The Geochemistry of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 105

Chapter5
The Geochemistry of the Earth
5.1 Introduction
In this chapter a comprehensive analysis of the geochemistry of the Earth is un-
dertaken as the starting point for assessing its thermodynamic properties. The
geochemical features of each layer of the Earth are described: the atmosphere; hy-
drosphere with the oceans, surface and ground waters as well as ice sheets; and the
crust, with a focus on its upper part.
5.2 The bulk Earth
The Earth is an approximately spherical body, 12.756 km in diameter and 5:98
10 24 kg (Beichner et al., 2000). Its physical and chemical peculiarities have allowed
the existence of planetary life. The solid Earth is divided into the crust (continen-
tal and oceanic), mantle and core. The lithosphere, meanwhile is the term used to
describe the rigid strong outer layer of the Earth. At approximately 100 km thick,
it consists of the crust and upper mantle.The external layers above the crust are the
hydrosphere and the atmosphere. The mantle and core are the largest, accounting
for 67 and 33% respectively of the total planetary mass (Javoy, 1999). The crust,
hydrosphere and atmosphere together, make up less than 1%, yet provide the nur-
turing and nourishing habitat in which the human species lives. Incidentally it is
only this small fraction which is available for direct study and analysis. Therefore
it is necessary to use indirect methods to estimate the Earth's inner composition.
5.2.1 The composition of the Earth
In terms of materials, the Earth can be considered as a closed system containing a
finite number of substances, except for the very occasional contribution of meteorites
(Mason, 1966). The Earth's spheres are large reservoirs and between them there
are material flows that balance out and thus keep their compositions near constant,
as is the case for the atmosphere, hydrosphere and continental crust.
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