Geology Reference
In-Depth Information
Table 6.1.
Contributions to global heat flow.
Area
Mean heat
Total heat
Percentage
(10
8
km
2
)
flux mW/m
2
flow (TW)
of global
1. Sea floor
3.1
100
31
76
2. Continental crust
2.0
50
10
24
a. Crustal radiogenic
-
25
5
12
b. Mantle
-
25
5
12
3. Total mantle (1
+
2b)
5.1
70
36
88
4. Total global (1
+
2a
+
2b)
5.1
80
41
100
Another way to get to this result is to calculate the average heat flux,
q
, required
to release the heat
H
over the time,
t
s
=
100 Myr, that the plate material takes to
reach a subduction zone:
q
=
H/t
s
.
(6.4)
10
14
J/m
2
10
15
s, so
q
=
Using the above values,
H
=
2.1
×
and
t
s
=
3
×
70 mW/m
2
. The surface area of the sea floor is 3.1
10
8
km
2
, so the total heat
×
loss through the sea floor is
Q
22 TW. This is essentially the same answer as
obtained from Eq. (6.3), apart from rounding errors, as it should be because the
two approaches are equivalent.
This rough estimate is a little lower than what is observed. The average heat flux
through the sea floor is measured to be about 100 mW/m
2
, which gives a total heat
flow of 31 TW [11]. A more sophisticated mathematical analysis will get closer
to the observed value. The point of our rough estimate is, as usual, to keep the
physics clearly in view. Thus we can conclude that the cycle of plate formation and
subduction removes around 30 TW of heat from the mantle.
We can compare this estimate with the total heat loss of the Earth, and the total
heat loss of the mantle. The total heat flow out of the Earth is 41 TW [11]. Some of
this heat is generated in the upper continental crust, and so is not part of the mantle
heat budget. The continents have an average heat flux of about 50 mW/m
2
=
and an
10
8
km
2
, so the total heat flow out of continents is about 10 TW. About
half of this is generated by the radioactivity concentrated in the upper 10 km or so of
the continental crust [1], and this heat conducts directly to the surface and so plays
no part in driving mantle convection. The other half of the continental heat flow must
then come from the mantle, conducting into the base of the continental lithosphere.
Thus about half of the total continental heat flux, 25 mW/m
2
, is emerging from the
mantle through the continents, or about 5 TW in total.
These relationships are summarised in Table 6.1. About 75% of the Earth's heat
loss occurs through the sea floor as a result of plate-scale convection. This is nearly
area of 2
×
