Geology Reference
In-Depth Information
growing ice surface pushes the dissolved air to the growing
front. In columnar‐grained ice, the air is also pushed to
some extent to the longitudinal boundaries between the
grains. At a certain stage the air particles are numerous
enough to form bubbles, which are generally trapped within
the grain boundaries of the columnar grains. They are usu-
ally long because the grains are oriented with their long
axis in the vertical or growth direction. Air bubbles are
readily visible in directionally solidified (DS) columnar‐
grained freshwater ice as well as translucent glacier ice.
Figure 2.17 shows air inclusions in the top 0.28 m of Lake
Resolute in May 1993 (before onset of melting). Rounded
air bubbles of diameters ranging from fractions of a mil-
limeter to a few millimeters are visible along with elongated
bubbles of a few millimeters in length. The shape and dis-
tribution of air bubbles in freshwater ice depend on the
directional growth of the crystals and their type, namely
whether they are S1 or S2 crystal type (see definitions in
Table 4.1).
The S1 type of ice covers develop in freshwater bodies
under calm conditions without any snowing activities
before and during the start of the surface freezing.
Following the nucleations at the surface level, the crystals
in S1 type of ice grow rapidly in the horizontal plane until
there is no room for further growth. The size of the
crystals or the grains depends on the number of available
sources of nucleation at the surface and air temperature
adjacent to the surface of the water. During the processes
of the initial surface growth, the dissolved air is segre-
gated and pushed away from the nucleation point or
center of the grains toward the boundaries of the crys-
tals. Naturally, the air pockets develop at the boundaries
of the crystals, commonly described as “grain bounda-
ries.” The size of the air pockets depends on the size of
the gains and hence the rapidity of surface growth. Once
the surface is covered fully, the crystals have no choice,
but to grow in the vertical direction like columns. As the
columns grow downward, the dissolved air is continu-
ously pushed toward the long vertically oriented grain
boundaries. This growth conditions leads to relatively
clear (almost transparent) ice with vertically oriented
long air bubbles.
Ice of glacier origin (see section 2.6.5) is also rich in
bubbles. Cracks and crevices are also common phe-
nomena in glaciers. They are often filled with meltwa-
ter or rain that eventually solidifies, leaving behind
large gaps of air bubbles. The cracks also heal with
time due primarily to the high thermal state of ice lead-
ing to sublimation and solidification in the narrow
spaces inside the cracks. This leads to formation of
rows of bubbles, another mechanism that contributes
to the bubble‐rich medium of glacier ice. Examples of
such cracks in iceberg ice in the Labrador Sea off
Newfoundland can be seen in
Barrette and Jordaan
[2002]. Healing of different cracks lead to different
families of bubbles, and they are usually visible even by
the naked eye when exposed surfaces of floating ice-
bergs are examined. Close examinations of ice sampled
from glaciers and/or icebergs reveal the details of the
families of healed cracks. A method for examining
detailed microstructure and texture of glacier ice, using
solid‐state thin sectioning combined with observations
by cross‐polarized, parallel‐polarized, and scattered
light techniques are described in section 4.3.3.7.
Figure 2.18a shows a photograph of a small ice island,
with exposed fractured surface, spotted in Wellington
Channel near Resolute in the Canadian central Arctic in
May 1993. Note the huge freeboard above the surround-
ing FY sea ice. The layered structure of the shelf ice can
also be clearly seen in the exposed fractured surface. This
ice island was identified to be a part of one of the larger
islands calved from the Ward Hunt Ice Shelf (WHIS) at
the northern tip of Ellesmere Island, Canada. The lead-
ing edge of WHIS also fractured into several pieces in
1982 and the largest one, named Hobson's Choice, was
used as a floating scientific base for geophysical research
for many years, as will be seen in section 5.2. WHIS is still
the largest ice shelf in the Northern Hemisphere. More
than 50 years ago, it used to be a strip about 250 km long
Figure 2.17
Photograph of a 100 mm diameter core of S1
ice, with elongated air bubbles, from Lake Resolute, Nunavut,
Canada, obtained in May 1993 (photo by M. Shokr,
unpublished).
