Geoscience Reference
In-Depth Information
Chapter 7
Heat
7.1 Introduction
Volcanoes, intrusions, earthquakes, mountain building and metamorphism are all
controlled by the transfer and generation of heat. The Earth's thermal budget con-
trols the activity of the lithosphere and asthenosphere as well as the development
of the innermost structure of the Earth.
Heat arrives at the Earth's surface from its interior and from the Sun. Virtually
all the heat comes from the Sun, as any sunbather knows, but is all eventually
radiated back into space. The rate at which heat is received by the Earth, and re-
radiated, is about 2
10
2
Wm
−
2
.
Compare this value with the mean rate of loss of internal heat from the Earth,
4.4
10
17
Wor, averaged over the surface, about 4
×
×
10
−
2
Wm
−
2
); the approximate rate at which energy is
released by earthquakes, 10
11
W; and the rate at which heat is lost by a clothed
human body on a very cold (
10
13
W (or 8.7
×
×
30
◦
C), windy (10 m s
−
1
) Canadian winter day,
−
10
3
Wm
−
2
.From a geological perspective, the Sun's heat is important because
it drives the surface water cycle, the rainfall and, hence, erosion. However, the heat
source for igneous intrusion, metamorphism and tectonics is within the Earth,
and it is this internal source which accounts for most geological phenomena. The
Sun and the biosphere have kept the surface temperature within the range of the
stability of liquid water, probably 15-25
◦
Caveraged over geological time. Given
that constraint, the movement of heat derived from the interior has governed the
geological evolution of the Earth, controlling plate tectonics, igneous activity,
metamorphism, the evolution of the core and hence the Earth's magnetic field.
Heat moves by conduction, convection, radiation and advection.
Conduction
is the transfer of heat through a material by atomic or molecular interaction within
the material. In
convection
, heat transfer occurs because the molecules themselves
are able to move from one location to another within the material; it is important
in liquids and gases. In a room with a hot fire, air currents are set up, which
move the light, hot air upwards and away from the fire while dense cold air moves
in. Convection is a much faster way of transferring heat than conduction. As an
example, when we boil a pan of water on the stove, the heat is transferred through
the metal saucepan by conduction but through the water primarily by convection.
2
×
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