Slope processes and forms 167
Table 7.3 Atteřb€rg limits for various clay mineťals
SURFACE AREA PLASTIC LIMIT
(m' kgJ) (%,J
SHR|NKAGE LIMIT VOLUME CHANOE
(c.\
LIQUID LIMIT
(qa)
smectite 800 000
Il|ite 80 txx)
Kaolinite 15 000
50_100
35 60
25 4I)
100-900
60_120
30-110
8,5_15
l5_17
3-15
High
Medium
Low
solř.": Modified from M.J. selby (1982), Hitlslope Materials and Processď. oxford University Řess,
oxíord. Table 4.8, p.79; based on data in R. E. G:.|n,1968,Clay Mineralo8}. Mccraw-Hill, New Yoík,
watel content on soil behaviour. They are expressed in
terms of the weight of contained water as a percentage of
the weight of dry soil.
When subject to the same stress different soils with an
identical moistule content may fail by brittle fIacture. deform
plastically or behave as a viscous fluid. This is
because of the varying abilities of different clay minelals to
absolb water (Table 7.3). Smectite geneIally has the highest
plaslic and liquid limits b€cause its cryslal srruclure enables
it to provide an enormous surface area for wateT absorPtion.
Consequently, smectite-dch soils swell and shrint significantly
when they ale wetted and dried. Kaolinite, by contlast,
has a much more limited Water absorption capacity.
An important parameter derived from Atterber8 limits is
the pIasticity index. This is defined as the liquid limit
minus the plastic limit and indicates the range of moisfuIe
content over which a soil exhibits plastic behaviour. It is an
important indicatol of the potential instability of the soil
since the higher the plasticity index the less stable slope
materials will be.
Two further aspects of soil ploperties need to be mentioned
briefly. Certain soil§ have an open'honeycomb'
structure which allows them to retain Water at píoportions
in excess of the liquid limit. The structure of these soils.
termed sensitiv€ soils, is potentially unstable. If they are
subject to high shear stresses (§uch as those induced by an
earthquake), or high compressive stresses (such as those
arising from loading by burial), they can collapse catastlophically
as the water is squeezed out and the soil becomes a
fluid. Such soils are sometimes called quick clays and are
often associated with major and rapid flows of slope
matedals.
sands can also act in a fashion similar to sensitive soils
under certain conditions. In a saturated mass of sand most
of the stlength arises from point to point contacts between
the solid sand gIains. If the sand is shaken violently, by a
seismic shock for insíance, all the effective stresses can be
transferred fŤom grain-to-gíain contacís to the pole water,
an effect known as |iquefaction. All the stlength from
interpaíicle friction is thereby lost, the sand mass consequently
has no re§istance to sheal shess and liquid
deformation occurs-
7.2 Mass movement
Mass movemení is lhe downslope movemenl of slope
material under the influence of the gravitational force of the
matelial itself and without the assistance of moving wateI,
ice or air. The distinction between mass movement and the
tlansPort of material by othel denudational processes is,
however, not always cleaí-cut in practice since mass
mov€ment§ involving material with a high watel content
grade into fluvial tíansport where streams carry very large
loads of fine sediment. In a very real sense glacier flow
itself is a form of mass movement, involYing as it does the
downslope movement of coherent masses of ice; nevertheless,
glaciers have particular characteíistics which merit
special consideration and they are examined separately in
the context of glacial landscapes (see Chapter 1l). Very
large-scale movement§ of rock which are tlansitional to
tectonic processes can also occur under gravity; we discuss
these bdefly in Section 7.3. Thd term mlss wasting i§ often
regarded as synonymous with mass movement. but it is also
used in a broader sense to encompass all processes involved
in the lowering of the landscape. Before we consider the
vadous mechanisms of mass movement we will look
briefly at the conditions which give rise to them.
7.2.1 Slop€ stability
The stability of a slope can be expressed in terms of the
relationship between those stlesses tending to disturb the
slope material and cause it to move and those forces tending
to lesist these drividg strcsse§. Clearly, movement will
occur where driving forces exceed re§isting forces and this
relationship is repre§ented a§ the safety factor for a slope.
This is expressed as the ratio between shear §trength and
shear shess (Box 7.2).
