Matters of state and motion - Physical Processes in Earth and Environmental Sciences - page 25

Pressure (bars)

0 250 500 750 1000

Quantity of matter is the
measure of it arising from its
density and bulk conjointly

1080

FRESHWATER

Constant T
20°C

1040

REPRESENTATIVE DENSITIES
(all in kg m -3 )

Air at top Everest 0.467 1000

Air at sea level 15 ° C 1.225

Water at 20 ° C 998 Constant P

1 bar Seawater at 0 ° C 1,028

960

Ice 917

Average crustal rock
at surface 2,750

920

Average mantle rock
at surface

0 25 50 75 100

3,300

Temperature (°C)

Mean solid Earth 5,515

Pressure (bars)

0 250 500 750 1000

Typical basalt magma
at 90 km depth

1.6

3,100

AIR

Ditto near surface 2,620

Fig. 2.8 Density may vary with state, salinity, temperature, pressure,
and content of suspended solids.

1.2

Constant P
1 bar

0.8

confusing term. Density is regarded as a material property
of any pure substance. The magnitude of such a property
under given conditions of temperature and pressure is
invariant and will not change whether the pure substance
is on Moon, Mercury, or Pluto, as long as the conditions
are identical. Neither does the value change due to any
flow or deformation taking place.

0.4

Constant T
20°C

0

0 125 250 375 500

Temperature (°C)

Fig. 2.9 Variation of density of freshwater and air with temperature
and pressure.

2.3.3 Controls on density

also has significant effects on both liquid (Fig. 2.9) and
solid density whereas pressure has smaller to negligible
effects upon liquid and solid density in most near-surface
environments, becoming more important at greater
depths. There are also important effects to consider in
cold lakes due to the anomalous expansion of pure water
below approximately 4

Note the emphasis on “given conditions” in Section 2.3.2,
for if these change then density will also change.
Temperature ( T ) and pressure ( p ) can both have major
effects on the density of Earth materials. We have already
sketched the magnitudes of temperature change with
height and depth in the atmosphere, ocean, and within
solid Earth (Section 2.2). These variations come about
due to variable solar heating by radiation, radioactive heat
generation, thermal contact with other bodies, changes of
physical state, and so on. Pressure varies according to
height or depth in the atmosphere, ocean, or solid Earth
(Section 3.5). All of these factors exert their influence on
the density of Earth materials. Why is this? Referring to
Section 2.1, you can revisit the role of molecular packing
upon the behavior of the states of matter. The loose
molecular packing of gases means that they are compressi-
ble and that small changes in temperature and pressure
have major effects upon density (Fig. 2.9). Temperature

C. This means that water is less
dense at colder temperatures. As salinity increases to that
of seawater the temperature of maximum density falls to
about 2 C. In the deep oceans and deep lakes, for example,

Lake Baikal, an additional effect must be considered, the
thermobaric effect . This is the effect of pressure in decreasing
the temperature of maximum density.
The case of seawater density is of widespread interest in
oceanography since natural density variations create buoy-
ancy and drive ocean currents. Its value depends upon
temperature, salinity (Fig. 2.10), and pressure. The covari-
ation with respect to the former two variables is shown in
Fig. 2.11. It is convenient to express ocean water density,