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be present for the occurrence of large rockfalls. Even
jointing is not a guarantee that rockfalls will eventuate.
Blocks separating from cliffs along vertical lines of
weakness must also undergo disintegration for rockfalls
to take place. Otherwise, they simply glide downslope
intact.
Globally, rockfall is the major process for the
removal of debris from cliffs in scarped or steep
mountainous regions affected by periglacial activity.
While many of the factors that trigger landslides
initiate rockfalls, the majority occur because of earth-
quakes. Many of the types of landslides discussed
above, especially those occurring in the European
Alps, began as rockfalls. In deeply snow-covered
mountains, rockfalls are a major triggering mechanism
for avalanches.
entrained boulders the size of houses. As it traveled
down the valley at speeds of 320 km hr -1 , it picked up
more debris each time it crossed a glacial moraine. At
the bottom of the valley, the flow had incorporated
enough fine sediment and water to become a mudflow
with a 1 km wide front. Eyewitness accounts describe
the flow as an enormous, 80 m high wave with a curl
like a huge breaker as it approached the town of
Yungay. Ridges over 140 m in height were overridden,
and boulders weighing several tonnes were tossed up
to 1 km beyond the rims of the flow. Within four
minutes, 50-100 million m 3 of debris completely
obliterated Ranrahirca and the much larger town of
Yungay. The flow then continued down the Rio Santa
reaching the Pacific Ocean 160 km downstream. Over
25 000 lives were lost. Only those who had raced to
the tops of ridges survived. The disaster ranks with
the Mt Pelée eruption of 1902 for the completeness
of its destruction.
Many large landslides described as such in the liter-
ature should in fact be classified as debris avalanches.
Almost all have originated in steep mountain areas. For
instance, the Bialla slide referred to above as a mega-
slide, and shown in Figure 12.14, is technically a debris
avalanche. Historically, many large landslides fit into
this category. At Elm, Switzerland, on 11 September
1881, about 10 million m 3 of mountain slipped onto
the town, burying it to a depth of 7 m and killing 115
people within 55 seconds. A debris avalanche of similar
size at Goldau in Switzerland, in 1806, buried four
villages and killed 457 people. On 4 September 1618,
debris avalanches in the Chiavenna Valley in Italy
killed over 2400 people. Larger disasters have also
taken place. Reportedly, the total population of 12 000
in the town of Khait, Tajikistan, was killed in 1949 by
converging avalanches triggered by earthquakes in the
Pamir mountains. The largest amount of debris ever
involved in a recorded failure occurred in the same
mountains at Usoy in 1911. Approximately 2.5
Deb ris avalanches
(Varnes, 1978; Voight, 1978)
Debris flows and mudflows can take on catastrophic
proportions several orders of magnitude greater
in size and speed than described above. The term
'debris avalanche' is more appropriate to these types
of flows. Many of these events begin as landslides or
rockfalls. All appear eventually to entrain a variety of
particle sizes and behave as a fluid flow. The high
velocities appear to be achieved by lubrication
provided by a cushion of air entrapped beneath the
debris. The flow literally travels like a hovercraft. In
some instances, there is little evidence of water
involved in the avalanche and, in other cases, masses
of debris appear to have traveled with minimal defor-
mation or re-alteration. By far the most destructive
debris avalanches in recent times occurred around
Mt Huascarán, Peru, in 1962, and again in 1970. The
1962 event started as an estimated 2 million m 3 of ice
avalanched from mountain slopes. This ice mixed with
mud and water, and turned into a much larger
mudflow with a volume of 10 million m 3 . It swept
down the Rio Shacsha Valley killing 4000 people,
mainly in the town of Ranrahirca. Material, some of
which consisted of boulders over 2000 tonnes in
weight, traveled down the valley at 100 km hr -1 and
100 m up valley slopes. Having survived that event -
and believing that another, similar, catastrophe was
unlikely - residents of the area then experienced an
earthquake on 31 May 1970, which caused the icecap
of Mt Huascarán, together with thousands of tonnes
of rock, to cascade down the valley. This debris flow
10 9 m 3
of rock failed, damming the Murgab River and forming
a lake 284 m deep and 53 km long. The debris
avalanche was triggered by an earthquake registering 7
on the M s scale that destroyed the town of Usoy, killing
54 inhabitants. A failure one-tenth this size, containing
0.25
10 9 m 3 of soil and rock, fell into a reservoir
across the Vaiont Valley of Italy on 9 October 1963.
The impact sent a wave of water 100 m high over
the top of the dam and down the valley, killing 2000
people.
 
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