Geology Reference
In-Depth Information
Box 15.4
PHILLIPS'S 11 'PRINCIPLES OF EARTH SURFACE SYSTEMS' APPLIED TO
GEOMORPHIC SYSTEMS
These principles are not easy to appreciate fully with-
out a background in non-linear dynamical theory (see
Stewart 1997 for an excellent and accessible introduc-
tion to the subject). However, the tenderfoot geomor-
phologist should be able to grasp the general thinking
involved. It may help to define a few terms first. An
unstable system
is susceptible to small perturbations
and is potentially chaotic. A
chaotic system
behaves in
a complex and pseudo-random manner purely because
of the way the system components are interrelated, and
not because of forcing by external disturbances, or at
least independently of those external factors. The chaos
is not generated by chance-like (stochastic) events but is
determined by the equations describing the system and
is said to be deterministic. Systems displaying chaotic
behaviour through time usually display spatial chaos,
too. So a landscape that starts with a few small pertur-
bations here and there, if subject to chaotic evolution,
displays increasing spatial variability as the perturba-
tions grow.
Self-organization
is the tendency of, for
example, flat or irregular beds of sand on streambeds
or in deserts to organize themselves into regular spaced
forms - ripples and dunes - that are rather simi-
lar in size and shape. Self-organization is also seen
in patterned ground, beach cusps, and river channel
networks.
Self-destruction
(
non-self-organization
)is
the tendency of some systems to consume themselves,
as when relief is reduced to a plain.
Now, here are Phillips's principles:
2
Geomorphic systems are inherently orderly. An
'attractor' that constrains the possible states of the
system may govern deterministic chaos in a geo-
morphic system. Such a geomorphic system does
not behave randomly. In addition, dynamic insta-
bility is bounded. Beyond these bounds, orderly
patterns emerge that contain the chaotic patterns
inside them. Thus, even a chaotic system must
exhibit order at certain scales or under certain
circumstances. At local scales, for instance, soil
formation is sometimes chaotic, with wild spa-
tial variations in soil properties, but as the scale
is increased regular soil-landscape relationships
emerge. In like manner, chaotic turbulence in flu-
ids does not prevent water from flowing downhill
or wind from blowing according to pressure gradi-
ents; and, further, does not prevent scientists from
predicting the aggregate flows of water or air.
3
Order and complexity are emergent properties
of geomorphic systems. This principle means
that orderly, regular, stable, non-chaotic patterns
and behaviours and irregular, unstable, and chaotic
behaviours appear and disappear as the spatial or
temporal scale is altered. In debris flows, collisions
between particles where the flow is highly sheared
are governed by deterministic chaos and sensitive to
initial conditions and unpredictable. However, the
bulk behaviour of granular flows is orderly and pre-
dictable from a relationship between kinetic energy
(drop height) and travel length. So, the behaviour
of a couple of particles is perfectly predictable using
basic physics. A collection of particles interacting
with each other is chaotic. The aggregate behaviour
of the flow at a still broader scale is again pre-
dictable. Accordingly, in moving up or down the
hierarchy from a few particles to many particles to
the aggregate behaviour of particles, order and pre-
dictability or complexity and unpredictability may
appear or disappear.
1
Geomorphic systems are inherently unstable,
chaotic, and self-organizing. This behaviour is seen
in the tendency of many, but emphatically not all,
geomorphic systems to diverge or to become more
differentiated through time in some places and
at some times. This happens, for example, when
rivers dissect a landscape, so increasing relief, or
when an initially uniform mass of weathered rock
or sediment develops distinct horizons.
