Geology Reference
In-Depth Information
(a)
(b)
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Snow
settling
c
c
c
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c
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c
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Growth
competition
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c
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Ice
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c
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Elongated air bubbles
Water
Figure 4.15
Nucleation of crystals by snow deposition and geometric selection leading to (a) columnar growth
of S2 ice (courtesy Paul Barrette); details of competition zone and domination of crystals with
c
axis (irrespective
of dependence on
a
axes) in the (b) horizontal plane [N. K. Sinha, unpublished].
since this is the direction of the maximum heat flow from
the surface to the atmosphere. However, not all the nucle-
ated crystals can grow freely. The growth becomes prefer-
able depending on their crystallographic orientations with
respect to the heat flow and hence the surface. If a crystal
is nucleated by a snowflake falling on the water surface
with its flat surface oriented in the vertical direction so
that the basal planes is parallel to the growth direction,
then that crystal is in the best position to grow.
As illustrated in Figure 4.15b, a layer of crystalline
competitions naturally develop under the snow deposit.
Since the growth is preferable in the basal plane, nucle-
ated crystals with
c
axis in the horizontal plane, irre-
spective of the orientation of the three
a
axes, become
the preferred grains to grow in comparison to others.
Crystals with
c
axis titled to the horizontal can still grow
while tilted to the vertical, but their growth is inter-
rupted by the faster growth of neighboring crystals with
the preferred horizontal direction of the
c
axis. Thus
the neighboring crystals “compete” against each other
as they grow, leading to the survival of the crystals that
have their
c
axis horizontal. Thus, crystals with their
c
axis in the horizontal plane simply choke the others
from growing. This process is known as “geometrical
selection” of crystal growth and was critically investi-
gated by
Perey and Pounder
[1958] at McGill University,
Canada. They found that the competition is often con-
cluded within about 10-15 mm of growth, and clear ice
with narrow elongated air/gas bubbles along the grain
boundaries develops thereafter.
The presence of transversely isotropic columnar‐
grained S2 ice in the bulk of an ice cover is a clear indica-
tion that the freezing was initiated during a calm period
when the air temperature was low and there were snowfall
activities. The snow deposition must be light to heavy, so
that after some initial melting, the water surface tempera-
ture was decreased to the freezing point. The cross‐
sectional grain size of S2 ice ranges from fine to large
with usually straight boundaries. The cross-sectional area
increases slightly with the increase in depth due mostly to
the continuation of the selection processes. However, as
the freezing front progresses, dissolved impurities includ-
ing air are also pushed down. This leads to the entrap-
ment of air bubbles as illustrated in Figure 4.15b. Nearly
transparent ice covers (below the white snow ice, if any)
composed of columnar grains start to exhibit air bubbles,
often in the form of long cylinders, below a certain depth.
Example of a horizontal section across the length of
the grains of pure S2 ice and a vertical section of the same
ice parallel to the long axis of the columns are shown in
Figure 4.16. In this case, the pure and transparent ice was
made in a laboratory using double‐distilled, deairated,
and deionized water [
Sinha,
1978b]. Randomness in the
c
‐axis orientation of the crystals in a plane normal to the
long axis of the grains in S2 ice can be visualized from
the even distributions of “dark or black” and “bright or
white” grains in cross‐polarized image, like the example
shown in Figure 4.16a. The inclusion‐free clarity of the
intragranular areas of the grains is particularly noticea-
ble. It is appropriate to point out here that the clarity in
the characteristics of the grain boundaries and surfaces
of the absolutely pure ice, however, is maintained primar-
ily because of the use of cold‐state, double microtoming
technique (DMT) in making the thin sections (described
in section 6.2.2). Conventional hot‐plate techniques for
making thin sections are convenient, but leave residues
