Geology Reference
In-Depth Information
C
HAPTER
A4
Water Cycles
The circulation and residence time of water below ground are one step
in an endless process, the water cycle. Through precipitation and runoff,
a fraction of the water caught up in this cycle seeps into the ground. By
renewing the water in underground reservoirs, infi ltration feeds the deep
circulations that then supply springs. But there is also an internal water
cycle, which captures a small percentage of surface water and carries it
to depth, through subduction zones, and then restores it to the surface
through volcanism.
1 THE INTERNAL CYCLE
1.1 The origin of water on Earth
The detection of water elsewhere in the Universe is made diffi cult by
the Earth's atmosphere, which absorbs much of the incoming radiation.
Instruments must therefore be placed outside the atmosphere. The data
thus collected, particularly by the European Space Agency's infrared
telescope, ISO (Infrared Spatial Observatory), and the American satellite
SWAS (Submillimeter Wave Astronomy Satellite), show that water exists
almost everywhere in our galaxy. It is widely distributed throughout the
universe.
Hydrogen has been present since the fi rst moments after the Big Bang.
The oxygen atom in the water molecule originated some time later, in
nuclear fusion reactions within stars. Water exists in the universe as gas,
ice, or hydrates, and also in liquid form, on planets (Reeves, 1981). Thanks
to its distinctive spectral signature, water has been detected on the surface
and in the immediate surroundings of low-temperature stars, as well as
in interstellar molecular clouds (Figure 2). The Earth was born, around 4.5
billion years ago, out of dust and gas aggregates, which already contained
hydrated silicates and ice. These grains formed larger, kilometer-scale
Search WWH ::
Custom Search