H16 Unsafe ground: Landslides and other mass
movements
H16.1 introduction
Substrate: a general term for material just below the ground surface.
Mass movement: the gravitatonally caused downslope transport of rock, regolith, snow, or ice.
Natural hazard: a natural feature of the environment that cause injury to living organisms and/or
damage to buildings and the landscape.
Becomes more of a threat every year because as the world’s populaton grows, cites expand int\
areas of unsafe ground.
16.2 Types of mass movement
Landslides:
1) The type of material involved (rock or regolith)
2) The velocity of movement (slow, intermediate, fast)
3) The character of the moving mass (coherent or chaotc, wet or dry)
4) The environment which the movements takes place (subaerial or submarine)
Avalanche: mass movement that moves like a turbulent cloud.
We classify them so we can be beter prepared.
Creep and soliflucion
Creep: refers to slow, gradual downslope movement of regolith on a slope.
During freezing or wetng, the regolith expands and its partcles move outward, perpendicular to the
slope. During the thawing or drying, the regolith contracts and gravity makes the partcles sink
vertcally and thus migrate downslope slightly. Cannot be seen by the naked eye, over tme you can
see it by cracked walls or curving trees.
Solifluction: the type of creep characteristc of tundra regions; during the summer, the uppermost
layer of permafrost melts, and the soggy, weak layer of ground then fows slowly downslope in
overlapping sheets.
Rouk glauiers
A slow moving mixture of rock fragments and ice. There are ice-dominated glaciers and rock glaciers:
are breccia’s cemented by ice. The combined mass can slowly move downslope. Form in 2 ways:
1) Snow or rain percolates down into pile of rock debris, that has accumulated above the
permafrost at the base of a clif. Water can’t infltrate into the frozen ground, so it freezes
and form ice in the pores between the clasts within the debris pile
2) Ice-dominated glacier already containing abundant rock debris begins to melt. When enough
meltng takes place, the proporton of rock increases.
Sllmps
Slumps: downslope movement in which a mass of regolith detaches from its substrate along a spoon-
shaped, sliding surface and slips downward semi coherently.
, Moving mass itself is a slump block. Slides down a failure surface: a weak surface that forms the base
of a landslide.
The exposed upslope edge of a failure surface Head scarp: the distnct step along the slope edge of a
slump where the regolith detached.
Downslope end called toe of slump / blocks toe. The upslope and downslope ends of a slump block
may break into a series of discrete slices, each separated from its neighbor by a small sliding surfaces.
At the toe may go over each other.
Mldfows, Debris fows and lahars
Mudflows: a downslope movement of mud at slow to moderate speed.
can be the result of a saturaton of the regolith, due to heavy rain fall.
Debris flows: a downslope movement of mud mixed with larger rock fragments.
Any slope underlain by poorly consolidated material can give wat in a mudfow or debris fow during
or following heavy rain. The speed of a fow depends on the slope angle and the water content.
Usually follow channels and at the base they will spread out.
Lahars: a thick slurry formed when volcanic ash and debris mix with water, either in rivers or from
rain or meltng snow and ice on the fank of the volcano. (devastatng mudfows spill down the river
valleys boarding volcanoes.)
Roukslides and debris slides
Rockslides: a sudden downslope movement of rock. A non-vertcal slope.
Debris slides: sudden downslope movement of material only consistng of regolith.
Happens when bedrock and/or regolith detaches from a slope , slips rapidly downhill on a failure
surface, and breaks up into a chaotc jumble. More likely to occur where a weak layer of rock or
sediment at depth below the ground parallels the land surface. They are partcularly fast when a
cushion of air gets trapped beneath, in which case there is virtually no fricton between the slide and
its substrate, and the mass moves like a hovercraf. ((aiont Dam)
Avalanuhes
A turbulent cloud of debris mixed with air that rushes down a steep hill slope at high velocity; the
debris can be rock and/or snow.
Snow avalanche: a chaotc jumble of snow surging downslope.
When snow melt, and later freezes again it forms a hard, icy crust. When the top is snowed under
again. The icy crust starts to become a failure surface, creatng a snow avalanche.
Wet-snow avalanches, pick up rocks on their way down. Involves snow that started to melt and thus
contains some liquid water and litle air.
Dry-snow avalanches: cold, powdery snow that disintegrates into a turbulent, air rich cloud at speeds
of 250 km/h.
Both can become powerful.
What triggers it?
