Water Cycles - Hydrogeology: Objectives, Methods, Applications - page 17

Figure 3 Spectral signature of water in the Hale-Bopp comet, measured by ISO (from Crovisier
et al ., 1997).
• water, when it freezes, expands and therefore fl oats. It forms an
insulating layer, which protects the liquid water below from freezing.
If the opposite were true, ice would sink to the bottom of the oceans
and the Earth could be much colder.
It is estimated that around 4.4 billion years ago, the Earth was already
at its current size, and had an atmosphere and a hydrosphere.
Water is present in both the continental and the oceanic crusts. In the
Earth's interior, mantle rocks today contain approximately 0.3% water,
which represents a volume one to two times greater than that held by the
oceans. This deep reserve gives birth to an estimated annual volume of a
few km 3 of young waters, a negligible amount in the short term (Castany,
1998), but one which, through accumulation over hundreds of millions of
years, made a signifi cant contribution to the creation of the hydrosphere.
1.2 The internal water cycle
Water in the mantle is returned to the surface in part by basaltic magmas,
or directly through hot springs and highly mineralized vents such as the
black and white smokers found at mid-oceanic ridges. Analyses of the
material issuing from the upper part of the mantle indicate the presence of
water. Hawaiian lavas (USA) can contain up to 0.45% water, and lavas in