Geoscience Reference
In-Depth Information
Table 4.1
Plate parameters
0.9
Area
Growth rate
(km 2 /100
(millions
of km 2 )
Plate
years)
Pacific
PAC
108
52
0.8
Africa
AFR
79
30
Eurasia
EUR
69
6
Indo-Australia
INA
60
35
N. America
NAM
60
9
Antarctic
ANT
59
55
0.7 0
20
40
60
80
S. America
SAM
41
13
Number of Plates
Nazca
NAZ
15
7
Fig. 4.6 When tiles of a given size and shape are packed on
a sphere, without overlap, the packing density depends on the
number n . The optimal packing density is achieved for n = 12.
This figure is for spherical caps but other shapes give similar
results. The voids between the tiles have dimensions of order
10--15% of the tile dimensions so only small tiles or plates can
be accommodated in the interstices (after Clare and Kepert,
1991).
Arabia
ARA
4.9
2
Caribbean
CAR
3.8
0
Cocos
COC
2.9
4
Philippine
PHI
5
1
Somalia
SOM
Juan de Fuca
JdF
Gorda
GOR
Scotia
SCO
SE Asia
SEA
the system loses rigidity, typically at 15% void
space.
The Indo-Australian (IAU) plate can be divi-
ded into an Indian (IND), Australian (AUS) and
Capricorn plate (CAP) and diffuse zones of
compression. There are few earthquakes or volca-
noes to mark these boundaries. The Pacific plate
has several bands of earthquakes and volcanoes
that could be cited as possible diffuse or incipi-
ent plate boundaries. One of these zones extends
from Samoa and Polynesia to the East Pacific Rise
(EPR) -- and then to Chile -- and includes most
of the intraplate earthquakes and active volca-
noes in the Pacific. There is little evidence for
relative motion between the north and south
Pacific plates but current motions between EUR,
Antarctica (ANT), and AFR are also very slow.
Therefore, there are somewhere between 8 and
20 plates with 12 being a frequently quoted num-
ber. Plate reconstructions in the past also recog-
nize about 12 persistent plates. Each plate, on
average, has about five nearest neighbors and five
next nearest neighbors.
Ridges and trenches account about equally for
80% of the plate boundaries, the rest being trans-
form faults. Most plates are not attached to a slab
or bounded by a transform. Therefore, in the real
Total
507.6
0
deformation. Eurasia (EUR) and North America
(NAM) are collages of accreted terranes ;the
large Pacific plate (PAC) grew by annexing neigh-
boring plates but it is now shrinking. Africa (AFR)
and PAC are the largest plates; AFR is growing,
by the migration of ridges, and PAC is shrinking
by the encroachment of continents and slab roll-
back. Table 4.1 gives the parameters of some of
the current plates.
Three of the larger plates have large fractions
of their areas occupied by diffuse deformation
zones and do not qualify in their entirety as rigid
plates. In fact, at least 15% of the Earth's sur-
face violates the rules of rigid plates and local-
ized boundaries. It is interesting that packing
of similar sized polygons or circles on a sphere
leaves about 15% void space (except when n is
6 or 12) (see Figure 4.6). The minor plates, in
aggregate, are smaller than the smallest major
plate. In a close-packed or random assemblage
of discs or spherical caps on a sphere the num-
ber of contacting neighbors decreases as the size
disparity of the discs increases. At some point
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