Geology Reference
In-Depth Information
4500
15
(b)
(a)
4000
Core (
T
)
C
14.5
3500
14
3000
Plates (
Q
)
S
Lower Mantle (
T
)
L
2500
13.5
Heat generation (
Q
)
R
2000
13
Upper Mantle (
T
)
U
Plumes (
Q
)
C
1500
1000
12.5
0
1
2
3
4
0
1
2
3
4
Time, Gyr
Time, Gyr
1800
(c)
1700
1600
1500
Upper Mantle (
T
)
U
1400
1300
0
1
2
3
4
Time, Gyr
Figure 9.1. A reference thermal evolution of the mantle and core. Details of the
parameters and outputs are given in Section B.2.
have started equal to
T
L
. It was necessary to adjust the Urey ratio to match the
present surface heat flow, the efficiency of plate heat flow to match the present
mantle temperature, and the efficiency of plume heat transport to match the present
plume heat flow. The 'efficiency' factors are effectively the not well-determined
numerical factors in the formulae for
Q
S
and
Q
C
.
The thermal evolution in Figure 9.1(a) looks fairly simple: everything declines
monotonically. However, there are a number of features to comment on. There are
two phases of cooling, an early rapid cooling lasting about 500 Myr and then a
much slower cooling. The transient early phase occurs because the assumed initial
mantle temperature is higher than the temperature that would be maintained by the
radioactive heating. This high starting temperature was chosen to reflect the high
temperature expected to have been generated by the formation of the Earth and the
separation of the mantle and core materials. We don't have an accurate estimate
of what that early temperature might have been, but the calculation makes it clear
