Geology Reference
In-Depth Information
Box 1.3
THRESHOLDS
A threshold separates different states of a system. It
marks some kind of transition in the behaviour, oper-
ation, or state of a system. Everyday examples abound.
Water in a boiling kettle crosses a temperature thresh-
old in changing from a liquid to a gas. Similarly, ice
taken out of a refrigerator and placed upon a table
in a room with an air temperature of 10
◦
C will melt
because a temperature threshold has been crossed. In
both examples, the huge differences in state - liquid
water to water vapour, and solid water to liquid water -
may result from tiny changes of temperature. Many
geomorphic processes operate only after the crossing
of a threshold. Landslides, for instance, require a criti-
cal slope angle, all other factors being constant, before
they occur. Stanley A. Schumm (1979) made a power-
ful distinction between
external
and
internal system
thresholds
. A geomorphic system will not cross an
external threshold unless it is forced to do so by a
change in an external variable. A prime example is the
response of a geomorphic system to climatic change.
Climate is the external variable. If, say, runoff were
to increase beyond a critical level, then the geomor-
phic system might suddenly respond by reorganizing
itself into a new state. No change in an external vari-
able is required for a geomorphic system to cross an
internal threshold. Rather, some chance fluctuation
in an internal variable within a geomorphic system
may take a system across an internal threshold and
lead to its reorganization. This appears to happen
in some river channels where an initial disturbance
by, say, overgrazing in the river catchment triggers
a complex response in the river channel: a compli-
cated pattern of erosion and deposition occurs with
phases of alluviation and downcutting taking place
concurrently in different parts of the channel system
(see below).
change is seen as a simple response to an altered input.
It shows that landscape dynamics may involve abrupt
and discontinuous behaviour involving flips between
quasi-stable states as system thresholds are crossed.
The latest views on landscape stability (or lack of it)
come from the field of
dynamic systems theory
,
which embraces the buzzwords
complexity
and
chaos
.
The argument runs that steady states in the landscape
may be rare because landscapes are inherently unsta-
ble. This is because any process that reinforces itself
keeps the system changing through a positive feed-
back circuit and readily disrupts any balance obtain-
ing in a steady state. This idea is formalized as an
'instability principle', which recognizes that, in many
landscapes, accidental deviations from a 'balanced' con-
dition tend to be self-reinforcing (Scheidegger 1983).
This explains why cirques tend to grow, sinkholes
increase in size, and longitudinal mountain valley profiles
become stepped. The intrinsic instability of landscapes
is borne out by mathematical analyses that point to the
environmental changes or random internal fluctuations
that cause the crossing of internal
thresholds
(Box 1.3),
a landscape will respond in a complex manner (Schumm
1979). A stream, for instance, if it should be forced away
from a steady state, will adjust to the change. However,
the nature of the adjustment may vary in different parts
of the stream and at different times. Douglas Creek in
western Colorado, USA, was subject to overgrazing dur-
ing the 'cowboy era' (Womack and Schumm 1977). It
has been cutting into its channel bed since about 1882.
The manner of incision has been complex, with discon-
tinuous episodes of downcutting interrupted by phases
of deposition, and with the erosion-deposition sequence
varying from one cross-section to another. Trees have
been used to date terraces at several locations. The ter-
races are unpaired (p. 236), which is not what would
be expected from a classic case of river incision, and
they are discontinuous in a downstream direction. This
kind of study serves to dispel for ever the simplistic
cause-and-effect view of landscape evolution in which
