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GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Plate Tectonics at Work
Today
•  Lecture:
–  Plate Boundaries and Vector Diagrams
•  Recommended reading:
–  Lowry 1.2
•  Bring to Lab on Thursday: ruler, protractor,
calculator, graph paper
4/14/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
4/14/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Three Types of Plate Boundaries
Constructive PB: Vector Diagram
1.  Divergent/
Constructive
2.  Convergent/
Destructive
Constructive (ridge)
–  Double line = ridge axis
–  Numbers in (a) = half spreading rate (will be
explicitly stated otherwise)
BvA = (vector) velocity of plate A with respect to plate B = -AvB
3.  Transform/
Conservative
4/14/15
Cal Poly Pomona
4/14/15
Cal Poly Pomona
1
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Constructive PB: Cartoon
Constructive:
•  Pull-apart motion
•  Rocks fail in tension
•  Mainly small
earthquakes (6 or
greater is rare) and
shallow (a few kms)
4/14/15
Constructive PB
Features
•  For example: midocean ridge
–  Generates new ocean
floor
–  Bathymetric high
•  Lithosphere spreads
away from ridge,
cools, contracts and
sinks
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
4/14/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Continental Rifting
Iceland sits atop a divergent plate
boundary where continental
rifting occurs
4/14/15
Cal Poly Pomona
4/14/15
Cal Poly Pomona
2
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Destructive PB: Cartoon
Destructive PB: Vector Diagram
Destructive (subduction zone)
–  Barbed (or hashed) line: barbs on side of overriding plate,
pointing away from downgoing plate
BvA
Destructive:
•  Subduction zones or
collisions
•  Greatest earthquakes,
can be deep or
shallow (down to
about 700 km depth)
= (vector) velocity of plate A with respect to plate B = -AvB
4/14/15
Cal Poly Pomona
4/14/15
GSC307
Introduction to Global Geophysics
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Destructive PB Features
•  Denser plate sinks
•  May sink to great
depths (CMB?)
•  Formation of
continental or
island arc in case of
subduction
•  Deep trench
a. Ocean-continent
c. Continent-continent
Subduction Zone Seismicity
Earthquake locations define
descending slab of oceanic
lithosphere
A
B
C
D
,/7342%.'4(
).4%2-%$)!4%342%.'4(
()'(342%.'4(
b. Ocean-ocean
4/14/15
Cal Poly Pomona
4/14/15
Cal Poly Pomona
3
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Tonga Subducting Slab
Conservative PB: Vector Diagram
Conservative (transform fault)
–  Single line = transform fault
BvA
4/14/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
= (vector) velocity of plate A with respect to plate B = -AvB
4/14/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Conservative PB: Cartoon
Conservative PB Features
Conservative:
•  Slide-past,
horizontal, motions
•  Large earthquakes
•  Usually connect
ridge segments and
represent most
seismically active
parts of ridge system
•  Transform faults often connect ridge segments
•  Away from ridge they extend as fracture zones
4/14/15
Cal Poly Pomona
4/14/15
Cal Poly Pomona
4
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
The Pacific Plate
•  Largest plate in the
world
•  Created at spreading
centers (E and S edges)
•  Long transform faults
near western N America
•  Large subduction
earthquakes (along N
and W edges)
https://youtu.be/MSbIKp571Rk
4/14/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
4/14/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
Plate Motion
Plotting Objects on a Velocity Line
To better understand the past, present and future
motion of plates, their relative motions are plotted
on velocity diagrams, using vectors.
4/14/15
Cal Poly Pomona
4/14/15
Cal Poly Pomona
5
GSC307
Introduction to Global Geophysics
GSC307
Introduction to Global Geophysics
Plotting Objects on a Velocity Line
Plate Motion in Velocity Space
Plot BVG and GVB
and B itself
Relative motions of plates can be similarly described by plotting
them in velocity space.
Which of these three plate diagrams is inconsistent with the
velocity space diagram above?
4/14/15
Cal Poly Pomona
GSC307
Introduction to Global Geophysics
4/14/15
GSC307
Introduction to Global Geophysics
Two Plates on a Flat Earth
Western boundary of plate B
is ridge that is spreading
with half-rate of 2 cm/yr.
What happens at other
boundaries between B and
A?
4/14/15
Cal Poly Pomona
Cal Poly Pomona
Three Plates on a Flat Earth
Add third plate C, with plate
A subducting under C at
rate of 6 cm/yr.
To determine relative rate of
plate motion at B-C
boundary, use vector
addition:
CvB = CvA + AvB
Will B grow or disappear?
4/14/15
Cal Poly Pomona
6