Geoscience Reference
In-Depth Information
Table 7.3 Heat loss and heat flow from the Earth
Area
Mean heat flow
Heat loss
(10 6
km 2 )
(10 3
Wm −2 )
(10 12
W)
Continents (post-Archaean)
142
63
9.0
Archaean
13
52
0.7
Continental shelves
46
78
3.5
Total continental area
201
65 ± 1.6
13.1 ± 0.3
Oceans (including marginal basins)
309
101 ± 2.2
31.2 ± 0.7
Worldwide total
510
87
±
2.0
44.2
±
1.0
Note : The estimate of convective heat transport by plates is ∼65% of the total heat
loss; this includes lithospheric creation in oceans and magmatic activity on continents.
The estimate of heat loss as a result of radioactive decay in the crust is ∼17% of the
total heat loss. Although oceanic regions younger than 66 Ma amount to one-third of
the Earth's surface area, they account for over half the total global heat loss. About
one-third of the heat loss in oceanic regions is by hydrothermal flow. The estimate of
the heat loss of the core is 10 12 -10 13 W; this is a major heat source for the mantle.
Source : Pollack et al .(1993).
over the mid-ocean ridges and young crust, and the lowest values are found over
the deep ocean basins. In continental regions the highest heat flows are measured
in regions that are subject to the most recent tectonic activity, and the lowest
values occur over the oldest, most stable regions of the Precambrian Shield.
These oceanic and continental heat-flow observations and their implications are
discussed in Sections 7.5 and 7.6.
To apply the heat-conduction equation (7.15)tothe Earth as a whole, we need
to use spherical polar coordinates ( r ,
θ
,
φ
) (refer to Appendix 1). If temperature
is not a function of
θ
or
φ
but only of radius r , Eq. (7.15)is
r 2
T
d t =
k
ρ c P
r 2
1
T
d r
A
ρ c P
+
(7.48)
r
First let us assume that there is no internal heat generation. The equilibrium
temperature is then the solution to
1
r 2 T
r
r 2
= 0
(7.49)
r
On integrating once, we obtain
r 2 T
r = c 1
(7.50)
and integrating the second time gives
c 1
r + c 2
T
=−
(7.51)
where c 1 and c 2 are the constants of integration.
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