Slopes can exist in one of thlee states. Whele shear
strength is significantly larger than shear stress the slope is
described as stabl€ (§afety factor > 1.3). Where shear stress
exceeds shear strength (safety factor < 1) there will be
continuous or intermittent movement and the slope is described
as activ€ly unstable. Since shear strength can vary
over time, especially in response to changes in the water
content of slope materials, the third stability categoly is the
16E Exogenic proc€ss€s and landforms
Box7,2 Safety factor for a slope
The salety factor (F) is defined as
J
F=_
1
whele s is the total §hear strength along a specific shear
plane, and t the total amount of sheaí stress developed along
this plane. For shallow, translational slides F is defined as
óu cos2 p
- ,) tan o
1z sin P cos P
where . is cohesion, "í the unit weight of regolith, z the vertical
depth to the shear plane, p the angle of the shear plane,
the pore-water pressufe at the shear plane and (D the angle
of intemal fúction,
conditionally stable slope which has a safety factor of
1-1.3 and fails on occasion in response to transient changes
in shear strength. Numerous factols contribute to the
occuíTence of mass movements, and these are listed in
TabIe 7.4, They can be categorized as either pleparatory
factors or triggering factors. Preparatory factors make the
slope susceptible to movement without 3ctually initiating
failure by transforming it into a conditionally Stable state.
Triggering factols tlansform the slope from a conditionally
stable to an actively unstable state.
Although the slope §tability approach to analyzing mass
movements provides a good theoretical understanding of
Table ?.4 Factors contributing to the occurrence ofmass movernent
the factols which promote movement, it has limited
applicability to specific situations. This is because both
cohesion and pore-water pIessure are highly variable on
most natural slopes, even ovel short distances and brief
periods of time. FoI instance, fissures may tíaverse slope
material§ leading to drastic variations in pore-Water pressule
from place to place.
7.2.2 Mass movement proces§es
There have been numerous attempts to classify the divelse
modes of mass movement, none of them universally
satisfactory. Here we identify six fundamental types of
movement - cleep, flow, slide, heave, fall and subsidence.
Each of these can be subdivided into moIe specil]c forms of
mas§ movement (Table 7.5). Classifications of the various
processes of mass movement are valuable in indicating the
range of mechanisms and forms of motion, but it must be
appreciated that most movements in reality involve a
combination of processes. Debris avalanches, for example,
may begin as slides consisting of large masses of rock but
then rapidly break up to form flows a§ the material is
pulverized in tíansit. The compound natule of many forms
of mass movement is illustrated in Figure 7,4, which also
indicates how the different types of movement vary in theil
moistule content and velocity. Flows tend to be wet and
slides dry, while heave can occur ovel a faiIly bToad lange
of moisture conditions. Heave processes are invariably
slow, whereas both flows and slides tend to be lapid.
7.2.2.1 Creep
Creep is the Slow, plastic deformation of rock or soil in
Fig.7.4 Classifcatíon oímass moýements in terms of pure flow,
slíde and heaýe. Note lhe compound nature of some moýements.
(Aíter M. A. Carson and M. J. Kirkby ( 1972) Hillslope Form and
Ptocess. Cambridge Uniýersit, Prcss, Cambrídge, Fig. 5,2,
p. 100.)
Faclols 0 tl,ibuling to ínťreasedsheař slress
Remova| of larcíal
support thíough
undercutling oí slope
sleepening
Removal of underlying
support
Loading of slope
Latera] pressuíe
Weathering €ffects
Changes in pore-wat€r
pí€ssure
Changes of §tructure
oíganic eff€cts
Erosion by rivers and 8laciers, wave action,
faulting, previous rock falls or slides
UndeŤcutting by rivers and waves, subsurface
solution.loss of ýŤength by exrusion of
undeílying sedimenls
wei8ht of water, vegetation, accumulalion of
debris
Water in cracks, freezing in cracks, swelling
(especially throu8h hydration of clays).
pressuíe íelease
Eaíthquake§, movement of trees in wind
Faclors contíibutinq lo redued sheal suenslh
Disintegration of granular íocks, hydration
of clay minera]s, dissolution ofcementing
minerals in rock oí soi]
saturation. softening of material
Creation offissures in shale§ and clays.
