Earth’s geological structure and processes - Fundamentals of the Physical Environment - page 275

which comprise 75 per cent of the crust. Like the core, it is not internally homogeneous.
An inner solid mesosphere extends for 2,560 km to within 350 km of the crust, over-lain
by partially melted and viscous asthenosphere . Cool solid lithosphere , averaging 70 km
in thickness, forms the outer mantle and its overlying, recyclable crust . Despite
differences in mineral composition and density, which falls from 3·5 gm cm −3 in
lithospheric mantle to less than 3·0 gm cm −3 in crust, traditional mantle-crust distinctions
are less important than the lithosphere-asthenosphere boundary. Here, mobile rigid
crustal plates are decoupled from underlying viscous mantle and form a distinctive
surface architecture of global landforms based on mineralogical differences. Denser,
heavier basalt -rich oceanic crust (2·8-3·4 gm cm −3 ) is only 7-10 km thick, compared
with less dense, lighter granite -rich continental crust (2·7 gm cm −3 ) 25-75 km thick with
a mean of 35 km.
The outermost planetary layers are quite distinct from the 'solid' mineral Earth and not
considered traditionally as geological systems. Although the hydrosphere (97 per cent
ocean and 2 per cent glacier ice by mass) and atmosphere have their own distinct
character and behaviour, they originate from the same planetary fractionation processes
and continue to exchange and synthesize materials with the lithosphere. The hydrosphere
has the greater mass but, with ocean (saline) water and ice densities of 1·03 gm cm −3 and
0·9 gm cm −3 respectively, it forms an intermittent surface layer averaging just 4·0 km
thick. By comparison, atmospheric density is only 0·00012 gm cm −3 at Earth's surface,
falling by two-thirds within 10 km aloft, with 75 per cent of its mass lying below this
altitude. Both spheres are residues of the lightest, most volatile outgassed elements of the
Hadean Earth retained by gravity. Low temperatures in our planetary boundary layer
determine that hydrogen, helium, nitrogen, oxygen, methane, carbon dioxide and some
trace elements are found primarily as gases, and the precise range of surface temperature
ensures that H 2 O appears commonly as gas, liquid or solid. The unstable nature of both
systems has resulted in the loss of up to 40 per cent of water mass and major changes in
atmospheric composition since formation, as the lightest elements were exhaled into
space. They continue to be sourced by volcanic outgassing and to evolve compositionally
through coupled material transfers driven by crustal recycling, geomorphological,
pedological and biological
PLANETARY MATERIAL AND ENERGY SYSTEMS
systems
The sun formed at the centre of a rotating nebula of planetary matter (Figure 1).
Condensing around small clusters of matter or planetesimals , solid terrestrial planets
(Earth, Mars, etc.) eventually formed in inner, hotter parts of the nebula and gaseous
planets (Jupiter, Neptune, etc.) formed in outer, cooler zones. This occurred through
fractionation or segregation of the elements composing our solar system into distinct
assemblages, determined by their physical properties, which we see throughout planetary
geology. Controlled by the thermal and pressure environment, dense refractory or heat-
resistant materials such as nickel, iron, silicates and calcium condensed at higher
temperatures nearer the sun and dominate terrestrial planets including Earth Less dense