Geoscience Reference
In-Depth Information
D
E
F
Rifting
A
B
Mid-ocean
B-subduction
A-subduction
B-subduction
O
al
Figure 10.2
The supercontinental or Wilson cycle: (a) supercontinent A begins to rift at B, causing B-subduction and contraction
in the global ocean C; (b) ocean B floods and expands as fragmentary continents A
1
and A
2
drift apart; (c) ocean C closes and
continents A
1
and A
2
eventually collide, with B subduction giving way to A-subduction.
Source: After Keary and Vine (1996)
Planetary material and energy systems
SYSTEMS
The sun formed at the centre of a rotating nebula of planetary matter (
Figure 10.3
).
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, more volatile elements such as nitrogen,
oxygen and carbon condensed at low temperatures farthest from the sun, forming outer planets rich in gas-liquid-ice.
The International Astronomical Union declassified Pluto as a planet in 2006 after the discovery of increasing numbers
of similar 'dwarf planets' orbiting beyond Neptune.
Our embryonic Earth gained kinetic and thermal energy through the accretion of mass and additional thermal energy
from crustal radioactivity, creating high temperatures at the core and raising surface temperatures briefly as high as
8,000
C. As a result, planetary materials segregated according to their chemical and physical character,
and Earth's internal structure is a microcosm of the solar system. Its core is surrounded by five concentric,
progressively cooler, less dense and more unstable layers or geospheres (
Figure 10.4
).
Paralleling the distribution of
elements in our solar system outwards from the sun, high-density stable refractory elements (Ni, Fe) survived in the
core. More volatile elements formed the mantle (Fe, Mg, Si,) and crust (Mg, Si, Ca, Na, K, C). The most volatile
elements (H, N, O, S) were driven off to form the ocean-atmosphere systems. Some condensed as fluids (H
2
O),
others formed gases (O
2
, N
2
, CO
2
, CH
4
, NH
3
, NO
2
, SO
2
), whilst some part of the lightest elements were exhaled to
space (H, He). Many of these more volatile elements may be stored as unstable compounds in Earth's crustal rocks.
They are exchanged with the atmosphere or hydrosphere through the acid/base oxidation/reductionprocesses referred
to in
Chapter 1.
Human activity, intentionally or inadvertently, often accelerates the rate and extent of these
processes, with increasingly detrimental environmental impacts and climate change.
C to 10,000
