Geology Reference
In-Depth Information
material at extremely high temperatures. Recrystallization
may
also be caused by melting (even localized melting) of
the original ice and refreezing of the meltwater, but this
may not be called polygonized ice.
Skeletal layer:
The bottom of the ice sheet near the
ice‐water interface (see schematic in Figure 2.24). In
sea ice, it features dendritic ice that varies in length
(up to about 10 mm) with considerable portion of brine
included between the dendritic arms (see section 6.3.3
for detailed analysis of skeletal layer of Arctic sea ice).
Skeletal layer in sea ice harbors a high concentration
of phytopkankton during the spring and at the end
of the ice growing season, and this is presented in
section 4.5.4.
Subgrains
: Under external forces, creep and other
deformation processes may occur and break up crystals
(grains) into smaller units, called subgrains. This is also
known as polygonization in metallurgy. Recovery pro-
cesses in metals after high‐temperature deformation
leads to polygonization. Subgrains are also developed in
complex alloys by subjecting the material to complex
metallurgical processes and heat treatments. In sea ice,
however, subgrains develop as a result of the formation
of dendritic growth processes in impure melt that causes
the brine to be rejected; thermal or mechanical stresses
between the dendrites play, perhaps, only minor roles.
Sublimation
: The process of solid‐to‐vapor transforma-
tions bypassing the liquid phase. Being a high‐temperature
material, sublimation plays a vital role in all aspects of
surface‐related phenomena in ice. Thermal etching tech-
nique (Section 6.4.3) for revealing finer aspects of sea ice
depends on this property of ice being very close to the
melting point.
Tilt boundaries
: Slight bending within a single crys-
tal, a grain, or a subgrain may produce small‐angled
boundaries nearly parallel to the
c
axis of the crystal.
These are called tilt boundaries and consist of rows of
lattice dislocations. Tilt boundaries in sea ice can be
revealed by selective sublimation and are seen as short
segments in the form of straight lines parallel to the
minor dimension (width) of the subgrains (as illustrated
in Figures 2.30 and 2.31). These boundaries may not be
readily seen under cross‐polarized light (see Figure 2.32)
but can be revealed by thermal and/or chemical etching.
360
80
L
K
320
40
VII
III
280
0
IV
VI
240
-40
V
lh
200
II
-80
VIII
160
-120
120
-160
IX
80
-200
024
6810 12 14
Pressure (k bar)
16
18 20 22 24
Figure 4.4
Phase diagram of ice; the solid and long‐dashed
lines are directly measured stable and metastable lines, respec-
tively, whereas the dotted and short‐dashed lines represent
extrapolated or estimated stable and metastable lines, respec-
tively (from E. Whally of NRC, Ottawa, Canada. See also
Whalley
[1969]).
crystalline structures. The strangest aspect of water is the
fact that it develops more solid forms of different crystal-
line structures than any other known materials. Figure 4.4
is the phase diagram of solid phases of pure water. It
shows preferred phases of ice that can be formed under
different temperatures and pressures. Within each phase,
ice is uniform with respect to its chemical composition
and physical state. When water freezes under normal
atmospheric pressure and air temperature (between 0 and
−80 °C) the molecules are arranged to form the hexago-
nal ice, Ih (Ι
h
), or ordinary ice. Naturally, Ih is the most
common terrestrial form of ice, including solidification
of water vapor to form snowflakes or when deposited on
surfaces (i.e., frost) under the same temperature range.
Other forms of ice are produced by the application
of high pressures and lower temperatures, which result in
denser packing of molecules than in Ιh. If water vapor is
deposited onto a surface at a temperature between about
−80 and −130 °C, then solidification takes place leading
to a cubical crystalline structure. This allotropic form is
known as cubic ice or simply Ic (not shown in the figure
but should be located within the Ih domain). This is a
close variant to Ih. If freezing of water vapor takes place
at temperatures below −130 °C, another allotropic form
of ice develops that has no long‐range crystallographic
order. This is known as amorphous ice. If the pressure, is
much higher than atmospheric pressure, then a number
of crystalline structures of ice can develop, depending
upon the pressure and temperature. These are called
4.2. morPhology of ice
4.2.1. Form of Ice Crystals
When pure water freezes, it exhibits a fascinating range
of solid phases, all of which are referred to as allotropic
forms of ice. In this respect water indeed is a peculiar liq-
uid [
Dorsey,
1968]. Under different freezing temperatures
and atmospheric pressures, ice is formed into different
