Planet Earth and Earth systems - Physical Processes in Earth and Environmental Sciences - page 3

subparts of the whole. Layering might indicate active plate
tectonics. Signs of tectonics and earthquake activity would
come from surface zones of active faulting or folding, and
any volcanic activity from eruptive clouds or surface traces
of recent volcanic flows. Naturally, as Earthlings, we are
curious to know whether there is water around and we
would thus be looking for signs of erosion by flowing
water, ice, or snow formation and movement. Also,
whether the atmosphere contains any oxygen from photo-
synthetic processes. In addition to all these properties and
current processes, conscious of the vast span of time the
planetary systems have existed, we wish to know some-
thing of the history of the planet. We would try to “read
the rocks” as geologists do on Earth in order to see
whether the current planetary state has evolved over time.

The summaries given in Fig. 1.1 come from the results

of 4,000 years of study: from astronomy, astrophysical

and astrochemical analysis, satellite remote sensing, remote
sampling from sondes and probes, physical sampling from
remote landers. Recent spectacular discoveries (2003-5)
concerning the undoubted evidence for previous Martian
surface water flow, permafrost, and perennial polar ice caps
simply serve to make us humble in the face of ignorance
concerning the nature of our own Solar System. With this
in mind, we now turn to the physical nature of planet
Earth, again bearing in mind the oft-quoted fact that we
know more about the nature of the planetary surface of
Venus (from systematic satellite-based radar data) than we
do about the motions, equilibrium, and interactions of our
own oceans.

1.2 Unique Earth

We generalize here pointing out the major features of Earth
that combine to make it unique within the Solar System:
1 Solid Earth is multilayered , the various layers (Section 1.5)
fractionated according to chemical composition and physical
properties. The fundamental subdivisions into crust, mantle,
and core probably date from quite early in planetary history.
Of interest here is how the layers have preserved their identity
in the face of deep mixing during the operation of the plate
tectonic cycle over the past 3 Gy (Fig. 1.2).
2 Although the three states (phases) of matter, gaseous,
liquid, and solid, dominate in the atmosphere, oceans, and

solid Earth, respectively, there are mixtures of phases
everywhere. These mixtures are made at planetary layer
interfaces (Section 1.5) and their reactions are of funda-
mental importance in the workings of the Earth system.
We note dust particles and raindrop nuclei in the atmos-
phere, sedimentary particles in water, and gas volatiles in
magma and lava. Most Earth layers are thus more or less
multiphase . For this reason, they present special problems
in terms of investigating physical processes.
3 Many adjacent Earth layers move relative to each other.
Their interfaces are thus prone to mixing due to processes
like diffusion (Section 4.18) and bulk shearing caused by
relative motions. An important feature for Earth evolution
is how rapidly the different layers communicate and inter-
mix, for example, the reactions of the ocean to changes in
mean atmospheric temperature due to warming or cooling
(Fig. 1.3), tropical storm reaction to changes in ocean sur-
face temperature, and the physical and chemical effect of
descending cool lithospheric plates on the deep mantle.
4 A consequence of the tendency of layers to intermix is
that they must have evolved to some steady state with
time, or be still evolving.
5 Earth's atmospheric oxygen is a by-product of photo-
synthesis. Oxygen levels evolved rapidly about 2.5 Ga from
previously very low levels. Oxygen nurtures animal life but
at the same time is hostile to the many mineral phases of
rocky Earth that crystallized under anoxygenic conditions.
6 Earth has abundant surface water, stratified oceans, and
a thoroughgoing hydrological cycle that encompasses
abundant near-surface and surface life forms.
7 Earth's solid surface is dominated by horizontal and
vertical motions associated with the motion of external

Core

Core

Fig. 1.2 Global cycling of rock, sediment, and water by plate
tectonics. Here, lithospheric plates “sink” into the lower mantle
during numerical “experiments.”