The Geochemistry of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 119

ically cover smaller ones, < 5 10 4 km 2 . Ice masses constrained by topography
cover areas between 1 and 100 km 2 wide.

5.4.3.1 The composition of glacial runoff

The main source of water in most glacier systems is snow and/or ice melt. Some
is also derived from rain and a little stems from geothermal melting and internal
deformation (Paterson, 1994).
Table 5.12 shows the chemical composition of glacial runoff compiled by Brown
(2002). It also includes an estimation on the average composition of glaciers on
Earth and the average weighted sum of each region. Glacial runoff as a dilute solu-
tion containing ions Ca 2+ , HCO 3 , SO 2 4 , with variable Na + and Cl , is usually
more dilute than that of the global mean river 3 .

5.5 The continental crust

The solid Earth is composed of several layers. These can be classified into the core,
mantle and crust (see Fig. 5.1). The Earth's centre, the core, lies about 2,900 km
below the surface and is composed of an inner and outer part. The former is solid
and approximately 1,250 km thick. The latter (approx. 2,200 km thick) is composed
of dense molten metals, with a high concentration of iron (Wedepohl, 1971). The
core is surrounded by a solid mantle of iron-magnesium silicates and oxides. It
contains the inner mantle (between 300 and 2,890 km below the Earth's surface)
and the outer mantle (at a depth between 10 and 300 km). The outer shell covering
the mantle is the crust. According to Rudnick (1995), it constitutes only 0.6% of
the silicate Earth and is covered by geological rock formations and the oceans.
The crust can be separated into oceanic and continental. The oceanic crust is
on average 7 km thick and is composed of mafic rock types such as basalt. The
continental crust is thicker at 40 km and contains virtually every rock type known
on Earth. The structure of the continental crust is divided into the upper-, middle-
and lower layers. Sometimes the middle and lower crust are grouped together. The
deep continental crust is composed of granulite facies and begins on average at a
depth of 23 km. The middle crust extends down from 8 to 17 km. Estimates indicate
that it is composed of rocks in the amphibolite facies. The upper continental crust
is the reservoir of the main minerals and other natural resources deemed useful for
mankind. It is therefore, the principal object of study in this topic. Furthermore,
being the most accessible solid part of the planet, the upper crust has long been
the target of geochemical investigations. According to Yoder (1995), the mass of
the upper continental crust is about half that of the total, which corresponds to a
volume of 6:55 10 5 km 3 and a radius of about 544 km.

3 See Table 5.8 for comparisons between river and glacier compositions.