Geoscience Reference
In-Depth Information
Global heat flow data
120
250
100
GDH1
200
80
+
150
+
60
100
+
+
?
40
50
20
0
0
50
100
150
0
0
1
2
3
4
Observed/GDH1: Global data
Crustal age (10
9
yr)
1.5
N
=
4458
Fig. 26.1
Continental heat flow as a function of crustal age.
1
ts
=
65
±
10 m.y.
0.5
0
the continents, heat flow is often plotted as a
function of age (Figure 26.1) or the time since
the last tectonic or igneous event and the long-
term asymptotic value is taken to be the back-
ground heat flow. The transient effect has a
time constant of more than 200 Myr and this is
stretched out further by erosion, which strips off
the radioactive-rich outer layers. In some compi-
lations, the transient effect is discounted in esti-
mating continental and global heat flow. Crustal
radioactivity is a major contributor to continen-
tal heat flow and lateral variations in this heat
flow.
In the ocean basins the main contribution to
the observed heat flow is the transient effect,
the formation of the oceanic crust itself. Theo-
retically, heat flow should die off as the square-
root of age but it is nearly constant after the ini-
tial transient (see Figure 26.1). The background
oceanic heat flow is nearly the same as under con-
tinents, perhaps slightly larger.
There is lit-
tle evidence that hotspots or swells
are associated with high heat flow
.In
contrast to predictions from the plate and cool-
ing half-space models there is little correlation
of heat flow with age or depth (Figure 26.2). Mea-
sured heat flow is not a unique function of age.
This indicates that the mantle is not isothermal
(characterized by a single
potential temper-
ature
) or homogenous (in composition and ther-
mal properties).
0
50
100
150
Age (My)
Fig. 26.2
Measured heat flow and cumulative heat flow as a
function of age. If the shallow mantle is at the melting point,
or if there are fertile blobs in the mantle with low melting
points, then intrusion by dikes and sills may modify the heat
flow. The top panel shows heat flow vs age with one-standard
deviations (Stein and Stein, 1994). [
constraints on
hydrothermal heat flux
]
The cold outer shell of the Earth is not simply
a cooling boundary layer of uniform composition
and conductivity, losing heat by conduction
alone. Ocean bathymetry is more uniquely a func-
tion of square-root age, suggesting that some
process affects the near-surface thermal gradient
without affecting the integrated density of the
outer layers.
Global heat flow
Global heatflow compilations
are read-
ily available (www.heatflow.und.edu) (http://
www.geo.lsa.umich.edu/IHFC/heatflow.html).
The total heat flow through the surface of
the Earth from the interior, based on
measured
heat f low
, is about 30 TW. Various corrections
and adjustments are made to the data and some
workers think the adjusted total heat flow may be
closer to 44 TW but this is based on assumptions.
