Principles of terrestrial ecosystem ecology.pdf

52 3. Geology and Soils
meters to cubic kilometers. The probability of a
mass-wasting event depends on the balance
between the driving forces for downslope
movement and the forces that resist this movement.
Gravity is the major driving force for
mass wasting.The gravitational force (or stress)
can be divided into two components: one parallel
to the slope, which drives mass wasting,
and one perpendicular to the slope, which
increases the friction between the material and
the bedrock (Fig. 3.5).The steeper the slope, the
greater the downhill component of the force
and therefore the greater the probability of
mass wasting.
Many factors influence the strength of a soil
mass (i.e., the amount of force required to
initiate slope failure) (Selby 1993). These
include the sliding friction between the
material and some well-defined plane and
the internal friction caused by the friction
among individual grains within the soil matrix.
In some cases, there is a well-defined plane
along which materials can slide, such as the
movement of soils over a frozen soil layer, but
commonly it is the internal friction that largely
determines the resistance to mass wasting.
Cohesion among soil particles and water molecules
enhances the internal friction that
resists mass wasting. A small amount of water
enhances cohesion among particles, explaining
why sand castles are easier to make with moist
than with dry sand. A high water content,
however, exerts pressure on the grains, making
them more buoyant and reducing the frictional
strength. They become unstable, leading to liquefaction
of the soil mass, which can flow
downslope. Fine-particle soils have lower slope
thresholds of instability and are more likely to
lead to slope failure than are coarse-textured
soils. Roots also increase the resistance of soils
to downslope movement, so deforestation and
other land use changes that reduce root
biomass increase the probability of landslides.
Mass wasting on soil-mantled, well-vegetated
gentle slopes occurs slowly through soil creep.
Displacement of surface soil particles by
freeze–thaw events or the movement of soils
brought to the surface by burrowing animals,
for example, is likely to cause a net downslope
movement of soil. These small-scale pro-
Fn
Fn
Ft
cesses contribute to erosion rates of 0.1mm
yr -1 or less.
The pathways by which water leaves the
landscape strongly influence erosion. Water
typically leaves a landscape by one of several
pathways: groundwater flow, shallow subsurface
flow, or overland flow (when precipitation
rate exceeds infiltration rate) (see Fig. 14.6).
The relative importance of these pathways is
strongly influenced by topography, vegetation,
and material properties such as the hydraulic
Ft
F p
Fp
Figure 3.5. Effect of slope on the partitioning of the
total gravitational force (F t) into a component that
is normal to the slope (F n)—and therefore contributes
to friction that resists erosion—and a component
that is parallel to the slope (F p)—and
therefore promotes erosion. Steep slopes have a
larger F p value and therefore a greater tendency to
erode.