Geoscience Reference
In-Depth Information
x
=0
ridge
t
=
x
u
(a)
250
(c)
T
=
T
0
q
q
q
200
T
=
T
1
u
u
u
150
t
=
t
0
t
=
t
1
t
=
t
2
t
(Myr)
100
(d)
0 50 100 150
0
200
50
50
100
0
0 50 100 150 200
t
(Myr)
150
u
(b)
Ridge
T
=
T
0
x
=0
Surface
Lithosphere
T
1
Asthenosphere
z
x
Fig. 5.40
Thermal matters for oceanic plate (a) Heat flow oceanwards from a midocean ridge, with distance expressed as plate age (derived
from ocean magnetic anomalies). Dots are measurements, curves are various theoretical estimates based on the erfc argument. (b) The
mechanical situation, with hot upwelling asthenosphere cooling laterally to define the plate thermal boundary layer above the isotherm at
c
.1,000
C. (c) The physical situation, with heat flow,
q,
conducting vertically through the ocean floor from the thickening plate above the
c
.1,000
C isotherm.
q
decrease with time and the plate thickens with time according to the erfc argument (see text). (d) The proof of the
pudding: data on plate thickness (dots) from seismic surveys versus estimates of plate thickness to the 1,000
C isotherm from heat conduction
theory.
Cookie 20) tell us that the thickness,
z
t
, of this thermal
boundary layer changes in proportion to t
he
square root
of time,
t
, as the simple expression
envisaged is buoyantly unstable is also axiomatic. Also, global
continuity tells us that creation of oceanic lithosphere in one
place must be accompanied by destruction elsewhere if the
Earth is to maintain constant volume. The fate of oceanic
plate is therefore determined; it has to be destroyed. Pushing
cold slab into the hot mantle (Fig. 5.41) creates a thermal
anomaly, that is, the lithosphere is cooler than it should be
for the depth it has reached. This has the effect of raising the
olivine : spinel transition (Section 4.17.4) by several tens of
kilometers in the slab (Fig. 5.41) and creating additional
negative buoyancy that adds to the slab pull force (explained
is
the thermal diffusivity (Section 4.18.3). The square root
term is a characteristic thermal diffusion distance and
z
t
refers to the thermal boundary layer thickness defined as
the thickness appropriate to a base lithosphere tempera-
ture of 90 percent of steady state value
c
.1,000
2.32
t
, where
C.
So, ignoring all the geological differences, we know
that the lithosphere can be regarded simply as a cold, dense
layer lying above warmer asthenosphere. That the situation
Search WWH ::
Custom Search