Various lines of evidence suggest that volatile elements escaped (degassed)
from the mantle by volcanic eruptions associated with crust building. Some of
these gases were retained to form the atmosphere once surface temperatures were
cool enough and gravitational attraction was strong enough. The primitive
atmosphere was probably composed of carbon dioxide (CO 2 ) and nitrogen gas
(N 2 ) with some hydrogen and water vapour. Evolution towards the modern oxi-
dizing atmosphere did not occur until life began to develop.
1.3.2 The hydrosphere
Water, in its three phases, liquid water, ice and water vapour, is highly abundant
at the Earth’s surface, having a volume of 1.4 billion km^3. Nearly all of this water
(>97%) is stored in the oceans, while most of the rest forms the polar ice-caps
and glaciers (Table 1.1). Continental freshwaters represent less than 1% of the
total volume, and most of this is groundwater. The atmosphere contains com-
paratively little water (as vapour) (Table 1.1). Collectively, these reservoirs of
water are called the hydrosphere.
Introduction 5
Density
(g cm–3)
2.90
3.3
5.5
10.0
12.3
13.3
Rapid
increase
in density
Gradual
increase
in density
4.3
13.6
Depth
(km)
Crust 0
40
200
700
2900
Siliceous rocks
Basic rocks of olivine and pyroxene
Dense Mg and Fe silicates
Dense oxides?
MgO and SiO 2
Liquid metal
Fe + Ni + Si
Solid metal
5150
6371
Upper
mantle
Lower
mantle
Outer
core
Inner
core
Fig. 1.2Schematic cross-section of the Earth. Silica is concentrated in the crust relative to
the mantle. After Raiswell et al.(1980).