The source of water for the formation of the hydrosphere is problematical.
Some meteorites contain up to 20% water in bonded hydroxyl (OH) groups,
while bombardment of the proto-Earth by comets rich in water vapour is another
possible source. Whatever the origin, once the Earth’s surface cooled to 100°C,
water vapour, degassing from the mantle, was able to condense. Mineralogical
evidence suggests water was present on the Earth’s surface by 4.4 billion years
6 Chapter One
Oxygen 46.6%
Others 1.4%
Magnesium 2.1%
Potassium 2.6%
Sodium 2.8%
Calcium 3.6%
Iron 5.0%
Aluminium 8.1%
Silicon 27.7%
Fig. 1.3Percentage of major elements in the Earth’s crust.
Table 1.1Inventory of water at the Earth’s surface. After Speidel and Agnew (1982).
Reservoir Volume (10^6 km^3 ) Percentage of total
Oceans 1350 97.41
Ice-caps and glaciers 27.5 1.98
Groundwater 8.2 0.59
Inland seas (saline) 0.1 0.007
Freshwater lakes 0.1 0.007
Soil moisture 0.07 0.005
Atmosphere* 0.013 0.001
Rivers 0.0017 0.0001
Biosphere 0.0011 0.00008
Total 1385.9 100
* As liquid equivalent of water vapour.