Geology Reference
In-Depth Information
Fig. 12.8
Cooling of the
oceanic lithosphere. If
v
is
the spreading rate, a
column of asthenospheric
material moves away from
the ridge at half-spreading
rate ½
v
and attains a
thickness
z
T
at distance
x
½
vt
from the ridge
crest
D
1mm
2
s
-1
,
T
0
D
10
ı
C,
Fig. 12.9
Isotherms of the oceanic lithosphere, determined inverting (
12.45
) and assuming ›
D
1,280
ı
C. The shape of the TBL at any time (
thick black line
) has been calculated using (
12.43
)
and
T
a
D
T.
z
;t/
D
.T
0
T
a
/
erfc
!
heat. The conductive TBL where
T
<
T
P
defines
the oceanic lithosphere. Let
T
a
be the initial
temperature of the asthenosphere that leaves a
melting regime, and let us assume that seawater
instantaneously cools and maintains the surface
of the residual column to the temperature
T
0
.In
this instance, neglecting the horizontal compo-
nents of heat conduction, we can apply the so-
lution found in the previous section. Figure
12.8
illustrates an idealized cross-section through the
ridge crest of a cooling oceanic plate.
If
v
is the spreading rate and
x
is the offset of
an asthenospheric column from the ridge crest,
then the temperature distribution at any depth
z
below the sea floor can be written as follows:
z
2
p
2›x=
v
C
T
a
D
.T
a
T
0
/
erf
!
z
2
p
2›x=
v
C
T
0
(12.45)
Figure
12.9
shows some isotherms beneath
the ocean surface vs the ocean floor age. We
can use (
12.43
) to estimate the thickness of the
oceanic lithosphere of any age. This quantity is
also displayed in Fig.
12.9
and coincides with the
depth to the oceanic LAB minus the ocean floor
depth.
