ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

of which there are preserved polished tectonical surfaces.
If there was an earthquake connected with dislocation
rejuvenation there are created fall-outs and slope goes
backwards partially or the whole zone of tectonical loosenings
is destroyed.
In the second stage slope destruction is accordant to joint
fissures. There are chutes on walls and at the foothills talus
cones overlap each other and make accumulative part of
the slope. Then chutes change into corrasional valleys and
in the zones of tension or with greater density of cracks
there are formed valleys with zig-zag or perpendicular pattern.
Faces of those scarps in the first stage are of a trapezium
and then of a triangle shape.
In the third stage slopes are more gentle and inclination of
them does not exceed 45-50° and waste cover is created on
them. The cover is taken away episodically during disastrous
wash-outs. The material washed out from the upper
and middle part of slopes covers talus cones which are
changed into torrential cones. They are much longer and
th-eyhave gentle concave profiles. In that stage of development
young slopes become mature ones.
Within the Gobi Altay and Khangai foreland slopes of
young, early and late mature stages can be distinguished.
Young slopes consist of 2 parts - rocky wall in which tumbling
and breaking off take place and talus cones which
are results of material deposition.
Early mature slopes consist of 3 elements. There are convex,
upper, parts of slopes which are modelled by cutting.
Middle parts of slopes are regularly inclined and transportation
and washing-out take place. Pediment is built at the
foot and material deposition and transportation takes place
there.
Late mature slopes consist of 4 elements. They consist of a
very short convex part which is modelled by cutting, long
concave part on which transportation and furrowy washing-out
take place. The third part is made by cryptopediment
on which deposition and transportation takes place.
The fourth part is made by washing-out pediment which
often in its lower part becomes parapediment.
During the earthquake in 1972 a fault 600 km long and
2-16 m high was formed. It transversally cut pediments
and cryptopediments in the northern part of the Altay. Some
dislocations in the Khangay were rejuvenated. The surfaces
of younger dislocations are accordant to older tectonical
zones and new tectonical scarps are formed. Their
height is different but does not exceed 45 m. Those scarps
transversally cut hanged valley bottoms and local erosional
bases are formed. Scar or frontal landslides were formed
above tectonical dislocation zones. Common pattern of
those forms and their greater density is clearly connected
with the amplitude of uplifting. Greater density of landslides
occur in places of a single greater uplifting or subsidence
of the surface.
At the foothills of tectonical slopes within pediments and
cryptopediments there are clear new scarps connected
with present tectonical movements. Those young dislocations
take place not only in the zones of older tectonical
loosenings but there are also formed new dislocations on
the foreland of the old ones. They are marked by zones of
frontal landslides. Such pattern of present dislocations
leads to creation of local erosional bases and in the same time
to zonality of the present erosional and denudative processes.
Tectonical dislocations are accompanied with eartquakes
what is confirmed by deep landslides which are situated
not only in the dislocation zone.
MARKUS N. ZIMMERMANN
Morphological changes following the 1991 eruption
of Mt. Pinatubo, Philippines
Geo7, Geoscientists, Neufeldstrasse 3,3012 Bern, Switzerland
The June 1991 eruption of Mt. Pinatubo Volcano, Philippines,
spewed some 8 to 9 billion cubic meters of pyroclastic
sediments and ash into the atmosphere. Approximately
6 to 8 billion cubic meters were deposited in 8 major river
basins, which drain the slopes of the volcano in all directions.
It is supposed that within 10 to 20 years about half
of this volume will be transported downstream by lahars
and hyperconcentrated flows. These processes occur regularly
during heavy rains. The first five rainy seasons caused
major lahar activity in all river basins. A total of about 2
billion cubic meters has been already washed down. The
pyroclastic flows as well as the subsequent lahars caused
major morphological changes in the headwaters and in the
lower river reaches.
The pyroclastic flows travelled as far as 15 km from today's
crater. Accumulations of up to 250 m thickness occurred.
Whole valleys were filled and new river networks developed.
Even 5 years after the eruption the pyroclastic sediments
are still hot. The infiltration of rainwater causes secondaryexplosions
(phreatic explosions). As a consequence
large secondary pyroclastic flows can occur. Due to such
flows the river network in the headwaters is frequently
changing. A large secondary pyroclastic flow during
Typhoon Kadiang (October 4 to 6, 1993) caused a major
impact on the whole area: about 21 square kilometres of
primary sediment sources shifted from the Sacobia River
to the Pasig-Potrero River. The shifting of such large volumes
had dramatic consequences for the Pasig-Potrero River
during the rainy seasons 1994 and 1995. The development
of the piracy point showed that vertical downcutting
contributed almost the total volume for the lahars; the sediment
delivery from adjacent slopes is minor.
The lahars which transport the sediments to the lower river
reaches are largely controlled by the occurrence of
typhoons. Volumes of 5 to 30 million cubic meters per
event were observed. In the upper reaches of the alluvial
fans the sticky, debris flow-type lahars deposited sediments
up to 40 m thick. Rivers, deeply incised in the alluvial fan
prior the eruption, were completely filled and the subsequent
lahars started to shift over parts of the fan. In the
Pasig- Potrero River a reverse process started at the end of
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