Geology Reference
In-Depth Information
4
Polycrystalline Ice Structure
Sea ice floating in the oceans varies significantly in
both space and time depending on the history of oceanic
and environmental conditions. During the early stages,
the sea state, including the wind, water current and water
salinity, air temperature, and deposition of snow, affects
the nucleation of ice crystals at the surface and subse-
quent subsurface growth. Then come the periods of con-
solidation and further growth, influenced by a number
of complex processes, during the entire first winter. This
is followed by the decay during the summer. The cycle of
growth and decay continues year after year. Thickness
of ice is one of the most important factors that can be
quantified relatively easily. Many types of sea ice, there-
fore, have been broadly defined on the basis of thickness
and quality of consolidation and age, without much con-
sideration to microstructural details. The broad aspects
of ice classes and regimes are presented in section 2.6.
Microstructural details, however, determine the physical
properties of sea ice including strength and deformation
characteristics, which is of direct relevance to marine ves-
sels (because mechanical strength increases as the salinity
and porosity of sea ice decreases).
In most ice engineering and geophysics problems, efforts
for describing the ice regime are confined within the
apparent features of the state of the ice involved. For ice
engineering applications, no effort is generally made in
performing any forensic‐type crystalline microstructural
investigations for understanding the micromechanics of
rate‐dependent failure processes in ice particularly under
compression. It should be admitted that a lot of the time
invested in studying a large number of apparently unrelated
facts on ice and ice‐related problems is often underused or
wasted. For ice geophysical applications, on the other hand,
the main interest in studying and monitoring large-scale
phenomena such as age‐based ice types, thickness, and
drift, especially in relation to the current issues of the
impact of global warming on polar ice, has deemphasized
the support for the ice microstructural studies. It finds use
only in modeling microstructure‐property relationships
related to constitutive equations and the emission and
scattering measured by remote sensing instruments.
Information on natural types of polycrystalline ice at
grain scale, let alone lattice or molecular scale, is easily
forgotten. Moreover, it is not commonly recognized that
yesterday's methods of gathering facts on ice in nature or
ice involved in engineering problems are not necessarily
those of today or tomorrow.
Nevertheless, both experimental and theoretical
approaches to understand ice as a material are signifi-
cantly advanced. However, information is presented on
the belief that the fundamental physical properties of
ice are very complex and will not be of any interest to
the glaciologists dealing with glaciers or engineers deal-
ing with ice‐induced engineering problems. An objective
of this chapter is to discuss ways of finding the needed
interrelations and establish familiarity with major devel-
opments in our understanding of polycrystalline ice
structure. This is mainly an educational purpose. The
specific objectives are:
•To identify the composition and properties of natural ice
• To explain basic “metallurgical phenomena” related
to polycrystalline structure of ice found in nature
• To provide current knowledge on the processes of
formation (nucleation), growth, and decay of ice,
particularly sea ice, in nature
The chapter starts with some terms and definitions rel-
evant to polycrystalline structure of ice described in sec-
tion 4.1. followed by a brief introduction to the morphology
of ice in section 4.2. A microstructure‐based classification
of natural types of ice, originally developed in Canada at
Laval University, for freshwater lake and river ice has now
been formally adopted and extended to sea ice. This is
