Geoscience Reference
In-Depth Information
Concentration
Speed
Distance along the highway
Henry's law
behavior
Speed
decreases
Concentration
increases
Concentration
decreases
Speed
increases
Figure 5.7
Traffic traveling waves. As long as vehicles are few, their speed is independent of their
concentration on the lane. This is Henry's law regime. When more cars join the traffic, car spacing
decreases and drivers reduce their speed which in turn increases concentration until cars pack to a
jam. Ahead, traffic lightens up and cars are allowed to move faster again. Such alternating zones
of light and dense traffic move up the highway. Chromatographic waves are produced by
concentration-dependent velocities.
permitted. Concentration is the number of cars per km and flux is the product of concen-
tration by speed. When traffic is light, car spacing exceeds the safety distance between cars
and the drivers presumably adjust to a speed close to the speed limit. The speed is indepen-
dent of the concentration of cars: this is Henry's law regime. When the density of traffic
increases, each driver must reduce the car speed to avoid crashing into the vehicle ahead:
velocity now decreases as car concentration increases. New cars merging into the highway
make spacing shorter and shorter and progressively bring traffic to a halt. After a while, as
the cars ahead of the jam move away, the traffic lightens up thereby increasing the distance
and therefore the speed. Under conditions of heavy traffic, the alternating zones of light and
dense traffic move up the highway as traveling waves. This is mechanical chromatography!
Let us now go back to our moving fluids and the elements they transport and see how
“traffic waves” occur in nature. Suppose that
D
i
decreases when the concentration in
i
increases. In this case, the greater the concentration of the element, the less reactive it
will be. By applying
(5.20)
, it can be seen that the ions of highly concentrated zones will
catch up with those of less concentrated zones (
Fig. 5.8
), thus forming what are known
as metasomatic fronts or even more complicated geometric patterns. The description of
magma migration is far more complex still because, contrary to the groundwater table
where the water itself reacts very little with the rock matrix, all the major elements of the
magma exchange intensely with the solid residue. In other words, incompatible elements
cannot scurry ahead of the melt, which is of course made of compatible major elements.
Chromatography theory applies to other cases of two-phase transport. An impor-
tant concept in oceanography is the entrainment - or scavenging - of elements by
particles such as dead organisms, waste matter, or atmospheric dust. In this case, the
