Water Cycles - Hydrogeology: Objectives, Methods, Applications - page 18

the Deccan Traps (India) up to 0.6%. Similarly, water is present in peridotite
fragments originating from deeper in the mantle.
At the bottom of the oceans, sea water seeps into the crust and hydrates
the rock. This altered crust can have a water content of up to 1 to 2 %. In
subduction zones, the crust is dragged down into the mantle, which then
becomes water-enriched.
It is estimated that the amount of water entering the mantle falls
between 5 and 16·10 11 kg per year. The balance is diffi cult to quantify. It is
considered to be in equilibrium since sea level remains relatively stable.
The presence of water in the mantle is very important, as it changes
the physical and chemical properties of rock and certainly plays a role in
the convection mechanisms responsible for plate tectonics (Gillet, 1993) by
modifying the melting points and the viscosities of minerals.
2 THE DISTRIBUTION OF WATER
Water exists as a gas in the atmosphere, as a liquid in oceans, rivers, and
aquifers, and as a solid in snow, glaciers, ice caps, and ice fl oes, but it is
also present in most rocks:
• compositional water, which is part of a mineral's chemical formula;
• water present in closed pores (pumice, peat);
• adsorbed water, electrically fi xed to ionic surfaces, and in some cases
extractable by plant roots;
• fi nally, gravitational water, which can circulate in pores and open
discontinuities in rock. This is the domain of hydrogeology. The
important volumes of gravitational water present below ground
constitute more and more sought-after reserves (Figure 4).
Figure 4 Different types of water on the planet.