Environmental Engineering Reference
In-Depth Information
of grain contacts, porosity) of the rocks. This results in a mean heat flow density
of 65 mW/m 2 at the earth's surface for the continental earth crust.
The radiogenic heat generation q . rad (e.g. the proportion of radiogenic heat pro-
duction in the entire heat flow density) of an area at a particular depth results from
the layer thickness ∆z and the heat production H according to Equation (2.29).
rad qHz
&
=
(2.29)
Radiogenic heat production has a minor role in the Earth's mantle due to the
lack of radioactive isotopes. By Equation (2.29) it can be estimated that the heat
flow density resulting from radioactive decay is approximately 35 mW/m 2 at a
depth of ca. 30 km, where the boundary between the crust and the mantle is lo-
cated. This represents a mean heat production rate of 1 µW/m 3 and thus, a rather
common value for the earth's crust. The main part of geothermal heat supplied at
the earth's surface is therefore generated in the earth crust during the decomposi-
tion of radioactive elements.
The oceanic earth crust mainly consists of basaltic rock with low level of heat
generation. Nevertheless the average heat flow density is around 65 mW/m 2 . Heat
from the convective surge of hot rock masses from the earth's coat at the rims of
the lithosphere play an important role for the heat flow here, as above-average
temperatures can be reached due to this process.
Heat balance at the surface of the earth. The heat flux density of 65 mW/m 2
results in a radiation power of the earth of approximately 33 10 12 W; thus the
earth provides energy of 1,000 EJ per year to the atmosphere. In contrast, solar
radiation incident on the surface of the earth is around 20,000 times more than the
energy released by the terrestrial heat flow. The released and the absorbed heat
radiation define the observed temperature balance of approximately 14 °C at the
surface of the earth.
This can also be concluded from Fig. 2.57. It has to be assumed that the tem-
perature very close to the surface of the earth (i.e. to a depth of some meters) is
dominated by the heat energy radiated from the sun. Among other things, this
reveals also the fact that the soil is frozen up to depths of several meters at some
places and is heated up to sometimes very high temperatures during the summer
on the other side (sometimes up to 50 °C and more at a correspondingly high solar
radiation incident). This is exclusively caused by the seasonal differences of the
solar radiation supply and the resulting temperature level in the atmospheric layers
close to the surface of the earth. Solar radiation influences the temperature course
within the earth up to a depth of 10 to 20 m (annual course).
In the top layer of the earth's crust, the proportion of the entire geothermal
flow, resulting from the geothermal heat and the incident solar energy, is influ-
enced by a number of very different effects. One main influencing parameter is
rain. The resulting surface and groundwater is "heated up" by solar energy, and
transports the incident solar energy to shallow layers of the earth. Heated surface
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