Jökull - 01.12.1981, Side 9

the time of formation of these fractures is not
yet clear.
T heoreticall)' a Iarge number of factors
could affect the behaviour of the clay minerals
ln the bonding material and subsequent
weathering processes. It is likely that vermi-
culite is sensitive to changes in adsorbed
water, if not interlayer vvater, at temperatures
below 50°C and this would lead to fracturing
ln the bonding material in the manner desc-
nbed above. The swelling effect of some
smectite minerals is well documented and free
swelling tests on room dry cubes of the Irish
basalts show linear expansion of up to 1.0%
(Douglas 1972). This compares with 0.02% for
dense basalt {Neþþer-Christensen 1965). How-
ever swelling is also dependent upon previous
slight changes due to, dehydration and if
moisture is uniform for some time even swel-
ling clay types can be very stable {Brekke 1965).
Swelling is also dependent upon the type of
cations in the interlayer and therefore upon
cation exchange capacity. For example swel-
ling montmorillonite can become stable in this
'vay if Ca+1! and Mg+2 substitute for Na+ in
octahedral sites (e. g. Cole and Lancucki 1976).
Smectite and vermiculite minerals have high
cation exchange capacity and experimental
determination of the nature of this in the
Figure 3. (a) Typical discontinuity with dis-
tinct edges. (b) Sinuous discontinuity with
„necking“ and fluffy edges. (c) Zoning within
linear discontinuity. (d) Transverse lines in
broad discontinuity. (e) Narrow discontinuity
entering plagioclase phenocryst which acts as
stress dissipator. (f) Electron micrograph of
bonding material from Iceland. Irregular
flakes. Fractures are indicated by arrows at
top. Scale in millimeters.
Mynd 3. (a) Dœmigerð smásþrunga, sem sýnir skýra
kanta. (b) Bugðótt smásþrunga, sem sýnir
,,flöskuhálsa “ og óskýra kanta. (c) Bein sþrunga með
beltaskiþtingu. (d) Þverlínur í breiðri smásþrungu.
(e) Mjó smásþrunga stoþþar innan feldsþat díls, sem
dreifir sþennu. (f) Rafeindasmásjármynd af fylliefni
frá Islandi. Öregluleg blöð. Örvar sýna sþrungur efst.
bonding material would be very significant in
developing a model for weathering processes.
The pH of freeface waters is somewhere bet-
ween 7.0 and 9.0 and these waters contain
dissolved elements in significant amounts.
Iron in either oxidation state has been consid-
ered to have a clogging effect on the cation
exchange position {Dion 1944).
Conclusion
The results permit a framework model for
one type of present day weathering process at
a freeface, the main stages of which would be:
1. Development of fractures under low stress
conditions following the main jointing
period by thermally induced stress.
2. A low temperature hydrothermal period
when the fractures are filled with the
bonding materials.
3. Changes occur under subaerial conditions.
These may include ion exchange,
hydration changes, and formation of new
fractures in various order.
4. Failure, largely along the bonded discon-
tinuities, produces fragments to scree
slopes.
It is not suggested that this is the only
process operating in the present environment,
but rather that the discontinuities are a fun-
damental controlling factor in any present
day weathering process. It has been shown
that in some localities fairly rapid small scale
removal of material from the freeface goes on
all the year round, while at certain times there
are peaks on a larger scale (Douglas 1980). This
strongly suggests that factors other than frost
for example play an important part. The facts
better fit the ideas of Gerber and Scheidegger
(1973) who believe that stress conditions
together with some trigger factor produce the
landscape.
ACKNOVVLEDGEMENTS
The author gratefully acknowledges the helþ recewed
from Hrefna Kristmannsdóttir who provided the
X-ray diffraction data at the National Energy
JÖKULL 31. ÁR 7