Geology Reference
In-Depth Information
It was pointed out before that for fine‐grained materials
like snow and frazil ice, it is not convenient to use the con-
ventional polarized light and universal stage method to
examine the fabric of the ice. Etching and replication tech-
nique described in section 6.4.2 was found to be ideal for
frazil S5 ice. Its application for observing basal dislocations
is described in Sinha [1978b]. Micrographs of thermally
etched surface of young frazil ice are shown in Figures 4.30a
and 4.30b using an optical microscope. They show grain
boundaries in the horizontal and vertical sections. A micro-
graph of a replica of chemically etched and replicated sur-
face (vertical section) using scanning electron micrograph
is shown in Figure 4.30c. The horizontal orientation of the
c axis (< c >) of the vertically oriented frazil ice crystals are
revealed by tiny elongated dislocation etch pits as indicated
by the doubled‐headed arrows.
As the etching and replicating processes were carried
out at −10 °C, there was brine (liquid) not only in the
brine pockets primarily at the grain boundaries but also
smeared on the surface prepared by microtoming. It was
found, however, that brine on the surface interferes with
the etching processes, although not sufficiently to prevent
etching of grain boundaries, as may be seen in the fig-
ure.  Figure  4.30c also illustrates that the c axis of the
crystals tended to be normal to the long axis of the grains
and therefore parallel to the surface (horizontal plane) of
the ice cover. This is indicated by the long direction of the
elongated etch pits, corresponding to the intersection of
the basal dislocations with the surface under observation
[ Sinha, 1977b, 1978a]. The c axis of the grains, however,
was found to be randomly oriented in the horizontal
plane. Because of this, not all the grains in Figure 4.30c
showed elongated pits, which develop only if the surface
under etching coincides with one of the prismatic surfaces
of the crystals.
0.00
0.05
0.10
horizontal
sections
20 mm
0.15
0.20
0.25
0.30
0.35
Figure 4.29 Thin sections of young ice of S5 type viewed
under cross‐polarized light: ( left ) vertical section from top to
the bottom and ( right ) horizontal sections at depth of 50 mm
and 150 mm. At the bottom right is an insert of the frazil‐
columnar ice interface at about 0.27 m depth [ Sinha, 1986].
A striking observation in Figure 4.29 is the sharp inter-
face at about 0.27 m depth between the frazil ice and the
underneath columnar ice (shown clearly in the insert at
the bottom right of the figure). The vertically oriented
frazil crystals with their c axis in the horizontal plane (i.e.,
basal planes oriented in the vertical plane) served well as
the nucleating agents for new ice at the ice‐water interface
at the bottom of the frazil layer. This was indeed a classic
case of nucleation from seed crystals. The processes of
crystallographic selections were already made for the
columnar grains to develop. It is not a wonder, therefore,
that columnar‐grained ice grew immediately at the bottom
of the frazil layer. Structural details of this interface are
shown in Figure 4.30. Since the frazil ice was transversely
isotropic, it must have initiated S2‐type structure. However,
examinations of the bottom ice revealed S3 structure. This
indicated that the only frazil crystals that have a chance to
grow are those with their c axis favorably oriented along
the water current.
4.4.1.3. Young Sea Ice: Agglomeration or Various
Crystallographic Shapes (S5 Type)
Young ice in highly dynamic areas in southern latitudes
can be subjected to intensive turbulent oceanic condi-
tions during its formation as well as melting and refor-
mation. Growth processes are disturbed by mechanical
or meteorological interferences. Grain size varies signifi-
cantly in size and orientation with irregular boundaries.
This situation is often encountered in the Labrador Sea
and the Gulf of St. Lawrence, both on the eastern coast
of Canada. An example of thin ice (nearly 80 mm)
from the Labrador Sea on 10 March, 1994 is shown in
Figure  4.31a. That was the thinnest ice that could be
walked on safely but with great caution (see Figure 1.12
for an idea on the bearing capacity of sea ice covers). The
general view of the ice cover featured nilas and gray ice (as
identified in the operational ice chart by the Canadian Ice
Service of Environment Canada). The surface roughness
Search WWH ::




Custom Search