Euler Poles and Triple Junctions - Myweb @ CW Post

GLY 47/511 - Plate Tectonics
Framework of Plate Tectonics
Lecture Summary
Chapter 2 in Frisch et al. (2011), Section 5.3 & 5.8-5.11 in Kearey et al. (2009)
See also Cox & Hart (1986)
Poles of Rotation - Euler Poles
If the Earth were flat, we could describe the longterm motion of a plate as some straightline
direction (e.g., NE or 53 degrees from due north). But because of the curvature of
the Earth it is not that simple. Think, for example, how a plate may drift over the north
or south pole. How, then would you describe its direction?
One can describe the motion of a plate (or of a point) on a spherical Earth by way of a
pole of rotation or Euler* pole (commonly pronounced "oiler"). Imagine an Euler pole
as a pivot point. A plate moves about that pivot point with a set angular speed. It's like
a dog chained to a peg anchored into the ground if it just runs around in a circle at the
end of its chain.
* Leonhard Euler was an 18th century, Swiss-born mathematician
The part of a plate closest to the Euler pole moves slowest (in terms of distance in km)
and the part of the plate farthest from the Euler pole moves the fastest. A point directly
at the pole of rotation would rotate in place without actually moving off the spot. The
velocity of a point at a given angular distance between the equator of rotation and the
Euler pole of rotation of is simply the cosine of that distance multiplied times the
maximum plate velocity at the equator of rotation,
V α
= V 0
cosα
where V0 is the maximum plate velocity at the equator of rotation
α is the angular distance from the equator of rotation
Note, just as the Earthʼs spin axis has 2 poles, there are 2 Euler poles of rotation on
opposite sides of the Earth. The equator of rotation lies midway between these.
On the Earthʼs surface, points on a plate (or midocean ridge separating 2 plates) that
are less than 90° away from the pole of rotation move along small circles. Their motion
looks like a curving path on the Earth. Points that are 90° from the pole of rotation
move along a great circle path. Great circle paths are the shortest path between two
points on the globe. They appear as straight line motion.

The Euler pole that defines the motion of a plate can be defined from the orientations of
fracture zones. Spreading can be oblique to midocean ridge spreading centers so they
are not good for defining Euler poles. The fracture zones themselves are flow lines.
They show the direction(s) that a plate moved through time. Arcs drawn perpendicular
to the fracture zones on a given plate will intersect at the plate's Euler pole. (see Fig.
2.2 in Frisch et al. (2011) or Fig. 5.3 in Kearey et al. (2009)).
Relative Plate Motion Directions
midocean ridges: relative motion between 2 plates that spread away from one another
at a ridge may be perpendicular to the ridge or oblique to the ridge
trenches: the motion of a plate as it converges and subducts at a deep ocean trench
may be perpendicular to the trench or oblique to it (oblique subduction)
transform faults: the motions of 2 plates at a transform fault is almost always exactly
parallel to the transform. (rarely, one may find slightly oblique motion - “leaky
transforms”)
Triple Junctions
Typically, we consider the motion of two plates relative to one another. We can
determine the direction of relative plate motion from the orientation of transform faults/
fracture zones along midocean ridges that separate the two plates. We can determine
the rate of separation of the two plates using the pattern of marine magnetic anomalies
that parallel the spreading ridges (using the known age of a reversal and the distance of
the reversal from the midocean ridge).
Commonly we are concerned with the relative motions of more than two plates. And
since there are more than two plates on the Earthʼs surface (there are 12 major and a
number of minor plates), there are points where three plates meet. These are called
triple junctions. For example, three midocean ridges meet in the Indian Ocean where
the African, Indo-Australian, and Antarctic plates meet.
Ridge-Ridge-Ridge (RRR) triple junctions are always stable, whether the spreading
rates on the 3 ridges are the same or different (e.g., Figs. 2.5 - 2.8 in Frisch et al.,
2011)). The relative motion between three plates will be diagonal (oblique to the ridges)
rather than at approximate right angles to the ridges (e.g., Fig. 2.5, Frisch et al., 2011).
Other types of triple junctions may be stable (under given conditions) or unstable
(regardless of exact configuration) and temporary and quickly transform into a different
stable plate boundary configuration (Fig 5.27 in Kearey et al., 2009). Many triple
junction combinations exist, including Trench-Trench-Trench (TTT) such as in the
western Pacific, and Ridge-Trench-Transform Fault (RTF) such as along western North
America.
For the three plates (plates A, B, C) that meet at a triple junction, their three Euler poles
(relative motion of A-B, B-C, and C-A) will fall on a great circle.