remoulding oi sand and s€nsitive clay§
Burrowing ofanimats, decay oftree řoots
Slope processes and forms 1ó9
Table 7.5 Cla§sifrcation and characteristics of the majoí t}pes of mass movement
Jo'iP: Based lalgelv on D, J, varne\lto/8l.in, R. L, sch_usteí and R.J, í*rizek leds, Laadstlde Analy\I\ ond conlrcl. rransDonarionResealch Board Special Reporl l7ó, \alional a*a..y .r š.li*.i w""r,l"gil". óř. ii-.l:
MAss MoVEMENT MATERIAL§ lN
TYPE MOTION
TYPE OF STRAIN ÁND
NATURE OF MOVEMENT
Rock (e§peciall], íeadily
delormábl€ lÝDes such as
shales and clay'
slo$ plasLic defoímalion of very slos to
rocK. or soll producln8 a exLreíne]y s]os
tanely ol lórms includinp cam_
berin8, válle} bulging anď our_
cíop bedding curvalúre
Dry now
solifluclion
celiíluction
Mud ílow
slow earthllow
Rapid earthflow
Debris now
D€bris (rock)
(sturzslíom)
slush avalanche
High
Hich
Exrremely high
Very high
Hi8h
Slow
slow
very rapid
very íapid
Extremely rapid
sand or siit
soil
soil
>80% clay_§ized
>80% sand_§ized
soil conlaining
sensilive clays
Mixlure of frne and
coařse debíis í20-80% of
particle§ coarsěr than
sand-sized)
Rock debris, in som€
snow and ic€, in some
cases wilh íock debřis
waler_salu.ated snow
FunDeIled flow down steeD
slopes of non-cohe§ive
widespíead flow of §aturated
soil oÝer low to moderate
angle slope§
widespread flow of sea§onallv
saluraled soil over Deímanen v
frozen subsoil
confined elongaled ílow
Conlined elongated flow
Rapid collapse and ]areíal
spreading of soil following
disturbance. oft€n bv an inifi,l
slide
Flow usuallv focused intó
píe_exi§tin8-díainag€ lines
catastrophic low friction
movement of up to s€veral kilo_
metres, U§ually precipitated bv a
majoí _rock lall and capable of
overrlolng st8nlhcaDt lopo_
Catastřophic low fíiction
molemenl precipitaled by fal] or
FIow along eris!ing drainage
extremely rapid
Modeřate
rapid
s]ow
Rock slide
Rock block slide
Debris/eaíth slide
Debíis/earth block
sIide
unfraclured íock mass
Rock debris or soil
Rock debřis oí§oil
shallow ďide aDoroximatel!
paralJel to grouira surface Óf
coherenl rock mass alonp
sing]e fracture
slide approximately paralle] lo
ground suďace of fractured rn.L
shallow slide of deformed
masse§ of soil
shal]ow slide of laípelv
uDdelormed masses'of soil
Rock slump Rock
Debris/earth dump Rock debfis or §oil
Rotational movement alon8 Extremely §low to
concave failure plane morterate'
Rotationa| molemeni a|on8 slow
concave la||ure D]ane
soil
Rock debris
widespread incremental ,_ E\treme\ slow
downs|ope movemenl of soil or
Rock fal]
Debřis/earlh
(loppl€)
Fall of,individual blocks from F\tremel} ídpld
!ertrcál lá.e(
Toppling ol cohesite unlls or Very íapld
§oll lrom neaFvert|cal láces súch
Detached tock joint
Delached cohesive units
of §oil
Collapse of rock o, soil inlo Ven raDld
underground cavities such
limeslóne cáves or ]ava lubes
Loweíing of surface due to slow
8round comDaclion usuállv
resulring from wirhdíaýa1-of
170 Exogenic processes and landforms
Iesponse to stless generated by the weight of oveTbulden. It
begins once the yield stress of the slope material is exceerled_
In rock it can extend to hundreds of metres below
the surface. It occurs at very slow rates, typically
l mm to l0 m a-l and is likely to be especially active where
weakly competent materials, such as clays, are overlain by
more competent beds. Cleep is often a precursor of slidetype
movements, but it can also cause specific observable
effects. The bending of the lower parts of tíee trunks iS
offen cited as evidence of creep, but this phenomenon can
also be caused by other mechanisms. More substantial
evidence of creep is provided by the downslope curvature
of strata near the suďace. Another consequenc€ of creep is
cambering which involves the extrusion of weak rocks
(usually clays), either lying below, or interbedded with,
mole rigid strata which causes valley sides to bulge.
lt is imponant ro distinguish the type of creep described
here from soil creep and talus creep. The former acts solely
under gravity, whereas the lattel involve heave and are consequently
considereó h Sectiol 7.2.2.4.
