Inner Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 226

(Figs 1.9, 4.109, and 4.142), Dead Sea fault, Jordan
(Fig. 4.110), and North Anatolian fault (Fig. 2.16).
In all the above examples, we discussed the nature of
binary plate boundaries, that is, where two plates meet.
However, it is theoretically possible to imagine multiple
junctions meeting at topological points. In fact, points
where three plates meet, termed triple junctions, are the
most common. These involve various combinations of
ridge, trench, and transform boundaries. The interesting
thing about them is that they may migrate with time.

r

PLATE B

u B

A

u B

A

u B

A

5.2.5 Describing the kinematics of plate motion -
plate vectors, Euler poles, and rotations on a sphere

.
Euler
rotational pole

= u ω
A

A

Since we all live on one of the moving plates, any state-
ment concerning the directional vectors of the motion we
undergo year upon year must be done with care. A vivid
example comes from kinematic representations of the sym-
metrical pattern of sea-floor magnetic anomalies.
Figure 5.36 shows the usual explanation for this, that of
symmetrically diverging plates with equal speeds but
opposite directions, that is, u A

B

B

PLATE A

Fig. 5.38 Sketch to show Plate B rotating with respect to Plate A.
Plate is generated at the spreading ridge and rotates as a solid body
about circular arcs. The northern boudary to Plate B is a transform
fault, an orthogonal line from which defines a great circle upon
which the Euler pole lies. The linear velocity vectors are the velocities
of Plate B moving with respect to Plate A. The length of the arrows
is proportional to velocity magnitude which varies with the rotational
radius, r , about the Euler pole shown. A typical angular velocity,

u B . In fact, the sym-
metrical spreading can equally well be achieved by motion
of plate B, with plate A fixed, as long as the spreading axis
also migrates in the direction of B at a velocity of

, is

about 10 8 radians per year.

0.5 B u .
This kind of relative motion is entirely possible since plates
are self-driven entities and spreading ridges do not have to
overlie upwelling limbs of convection cells fixed in
asthenospheric mantle space.
We may most generally express plate velocities as relative
velocities, that is with respect to adjacent plates, which have
boundaries with the plate in question; Fig. 5.38 shows a
simple two-plate example where the velocity of plate A with
respect to plate B is minus the velocity of plate B with
respect to plate A, that is, B u A A u B . To do more com-
plicated three-plate problems, we can use the techniques of
vector addition and subtraction (Fig. 5.39). Sometimes a
fixed internal or external reference point is used to express
the plate velocity vector. It is generally held that certain
“hot-spots,” the surface expressions of rising mantle
plumes (the Hawaiian islands are the best known example)
may approximate to such stationary points. Another trick
relevant to some geographical situations is to fix one plate
and relate other plate velocities with respect to that.
The linear speed, u , of any rotating plate on the spherical
surface of the Earth is a function of both angular speed of
the motion and the radius of the motion, r , from its rota-
tional pole, the linear speed increasing as the length of the
arc increases (Fig. 5.38). Linear speed is thus given simply
by u

5.2.4 Plate boundaries, earthquakes, and volcanism

Plate boundaries are described as constructive when new
oceanic plate is being added by upwelling asthenospheric
melt at the midocean ridges. Remember that this melting is
due to adiabatic upwelling of mantle peridotite (Section 5.1).
The volcanism is accompanied by voluminous outpourings
of hot fluids along hydrothermal vents (Section 1.1.3).
Shallow and relatively minor normal faulting (extensional)
earthquakes accompany this plate creation along the ridge
axis. Destructive boundaries occur at the ocean trenches
where plate is lost to the deep mantle by subduction.
The process is manifest by arrays of deep earthquakes
(Section 4.17) along Benioff-Wadati zones and below
island arcs. These latter form as water from the descending
slabs dehydrate the water fluxing mantle of the overriding
plate so that the mantle geotherrn intersects the peridotite
solidus (Section 5.1). Conservative boundaries are those
where no net flux of mass occurs across them, the plates
simply slide past each other. In the oceans this occurs
along the active parts of oceanic fracture zones, called
transform faults (Fig. 5.37). Strike-skip faults in continen-
tal lithosphere may also mark conservative plate bound-
aries; examples are the San Andreas Fault, California

r . The rotational pole is commonly termed the