Geoscience Reference
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boundaries. These transient effects (large igneous
provinces, continental flood basalts, seaward dip-
ping reflectors, volcanic chains) may reflect natu-
ral transient plate-tectonic events associated with
stress changes in the lithosphere. If the astheno-
sphere is close to the melting point and vari-
able in fertility then the location of volcanism
is controlled by plate stress and architecture,
and mantle composition, rather than by absolute
temperature.
of the major sources, appear to be in the range of
the
'missing heat-source paradox'
which
has led to a series of speculative papers on hid-
den radioactive heat sources in the deep mantle
or core. The 'missing heat' is also not a problem
if the hydrothermal contribution to plate cool-
ing has been over-estimated. The magnitude of
the 'missing energy' is comparable to the upward
adjustment of the measured heat flow due to
its mismatch with theoretical expectations from
simple plate-cooling models.
Minor heat sources
The helium--heat flow paradox
Radioactive heating and secular cooling are the
major contributors to the energy budget of the
Earth's interior. There are many other contribu-
tions that are often overlooked. The progressive
differentiation of the mantle and core releases
heat. As the Earth cools and differentiates the
mantle generates about 3 TW of gravitational
energy. There is a release of gravitational energy
and latent heat by growth of the inner core.
This contributes about 1.2 TW. The total power
from the core has been estimated to be 8.6 TW.
Solid Earth tides contribute slightly to the heat-
ing of the mantle. This energy source would have
been greater in the past when the Moon was
closer to the Earth. Past tidal heating may con-
tribute to present-day heat flow. The Earth, as it
cools, releases gravitational energy by contrac-
tion. Some fraction of this may be released by
earthquakes. The change in gravitational energy
associated with earthquakes has been estimated
to be as high as 2 TW. A very speculative energy
source,
UandThgenerate
4
He and anti-neutrinos as well
as heat. The observed flux of
4
He to the oceans
10
5
kg/a. The flux
predicted from a mantle with 21 ppb U and
95ppbThis3
from the mantle is about 3
.
2
×
10
6
kg/a. For comparison, the
4
He flux predicted from the continental crust is
about 10
6
kg/a. The fact that the current flux of
4
He from the oceanic mantle is an order of mag-
nitude less than predicted from the mantle U
and Th abundance is known as the
helium-
heat flow paradox
, just one of many para-
doxes associated with the decay chain of ura-
nium. The discrepancy is even larger if there is
a substantial delay in the transport of
4
He from
the source to the surface. On the other hand, He,
and CO
2
, may be trapped in the shallow man-
tle. The amount of
40
Ar in the atmosphere com-
pared to that released by
40
Kovertheageofthe
Earth indicates that, on average, the mantle is
efficiently outgassed. Because of the short half-
life of
40
K a large fraction of the Earth's
40
Ar was
produced in early Earth history. The production
of
4
He decreased with a longer time constant.
Argon does not escape from the atmosphere so
the atmospheric inventory of
40
Ar can be used
to give a lower bound on K in the Earth. Helium
escapes so we cannot bound the U and Th in this
way.
Helium is degassed, along with its main car-
rier, CO
2
, as basalts rise toward the surface.
Helium differs from argon in being highly sol-
uble in magmas. Total degassing requires erup-
tion or intrusion near the surface and even then
quickly quenched glasses retain substantial quan-
tities of CO
2
and helium. The missing helium-4
.
4
×
3 TW, is thermonuclear reactions ('natu-
ral nuclear reactors') in regions where radioactiv-
ity has been concentrated by natural processes.
These minor, and in some cases, speculative,
energy sources may account for up to some 17--
22 TW of the total heat flow. This is of the order of
the 'missing energy source' which has prompted
many suggestions for hidden radioactivity in the
deep mantle. In addition, heat flow may not be a
steadily decreasing function of time if plate reor-
ganizations modulate the heat flow. Current heat
flow, for example, may be 5% higher than the
mean because of the relatively recent breakup
of Pangea. The cumulative contribution of these
sources, and the uncertainties in the magnitudes
∼
