Geology Reference
In-Depth Information
2.2. Ice Growth
Understanding the processes of ice growth, particu-
larly during the early seasons of freezing and growth, is
important both from the climatic and remote sensing
viewpoints. Ice grows mostly thermodynamically due to
the colder temperature of the atmosphere with respect
to the temperature of the seawater. It can also grow, to a
much lesser extent, mechanically as a result of its mobility.
Mobility of ice sheets enhances the possibility of cracking
the sheets or breaking the edges of ice floes on collision
and piling up of the broken ice blocks.
The rate of thermodynamic growth of sea ice depends
mainly on three factors that can be measured: air tem-
perature, ice thickness, and snow cover. Other factors
include the solar radiation, wind conditions, and the den-
sity and albedo of snow. Some of these factors are often
difficult to quantify. The apparent or macroscopic and
crystallographic form of ice at any growth stage particu-
larly during the early growth is determined mainly by the
oceanic conditions, whether calm or turbulent. Wind and
ocean current are also active factors because they deter-
mine the mobility of floating ice. One of the most impor-
tant processes that occur during sea ice growth is the
brine rejection to the underlying seawater and the brine
entrapment within the ice mass. After the initial forma-
tion of ice discoid with its dendritic boundaries as
described earlier, the ice growth commences laterally as
the minute ice discoid grow sidewise or frazil crystals
herd horizontally. The growth will then proceed vertically
in the direction of maximum heat flow.
Frazil ice streaks
Ice floe
Frazil
Figure 2.7
Frazil ice blowing into bands and accumulated
next to an ice floe (adapted from the collection of Dr. Pablo
Clemente‐Colon, National Ice Center).
blowing into bands and accumulating against the edge of
an ice floe.
Due to the elongated shape, frazil needles may not cre-
ate enough buoyancy force to keep them floating and
can be carried by the turbulent currents to areas below
the water or ice surfaces. It is, therefore, not uncommon
for frazil crystals to exist as suspended elements in deeper
waters or solid masses below ice sheets. When frazil crys-
tals form a layer at the ice‐water interface at the bottom
of ice sheets, the crystalline growth habit of the ice cover,
such as columnar structure, is interrupted. However, as
will be seen in Chapter 4 dealing with microstructural
aspects of natural ice sheets, crystals of frazil ice may also
act as seeds for new growth of columnar grained ice.
During the early part of the growth season, frazil
crystals in fjords and channels can also be herded and
pushed by the wind toward the shores, which eventually
consolidate to form thick layers of vertically oriented
crystals. Microstructure and strength properties of this
type of frazil ice have been examined in detail by
Sinha
[1986], and a brief description of the structural aspects
of this type of frazil ice is given in section 4.3.3.2.
Before closing this section, it should be mentioned that
the initial ice cover can also develop from snow deposi-
tion on open‐water surface and eventual freezing of
water‐saturated snow. Relatively thin ice covers can also
be thickened by the solidification of flooded snow over-
laying the existing ice cover. This commonly occurs
when a snow cover becomes heavy enough to depress the
ice surface below its freeboard. The influx of seawater
through the permeable snow saturates the snow mass,
which subsequently freezes into what is known as granu-
lar or snow ice. Microstructural aspects of granular, snow
ice will be presented in section 4.3.3.1.
2.2.1. Lateral Ice Growth
The general characteristics of thin ice covers depend on
the state of the ocean surface, namely whether it is quies-
cent or turbulent. For relatively calm atmospheric and
oceanic conditions, ice discoids and frazil crystals or fra-
zil streaks that are formed during the onset of freezing
continue to grow sideway until they touch each other and
cover the entire surface area of the water. Under a quies-
cent water surface, frazil crystals can also be herded to
form streaks of ice. The ice particles eventually consoli-
date if cold temperature persists and the water surface
continues to be calm, leading to the formation of nila.
This is a continuous but flexible sheet and translucent in
the beginning. Initially, it has a dark appearance but it
takes on a gray appearance as the sheet continues to
thicken (Figure 2.8). Nilas may grow up to the thickness
range of 10-20 mm if the water surface remains calm, but
this rarely happens in open seas. In nature, even under rela-
tively calm weather conditions, there could be oceanic cur-
rents and low‐amplitude waves generated by convections
or very light breeze. Nilas are usually broken into large
