Geoscience Reference
In-Depth Information
Table 5.1
Plate statistics, see Fig. 5.35 for map. Asterisked plates have long trench boundaries and are fastest due to the importance of slab pull
forces in generating steady plate motion (see Section 5.2.7).
Plate
Total area
Land area
Speed
Periphery
Ridge
Trench
10
6
km
2
10
6
km
2
mm year
1
10
2
km
length
length
10
2
km
10
2
km
NA
60
36
11
388
146
12
SA
41
20
13
305
87
5
PAC*
108
0
80
499
152
124
ANT
59
15
17
356
208
0
IND *
60
15
61
420
124
91
AF
79
31
6
418
230
10
EUR
69
51
7
421
90
0
NAZ*
15
0
76
187
76
53
COC*
3
0
86
88
40
25
CAR
4
0
24
88
0
0
PHIL*
5
0
64
103
0
41
ARAB
5
4
42
98
30
0
ANATOL
1
0.6
25
28
0
8
Plates: NA - North America, SA - South America, PAC - Pacific, ANT - Antarctica, IND - India, AF - Africa, EUR - Eurasia, NAZ - Nazca, COC - Cocos, CAR - Caribbean, PHIL - Phillipine,
ARAB - Arabian, ANATOL - Anatolian.
-40
B
V
A
Plate A
Plate A
A
V
B
+40
+40
A
V
B
C
V
A
+30
= +50
C
V
B
A
V
C
C
V
A
-30
+30
Plate B
Plate B
Fig. 5.39
There are three plates and alleline. The velocities of A and B and A and C with respect to each other (in millimeter year
1
) are
known. We want to know the velocity of B with respect to C. This is given by vectorial addition, as shown on the right. Velocity vector codes
like
B
V
A
read “the velocity of A with respect to B.”
Euler pole
and is most easily found by drawing orthogonal
lines from
transform faults
(see below), the latter being arcs
of small circles on the global sphere. The Euler pole is on a
great circle perpendicular to the trend of the transform fault.
advecting mantle generates heat at the ridge due to adia-
batic decompression. The associated melting produces
new oceanic crust at the ridge axis and defines a
thermal
boundary layer
in the form of cooling plate mantle that thick-
ens away from the point of upwelling. It is thus axiomatic
that lithospheric mantle above the top-asthenospheric
1,000
5.2.6
Thermal aspects of plates and slabs
C isotherm must gradually thicken laterally due to
conduction of the adiabatic heat released out through the
upper surface of the new ocean crust into the ocean
(Fig. 5.40). We ignore here the undoubted highly efficient
convection witnessed at the ridge axis by hydrothermal
systems responsible for “black smokers” (Fig. 3.6). In
physical terms, our example means that temperature is
changing with time and distance from the ridge
(Fig. 5.40). However, our most complicated heat conduc-
tion scenarios to date (Section 4.18; Cookie 20) say that
T
only changes with distance! Advanced sums (hinted at in
Consider first of all the likely temperature distribution in
the upper 1 km of the lithosphere in a lateral transect from
the mid-Atlantic ridge in Iceland to New York. At the
ridge there is abundant evidence in the form of submarine
volcanic activity and from heat flow measurements that
temperatures in the upper crust are high and that overall
heat flow is high (Fig. 5.40). In the case of offshore New
York the opposite is true. Divergent plate boundaries, like
this North Atlantic example, obey the simple rule that an
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