Page 80 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 63

Shrinkage limit Plastic limit Liquid limit Liquid Mass movements

Plastic Mass movements may be classified in many ways.
solid
Elastic Table 3.2 summarizes a scheme recognizing six basic
solid
Brittle types and several subtypes, according to the chief mech-
solid
anisms involved (creep, flow, slide, heave, fall, and
volume AIR WATER subsidence) and the water content of the moving body
(very low, low, moderate, high, very high, and extremely
Sample high):

1 Rock creep and continuous creep are the very slow
plastic deformation of soil or rock. They result from
SOLIDS stress applied by the weight of the soil or rock body
SOLIDS
and usually occur at depth, below the weathered
mantle.They should not be confused with soil creep,
which is a form of heave (see below).
0 100
Moisture content (per cent) 2 Flow involves shear through the soil, rock, or snow
and ice debris. The rate of flow is slow at the
Figure 3.5 The composition of soil, ranging from air-filled base of the flowing body and increases towards the
pores, to water-filled pores, to a liquid. The Atterberg surface. Most movement occurs as turbulent motion.
or soil limits are shown. Flows are classed as avalanches (the rapid down-
Source: Adapted from Selby (1982, 76) slope movement of earth, rock, ice, or snow), debris
flows, earthflows,or mudflows, according to the
predominant materials – snow and ice, rock debris,
moisture content, the soil becomes a suspension of par- sandy material, or clay. Dry flows may also occur;
ticles in water and will flow under its own weight. The water and ice flow. Solifluction and gelifluction –
three limits separating different kinds of soil behaviour – the downslope movement of saturated soil, the lat-
shrinkage limit, plastic limit, and fluid limit – are known ter over permanently frozen subsoil – are the slowest
as Atterberg limits, after the Swedish soil scientist flows. A debris flow is a fast-moving body of sedi-
who first investigated them (Figure 3.5). The plastic- ment particles with water or air or both that often has
ity index, defined as the liquid limit minus the plastic the consistency of wet cement. Debris flows occur as
limit, is an important indicator of potential slope insta- a series of surges lasting from a few seconds to sev-
bility. It shows the moisture range over which a soil will eral hours that move at 1 to 20 m/s. They may flow
behave as a plastic. The higher the index, the less stable several kilometres beyond their source areas (Figure
the slope. 3.6a).Somearepowerfulenoughtodestroybuildings
Some soils, which are referred to as quick clays or and snap off trees that lie in their path. Mudflows
sensitive soils, have a honeycomb structure that allows triggered by water saturating the debris on the sides of
water content to go above the liquid limit. If such soils are volcanoes are called lahars. When Mount St Helens,
subject to high shear stresses, perhaps owing to an earth- USA, exploded on 18 May 1980 a huge debris
quake or to burial, they may suddenly collapse, squeezing avalanche mobilized a huge body of sediment into
out water and turning the soil into a fluid. Quick clays a remarkable lahar that ran 60 km from the volcano
are commonly associated with large and swift flows of down the north and south forks of the Toutle River,
slope materials. A violent shaking, as given by a seismic damaging 300 km of road and 48 road bridges in the
shock, may also liquefy a saturated mass of sand. process.