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Figure 4.10 ( Left ) Vertical and ( right ) horizontal thin section of vertically oriented S5 type of frazil sea ice, with
c axis randomly oriented in the horizontal plane, photographed between crossed polarizers [ Sinha et al., 1996].
(For color detail, please see color plate section).
south and thereby induces north‐south water current in
the sound. However, it was not the tidal current, but the
westerly strong wind (recorded at the weather station) that
pushed the frazil crystals toward the eastern shoreline
where they congealed and eventually solidified. Since there
was no information available on the microstructural and
strength properties of this type of sea ice, up to that date,
it was decided to perform a detailed study on this type of
ice [ Sinha, 1986]. Microstructural studies including the
scanning electron micrography (SEM) and the rate sensi-
tivity of compressive strength in a wide range of strain rate
were carried out on this ice. An example of the structural
features seen in vertically oriented S5 type of frazil sea ice
is shown in Figure 4.10. Since the discovery of this type of
S5 ice in 1981, together with its recorded growth history,
this type of ice has been noticed and recognized in FY and
old MY sea ice in many places in the Arctic. Thus S5 type
of ice is a common microstructural feature of sea ice.
In agreement with the observations of Sinha [1986]
who found that strong and persistent winds caused herd-
ing of oriented frazil ice, Lawson and Brockett [1990]
stated that frazil deposit characteristics reflect processes
and properties of their source such as depositional mech-
anisms and the history of ice sedimentation. Hence, their
appearance in thin‐section photographs can also be inter-
preted in terms of deposit evolution and used as proxy
data on the nature of subice deposition.
under calm conditions with no deposition of snow. The
individual crystals grow rapidly in all directions along
their basal planes at the surface level until the entire sur-
face is covered with ice. Solidification of the entire sur-
face layer stops the horizontal growth. Since the grains
are constrained by the boundaries of the neighboring
crystals, they have no choice but to grow downward. Each
crystal is forced to grow in the direction of maximum heat
flow and in a direction parallel to their c axis. This leads to
vertically oriented columnar‐grained crystals with their
long axis parallel to the direction of growth that coincides
with the maximum heat floe. As the ice cover thickens, the
c axis of the columnar grains remain in the vertical plane
parallel to the length of the grains. The bulk of the ice sheet
thus exhibits marked anisotropy with large cross‐sectional
dimensions. Due to the large grains, the ice also looks very
clear and transparent. This type of ice mass was classified
as S1 type as illustrated schematically in Figure 4.11a.
Purity of the water determines the quality of S1‐type
ice because it allows crystals to grow into relatively bigger
sizes with large planar boundaries as there is no “resist-
ance” to the growth in the horizontal plane during the
initial growth. Crystals can have their cross‐sectional
diameters as large as 200 mm. It will be seen that the large
cross‐sectional areas of the grains allow the formation of
substructures (subgrains) with small mismatches (mainly
in their a axis) in the orientation of the ice lattice. The
characteristics of subgrains are illustrated schematically
in Figure 4.11b. A photograph of a thin section, placed
between cross polarizers, exhibiting a large grain of S1 ice
with incursion of a second crystal is shown in Figure 4.12.
The grains in S1 ice rarely consist of pure single crys-
tals. As the grain grows, local disturbances develop
4.3.3.3. Columnar‐Grained with c‐Axis Vertical (S1) Ice
A freely floating ice crystal grows more rapidly in any
direction in the plane parallel to the basal plane or nor-
mal to the c axis. This is the situation if nucleation of ice
crystals occurs at the surface of a large body of water
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