7,2 -2 -2 Flow
In a pure flow, shear occurs throughout the moving mass of
material and there is no well-defined shear plane (Fig.
7.5(A)). Flow is distinguished from creep by having discrete
boundaries or narow peripheral zones experiencing
shear. shear is at a maximum at the base of the flow. but
here the late of flow is relatively slow and nearly all the
movement occul§ a§ tulbulent motion within the body of
the flowing mass. \ ílow§ can occur, but abundant watgr is
usually present. They are often initiated by falls or slides,
becoming flows when the moving soil or rock mass breaks
up. Flows are categorized as avalanches, debris flows,
earthflows or mudflows depending on whethel they
consist of pledominantly snow and ice, rock fragments,
sand-sized material or clay (Fig. 7.6).
Where the flow has a high water content it may extend a§
a long, narrow tongue well beyond the base of the slope
from which it originated. Such flows are usually more or
less confined to existing drainage lines (Fig. 7.7) and there
is jn fact a transition between mudflows and streams laden
with abundant fine sediment. Earthflows involve the extrusion
of lobes downslope and are usually slow moving.
Where the slope matelial is composed of sensitive soils,
however, an initial disturbance can cause an instantaneous
loss of shear Strength and plomote a rapid earthflow. The
slowest type of flow is solifluction which involves the
downs|ope movement of Safuraled soil, Solifluction can
occur al slope angles a5 low as l" and is paflicularly aclive
in periglacial envilonmgnts. Here abundant moisture is
made available by seasonal thawing of soil above a frozen
subsurface, and this form of solifluction is termed
gelifluction (see Section l2.2,3). Although solifluction can
be regarded as a distinct mass movement process it
A PURE FLoW
B PURE sLlDE
C PURE HEAVE
Fig.7.5 Velocity pt,ofles íor ideal types of mass moyement: (A)
pure flow, (B) pure slide anel (C) pure heave. (Modijedfron
M. A. Carson and M. J. Kit,kby ( l972) Hillslope Form and
Process. Cambridge University Press, Cambridge, Fi8. 5.1 ,
p. l00).
frequently occurs in close association with soil creep. Many
hillslopes show the combined effects of both plocesses
(Fig. 7.8).
In addition to the transport of material ftom higher to
lower elevations, flows can be effective erosional agents,
especially the more ener8etic varieties. This is particularly
true for debris flows and for debris and snow avalanches
which travel over the ground, rather than predominantly
<".,N
Ť§\Sҧ:
N,..i.,ixšž
Slope processes and forms l71
Fig.7.7
, Deb_risflow on western flank of the Southern Alps.
Souh Island, Nerý zealand
F,ig. 7.6
.
Uaieties oíílow-typemass hovement. ( Based on D. J.
Varnes ( 1978) in R. L. Sl.huster and R. J. Krizek (eds) Laiddid;si
Analysisjnd conlrol. Transportation Resear(h Boar.l sPecial
Report l7ó. National Al ad?my or scicnces. Washington', DC,
FiR.2.1.)
though the air. The major geomorphic effects of avalanches
are the removal of debris fróm gullies and slope
faces, the excavation of lock to form avalanche chutes, tire
elosion and redistribution of unconsolidated slope deposits,
and the deposition of snow and/or rock debris.
Major ayalanches can be one of the most violent and
destructive forms of geomorphic activity. Two particularly
catastíophic avalanches, both triggered by earthquakes,
crashed from the the mountain peak of Huascaran in the
Peruvian Andes in 1962 and 1970. In the 1962 event 3 Mt
of ice and 9 Mt of rock Were transported at speeds well in
excess of 100kmh,l over a holizontal distance of 20km.
An estimated 3500 people were kiiled. In 1970 a second
avalanche occurred which initially moved along the same
route as ďle 1962 event, but aftel travelling some 16 km at
an average speed of around 300 km h,t PaIt of the furbulent
flow oírock debris and ice jumped aiross a 300m high
ridge and buried the town of Yun8ay beyond. on this
occasion the death toll was possibly as high as 40 00Ó.
