Geoscience Reference
In-Depth Information
the
heat
loss
through
the
surface
to
44
TW
Table 26.1
Summary of heat flow observa-
[
mantleplumes heat flow
].
The local heat flow from the interior is esti-
mated by drilling holes and measuring tem-
perature gradients and thermal conductivity.
Clearly, the surface of the Earth is not densely
or uniformly covered by such holes. The most
straightforward way of estimating the global ter-
restrial heat flow is simply to average the data in
an appropriate way. A spherical harmonic expan-
sion of heat flow data smooths it, and serves as
an interpolation scheme; however, it is not nec-
essarily appropriate for heat flow, tomography,
bathymetry or other functions that are not poten-
tial functions. Data can be binned (by region,
age, tectonomagmatic age and so on) to minimize
the uneven spatial distributions of the measure-
ments. In practice, averages are calculated in var-
ious tectonic provinces since the global dataset
is not uniformly dense. Various 'corrections' are
applied to the raw data so that estimates of
global power are model dependent. Examples
of these corrections are: replacing oceanic mea-
surements with predictions from a theoretical
cooling model, adding in an arbitrary or the-
oretical amount of hydrothermal heat flow --
which is well known only near ridges, remov-
ing transient effects from tectonic or magmatic
events, and eliminating data from areas thought
to be affected by hotspots. Some workers argue
that it is
preferable to base surface heat
flow analysis not only on the exten-
sive measurements
but also on processes that
are thought to bias the measurements. This
has become a contentious issue.
Global heat
flow maps
show a strong age dependency that
is lacking in the data; this is a result of the
correction.
The dramatic effects of hydrothermal circu-
lation
tions
Input
TW
Potential energy contributions
Mantle differentiation and contraction
3
Heat from core
8
Core differentiation
1.2
Conduction down adiabat
6
Inner core growth
0.5
Earthquakes
2
Tidal friction
1
Current radiogenic (BSE)
28
Delayed radiogenic (1--2 Gyr)
5--15
Secular cooling (50--80 K/Gyr)
9--14
Total
42--57
Radiogenic
+
other
56--71
Output
Global heatflow (observed)
30--32
Cooling plate model (theoretical)
44
Regions of excess magmatism
2.4--3.5
About 28 TW are generated by radioactive decay
in the interior. There are about 10 TW of non-
radiogenic heat sources such as cooling and
differentiation of the core, contraction of the
mantle, tidal friction and so on. On a convecting
planet one expects temporal variations in heat
flow of at least 10%. The secular cooling of the
Earth contributes somewhere between 30--60% of
the measured heat flow. Thus, there is either a
good match between heat production and heat
flow, or there is a deficit or a surplus of heat.
Some workers have declared an energy crisis, or
a missing heat-source problem. This crisis is simi-
lar to the crisis precipitated by Lord
Kelvin and
his age of the Earth
.
A summary of the energy inputs and outputs
of the mantle and core are given in Table 26.1.
The total radiogenic and secular cooling amounts
to 42--57 TW, while the current radiogenic pro-
duction is only 28 TW. The observed conducted
heat flow loss is 30--32 TW. Of this, about 2.4--3.5
TW is from the vicinity of hotspots. An unknown
amount of heat loss is due to hydrothermal cir-
culation. A generous allowance for this brings
on
surface
heat
flux
have
been
exten-
sively documented on young (
20 My) seafloor
but theory and data are lacking for old seafloor.
The magnitude of the assumed hydrothermal
correction to measured values of heat flow is
essentially the same as the so-called missing heat
source.
Continental and oceanic heat flow
data
are treated differently. The secular decay of
the heat flow in continents is often considered
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