Geology Reference
In-Depth Information
a distance of 3000 kilometres over
a period of 150 million years. Thus
South America and the western half of
the South Atlantic, on the one hand,
and Africa and the eastern half of the
South Atlantic on the other, could be
considered as separate blocks, which
moved as units. The Atlantic oceanic
age pattern (Figure 3.6A) confirms this.
It was then realised that the linear
zones of earthquakes that follow the
ridge-trench network (Figure 2.6)
must be related to the movements
taking place along the boundaries of
the relatively stable blocks, and that
the margins of these blocks could be
mapped out by following the earth-
quake zones. The term ' tectonic plate '
was introduced to describe these blocks.
The earthquake zones completely
surround the stable plates, whose
boundaries could now be seen to be of
three types: ridges , trenches and faults
(Figure 3.7). Moreover, since the ridges
must mark sites of production of new
ocean crust ( constructive bounda-
ries ), and the trenches mark sites of
destruction of ocean crust ( destructive
boundaries ), the faults must corre-
spond to zones where one block merely
slides past its neighbour without either
creation or destruction of crust taking
place. The faults that form parts of the
boundary network link constructive and
destructive boundaries and are termed
transform faults , since they 'transform'
one type of motion (e.g. convergent) to
another (e.g. divergent). Because plate
is 'conserved' at these boundaries, they
are known as conservative boundaries .
The fact that transform faults, by
definition, must be parallel to the
direction of relative motion between
two adjacent plates can be used to
Lat.
o
80
Eurasian
plate
Americas
plate
o
Iceland
60
o
40
A
o
20
Ph
Pacific
plate
Ca
Afar
Co
o
0
African
plate
Na
o
20
Indian plate
40
o
60
o
Antarctic plate
constructive (ridges)
destructive (trenches & collision belts)
conservative (faults)
plate boundaries
Figure 3.7 The plates and their boundaries. The plates are separated by three types of boundary:
constructive - ocean ridges; destructive - ocean trenches; and conservative - faults. Small plates:
Na, Nazca; Co, Cocos; Ca, Caribbean; Ph, Philippine; A, Arabian. SAF, San Andreas fault. The
arrows give the direction of motion of each plate relative to the Antarctic plate.
the northern and eastern margins
of the Pacific plate. For example, the
Pacific plate must be moving in a
NNW direction relative to the North
American plate, parallel to the San
Andreas transform fault, and being
subducted along the Aleutian trench
in the north. However, the transform
faults within the Pacific plate tell us
that relative to the East Pacific ridge
(of which only part is now visible near
the American plate margin) this plate
is moving westwards! This only makes
sense when we realise that the E-W
transform faults indicate only the rela-
tive motion between the Pacific plate
and the Cocos plate, on the eastern side
determine the relative motion between
any two plates that are separated by
such a fault. In Figure 3.6A, the orienta-
tion of the transform faults tells us that
the direction of relative motion between
the American and African plates is
approximately E-W. Because the plates
are, in effect, pieces of a spherical shell
(we can imagine them as broken pieces
of egg shell) the transform faults must
be curved, as shown, for example,
on Figure 3.6. In fact, each transform
fault represents part of a circle whose
centre is the rotation axis of the plate
movement, as shown in Figure 3.8.
Figure 3.6B shows a good example
of the three types of boundary around
 
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