Geology Reference
In-Depth Information
safety of structures that have to withstand the impacts of
moving ice in case of collision and for the design of ice
breakers and ice-strengthened ships. Naturally, aspect 2
is related to the age‐ and thickness‐based categories
introduced in section 2.6.1, while presenting criteria of
operationally used ice classification. Certain aspects of 2,
such as thickness, distribution and seasonal variations,
are drawing attention to the long-term issues of marine
environment and climate change.
In this respect, there are marked differences in the
characteristic of pack ice floating in the Arctic and that
surrounding the continent of Antarctica. The pack ice in
the Arctic Ocean, except for a few fragments of shelf ice,
could entirely be a mixture of sea ice of different ages.
Icebergs, a common sight in the North Atlantic Ocean,
Baffin Bay, and Labrador Sea, are not seen in the Arctic
Ocean. On the other hand, beyond the shore‐fast ice in
the Antarctic, the pack ice could be a mixture of sea ice,
fragments broken off the shore‐fast ice, pieces of disinte-
grated land ice, including shelf ice, massive tabular, and
other icebergs, and bergybits. Moreover, the Antarctic sea
ice is mostly seasonal as opposed to perennial and usually
thinner than that in the Arctic and may not, therefore,
contain any second‐ or multiyear ice.
Ice in natural water bodies has a particular grain struc-
ture and texture that depends on the growth processes,
as well as thermal and mechanical histories. Inuit people of
the north use several dozen names for snow and ice. For
practical day‐to‐day life in Canada and in the northern
states of the United States, it was necessary to develop
names and descriptions for various type of ice that may be
encountered in lakes and rivers. Some of the general terms
are
clear ice
, which is self‐explanatory,
bubbly ice
for trans-
lucent ice,
snow ice
for white or milky opaque ice,
frazil ice
for needle‐shaped ice in fast flowing water, and
anchor ice
for ice at the bottom of shallow streams [
Pounder,
1965].
The first crystallographic structure‐ and texture‐based clas-
sification was introduced by
Michel and Ramseier
[1971].
They performed their pioneering studies on the microstruc-
ture‐properties relationship (specifically tensile and com-
pressive strengths) of lake ice and river ice for many years at
Laval University, Quebec, Canada. A detailed report on
classification systems for freshwater river and lake ice on
the basis of genesis, crystal structure, and texture was pub-
lished by
Michel and Ramseier
in 1971. Some of the detailed
analysis based on the physics and mechanics of ice that led
to this classification were described by
Ramseier
[1976] in
his doctoral thesis at Laval University.
It is essential to have complete records of the mete-
orological and hydrodynamical conditions at the time
of formation of the primary layer of ice and subsequent
growth if an understanding of the physical properties of
the growing ice sheet is to be developed. In reality, this
is not practical. The practical choice is to extract
information from the ice profiles and correlate that to
external events. This was the goal of the group at Laval
University. This group never worked on sea ice that
formed in the oceans and so the classification, entitled
Classification of River and Lake Ice, was never meant for
sea ice. However, the system of classifications they devel-
oped had universal appeal and provided the building
blocks for the classification of sea ice. The system is
adopted and used for sea ice microstructure‐ and texture‐
based classification in this topic. A summary of classes
and subclasses of polycrystalline freshwater ice is given
below in order to lay ground for further discussions
throughout the rest of this chapter.
4.3.1. Freshwater Ice Classification of Michel
and Ramseier
The characteristics of the constituent crystal structure
of an ice cover depend on the complexities of thermal
and mechanical conditions associated with nucleation
and growth. The most important fact that controls the
growth of ice after nucleation is that it grows more readily
in the basal plane, perpendicular to the
c
axis [
Hillig,
1958].
This particular behavior is responsible for the develop-
ment of a preferred crystallographic orientation of ice,
as proposed by
Perey and Pounder
[1958] from results of
their theoretical studies at McGill University, Montreal,
and observed by
Gold
[1960] in his laboratory studies at
the National Research Council (NRC), Ottawa. Some
details of the factors that control the growth of one grain
relative to its immediate neighbor have been studied by
Ketchum and Hobbs
[1967] and
Ramseier
[1968]. Naturally,
the classification takes into account the origin, structure,
and texture of ice crystals.
In comparison to the microstructure of lake and river
ice, the structure of sea ice is very complex, but the solid
part of even the most highly saline sea ice consists of pure
ice like the grains in freshwater ice. Moreover, the struc-
tural details of sea ice changes with time. In any case, just
like lake and river ice, sections of a sea ice cover can be
grouped into three major classes: primary ice (P), second-
ary ice (S), and superimposed ice (T). Each category of
ice can be classified into subgroups as shown in Table 4.1.
Primary ice is the first layer of ice or the horizontal skim
layer of an ice cover. In turbulent water it may consist of
frozen frazil slush, which can be very thick. If nucleation
occurs by snow, the resulting congealed snow slush would
also be part of the primary ice. Secondary ice forms under
the primary ice and grows parallel to the direction of heat
flow, which for floating ice is in the vertical plane. Its struc-
ture is mostly in the form of columnar ice, the texture of
which is entirely controlled by the primary ice. It can also
be in the form of frazil slush or snow slush, deposited under
a primary ice layer that after some time may become an
