Geology Reference
In-Depth Information
The geometric parameters that characterize ice crystals
include: (1) perimeter, (2) major and minor axes, (3) area,
(4) shape factor (defined as 4
π
× Area/
P
2
, where
P
is the
perimeter, to indicate how far the crystal shape is from
circle), (5) compactness (defined as
P
2
/Area, another indi-
cator of the degree of circularity of a crystal), and
(6) Feret diameter (diameter of a fictitious circular object
that has the same area as the crystal; given by the square
root of 4 × Area/
P
). A few of those parameters may be
required to uniquely identify a few shapes.
The three objects shown in Figure 5.44 have the same
perimeter. The object of irregular boundary at the left
could have been drawn to contain the same area and perim-
eter as the circular object at the middle. This means that
these two parameters will not differentiate between the two
objects. For that purpose, it is necessary to resort to a third
independent variable such as the length of the major axis
of the object. When the perimeter is divided by the Feret
diameter, it provides a scale ranging from 1 (for a circle) to
infinity (for a line). Crystals with an irregular contour but
with a blunt shape will lie near the minimum ratio. Crystals
with an elongated shape and a linear contour such as the
needle shape at the right will lie at the other end of the scale.
From the analysis of nearly 1000 delineated crystals,
Barrette and Sinha
(1994/6) found a lack of textural varia-
bility (which describes boundary morphology) with a depth
between 0 and about 30 m as illustrated in Figure 5.45. The
variation of the mean Feret diameter and the average crys-
tal area with depth was also erratic. The
c
‐axis orientation
defined a single‐maxima parallel to the length of the core
(i.e., vertical). There was no correlation between the angle
of the major axis of the crystal and its
c
axis.
5.2.3. Multiyear Rubble Field of Arctic Ice Islands
As shown in the schematic diagram of Hobson's Choice
in Figure 5.38, the island consisted of three sections—
corrugated shelf‐ice, “older” pre‐1982 shelf‐fast MY rub-
ble field, and “newer” post‐1982 MY rubble produced by
pack ice. This is essentially a common feature of all the
ice islands in the Arctic ocean because a sea ice rubble
field develops adjacent to the freshly created fracture sur-
face of the shelf ice after any calving incident, and as the
fragmented section floats away another rubble field forms
on the other exposed surface. Both rubble fields grow and
age as the islands drift year after year, but naturally one is
always older than the other.
Medium‐scale indentation strength tests were per-
formed in long trenches prepared in the newer MY ice of
Hobson's Choice. Tests were carried out in 1989 [
Frederking
et al.
, 1990a, 1990b] and in 1990 [
Jordaan, et al.
, 1992].
Never in the history of sea ice research have such long
trenches, as shown in Figure 5.46, been prepared in old
rubble or ridged MY sea ice fields.
In both the test years, the 65 m long, 3 m deep, and
3 m wide trenches prepared for the medium‐scale inden-
tation tests showed that the ice was fully consolidated
and free from any large‐scale voids. The trench walls
illustrated in Figure 5.47 were deep blue in colour indi-
cating solid mass of ice without any large scale voids.
Figure 5.43
Delineation of crystals in a vertical thin section of
an ice core from the Ward Hunt Ice Shelf, Canada. The dia-
gram at the right shows the configuration of the grain assem-
blage [
Barrette and Sinha
, 1994/96].
Figure 5.44
Three objects with very different shapes shown here have the same perimeter; the morphology of an
object may be expressed with reference to the shape of a circle.
