Matters of state and motion - Physical Processes in Earth and Environmental Sciences - page 20

4 Magma has small but important fractions of pressurized
dissolved gases, including water vapor.
5 River water contains suspended solids, while the atmos-
phere carries dust particles and liquid aerosols.
6 Seawater has c. 3 percent by weight of dissolved salts and
also suspensions of particulate organic matter.
Solid Earth substances may break or flow:
1 Ice fragments when struck, yet deformation of boreholes
drilled to the base of glaciers also shows that the ice there
flows, while cracking along crevasses at the surface.
2 Earth's mantle imaged by rapidly transmitted seismic
waves behaves as a solid mass of crystalline silicate minerals.
Yet there is ample evidence that in the longer term

( 10 3 years) it flows, convecting most of Earth's internal

heat production as it does so. Even the rigid lower crust is
thought to flow at depth, given the right temperature and
water content.

2.1.5 Timescales of in situreaction

The lesson from the last of the above examples is that we
must appreciate characteristic timescales of reaction of
Earth materials to imposed forces and be careful to relate
state behavior to the precise conditions of temperature and
pressure where the materials are found in situ .

2.2 Thermal matters

2.2.1 Heat and temperature

In each case temperature change signifies internal energy
change . Changes of state between solid, liquid, and gas
require major energy transfers, expressed as latent heats
(Box 2.1). We shall further investigate the world of ther-
modynamics and its relation to mechanics later in this topic
(Section 3.4).
Substances subject to changed temperature also change
volume, and therefore density; they exhibit the phenome-
non of thermal expansion or contraction (Box 2.1). This
arises as constituent atoms and molecules vibrate or travel
around more or less rapidly, and any free electrons flow
around more or less easily. If changes in volume affect only
discrete parts of a body, then thermal stresses are set up
that must be resisted by other stresses failing which a net
force results. Temperature change can thus induce motion
or change in the rate of motion. Stationary air or water
when heated or cooled may move. Molten rock may move
through solid rock. A substance already moving steadily
may accelerate or decelerate if its temperature is forced to
change. But we need to consider the complicating fact that
substances (particularly the flow of fluids) also change in
their resistance to motion, through the properties of vis-
cosity and turbulence, as their temperatures change. We
investigate the forces set up by contrasting densities later
in this topic (e.g. Sections 2.17, 4.6, 4.12, and 4.20).

Heat is a more abstract and less commonsense notion than
temperature, the use of the two terms in everyday speech
being almost synonymous. We measure temperature with
some form of heat sensor or thermometer. It is a measure
of the energy resulting from random molecular motions in
any substance. It is directly proportional to the mean
kinetic energy, that is, mean product of mass times velocity
squared (Section 3.3), of molecules. Heat on the other
hand is a measure of the total thermal energy, depending
again on the kinetic energy of molecules, and also on the
number of molecules present in any substance.
It is through specific heat, c , that we can relate temperature
and heat of any substance. Specific heat is a finite capacity,
sometimes referred to as specific heat capacity , in that it is a
measure of how much heat is required to raise the temper-
ature of a unit mass (1 kg) of any substance by unit Kelvin
( K 273). It is thus also a storage indicator - since

only a certain amount of heat is required to raise tempera-
ture between given limits, it follows that only this amount
of heat can be stored. In Box 2.1, notice the extremely
high storage capacity of water, compared to the gaseous
atmosphere or rock.
Temperature change induces internal changes to
any substance and also external changes to surrounding
environments, for example,
1 Molten magma cools on eruption at Earth's surface, turn-
ing into lava; this in turn slowly crystallizes into rock.
2 Glacier ice in icebergs takes in heat from contact with
the ocean, expands, and melts. The liquid sinks or floats
depending upon the density of surrounding seawater.
3 Water vapor in a descending air mass condenses and
heat is given out to the surrounding atmospheric flow.

C

2.2.2 Where does heat energy come from?

There are two sources for the heat energy supplied to
Earth (Fig. 2.4). Both are ultimately due to nuclear reac-
tions. The external source is thermonuclear reactions in
the Sun. These produce an almost steady radiance of
shortwave energy (sunlight is the visible portion), the