Geology Reference
In-Depth Information
For general descriptions in conjunctions with the crys-
tallographic classification, sea ice can be grouped, as
suggested by
Sinha
[1991], into three major categories:
granular, frazil, and columnar. The author also suggested
adaptation of names on the basis of classification for fresh-
water ice as deemed appropriate. Characteristics of each
category of freshwater ice and sea ice are introduced in
the following sections. The presentation does not cover
the crystallographic classes in the same order as listed in
Table 4.1. Instead, it starts with the three categories com-
monly found at the top of the ice (T1, S4, and S5), then
proceeds to the categories that make up the bulk of the
ice (S1, S2, and S3). It concludes with the agglomerated
type (R ice type). Several images of ice thin sections
photographed between crossed or parallel polarizers are
presented. More information on the thin sectioning tech-
nique is presented in section 6.2.2 and on photographing
thin sections in sections 6.3.2 and 6.3.3.
4.3.3. Crystallographic Classes of Natural Ice
4.3.3.1. Granular or Snow Ice (T1 Ice)
Below the blankets of snow, often snow ice or granular
ice is found at the top of ice covers in rivers, lakes, or
oceans. Granular ice in sea ice covers is equivalent to the
T1 ice described in Table 4.1. It may form from a number
of processes including freezing of wet snow or snow mixed
with ice particles (e.g., fractured frazil crystals) when satu-
rated with seawater in a slushy medium. Crushing of ice
covers or deformation‐induced solid-state transforma-
tion or recrystallization of secondary columnar‐grained
ice can also provide fine‐grained particles for flooding and
freezing. Snow‐ice features randomly oriented
c
axis with
rounded grains with diameters in the range of about 1 mm
(Figure 4.6).
Granular or snow ice forming in seawater traps not only
air but also a measurable amount of brine (Figure 4.7).
These inclusions are usually located at the grain boundaries.
The density of this ice can be relatively high (could be
925 kg/m
3
) within a few percent of that of single crystals
with density of 918 kg/m
3
, depending upon the amount of
trapped air and brine. Macroscopically, this type of ice is
isotropic (Figure 4.6) due to three dimensional randomness
of the crystallographic orientation of the constituent grains.
However, granular ice that may form from sintering of snow
and crushed ice, as found in the upper sections of glaciers,
ice caps, and ice shelf could exhibit marked anisotropy in
shape and size of the grains and their crystallographic ori-
entation. A detailed description of snow, firn (intermediate
stages of transformation), and ice related to glaciers is given
by
Paterson
[1969]. Firn is considered to be glacier ice when
the interconnecting air passages between grains (or aggre-
gate of several crystals) are sealed off at a density of
between 800 and 850 kg/m
3
. It should also be noted that,
in contrast to the commonly observed fine‐grained snow
c
c
c
T1 type
Figure 4.6
A three‐dimensional schematic diagram of isotropic
granular (snow) T1 ice [
Sinha,
1989].
10 mm
10 mm
Figure 4.7
Granular T1 type of ice formed from (
left
) freshwater and (
right
) seawater as seen in double‐microtomed
(DMT) thin sections photographed between crossed polarizers [
Sinha et al.,
1996].
