Geology Reference
In-Depth Information
the continuous transition from dry moving material to
muddy water.
the force makes the agency move (water for instance) and
in turn the moving agency exerts a force on the sediment
and tends to move it, as in sediment transport in rivers.
The chief forces that act upon geomorphic materials are
gravitational forces, fluid forces, water pressure forces,
expansion forces, global fluid movements, and biological
forces.
Transport mechanics
Geomorphic forces
The transport of all materials, from solid parti-
cles to dissolved ions, needs a force to start and
maintain motion. Such forces make boulders fall from
cliffs, soils and sediment move hillslopes, and water and
ice flow along channels. For this reason, the mechanical
principles controlling movement underpin the under-
standing of transport processes (Box 3.3).
The forces that drive sediment movement largely
derive from gravity, from climatic effects (heating and
cooling, freezing and thawing, winds), and from the
action of animals and plants. They may act directly, as in
the case of gravity, or indirectly through such agencies as
water and wind. In the first case, the force makes the sed-
iment move, as in landslides; while, in the second case,
1
Gravitational forces
. Gravity is the largest force for
driving geomorphic processes. It acts directly on bod-
ies of rock, sediment, water, and ice, tending to make
them move. Moreover, it acts the world over at a
nearly uniform magnitude of 9.81 metres per second
per second (m/s
2
), with slight variations resulting
from distance from the Earth's centre and latitude.
2
Fluid forces
. Water flows over sloping land surfaces.
It does so as a subdivided or uniform sheet or as
channel flows in streams and rivers. Water is a fluid
so that it moves in the direction of any force that
is applied to it, and no critical force is necessary.
So water flows downhill under the influence of its
Box 3.3
FORCE AND RESISTANCE
A body will not move unless a force is applied, and
its movement will not continue without the sustained
exertion of a force. Likewise, forces act on a body at
rest that are in balance while the body remains station-
ary. For this reason, forces are immensely important in
determining if the transport of sediments takes place.
A
force
is an action in a specified direction that
tends to alter the state of motion of a body. An equal
and opposite force called the reaction always balances
it. A boulder resting on the ground exerts a vertical
force on the ground due to its weight; the ground
exerts a force of the same magnitude in the opposite
direction on the boulder; and, if it did not do so, the
boulder would sink into the ground. Forces result from
the acceleration of a body. If a body is not subject to an
acceleration, then it cannot exert a force in any direc-
tion. At the Earth's surface, most bodies are subject to
the acceleration due to gravity and exert a force in the
direction of gravity, which is approximately vertically.
The magnitude of this force is generally the weight
of the body in a static condition (but, if the body is
moving, the force alters).
Forces have direction and magnitude. If two or
more forces are acting on a body, then the magnitude
and direction of a resultant force is determinable. For
example, a sediment grain entrained in flowing water is
subject to several forces: a vertical force pushing it ver-
tically upwards in the flow, the force of its own weight
dragging it down vertically, and the downstream force
of the flowing water carrying it along the river channel.
The magnitude and direction of all these forces dictate
the net direction in which the grain will travel and so
whether it will stay suspended or sink to the riverbed. If
a single force is known, its effects in different directions
