Geology Reference
In-Depth Information
Vertical
orientation
S1 type
Random
orientation
T1 type
T1 type
superimposed on
S1 type
Horizontal
orientation
S2 type
Aligned horizontal
orientation
S3 type
Figure 4.24 Fabric diagrams of a few common types of natural polycrystalline ice [ Sinha , 1991b].
N
N
N
W
E W
E
W
E
S
S
S
Figure 4.25 Fabric diagrams exhibiting c ‐axis orientations at depths of 0.35, 0.63, and 0.83 m (left to right) in a
FY ice core sampled in January 1978 from Eclipse Sound, Baffin Island, Canada. The two‐headed arrow indicates
the mean and the “bow tie” indicates the standard deviation [ Nakawo and Sinha, 1984].
optically orient a crystal in a thin section, has also been
incorporated in the design of a large‐diameter polari-
scope developed at the National Research Council (NRC)
of Canada to be described in section 6.3.1. This polari-
scope is capable of turning the entire stage (360°) with
respect to other components of the system. It is also
capable of handling thin sections up to a maximum diam-
eter of 300 mm as will be seen later in Figure 6.10.
Figure 4.24 shows graphical presentations of some ori-
entation fabrics of ice found in nature. Each segment in
the figure represents a horizontal circular cross section
of ice in the place of the ice surface. Each pair of dots
inside the circles represents the two ends of c axis pro-
jected on the surface. (Strictly speaking the dots should
be drawn as open or closed to represent the two ends.) If
the dots are spread uniformly over the circle, it means
that the projections of the c axes can have any length and
may run at any direction (randomly oriented c axis). This
is typically observed in isotropic, equiaxed granular T1
type of ice. If the dots are concentrated in a small area
at the center, it means that the projections of the c axes on
the surface are very short, implying that the optic axes
are almost vertically oriented—the main characteristic
of S1 ice. If the dots are located near the peripheral of
the circle, but in all directions, it means that the c axes
are randomly oriented in the horizontal plane, and this
characterises the S2 type of ice. S3 ice is represented by
the dots concentrated in two opposite areas close to the
peripheral.
Nakawo and Sinha [1984] examined the fabric diagram
of 15 cores of FY ice sampled from Eclipse Sound, Baffin
Island, Canadian Arctic, in January 1978. The samples
were recovered when the ambient air temperatures were
less than −20 °C. The orientation of the crystallographic
optic axis of crystals was measured in thin sections of the
cores using a universal stage. Figure 4.25 shows the distri-
bution of the c ‐axis orientation at three depth levels, plot-
ted on a Schmidt equal‐area net through the lower
hemisphere. It may be seen that the c axes of the crystals
are almost all in the horizontal plane but the plunges are
small. Moreover, they seem to concentrate approximately
in the NE‐SW direction. Neglecting the slight deviation
in plunge from the horizontal, i.e., assuming that the c
axes are in the horizontal plane, the mean and standard
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