The extremely high velocities achieved by some avalanches
clearly requie explanation. Some may ride on a
layer of compressed air tapped between the avalanche debris
and the gŤound surface. This has the effect of greatly reducing
thc frictional drag on the moving -asr. Otrr-o possiUte
explanations of high rates of movement include fluidization
where fine particles are kept in suspension by a flow of
ail, and cohesionless gmin flow in whiih particle motion is
§ustained by continuous collisions as in a fluid.
Fig.7.8 Valle, cul inro a Lhalk escarpment in Ken!, IJK. such
valleys on the chalk are known as coombes, and soliIuclion
deposits. known_as coombe rock, are ofien puuru á, ,|oli"y
lloors (eý,i.denl
,in
phorographftom theJlat bolom of the villey).
l, hese
.sol.úlu.clrcn
deposits probahly accumulated at the end of the
laý s,|acial l0000 l 5 000 a ar under periglacial tonditions 1and
are lhereJore more accurarely dest.ribed as lelŮu,.lion dcposils).
Term(e.tíes Can be s?en on úevalle!-sidc rlopžs and are'
probably related lo soil creeP.
172 Exogenic processes ánd landforms
Although spectacular, avalanches are not of widespread
significance. They are largely conflned to terain characterized
by high local relief and steep sloPes and are tius most
common in actiYe orogenic belts. on a global basis the
more ubiquitous but le§s spectacular slower forms of flow
aíe moíe significant dgnudational agents.
7 .2.2.3 Slide
Slide is an extremely widespread form of mass movement,
and the term landslide is part of our everyday vocabulary.
This presents problems when using it in a Specific technical
sense since landslide in general usage simply means the
rapid downslope movement of slope material. Applied in
this sense many landslides also involve fall and flow. In a
pure slide failure occurs along a well-defined sheal plane
(Fig. 7.5(B)). Resi§tance to movement falis sharply immediately
the initial failure takes place, and downslope movement
continues unlil lhere is a sufficient increase in resislance,
often related to a decrease in slope angle, to halt it.
Slides are nearly always long in relation to theil width
and depth, their length-width ratio typically being l0: 1.
They can be subdivided into translational slides, which
have predominantly planar shear surfaces, and Ťotational
slides in which the shear plane is concave-up (Fig. 7.9).
Rotational slides are most common where slopes consist of
thick, homogeneous materials, such as clays. The rotational
movement can Iesult in the upper part of the slumped mass
being back-tilted towaíds the failure surface (Fig. 7.10).
The material can moye as a single block, but usually it is
broken into sevelal discrete segments sepalated by transveíse
lissures. Movement at the base of rotational slides in clay
or similar cohesive material is often transformed into that
of an eanhflow and this gives rise to a chaolic. hummbcky
surfáco ol
s6párátion
Fig.7.10 Major features of a rotational slide (Modifiedfrom
D. J. Vames (]978) in: R. L. Schuster and R. J. Krizek (eds)
Landslides: Analysis an d Control. Tralsportation Re,earch
Board special Reporí 176. National Academ| ofSciences,
Washington, DC, Fig. 2.1.)
suďace. Both rotational and translational slides are precipi
tated by a temporary excess of shear stress over shear
strength within the slope (Box 7.3). The difference between
the two types of moyement is that in thick, relatively
homogeneous material the depth at which the ratio between
Box 7,3 Analysis of rotational slides
TRANSLATlONAL EARTH
Fig.7.9 varieties oí slide-type mass moýements. (Based on D.J.
Varnes, 1978, in: R- L- Schuster and R. J. Krže,t (eds) Landslides:
Analysis and Control. Transportation Research Board Specíal
Repoft 176, National Academy of Sciences, Washington, DC,
Fi8.2.1 .)
The shear plane of a rotational slide is cuíved artd therefore
the stability analysi§ outlined in Box 7.2 must be modified,
One way the stability of a rotational slide can be evaluated
is to divide the slide into a number of 'slices' of length L
and aggregate the forces acting of each of these slice§ (Fig.
B7.3). The weight (W) is taken a§ operating through the
centre of each slice. The angle of the shear plane ((r) is
calculated for each slice from the centre of rotation (o).
The effective noímal stress (o') at the base of each slice is
W cos (r and the shear strength (s) is lť sin q, The safety
factor (F) can then be defined as
A
F => IcL + (W
:os
(l
- uL) tu| l)
) w.in c
where c is cohesion, a die pore-water pressure at the ba§e of
lhe slice and lan o lhe angle of inlemal friction.
