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On the scale of 100 to
10-4 m, sea ice reveals a complex internal
structure. Crystals can assume a multitude of shapes,
orientations and sizes. Brine is included within the ice matrix
both between and within crystals. The amount of air and brine
inclusions, their orientation, as well as crystal morphology all
affect the bulk properties of sea ice. In Section A, we will
first take a look at the different textures that are found in sea ice
and introduce a scheme for classification of sea ice based on
variations in microstructure. In Section B, we will look at the
connection between microstructure and larger scale processes by
introducing another further classification scheme based on ice
formation mechanisms. In Section C, we concentrate on the
effect of porosity on sea ice strength and provide estimates of
uniaxial compressive strength for several ice types as defined in the
WMO nomenclature.
There are three primary crystal
textures: granular, columnar (which are established names) and blade
(which is a name I have chosen for the present discussion). The
crystal textures may occur on their own or as mixed
textures.
These textures are distinguished by the differences in crystal morphology.
Granular ice is composed of
crystals with equal dimensions. Grain size is usually less than
10-2m
(Eicken & Lange, 1989). Brine
inclusions exist as irregular pockets at grain boundaries.
Optical axes (or c-axes) are randomly oriented.
Columnar ice crystals are elongated in the vertical direction (hence, the columns). In the horizontal plane, there is a high degree of interlocking between grains. Grain size ranges from 10-2m to greater than 10-1m. Brine is included both within the crystals and along the grain boundaries. Within the columnar crystals, brine exists as uniform plate-like lamellae. Along the grain boundaries, brine exists in the form of irregular pockets.
Mixed texture can be a
combination of granular and columnar crystals or blade crystals in a
columnar or granuar matrix. Blade crystals are angular and
blade-like. Grain sizes range from under 10-2 to
10-1m. They occur within a matrix of columnar or
granular ice. Blade crystals are inclusion-free.
Commonly, such crystals have been termed platelets, forming platelet
ice. However, given the highly variable microsture of this ice
type, the term blade crystal has been employed here.
Microstructure:
Orbicular granular crystals are
equigranular and rounded. They usually have randomly oriented
c-axes. Brine and air are segregated at the grain
boundaries.
Microstructure:
Polygonal granular crystals are
equigranular with planar crystal contours. They usually have
randomly oriented c-axes. Brine and air inclusions are present
only at the grain boundaries.
Microstructure: Columnar ice crystals are elongated in the vertical direction. In the horizontal plane, there is a high degree of interlocking between grains.
All c-axes are oriented within a few degrees of the horizontal plane. It has been observed that c-axes can further be oriented within the horizontal plane, when a strong local oceanic current is present during the growth period (Weeks and Gow, 1980).
Brine is incorporated in the columnar
matrix, both within grains and along grain boundaries. Within
grains, brine is included in parallel lamellae structure. In
horizontal section, they look like parallel striations within the ice
crystal. Spacing between brine lamallae has been found to be
inversely proportional to the growth rate (Nakawo and Sinha,
1981). As the columns of ice crystals grow downwards brine
pockets can be trapped in between the crystal interface.
Microstructure: In the horizontal section, grain boundaries are irregular and interlocking. Vertical grain boundaries are planar and smooth. Horizonta grain boundaries are interlocking. Brine inclusions exist both within grains and along grain boundaries. Inclusions within grains can be regular lamallae or small droplets. C-axes can be randomly or horizontally oriented.
Microstructure: In the horizontal section, grain boundaries do not interlock. Brine inclusions exist as regular lamellae in columnar crystals. Brine pockets appear along the grain boundaries of both columnar and granular cyrstals. Depending on the formation mechanism, c-axes of columnar crystals may be aligned in the horizontal plane, tilted along a plane at angle to the horizontal, or there may be little alignment of the c-axes.
Microstructure: In the vertical and horizontal sections, angular, blade-like, inclusion free crystals are set in a matrix of columnar or granular ice. C-axes of columnar crystals are randomly oriented in the horizontal plane. C-axes of bladed crystals are randomly oriented.
To: Section B. Microstructure vs.
Formation mechanisms and Albedo
To: Section C. Parameterizing Uniaxial
Compressive Strength
To: Section D. References
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Sea-Ice Nomenclature Overview page
