Geoscience Reference
In-Depth Information
Salinity g kg
-1
1040
at 0°C
and 1 atm
20
30
40
AVER AGE
SEAWATER
30
30
1030
s
t
12
14
16
18
20
22
24
26
1020
1,028 kg m
-3
at salinity
35 g kg
-1
20
20
28
1010
1000
10
30
10
0
10
20
30
40
50
Salinity (g kg
-1
)
Fig. 2.10
Variation of seawater density with salinity.
0
0
20
30
40
Salinity (g kg
-1
)
, as the excess over that of pure water at standard condi-
tions of temperature and pressure. This is referred to as
Fig. 2.11
Covariation of seawater density (as
t
) with salinity and
t
temperature.
1,000) kg m
3
. This variation is usu-
ally quite small, since over 90 percent of ocean water lies at
temperatures between
and is given by (
C and salinities of
20-40 parts per thousand (g kg
1
) when the density
2 and 10
Freshwater
suspension
of solids, density
2,750 kg m
-3
1.5
t
ranges from 26 to 28 (Fig. 2.11). It is difficult to measure
density
in situ
in the ocean, so it is estimated from tables
or formulae using standard measurement data on temper-
ature, salinity, and pressure. Detailed measurements reveal
that the rate of increase in seawater density with decreasing
temperature slows down as temperature approaches freez-
ing: this is important for ocean water stratification at high
latitudes when it is more difficult to stratify the very cold,
almost surface waters without changes in salinity.
Finally, our definition of density deliberately refers to
the “pure” substance. As noted in Section 2.1, many
Earth materials are rather “dirty” or impure, due to nat-
ural suspended materials or human pollutants. The tur-
bid suspended waters of a river in flood, a turbidity
current, or the eruptive plume of an explosive volcanic
eruption are cases in point. The changed density of such
suspensions (see Fig. 2.12) is a feature of interest
and importance in considering the flow dynamics of such
systems.
1.4
1.3
1.2
1.1
Seawater density
reached by
fractional mass of
0.01 mineral solids
1.0
0
0.1
0.2
0.3
Fractional mass of mineral solids
Fig. 2.12
Variation of freshwater density with concentration of
suspended mineral solids.
2.4
Motion matters: kinematics
2.4.1
Universality of motion
hydrosphere. Glaciers and ice sheets move, as do the per-
mafrost slopes of the cryosphere during summer thaw. The
slow motion of lithospheric plates may be tracked by GPS
and by signs of motion over plumes of hot material rising
from the deeper mantle. Magma moves through plates to
reach the surface, inflating volcanoes as it does so. The
All parts of the Earth system are in motion, albeit at
radically different rates (Box 2.2); the study of motion in
general is termed
kinematics
. We may directly observe
motion of the atmosphere, oceans, and most of the
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