Fi?. 87.3 stabílítyanalJsis oía deep-seated slide usíng the
meíhod oí slices. (Aíter M. A. Catson aru1 M. J. Kil"khJ (1972)
Hillslope Form and Process. Cambridge University Press,
Cambritlge, Fig.7.1l, p. l67.)
Slope processes and forms 173
shear strength and §hear stress is at a minimum (that is, the
potential shear plane) forms an arc rather than a straight Úne.
_ The gleat majority of slides are small and ,t attŠ**iÁ
lengths of a few tens of menes and depths of 2-3 m. Slides
l T bedrock are less common. but may anain .n.;;;;
] dimensions and involve the movement of millions of cubic
l :De*:
of.material. Very large s|ides usually break up to
tbrm debris avalanches bul in some cases the rock travels a
significant distance as a coherent mass. This is most likely
to occuí_ where competent beds slide over incompetá
strata (often clay or mudstone) dipping steepty Ághly
parallel with the gíound slope. Such conditions
"ont
iřuó
to low shear strength and enhance the probability of slides
occurring.
FocK FALL
Probably the largest slide on Eanh is the Saidmarreh
slide located in south-westem Iran. Although it occuneJ
more than 10000 a sp it has suffered oňly superficial
modification by subsequent erosion. The á"p*it. -",crudely stratified, indicating that the movement was not
predominantly one of a furbulent debris avalanche. A mass
of limestone some 15 km long, 5 km wide and at least 3(Xi
m thick slid off the underlying interbedded marl and limestone
which dips at an angle of around 20o out of the slope.
The^initial vertical component of movement was only about
1000 m, but the slide travelled a total distance of 18 km,
crossing an 800 m high ridge en roure.
F.ig.7.1I,
.Varieties oífall-typ? mass moýemehls. (Based on D.J.
Varhes
.(
l978) in R. L. St-hustir and R.J. Krlrcr íeásl Landslider:
Analysis and Contíol. Trahsponaion Resear(h Board special
Report 176. Na|ional Academy of sciences, Washington|. DC,
Fi8. 2.1.)
*. o|":1$o: of soil creep because above an angle of
around 25'the soil cover is thin or absent. On st.ep,ira..covered
siopes flights of narow
".pr.
."lil i'"-ria"Ťň;,
are commonly present and it is likely that these are relatj
to §oil ffeep, although shallow landslides may also be instnrmental
in theř deve|opment (Fig. 7.8).
7.2,2.5 Fall
7.2 .2.4 Heave
In pure heave the slope material experiences cycles of
expansion and contracťon (Fig. 7.5(C)). Downslope movement
aíises from the fact that while expansioit occurs
normal to the sloping ground surface contraction under
$avity tends to be more nearly vertical. cohesion between
particles usually prevents a purely vertical refum movement
yd9r gravity. Two t}pes of heave can be distinguished on
the basis of the size of the constituent particles -ioil creep
and taIus creep; the lattel inYolves coarser material than
the former. Expansion and contraction can be caused bv
wetting and drying, freezing and thawing (in which case thá
process is described as frost creep - see Section 12.2.3),
temperature changes aId the burrowing activity of worms
and other olganisms.
T}e rat9
9f
soil or talus creep on a slope depends on a
number of factors. It will become greatei with increasing
slope angle since this increas€s the downslope
"ornpon"niof movement. It wi]l also be high, in .oil. .oniuining
abundant quantities of clays. such as smectite, whicň
expand significantly on weíing. or in silt-sized material
whlch is capable of subslantial ice accumulation. soil and
talus creep will, however, decrease with depth below the
§lope suďace, both because of the moderation of changes
associated with wetting and drying and freeze-thaw, ňrd
the increase in the weight of the overburden. There is some
support from experimental and field observations for these
expected relationships, although lhere is an upper timit to
Fall involves the downward motion of rock or. more rar.elvsoil
through the ail (Fig. 7.11). Soil i§ not rrequenttf ir1volved
for the §imple reason that free fall can á.ry á""*
from.very steep slopes or cliff faces which, of course, have
very linle soil cover. An exception is the loppling of slabs
ol eaflh along river banks. a process often iefeied to as
bank calving. This arises from the undercutting of banks
by_ streams and is a very common phenomenon iuhi"h aontnbu(es
large quanti(ies of sediment to river channels.
