability of TJ is assessed
by considering how it can
move along any of its
(three) plate boundaries
•  If three velocity lines for its
three plate boundaries
intersect, TJ is stable
•  If not, it is unstable and
will evolve into a different
plate geometry
B
A
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Cal Poly Pomona
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Cal Poly Pomona
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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Velocity Line for Trench
Velocity Line for Trench
•  Triple junctions migrate along boundaries between pairs of
plates as if they are marbles rolling parallel to boundaries
•  Marble (or triple junction) will remain on a plate boundary if
it has velocity corresponding to any point on dashed velocity
line ab, which describes the velocity of those marbles
•  Consider marbles rolling along trench at different velocities
•  Will plot on velocity diagram on line passing through velocity of
upper plate (because trench moves with overriding plate) and
trending in same direction as (so: parallel to) trench
4/21/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
–  does not require that convergence velocity be perpendicular to trench
4/21/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Velocity Line for Transform Fault
Velocity Line for Ridge
•  Consider marbles rolling along transform at different velocities
•  Will plot on velocity diagram on line passing through both plates
(because transform does not move with respect to either plate) and
trending in same direction as (so: parallel to) transform
•  Consider marbles rolling along ridge at different velocities
•  Will plot on velocity diagram on line trending in same
direction as (so: parallel to) ridge and (if spreading is
symmetrical and perpendicular to ridge) that is perpendicular
bisector of velocity vector between two plates
4/21/15
Cal Poly Pomona
4/21/15
Cal Poly Pomona
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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Triple
Junction
Stability
TJ lies at intersection of three lines: ab, bc and ac for three plate
boundaries involved.
If these three lines intersect, TJ is stable. If not, it is unstable and
will evolve into a different plate geometry.
4/21/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Triple Junction Stability
TJ lies at intersection of three lines: ab, bc and ac for three plate
boundaries involved.
If these three lines intersect, TJ is stable. If not, it is unstable and
will evolve into a different plate geometry.
4/21/15
GSC307
Introduction to Global Geophysics
Triple Junction Stability
TJ lies at intersection of three lines: ab, bc and ac for three plate
boundaries involved.
If these three lines intersect, TJ is stable. If not, it is unstable and
will evolve into a different plate geometry.
4/21/15
Cal Poly Pomona
Cal Poly Pomona
Another Example
Draw velocity space diagram and determine whether
this is a stable triple junction, or not. Keep C fixed.
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Cal Poly Pomona
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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Hot Spots: Hawaii
Volcanism Within Plates: Hot Spots
•  Hotspots are
stationary, deeply
anchored columns of
hot material
•  Lithospheric plate
moves over the
hotspot
•  Creates a row of
volcanoes
progressively older
toward one end
•  Mantle plumes and hotspots
–  Linear chains of volcanoes or volcanic islands
–  Systematic variation of age
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Cal Poly Pomona
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GSC307
Introduction to Global Geophysics
4/21/15
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GSC307
Introduction to Global Geophysics
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Cal Poly Pomona
N
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Cal Poly Pomona
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GSC307
Introduction to Global Geophysics
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GSC307
Introduction to Global Geophysics
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•  Dating
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velocities and directions
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MANTLE
PLUME
Cal Poly Pomona
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Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Gravity and the Shape of the
Earth
SPHERE
Earth’s Shape
•  Newton argued that
shape of rotating Earth
should be oblate
ellipsoid
REDUCED
PRESSURE
SUPPORTS
SHORTER
COLUMN
ELLIPSOIDOF
ROTATION
CENTRIFUGAL
FORCEREDUCES
GRAVITY
CENTRALPRESSUREIS
REDUCEDDUETO
WEAKERGRAVITY
–  somewhat flattened at
poles
–  bulging around equator
•  based on hydrostatic
equilibrium
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Cal Poly Pomona
4/21/15
Cal Poly Pomona
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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
What is Gravity?
•  Force that pulls two masses together
•  Infinite range, always attractive
•  Newton’s Law of Gravity:
Gravitational
Acceleration

mM
F = −G 2 rˆ
r
If M is mass of Earth, we can
determine gravitational
acceleration, g (or aG), from
Newton’s law.
•  Points towards center of Earth
•  On average on Earth’s surface
~9.8 m/sec2 = 980 cm/sec2 =
980 gal = 980 000 mgal
(independent of mass of object!)
where G is universal gravitational constant =
6.67 x 10-11 Nm2/kg2
4/21/15
Cal Poly Pomona
4/21/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Gravitational
Acceleration
•  Same equation is valid
everywhere outside the
sphere (Earth)
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Cal Poly Pomona
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