Geology Reference
In-Depth Information
Box 1.2
NEGATIVE AND POSITIVE FEEDBACK
Negative feedback
is said to occur when a change
in a system sets in motion a sequence of changes
that eventually neutralize the effects of the origi-
nal change, so stabilizing the system. An example
occurs in a drainage basin system, where increased
channel erosion leads to a steepening of valley-
side slopes, which accelerates slope erosion, which
increases stream bed-load, which reduces channel
erosion (Figure 1.9a). The reduced channel erosion
then stimulates a sequence of events that stabilizes
the system and counteracts the effects of the orig-
inal change. Some geomorphic systems also display
positive feedback
relationships characterized by an
original change being magnified and the system being
made unstable. An example is an eroding hillslope
where the slope erosion causes a reduction in infil-
tration capacity of water, which increases the amount
of surface runoff, which promotes even more slope
erosion (Figure 1.9b). In short, a 'vicious circle' is cre-
ated, and the system, being unstabilized, continues
changing.
Figure 1.9
Feedback relationships in geomorphic
systems. (a) Negative feedback in a valley-side slope-
stream system. (b) Positive feedback in an eroding
hillslope system. Details of the relationships are given
in the text.
system is Uranus and its moons. These structures
may be thought of as simple systems. In geomor-
phology, a few boulders resting on a talus slope
may be thought of as a simple system. The condi-
tions needed to dislodge the boulders, and their fate
after dislodgement, can be predicted from mechan-
ical laws involving forces, resistances, and equations
of motion, in much the same way that the motion of
the planets around the Sun can be predicted from
Newtonian laws.
2Ina
complex but disorganized system
, a vast num-
ber of objects are seen to interact in a weak and
haphazard way. An example is a gas in a jar. This
system might comprise upward of 10
23
molecules
colliding with each other. In the same way, the count-
less individual particles in a hillslope mantle could be
regarded as a complex but rather disorganized system.
In both the gas and the hillslope mantle, the interac-
tions are somewhat haphazard and far too numerous
to study individually, so aggregate measures must
be employed (see Huggett 1985, 74-7; Scheidegger
1991, 251-8).
3
In a third and later conception of systems, objects
are seen to interact strongly with one another to
form
systems of a complex and organized nature
.
Most biological and ecological systems are of this
kind. Many structures in geomorphology display
high degrees of regularity and rich connections, and
may be thought of as complexly organized systems.
A hillslope represented as a process-form system
could be placed into this category. Other examples
include soils, rivers, and beaches.
