Geology Reference
In-Depth Information
mountains in an effort to develop a standardized method
for investigations of snow cover and classify the snow on
textural information and interpret mechanical parame-
ters. The procedures have also been applied to study snow
compacted by aircraft tires on runways by
Klein‐Paste
et al.
[2004, 2007]
.
It is hoped that such schemes will be
used in the future for quantitatively structural aspects of
snow on sea ice covers.
Since snow is porous and cannot be processed by DMT,
the pores are filled by the method suggested by
Perla
[1982].
This is accomplished at the sampling site and at the ambi-
ent temperature by slowly releasing a small block of sam-
ple (cube with sides of 50 mm) in a paper cup containing
supercooled dimethyl phthalate. The phthalate is allowed
to penetrate into the pore space. This may take a long time
depending on the porosity and temperature. Once the sam-
ple is saturated with the fluid, the cup is stored below
−20 °C to solidify the liquid. Filling the pore space and
lowering the temperature also slows down the snow meta-
morphism and supports the snow crystals during further
handling. The paper cup can then be removed, and thick
sections are cut and thin sections are prepared essentially
following the DMT procedures described earlier for ice.
However, for the first stage of microtoming, it is better to
fix the thick section on the glass plate by using a few drops
of supercooled phthalate instead of water drops prescribed
for ice. This is because it is often found difficult to release
the sample from the glass plate after the first microtoming
sequence without damaging it. Frozen phthalate is softer
than ice and adheres less to the glass plate, and this reduces
the stresses on the sample while releasing it. The mounting
step before the second microtoming sequence is performed,
as prescribed for ice, with water to ensure a good sample
support during microtoming to the final thickness. The
final thickness for fine‐grained snow and snow ice has to be
around 0.1-0.2 mm. Sections of snow are to be cut both
parallel and perpendicular to the snow samples.
Since a single crystal of snow mass or a grain of snow ice
is a birefringent material, it allows detection of individual
crystals using polarized light. Consequently, their crystal-
lographic orientations in a polycrystalline mass of consoli-
dated snow and hence texture can be evaluated with no
difficulty. However, thin sections of snow mass or snow ice
with extremely fine grains must be thin enough (preferably
around 0.1-0.2 mm) to avoid problems created by the
slanted crystal boundaries. Such thin sections can be made
by DMT, but they will be white or most probably gray in
color as can be figured out from Figure 6.6, and individual
crystals cannot be delineated with ease. However, if a retar-
dation plate that will introduce a relative retardation of
about 400 nm is added to the light beam, for convenience
between the polarizer and the thin section, then interfer-
ence colors in cross‐polarized mode could be within the
desired zone (see also section 6.2.5.) between the first and
Figure 6.7
Double‐microtomed ultra-thin horizontal section
of snow subjected to equitemperature (ET) metamorphism
from Himalayan Mountain under cross‐polarized light using a
retardation plate to show colors. Grains with boundaries and
the background is the pour feeling dimethyl phthalate and
appears gray‐brown [
Satyawali et al.
, 2003]. (For color detail,
please see color plate section).
the second order of interference colors. In case of difficulty
in getting a proper retardation plate, a mica sheet from an
old iron can do the job. Mica sheets provide added advan-
tages. They consist of thin layers that can be peeled for
selecting the favorable thickness. This technique has been
exploited to view the texture of fine-grained, faceted grain,
and melt‐freeze snow [
Satyawali et al.,
2003;
Klein‐Paste
et al.,
2004, 2007]. The extracted information cannot oth-
erwise be obtained using other techniques. An example of
Himalayan snow is shown in Figure 6.7. Another applica-
tion of DMT to snow on runways is shown in Figure 6.8.
Microtomed surfaces of snow samples (using the DMT)
can be further processed using a thermal etching technique.
This combination of techniques has been shown to be the
best way to classify a snow sample by its texture [
Satyawali
et al.,
2003]. Comprehensive and the best up‐to‐date exper-
imental and theoretical investigations on microstructure‐
property relationships of metastable snow were carried out
by
Satyawali
[2009]. Further developments in this area of
snow pack characteristics can now be found in the recently
published topic by
Satyawali
[2012]. Developments on the
physics of mountain snow morphology are equally appli-
cable to those of snow covers on floating sea ice (albeit
with complications caused by upward migration of brine
from ice underneath).
6.2.4. Precautions for Thin Sectioning by DMT
Viewing of ice thin sections through polarized light
involves the passage of light through the glass plate and
the slice thickness. It is an integrated method. A polarized
