Geology Reference
In-Depth Information
Probably the most important aspect of columnar‐
grained sea ice is that the grain boundaries as well as the
intragranular subgrain boundaries are not only irregular
but may not be distinct as depicted in Figure 20b and
Figure 4.21. In fact, there are strong reasons to believe
that the concept of grain as a single crystal (the way it is
understood universally) with its distinctive boundary that
separates it from its neighboring crystals does not really
apply to sea ice [
Sinha,
1989
; Sinha et al.,
1995
; Zhan
et al.,
1996]. The most important structural aspects of sea
ice are its subgrains.
Subgrains are made of pure ice. Most of the subgrains
are separated by distinct boundaries, called subgrain
boundaries as presented earlier schematically in
Figure 2.25 and illustrated in Figure 2.26. These bounda-
ries are the result of small mismatches in the lattice of
the neighboring subgrains. The mismatch in the lattice is
small, say a few degrees. The mismatch is related to slight
differences in the orientations of either
a
axis or
c
axis or
both as can be visualized from the schematic diagram in
Figure 4.20b. Brine and air pockets are mostly located on
the grain and subgrain boundaries. They could be in the
form of rows of interconnected or isolated inclusions.
Often rows of isolated brine pockets are also seen with
apparently no subgrain boundaries joining them, as can
be seen in the micrograph of Figure 2.26. Unfortunately,
most of the popular knowledge on microstructural aspects
of sea ice is based on observations of thin sections through
polarized light. This conventionally used technique, how-
ever, does not provide any clue to the finer aspects of sea
ice microstructure. Etching and replicating techniques,
as well as controlled thermal etching in conjunction with
solid state double‐microtoming procedures for surface
preparations have been found to be extremely powerful for
forensic investigations on sea ice, as discussed in section 6.4
and exemplified in Figures 2.22, 2.26, and 2.27.
The geometry of the subgrains and the brine pockets is
the most important factor that determines the mechanical
properties of sea ice, such as creep behavior and strength
of sea ice [
Sinha et al.,
1995
; Zhan et al.,
1996
; Johnston
and Sinha,
1995]. It also affects the microwave scattering
from sea ice, but that effect is usually overturned by the
more dominant effects of surface roughness and salinity.
Figure 4.22
Vertical thin section of the top 0.12 m of R‐type
young ice from the Labrador Sea, Canada, in March 1994,
exhibiting grains of different shapes, sizes, and orientations
under cross‐polarized light (photo by M. Shokr, unpublished).
(For color detail, please see color plate section).
of different orientations,
c
‐axis directions, and sizes.
Figure 4.22 is an example of the R‐type young ice, as
revealed in a vertical section of the top 0.12 m of young
sea ice from the Labrador Sea. The figure shows agglom-
eration of ice crystals of different sizes and orientations
and signifies the impact of the highly turbulent condi-
tions that prevail during the ice formation and growth.
4.3.3.7. Ice of Land‐Based Origin
Land‐based ice originates from glaciers and ice caps
that form as a result of snow deposition, compaction, and
melting and freezing over hundreds of thousands of years.
Although ice shelves may go through all the morphologi-
cal changes mentioned above, the bottom of the shelves
have sea ice components. In the ocean, land‐based ice is
found in the form of icebergs and bergybits. Ice islands
come from ice shelves and strictly speaking are not glacier
based, but they are considered as part of land‐based float-
ing ice according to the terms and descriptions of
MANICE
[2005], as described in section 2.6.5.
An important feature of land‐based ice is the presence
of cracks and crevices. They are often filled with meltwa-
ter or rain that eventually solidifies, but this process leaves
behind different families of bubbles that vary in shape
and size. An example of microstructural analysis of gla-
ciers ice is presented in Figure 4.23. In this case, the thin
sections were prepared from an ice block sampled from
the Morris Jessup Glacier, Greenland, on 5 July, 1982
4.3.3.6. Agglomerate Ice with Discontinuous
Columnar‐Grained (R Type Ice)
Growth history affects the structure and texture of ice
in a profound manner. In oceans, the tidal currents, wave
actions, rafting, pressure ridging producing rubbles, and
the subice water conditions including the mobility of fra-
zil slush are some of the factors that change the habit of
growing crystals in sea ice. The agglomerate ice, R type,
is probably the most common characteristics of sea ice in
highly dynamic ice regimes. It features a mix of ice types
