Geoscience Reference
In-Depth Information
(a)
(b)
4
A
v
2
2
4
B
v
A
Plate B
Plate A
(c)
(d)
10
A
v
10
10
B
v
A
Plate B
Plate A
(e)
(f)
A
v
6
6
B
v
A
6
6
Plate B
Plate A
Figure 2.3.
Three possible boundaries between plates A and B.
(a) A
constructive
boundary (mid-ocean ridge). The double line is the symbol for
the ridge axis, and the arrows and numbers indicate the direction of spreading and
relative movement of the plates away from the ridge. In this example the
half-spreading rate of the ridge (half-rate) is 2 cm yr
−1
; that is, plates A and B are
moving apart at 4 cm yr
−1
, and each plate is growing at 2 cm yr
−1
. (b) The relative
velocities
A
v
B
and
B
v
A
for the ridge shown in (a).
(c) A
destructive
boundary (subduction zone). The barbed line is the symbol for a
subduction zone; the barbs are on the side of the overriding plate, pointing away
from the subducting or downgoing plate. The arrow and number indicate the
direction and rate of relative motion between the two plates. In this example, plate B
is being subducted at 10 cm yr
−1
. (d) The relative velocities
A
v
B
and
B
v
A
for the
subduction zone shown in (c).
(e) A
conservative
boundary (transform fault). The single line is a symbol for a
transform fault. The half-arrows and number indicate the direction and rate of
relative motion between the plates: in this example, 6 cm yr
−1
. (f) The relative
velocities
A
v
B
and
B
v
A
for the transform fault shown in (e).