Topples are distinguished from other types of fall by the
rotation of the block of material as it fatt, a*ay. roppte can
occur in rock especially where joints are veíically extenslve
Ur re|atlon to their width and where lhey diP out oÍ a
slope.
Rock can become detached as a result of various physical
weathering processes, including pressure release and
joínt Wid€ning by frost action, and the fragments produced
ar_e rapidly removed under gtavity. Rocr fills are common
phenomena in terrain charac(erized by high. ste€p rock slopes
and cliffs (Fig. 7.12). Where the disto;ged r.ugrn*t, 'ur"
large they accumulate a significant amount of kinetic
energy by^ thetime they impact on the §lope betow and ttreý
can therefore be an active erosive agent by detaching othe;
fra€ments. Large rock falls originating from a consijerable
hetght above (he ground spread Lheir debri§ over an extensive
area_unless the dispersal of material is confined by topoglaphy.
As already mentioned, large rock fals are otien
transfórmed into debris avalanches once they have made
theil initial impact. In sifuations where deep valleys are cut
into.hard rocks, such as gíanites and somó .-O.ton"r, ifglaciers or rapidly incising river channels, the releasó of
--)
_,..\
174 Exogenic processes and landťorms
Fig.7.12 Rock íall in a recently qlatiated ýalley in the Southern
Alps, South Island, New Zealand. The area of ftesh rock íace írotfi
which the large blocks ofrock have fallen is more than20 m high.
lateral coníining pressure along the valley walls can give
dse to plessure release and generate tension joints running
Iou8hly Parallel With the ground slope. These can promote
slab failure when the progressive widening of these joints
evenfually leads to the detachment of thick slices of fock
(Fig.6.2l).
7 .2.2.6 Subsidence
subsidence can occur either as the morg or less instantaneous
collapse of mateíial into a cave or other cavity
(cavity collapse) or as a progressive lowering of the
ground suďace (settlement). cavity collapse is largely
conflned to limestone teíains where the loofs of undeíground
cavities occasionally collapse. More rarely lava
tubes within lava flows may experience a similar fate.
Cavity collapse can also occur as a result of human activites
such as mining. settlement usually arises from the
lowering of Water tables and is most dramatically illustrated
in areas where tiere has been oil drilling or large-scale
abstractión of ground Water for iírigation. senlement can
also occur naturally where the volume of poorly compacted
matelials i§ decleased by the addition of watel (hydrocompaction)
or by Yibrations such as those generated by
earthquakes.
7.3 Gravity tectonics
Gravity tectonics is a useful term which covers a range of
píocesses extending from the very large-scale movements
of rock masses involved in the development of thrust§ and
nappes to smaller scale downdip translocations of matelial
which are transitional to landslides. Many of the massive
nappes occurring in intercontinental collision orogens such
as the AlPs are now considered to be vast gravity slides
moving away from their high, axial zones (see Section
3.4.1).
Gravity tectonics can involve both spreading and sliding,
the two processes being closely associated, but not identical.
Rocks located high up in mountain masses and
bounded by st€ep slopes gradually yield and move
downslope under gravity. This kind of motion can be
accommodated by intemal movements within the rock
(gravity spreading) or it may occur primarily through
gravity sliding of the rock mas§ oveT a few well-defined
planes composed of incompetent strata. Although the
movement is essentially downslope, intemal deformation
and the rotation of blocks along small-scale faults can
cleate chaotic pattems at the local scale (Fig. 7.13).
Such strucfuIes clearly have a combined endogenic and
exogenic ori8in. The initial energy input is endogenic,
involving as it does orogenic uplift. But the subsequent
sliding, which may take place over gradients as low as 0.5o,
ii
f
|{
Fig.7,1j Schematic fepresentation oí ?raýity.tectonics in the
northern APennines, near Flofence,Italy: (A) the oriqinal
depositional basín; (B) thrustinq from the ýýest; (C) gravity
sliding owr UpperTriassic eýaporites (solid shading) acting as a
décollement surface. (AJter P. Elter and L.Treýisan (}973) in:
K. A. De long and R. Schohen (eds) Gtavity and Tectonics, Wil?y,
Ne.,ý York, Fig. 15,p. ]87.)