Earth’s geological structure and processes - Fundamentals of the Physical Environment - page 199

Temperature of Primitive
Condensates/Fractionates (°C)

1000 - 1500

500 - 1000

250 - 500

<250

Terrestrial Planets

Mean distance = 6 w 10 9 km

Terrestrial Planets Jovian Planets

Low mass and high density

Low mass and high density High mass and low density

Figure 10.3 Earth and other planets of our solar system. Planetesimals condensed and concentrated in a rotating disc of
planetary matter around our sun, with early fractionation according to temperature zones within the disc. (Not to scale).

Pangaea c . 200 Ma ago, in the Mesozoic era of the
Phanerozoic aeon. Its global Panthalassic Ocean has been
replaced by the new, equatorially centred basins of the
Pacific, Atlantic and Indian Oceans, whilst its Tethys Sea
arm was closed as Africa converged with Eurasia. Modern
oceans are partially enclosed by Pangaea's fragmentation
into North and South America, Antarctica, Australia and
India and the emergence of South East Asia-Pacific Island
arcs. Our modern polar, landlocked Arctic micro-ocean
contrasts with the south polar Antarctic continent sur-
rounded by the Southern Ocean. We need to think of the
global map as mobile and dynamic, rather than fixed
in a position which we take for granted. Major topograph-
ical features which profoundly influence modern global
ocean and atmospheric circulation such the Panama
isthmus, linking North and South America, and the
Tibetan plateau are less than 3 Ma old. Closer inspec-
tion of Earth's crust reveals the global morphotectonic
landforms of current plate dynamics, clear evidence of
past rifts and collisions and the potential sites of future
ocean basins and mountain ranges. Plate tectonics provide
the framework for understanding the geological evolu-
tion of the crust. Its related supercontinental cycle and
rock cycle drive the formation, degradation and recycling
of rock material and create distinctive landform assemb-
lages. Most geographical references in the text refer to the
modern location and identity of crustal fragments, which
acquired their form and global position only recently.
The age of events in their geological history is indicated
where appropriate.

EARTH STRUCTURE AND INTERNAL
ENERGY

Core, mantle, crust, ocean, atmosphere
and biosphere

Plate tectonics perpetuates the geological distillation and
fractionation of planetary raw materials which began as
the planets condensed from interstellar gases and led to
the formation of Earth's six concentric geospheres ( Figure
10.4 ). The innermost core is formed by the separation of
a nickel-iron mixture from lighter silicon-rich material
and generates Earth's magnetic field. Its mean density of
10·7 gm cm -3 rises to almost 14 gm cm -3 at Earth's centre,
from which the core extends 3,460 km, concentrating
32·2 per cent of rock mass in just 16·9 per cent of planetary
volume. Seismic evidence described later (box, p. 209)
indicates that the inner core is solid for 1,300 km, with a
liquid outer core. Density falls sharply at its boundary
with the mantle , which extends for a further 2,970 km.
The mantle has a mean density of 4·5 gm cm -3 and is
composed of minerals transitional between the iron of the
core and lighter oxides of silicon and aluminium, 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,
overlain 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,