Geology Reference
In-Depth Information
conducted by
Perey and Pounder
[1958]. They analyzed
thin sections of the ice in a universal stage polariscope
(see descriptions in section 6.3.1) and determined the
percentage of the area having crystals with polar angles
(defined as the angle between the
c
axis of the crystals
and the vertical plane) within the range 0-10°. They
observed that 68% of the ice area near the surface was
covered with grains with polar angles in that range. This,
however, dropped to only 13% at a depth of about
130 mm. It is obvious that they did see S1 type of ice at
the top, but the growth habit changed as the ice thick-
ened. The observations led one to believe that irrespective
of surface nucleation mechanism, ice would eventually
grow with
c
axis of the crystals tending to be in the
horizontal plane. This interpretation from the labora-
tory tests led
Pounder
[1965], when using his observations
on seasonal sea ice, to conclude that all the ice crystals at
depths below about 200 mm had an essentially horizontal
c
axis. In his topic, Pounder also made several important
remarks on natural sea ice: (1) The bulk of ice covers
formed under calm conditions consists of crystals with
horizontal
c
axis. (2) The
c
axis is randomly oriented in
the horizontal plane (i.e., S2 type). (3) The tidal current
and unusual situations like rafting and pressure ridging
occurred during growth affect the structure. He also
provided an explanation on how an ice crystal grows
more rapidly in the basal plane than along the
c
axis and
thereby proposed a mechanism for preferred orientation
of
c
axis in the horizontal plane. Schematically, he showed
the preferred growth of crystals with their
c
axes inclined
to the vertical plane leading to the extinction of the
crystals with vertically oriented
c
axis. This mechanism
is now widely accepted.
Since the grains in S1‐type ice are very large, it is actu-
ally difficult to get statistically meaningful results from
laboratory studies using relatively small freezing tanks
as reported by
Perey and Pounder
[1958]. The heat flow
in small tanks may not be unidirectional as assumed.
Therefore, their inferences on the bulk of S1 ice in lakes
were not correct. In natural freshwater lakes and rivers
the crystals in S1 ice do maintain a continuity of verti-
cal
c
axis through the usually observed depths up to
about 1 m. However, one clarification should be made
here. If a natural ice cover is found to be clear, it is cor-
rect to assume that the growth occurred under still con-
ditions, but that does not necessarily mean that the
nucleation at the surface also occurred under clear
atmospheric conditions and that the ice is of S1 type.
Clear ice could also be S2 type. A light snowfall under
calm conditions just before the start of freezing in lake
waters can trigger nucleation of ice crystals at the sur-
face and this could lead to S2‐type ice. The type of clear
ice should never be assumed without conducting a
microstructural examination.
Michel and Ramseier
[1971] performed an extensive
survey of the ice conditions in St. Lawrence Ship Channel
for the Department of Transport, Canada. This shipping
channel handles all the overseas ships destined for
Montreal and the Great Lakes between Canada and the
United States. They noticed that S1 type of ice was a
common occurrence in the ship channel. This type can,
therefore, develop in rivers as well as in lakes, although it
is commonly seen in lakes [
Ashton,
1986]. Unfortunately,
clear lake ice is often assumed to be S1 type. Conversely,
often S1 ice is wrongly called Lake ice. The novice should
remember that S1 ice and Lake ice are not synonymous,
by any means, and should never be used as such.
4.3.3.4. Columnar‐Grained with c‐Axis Horizontal
and Random (S2) Ice
One of the most common type of river and lake ice cov-
ers features vertically oriented columnar‐grained material
with
c
axis of the grains in the horizontal plane (i.e., par-
allel to the ice surface) but randomly oriented within this
plane. This is classified as S2 type, and a schematic dia-
gram of this type of freshwater ice is shown in Figure 4.14.
The growth of S2 ice occurs after the nucleation of
crystals at the water surface by falling snow under rela-
tively calm conditions without any turbulence in the water
mass and may extend across the entire water surface.
Figure 4.15a shows a schematic of the nucleation of ice
crystals at the water surface and how an ice cover may
develop after deposition of snow on the surface with sur-
face temperatures close to the freezing point. At the inter-
face of water and the bottom of snow deposit, new crystals
are nucleated and each of them starts to grow downward,
as shown in Figure 15a. Ice crystals tend to grow vertically
c
c
c
c
S2 type
Figure 4.14
A schematic diagram of vertically oriented colum-
nar‐grained S2 ice with the
c
axis randomly oriented in the
horizontal plane, parallel to ice sheet surface [
Sinha,
1989].
