Geoscience Reference
In-Depth Information
INTRODUCTION
Beneath the tenuous atmosphere lies the geo-
sphere - Earth's most complex and largest con-
stituent mass by far, with a rock volume of 1·083
Earth's crust and surface provide the principal
key to understanding Earth history. Ours is the
only mature continental crust among the inner ter-
restrial planets. Its prototype formed within the
first 100 Ma and a 'stable' crust probably existed
by 3.8 Ga. However, enormous sensible and poten-
tial heat sources inherited from Earth's formative
processes drive persistent convection currents
towards the crust, where constant break-up and
recycling of outer Earth materials act as the
planet's radiator. Less than a century ago Alfred
Wegener co-ordinated the first coherent explana-
tion in the theory of continental drift and less than
fifty years ago sea-floor spreadingwas recognized
as the principal mechanism driving plate tectonic
motion in the crust. This revolutionised Earth sci-
ences, paving the way for the Earth Systems
Science paradigm, provided the unifying mecha-
nism for the geosphere and much besides, and is
a constant thread throughout Part Three.
Even so, to what extent can we cover 4.6 Ga of
Earth history (more obscure as we travel further
back in time) in the short span of this topic? How
much directly relates to human life? We attempt
this daunting task through four themes in Part
Three. First, that although plate tectonics has
operated over a number of long-term cycles,
the most recent 550 Ma is widely regarded as
an acceptable analogue for the previous 3.0 Ga
(
Chapter 10).
Second, tectonic processes 'rough
out' the shape and location of oceans and conti-
nents (
Chapters 10-
12)
. The latter are then etched
and polished by geomorphological, pedalogical
and biological processes at smaller 10
1-6
yr time
and spatial scales, forming the hills, valleys, rivers
and coastlines of familiar landscapes
(Chapters
character and operation of Earth's climate, oceans,
geomorphic processes and biosphere, developed
through a number of 'Boxes'. Fourth, the Quater-
nary period of just 2.6 Ma duration so far, contains
a wealth of evidence of these interactions (illus-
trated throughout Part Three and in
Chapters 23
and
28)
- and happens to coincide with human
evolution and impact at geological time scales we
can more readily understand.
10
12
km
3
and mass of 5·977
10
24
kg compared
with 1·4
10
9
km
3
and 1·40
10
21
kg of global
water and 5·13
10
18
kg of atmospheric gases.
With its outer surface crust bathed by the hydro-
sphere and supporting the biosphere, the
geosphere has undergone continuous transforma-
tion since it condensed from a collapsing cloud of
interstellar gas and dust 4·6 Ga (4·6 billion years)
ago. Earth is relatively small compared with giant
outer planets, just 40,000 km in circumference, as
highlighted by passenger jet capability of full cir-
cumnavigation in forty-eight hours and orbiting
satellites in ninety minutes. The average radius of
6,371 km from Earth's surface to the centre equals
the distance between London and Chicago. Yet the
mass, character and age of Earth's rocks can be
hard to comprehend, along with its origins in
astrophysical processes which formed our solar
system, through gravity concentration of matter
from a supernova explosion c. 6 Ga ago.
What, then, is the interest of geographers, Earth
scientists and environmental scientists in plane-
tary processes with remote origins, astronomic
time scales, tiny geothermal energy flows com-
pared with solar irradiation of the atmosphere and
vast but almost entirely concealed material
reserves? How far do they influence human lives
and habitat at Earth's surface - the only part of
this great mass we can study in any detail? We start
with some broad assertions, developed and justi-
fied in later chapters. Earth's atmosphere, oceans
and eventually its biosphere evolved from, and are
surviving portions of, volatile accretion compo-
nents of early Earth. Earth's crust continues to
actively exchange materials with those spheres and
the deep Earth, fuelled by solar and geothermal
energy. They provide the reservoirs and fluxes for
integrated, biogeochemical cycles constituting the
Earth system, which human activity taps to sustain
and enhance human life and disturbs to its detri-
ment. These attributes and processes are heavily
implicated in global climate and environmental
change. All we need is to identify a central, unify-
ing driving mechanism for all these processes.
