tripple junction (2)
TRANSCRIPT
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GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Todays Class
Lecture:Triple Junctions StabilityHot Spots
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Triple Junctions
Points where three plateboundaries meet are called
triple junctions.
Triple junctions may move(migrate) and change the
tectonic environment.
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Triple Junctions
R= Ridge F = transform Fault T = subduction zone
(Trench)
A triple junction is stable if
the relative motions of
three plates theirgeometry do not change
over time.
GSC307Introduction to Global Geophysics
Example: RTF Junction
In a circuit about atriple junction an
observer must return
to the starting point
the vector diagram ofthe interplatevelocities is a closedtriangle
AVB+BVC+CVA= 0
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GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Triple Junction Stability
Stability of a TJ is assessedby considering how it canmove along any of its (three)
plate boundaries
A TJ lies at the intersectionof the three velocity lines for
the three plate boundaries.
If these three lines intersect,the TJ is stable. Ifnot, it is
unstable and will evolve into
a different plate geometry.A
B
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Velocity Line for a Trench
Consider marbles rolling along a trench at different velocities They will plot on a velocity diagramon a line passing
through the velocity of the upper plate (because the trench
moves with the overthrust plate) and trending in the samedirection as (so: parallel to) the trench
does not require that convergence velocity be perpendicular to trench
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Velocity Line for a Transform Fault
Consider marbles rolling along transform at different velocities They will plot on a velocity diagram on a line passing through
both plates (because the transform does not move with respect
to either plate) and trending in the same direction as (so:parallel to) the transform
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Velocity Line for a Ridge
Consider marbles rolling along ridge at different velocities They will plot on a velocity diagram on a linetrending in the
same direction as (so: parallel to) the ridge and (if the
spreading is symmetrical and perpendicular to the ridge) thatis the perpendicular bisectorof the velocity vector between the
two plates
A
B
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Triple
Junction
Stability
A TJ lies at the intersection of the three lines: ab, bc and ac for thethree plate boundaries.
If these three lines intersect, the TJ is stable. Ifnot, it is unstable
and will evolve into a different plate geometry.
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Triple
Junction
Stability
A TJ lies at the intersection of the three lines: ab, bc and ac for thethree plate boundaries.
If these three lines intersect, the TJ is stable. Ifnot, it is unstable
and will evolve into a different plate geometry.
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GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Triple
Junction
Stability
A TJ lies at the intersection of the three lines: ab, bc and ac for thethree plate boundaries.
If these three lines intersect, the TJ is stable. Ifnot, it is unstable
and will evolve into a different plate geometry.
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Another Example
Draw the velocity space diagram and determinewhether this is a stable triple junction, or not. Keep
C fixed.
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Volcanism Within Plates: Hot Spots
Mantle plumes and hotspots Linear chains of volcanoes or volcanic islands Systematic variation of age
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Hot Spots: Hawaii
Hotspots arestationary, deeplyanchored columns ofhot material
Lithospheric platemoves over thehotspot
Creates a row ofvolcanoesprogressively oldertoward one end
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Formation of Hot Spot Volcanoes
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Big Island
Kauai
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GSC307Introduction to Global Geophysics
Plate
Velocities
Dating of basalt samples helps determine platevelocities and directions
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GSC307Introduction to Global Geophysics
Hot Spot
Reference Frame
Results from otherhotspots suggest
they form a
stationary network
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GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Movement on a Spherical Earth
Eulers theorem:Every displacement of a plate from one position to another on
the Earth's surface can be regarded as a rotation of that plateabout a suitably chosen axis, which passes through the centreof the Earth and cuts through the Earths surface at the Eulerpole or pole of rotation.
The Euler pole does not have to be the
North Pole!
GSC307Introduction to Global Geophysics
Geometry of constructive boundary at which two plates areseparating.
Note that whilst rotation angle remains same along length of split,width of gap increases with angular distance from pole of rotation.
What effect does this have on local spreading rates along length ofplate boundary?10/10/11 Cal Poly Pomona
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Rotation VectorsThe relative velocity v between two plates is:
v = R sin
And thus is 0 at the rotation poles and maximum ( R) at90 from the poles.
Given rotation vectors, you can calculate the relative motionat a plate boundary.
GSC307Introduction to Global Geophysics
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GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Relative Plate MotionsWhat does the change in spreading rate along the mid Atlantic
ridge tell you about location of the pole of rotation?
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Determination of Euler
Poles
The motion of plate 1 relative to plate 2must be a rotation about an Euler pole.
The strike of a transform fault is parallel to
the relative motion between the twoplates.
=> All transform faults on the plate
boundary must be small circles about
that pole.
=> Orientation of transform faults can be
used to locate the Euler pole
GSC307Introduction to Global Geophysics
10/10/11 Cal Poly Pomona
Determination
of Euler Poles
Lines (great circles) at right angles totransform faults will pass through the poleof rotation.
Using this method, the pole of rotation of theSouth American Plate can be shown to belocated in the North Atlantic, south ofGreenland.