Earth’s geological structure and processes - Fundamentals of the Physical Environment - page 200

Hydrosphere

Oceanic Crust

Atmosphere
Biosphere

Continental Crust
Lithosphere
Asthenosphere

Mesosphere

Mesosphere

Outer
Core

Mantle

Closing
ocean

Continental
Sutre

Inner Core

Mid-ocean
ridge

Triple junction

Transform
fault

lo o

o

d

Figure 10.4 Earth's internal structure and the relationship between outer spheres and crustal processes.

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 processes. The biosphere itself