ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

have been accreting at a noticeable rate over the past few
decades. On the other hand, the southeast (SE) barrier system
appears to be migrating landward more quickly than
the NW barrier system and eroding at the same time. Historical
evidence indicates that the road that used to follow
this barrier system washed away several times between
1915 and 1920 (it has never been replaced). Understanding
the recent geomorphic history of this area is complicated
by various types of human disturbance ranging from
the construction of dikes by the Acadian People in the
1700s to the present construction of the Confederation
Bridge.
CLIFFORD D. aLLIER
Geomorphic constraints on tectonic theories
Centre for Resource and Environmental Studies,
Australian National University, Canberre Act 0200, Australia
Too often geomorphologists have accepted tectonic hypotheses
from geologists and geophysicists instead of starting
from their own basic data. In contrast, tectonic geologists
and geophysicists often invoke uplift (for example
over subduction sites or thermal domes) without any consideration
of how such uplift will affect landforms. In reality
many landforms are older than the tectonic movement,
and severely constrain possible tectonic interpretations.
Changes in elevation and slope, effects on drainage patterns,
and distribution of regolith are the most obvious
constraints on tectonic theories. Planation surfaces indicate
long periods of tectonic stability. Where present such
surfaces deny simultaneous folding and uplift, commonly
assumed in hypotheses of compressional mountain building.
Through feedback processes such as isostatic response
and gravity spreading, some tectonic processes are actually
driven by erosional processes.
YUICHI ONDA \ CHISATO TAKENAKA2
& TAKAHISA MIZUYAMA3
The mechanism inducing the infiltration rate lowering of
Unzen Volcanic Ash
1 Lab. of Forest Hydrology & Erosion Control Engineering, School of
Agricultural Sciences, Nagoya University, Nagoya 464-01, Japan
2 Lab. of Forest Environment and Resources, School of Agricultural
Sciences, Nagoya University, Nagoya 464-01, Japan
3 Lab. of Forest Hydrology & Erosion Control, Faculty of Agriculture,
Kyoto University, Kyoto, 606-01, Japan
After volcanic eruption, surface infiltration rates of the
tephra deposits have been reported extremely low (e.g.,
Allen, 1981, Hendrayanto & alii, 1995), which sometimes
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causes debris flow (Yamamoto & alii, 1980) or rapid surface
erosion (Collins & Dunne, 1988). However, the mechanism
still remains unresolved. Here, the authors studied
the mechanism inducing the infiltration rate lowering of
the Unzen volcanic ash through permeability tests using
varying electrolyte concentration, Esp (Exchangeable Sodium
Percentage) measurement of the fallout deposits, and
Sem (Scanning Electron Microscope) observation.
The eruption of the Unzen volcano occurs from Nov. 1990,
and much volcanic ash deposited around the Mountains
(fig. 1). Debris flow from ash deposited area has been frequently
occurred. The experimental results of the permeability
tests showed that the permeability decreased with the
number of experiments using distilled water, whereas the
permeability kept constant using electrolyte (tab. 1). Sample
surface cone penetrometer strength became higher for
the distilled water case, whereas the surface remains soft for
high NaCI concentration cases (tab. 1). The Esp in the field
condition is sufficiently higher than the forest soil (tab. 2).
These results indicate that that the mechanism of infiltration
lowering of the Unzen volcanic ash is due to formation
of chemical crust by physico-chemical processes of the ash.
The observation of the Sem also support the results. The
experimental results suggests that infiltration rate of Unzen
volcanic ash could be increased by applying electrolyte for
reducing surface runoff and debris flow initiation.
FIG. 1 - Unzen Volcano (July 24, 1993, Ministry of construction).
TABLE 1. The permeability tests using varying electrolyte and the
strength of the soil surface after 13 permeability tests
Electrolyte
merit)
Distilled water
0.005N NaCl
0.05N NaCl
05N NaCl
Depth
(em)
2.0-7.0
7.0
Permeability Penetrometer strength
(First experiment) (13th experiment) (after the experi-
(mm/h) (mm/h) (kgf/cm 2)
6.00 4.94 18.0
6.78 6.83 9.5
758 6.61 8.0
559 7.08 55
TABLE 2. The result of chemical characteristics of the soil
volcanic ash
forest soil
CEC
(meq./100g)
0.74
8.00
ESP
(%)
10.0
0.9